is the scientific
study of life
It is a natural science
with a broad scope but has several unifying themes that tie it together as a single, coherent field.
For instance, all organisms
are made up of cells
that process hereditary information encoded in genes
, which can be transmitted to future generations. Another major theme is evolution
, which explains the unity and diversity of life.
Finally, all organisms require energy
to move, grow, and reproduce, as well as to regulate their own internal environment.
Biology deals with the study of life.
derives from the Ancient Greek
words of βίος romanized bíos
meaning 'life' and -λογία; romanized -logía
meaning 'branch of study' or 'to speak'.
Those combined make the Greek word βιολογία romanized biología
meaning 'biology'. Despite this, the term βιολογία as a whole didn't exist in Ancient Greek. The first to borrow it was the English and French (biologie
). Historically there was another term for biology
in English, lifelore
; it is rarely used today.
The Latin-language form of the term first appeared in 1736 when Swedish scientist Carl Linnaeus
(Carl von Linné
) used biologi
in his Bibliotheca Botanica
. It was used again in 1766 in a work entitled Philosophiae naturalis sive physicae: tomus III, continens geologian, biologian, phytologian generalis
, by Michael Christoph Hanov
, a disciple of Christian Wolff
. The first German use, Biologie
, was in a 1771 translation of Linnaeus' work. In 1797, Theodor Georg August Roose used the term in the preface of a book, Grundzüge der Lehre van der Lebenskraft
. Karl Friedrich Burdach
used the term in 1800 in a more restricted sense of the study of human beings from a morphological, physiological and psychological perspective (Propädeutik zum Studien der gesammten Heilkunst
). The term came into its modern usage with the six-volume treatise Biologie, oder Philosophie der lebenden Natur
(1802–22) by Gottfried Reinhold Treviranus
, who announced:
The objects of our research will be the different forms and manifestations of life, the conditions and laws under which these phenomena occur, and the causes through which they have been affected. The science
that concerns itself with these objects we will indicate by the name biology [Biologie
] or the doctrine of life [Lebenslehre
Scholars of the medieval Islamic world
who wrote on biology included al-Jahiz
(828–896), who wrote on botany,
(865–925) who wrote on anatomy
was especially well studied by Islamic scholars working in Greek philosopher traditions, while natural history drew heavily on Aristotelian thought, especially in upholding a fixed hierarchy of life.
Advances in microscopy
also had a profound impact on biological thinking. In the early 19th century, a number of biologists pointed to the central importance of the cell
. Then, in 1838, Schleiden
began promoting the now universal ideas that (1) the basic unit of organisms is the cell and (2) that individual cells have all the characteristics of life
, although they opposed the idea that (3) all cells come from the division of other cells. Thanks to the work of Robert Remak
and Rudolf Virchow
, however, by the 1860s most biologists accepted all three tenets of what came to be known as cell theory
Serious evolutionary thinking originated with the works of Jean-Baptiste Lamarck
, who was the first to present a coherent theory of evolution.
He posited that evolution was the result of environmental stress on properties of animals, meaning that the more frequently and rigorously an organ was used, the more complex and efficient it would become, thus adapting the animal to its environment. Lamarck believed that these acquired traits could then be passed on to the animal's offspring, who would further develop and perfect them.
However, it was the British naturalist Charles Darwin
, combining the biogeographical approach of Humboldt
, the uniformitarian geology of Lyell
writings on population growth, and his own morphological expertise and extensive natural observations, who forged a more successful evolutionary theory based on natural selection
; similar reasoning and evidence led Alfred Russel Wallace
to independently reach the same conclusions.
Darwin's theory of evolution by natural selection quickly spread through the scientific community and soon became a central axiom of the rapidly developing science of biology.
The basis for modern genetics began with the work of Gregor Mendel
, who presented his paper, "Versuche über Pflanzenhybriden
" ("Experiments on Plant Hybridization
"), in 1865,
which outlined the principles of biological inheritance, serving as the basis for modern genetics.
However, the significance of his work was not realized until the early 20th century when evolution became a unified theory as the modern synthesis
reconciled Darwinian evolution with classical genetics
In the 1940s and early 1950s, a series of experiments
by Alfred Hershey
and Martha Chase
pointed to DNA
as the component of chromosomes
that held the trait-carrying units that had become known as genes
. A focus on new kinds of model organisms such as viruses
, along with the discovery of the double-helical structure of DNA by James Watson
and Francis Crick
in 1953, marked the transition to the era of molecular genetics
. From the 1950s to the present times, biology has been vastly extended in the molecular
domain. The genetic code
was cracked by Har Gobind Khorana
, Robert W. Holley
and Marshall Warren Nirenberg
after DNA was understood to contain codons
. Finally, the Human Genome Project
was launched in 1990 with the goal of mapping the general human genome
. This project was essentially completed in 2003,
with further analysis still being published. The Human Genome Project was the first step in a globalized effort to incorporate accumulated knowledge of biology into a functional, molecular definition of the human body and the bodies of other organisms.
Atoms and molecules
Model of hydrogen bonds (1) between molecules of water
All organisms are made up of matter
and all matter is made up of elements
, and nitrogen
are the four elements that account for 96% of all organisms, with calcium
, and magnesium
accounting for the remaining 3.7%.
Different elements can combine to form compounds
such as water, which is fundamental to life.
Life on Earth began from water and remained there for about three billions years prior to migrating onto land.
Matter can exist in different states
as a solid
, or gas
Unlike ionic bonds, a covalent bond
involves the sharing of electron pairs
. These electron pairs and the stable balance of attractive and repulsive forces between atoms, when they share electrons
, is known as covalent bonding.
A hydrogen bond is primarily an electrostatic
force of attraction between a hydrogen
atom which is covalently bound to a more electronegative
atom or group such as oxygen. A ubiquitous example of a hydrogen bond is found between water
molecules. In a discrete water molecule, there are two hydrogen atoms and one oxygen atom. Two molecules of water
can form a hydrogen bond between them. When more molecules are present, as is the case with liquid water, more bonds are possible because the oxygen of one water molecule has two lone pairs of electrons, each of which can form a hydrogen bond with a hydrogen on another water molecule.
Organic compounds such as glucose
are vital to organisms
With the exception of water, nearly all the molecules that make up each organism contain carbon.
Carbon can form very long chains of interconnecting carbon–carbon bonds
, which are strong and stable. The simplest form of an organic molecule is the hydrocarbon
, which is a large family of organic compounds
that are composed of hydrogen
atoms bonded to a chain of carbon atoms. A hydrocarbon backbone can be substituted by other atoms. When combined with other elements such as oxygen
, and sulfur
, carbon can form many groups of important biological compounds such as sugars
, amino acids
, and nucleotides
The (a) primary, (b) secondary, (c) tertiary, and (d) quaternary structures of a hemoglobin
Molecules such as sugars, amino acids, and nucleotides can act as single repeating units called monomers
to form chain-like molecules called polymers
via a chemical process called condensation
For example, amino acids can form polypeptides
whereas nucleotides can form strands of deoxyribonucleic acid (DNA)
or ribonucleic acid (RNA)
. Polymers make up three of the four macromolecules
, and nucleic acids
) that are found in all organisms. Each macromolecule plays a specialized role within any given cell. Some polysaccharides, for instance, can function as storage material that can be hydrolyzed
to provide cells with sugar. Lipids are the only class of macromolecules that are not made up of polymers and the most biologically important lipids are fats
, and steroids
Proteins are the most diverse of the macromolecules, which include enzymes
, transport proteins
, large signaling
, and structural proteins
. Finally, nucleic acids store, transmit, and express hereditary information.
Every cell is enclosed within a cell membrane
that separates its cytoplasm
from the extracellular space
A cell membrane consists of a lipid bilayer
, including cholesterols
that sit between phospholipids
to maintain their fluidity
at various temperatures. Cell membranes are semipermeable
, allowing small molecules such as oxygen, carbon dioxide, and water to pass through while restricting the movement of larger molecules and charged particles such as ions
Cell membranes also contains membrane proteins
, including integral membrane proteins
that go across the membrane serving as membrane transporters
, and peripheral proteins
that loosely attach to the outer side of the cell membrane, acting as enzymes
shaping the cell.
Cell membranes are involved in various cellular processes such as cell adhesion
, storing electrical energy
, and cell signalling
and serve as the attachment surface for several extracellular structures such as a cell wall
, and cytoskeleton
Structure of a plant cell
Example of an enzyme-catalysed exothermic
All cells require energy
to sustain cellular processes. Energy is the capacity to do work
, which, in thermodynamics
, can be calculated using Gibbs free energy
. According to the first law of thermodynamics
, energy is conserved
, i.e., cannot be created or destroyed. Hence, chemical reactions
in a cell do not create new energy but are involved instead in the transformation and transfer of energy.
Nevertheless, all energy transfers lead to some loss of usable energy, which increases entropy
(or state of disorder) as stated by the second law of thermodynamics
. As a result, an organism requires continuous input of energy to maintain a low state of entropy. In cells, energy can be transferred as electrons during redox (reduction–oxidation)
reactions, stored in covalent bonds, and generated by the movement of ions (e.g., hydrogen, sodium, potassium) across a membrane.
is the set of life
-sustaining chemical reactions
. The three main purposes of metabolism are: the conversion of food to energy
to run cellular processes; the conversion of food/fuel to building blocks for proteins
, nucleic acids
, and some carbohydrates
; and the elimination of metabolic wastes
. These enzyme
-catalyzed reactions allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolic reactions may be categorized as catabolic
– the breaking down of compounds (for example, the breaking down of glucose to pyruvate by cellular respiration
); or anabolic
– the building up (synthesis
) of compounds (such as proteins, carbohydrates, lipids, and nucleic acids). Usually, catabolism releases energy, and anabolism consumes energy.
The chemical reactions of metabolism are organized into metabolic pathways
, in which one chemical is transformed through a series of steps into another chemical, each step being facilitated by a specific enzyme
. Enzymes are crucial to metabolism because they allow organisms to drive desirable reactions that require energy
that will not occur by themselves, by coupling
them to spontaneous reactions
that release energy. Enzymes act as catalysts
– they allow a reaction to proceed more rapidly without being consumed by it – by reducing the amount of activation energy
needed to convert reactants
. Enzymes also allow the regulation
of the rate of a metabolic reaction, for example in response to changes in the cell's
environment or to signals
from other cells.
is a set of metabolic
reactions and processes that take place in the cells
to convert chemical energy
into adenosine triphosphate
(ATP), and then release waste products.
The reactions involved in respiration are catabolic reactions
, which break large molecules into smaller ones, releasing energy because weak high-energy bonds, in particular in molecular oxygen
are replaced by stronger bonds in the products. Respiration is one of the key ways a cell releases chemical energy to fuel cellular activity. The overall reaction occurs in a series of biochemical steps, some of which are redox
reactions. Although cellular respiration is technically a combustion reaction
, it clearly does not resemble one when it occurs in a cell because of the slow, controlled release of energy from the series of reactions.
Sugar in the form of glucose
is the main nutrient used by animal and plant cells in respiration. Cellular respiration involving oxygen is called aerobic respiration, which has four stages: glycolysis
, citric acid cycle
(or Krebs cycle), electron transport chain
, and oxidative phosphorylation
Glycolysis is a metabolic process that occurs in the cytoplasm whereby glucose is converted into two pyruvates
, with two net molecules of ATP being produced at the same time.
Each pyruvate is then oxidized into acetyl-CoA
by the pyruvate dehydrogenase complex
, which also generates NADH and carbon dioxide. Acetyl-Coa enters the citric acid cycle, which takes places inside the mitochondrial matrix. At the end of the cycle, the total yield from 1 glucose (or 2 pyruvates) is 6 NADH, 2 FADH2
, and 2 ATP molecules. Finally, the next stage is oxidative phosphorylation, which in eukaryotes, occurs in the mitochondrial cristae
. Oxidative phosphorylation comprises the electron transport chain
, which is a series of four protein complexes
that transfer electrons from one complex to another, thereby releasing energy from NADH and FADH2
that is coupled to the pumping of protons (hydrogen ions) across the inner mitochondrial membrane (chemiosmosis
), which generates a proton motive force
Energy from the proton motive force drives the enzyme ATP synthase
to synthesize more ATPs by phosphorylating ADPs
. The transfer of electrons terminates with molecular oxygen being the final electron acceptor
If oxygen were not present, pyruvate would not be metabolized by cellular respiration but undergoes a process of fermentation
. The pyruvate is not transported into the mitochondrion but remains in the cytoplasm, where it is converted to waste products
that may be removed from the cell. This serves the purpose of oxidizing the electron carriers so that they can perform glycolysis again and removing the excess pyruvate. Fermentation oxidizes NADH to NAD+
so it can be re-used in glycolysis. In the absence of oxygen, fermentation prevents the buildup of NADH in the cytoplasm and provides NAD+
for glycolysis. This waste product varies depending on the organism. In skeletal muscles, the waste product is lactic acid
. This type of fermentation is called lactic acid fermentation
. In strenuous exercise, when energy demands exceed energy supply, the respiratory chain cannot process all of the hydrogen atoms joined by NADH. During anaerobic glycolysis, NAD+
regenerates when pairs of hydrogen combine with pyruvate to form lactate. Lactate formation is catalyzed by lactate dehydrogenase in a reversible reaction. Lactate can also be used as an indirect precursor for liver glycogen. During recovery, when oxygen becomes available, NAD+
attaches to hydrogen from lactate to form ATP. In yeast, the waste products are ethanol
and carbon dioxide
. This type of fermentation is known as alcoholic or ethanol fermentation
. The ATP generated in this process is made by substrate-level phosphorylation
, which does not require oxygen.
Photosynthesis changes sunlight into chemical energy, splits water to liberate O2, and fixes CO2 into sugar.
Photosynthesis has four stages: Light absorption
, electron transport, ATP synthesis, and carbon fixation
Light absorption is the initial step of photosynthesis whereby light energy is absorbed by chlorophyll
pigments attached to proteins in the thylakoid membranes
. The absorbed light energy is used to remove electrons from a donor (water) to a primary electron acceptor, a quinone
designated as Q. In the second stage, electrons move from the quinone primary electron acceptor through a series of electron carriers until they reach a final electron acceptor, which is usually the oxidized form of NADP+
, which is reduced
to NADPH, a process that takes place in a protein complex called photosystem I
(PSI). The transport of electrons is coupled to the movement of protons (or hydrogen) from the stroma to the thylakoid membrane, which forms a pH gradient across the membrane as hydrogen becomes more concentrated in the lumen than in the stroma. This is analogous to the proton-motive force generated across the inner mitochondrial membrane in aerobic respiration.
During the third stage of photosynthesis, the movement of protons down their concentration gradients
from the thylakoid lumen to the stroma through the ATP synthase is coupled to the synthesis of ATP by that same ATP synthase.
The NADPH and ATPs generated by the light-dependent reactions
in the second and third stages, respectively, provide the energy and electrons to drive the synthesis of glucose by fixing atmospheric carbon dioxide into existing organic carbon compounds, such as ribulose bisphosphate
(RuBP) in a sequence of light-independent (or dark) reactions called the Calvin cycle
Cell communication (or signaling
) is the ability of cells
to receive, process, and transmit signals with its environment and with itself.
Signals can be non-chemical such as light, electrical impulses
, and heat, or chemical signals (or ligands
) that interact with receptors
, which can be found embedded
in the cell membrane
of another cell or located deep inside
There are generally four types of chemical signals: autocrine
, and hormones
In autocrine signaling, the ligand affects the same cell that releases it. Tumor
cells, for example, can reproduce uncontrollably because they release signals that initiate their own self-division. In paracrine signaling, the ligand diffuses to nearby cells and affect them. For example, brain cells called neurons
release ligands called neurotransmitters
that diffuse across a synaptic cleft
to bind with a receptor on an adjacent cell such as another neuron or muscle cell
. In juxtacrine signaling, there is direct contact between the signaling and responding cells. Finally, hormones are ligands that travel through the circulatory systems
of animals or vascular systems
of plants to reach their target cells. Once a ligand binds with a receptor, it can influence the behavior of another cell, depending on the type of receptor. For instance, neurotransmitters that bind with an inotropic receptor
can alter the excitability
of a target cell. Other types of receptors include protein kinase
receptors (e.g., receptor
for the hormone insulin
) and G protein-coupled receptors
. Activation of G protein-coupled receptors can initiate second messenger
cascades. The process by which a chemical or physical signal is transmitted through a cell as a series of molecular events
is called signal transduction
In meiosis, the chromosomes duplicate and the homologous chromosomes
exchange genetic information during meiosis I. The daughter cells divide again in meiosis II to form haploid gametes
The cell cycle
is a series of events that take place in a cell
that cause it to divide into two daughter cells. These events include the duplication of its DNA
and some of its organelles
, and the subsequent partitioning of its cytoplasm into two daughter cells in a process called cell division
, and protist
cells), there are two distinct types of cell division: mitosis
Mitosis is part of the cell cycle, in which replicated chromosomes
are separated into two new nuclei. Cell division gives rise to genetically identical cells in which the total number of chromosomes is maintained. In general, mitosis (division of the nucleus) is preceded by the S stage of interphase
(during which the DNA is replicated) and is often followed by telophase
; which divides the cytoplasm
and cell membrane
of one cell into two new cells
containing roughly equal shares of these cellular components. The different stages of mitosis all together define the mitotic phase of an animal cell cycle—the division of the mother cell into two genetically identical daughter cells.
The cell cycle is a vital process by which a single-celled fertilized egg
develops into a mature organism, as well as the process by which hair
, blood cells
, and some internal organs
are renewed. After cell division, each of the daughter cells begin the interphase
of a new cycle. In contrast to mitosis, meiosis results in four haploid daughter cells by undergoing one round of DNA replication followed by two divisions. Homologous chromosomes
are separated in the first division (meiosis I
), and sister chromatids are separated in the second division (meiosis II
). Both of these cell division cycles are used in the process of sexual reproduction at some point in their life cycle. Both are believed to be present in the last eukaryotic common ancestor.
) can also undergo cell division (or binary fission
). Unlike the processes of mitosis
in eukaryotes, binary fission takes in prokaryotes takes place without the formation of a spindle apparatus
on the cell. Before binary fission, DNA in the bacterium is tightly coiled. After it has uncoiled and duplicated, it is pulled to the separate poles of the bacterium as it increases the size to prepare for splitting. Growth of a new cell wall begins to separate the bacterium (triggered by FtsZ
polymerization and "Z-ring" formation)
The new cell wall (septum
) fully develops, resulting in the complete split of the bacterium. The new daughter cells have tightly coiled DNA rods, ribosomes
, and plasmids
depicting a cross between two pea plants heterozygous for purple (B) and white (b) blossoms.
is the scientific study of inheritance. Mendelian inheritance
, specifically, is the process by which genes and traits are passed on from parents to offspring.
It was formulated by Gregor Mendel
, based on his work with pea plants in the mid-nineteenth century. Mendel established several principles of inheritance. The first is that genetic characteristics, which are now called alleles
, are discrete and have alternate forms (e.g., purple vs. white or tall vs. dwarf), each inherited from one of two parents. Based on his law of dominance and uniformity
, which states that some alleles are dominant
while others are recessive
; an organism with at least one dominant allele will display the phenotype
of that dominant allele.
Exceptions to this rule include penetrance
Mendel noted that during gamete formation, the alleles for each gene segregate from each other so that each gamete carries only one allele for each gene, which is stated by his law of segregation
individuals produce gametes with an equal frequency of two alleles. Finally, Mendel formulated the law of independent assortment
, which states that genes of different traits can segregate independently during the formation of gametes, i.e., genes are unlinked. An exception to this rule would include traits that are sex-linked
. Test crosses
can be performed to experimentally determine the underlying genotype
of an organism with a dominant
A Punnett square
can be used to predict the results of a test cross. The chromosome theory of inheritance
, which states that genes are found on chromosomes, was supported by Thomas Morgans
's experiments with fruit flies
, which established the sex linkage
between eye color and sex
in these insects.
In humans and other mammals (e.g., dogs), it is not feasible or practical to conduct test cross experiments. Instead, pedigrees
, which are genetic representations of family trees,
are used instead to trace the inheritance of a specific trait or disease through multiple generations.
Bases lie between two spiraling DNA strands.
is a unit of heredity
that corresponds to a region of DNA
that influences the form or function of an organism in specific ways. DNA is found as linear chromosomes
, and circular chromosomes in prokaryotes
. A chromosome is an organized structure consisting of DNA
. The set of chromosomes in a cell and any other hereditary information found in the mitochondria
, or other locations is collectively known as a cell's genome
. In eukaryotes, genomic DNA is localized in the cell nucleus
, or with small amounts in mitochondria
In prokaryotes, the DNA is held within an irregularly shaped body in the cytoplasm called the nucleoid
The genetic information in a genome is held within genes, and the complete assemblage of this information in an organism is called its genotype
Genes encode the information needed by cells for the synthesis of proteins, which in turn play a central role in influencing the final phenotype
of the organism.
is the process by which information from a gene is used in the synthesis of a functional gene product
that enables it to produce end products, protein or non-coding RNA
, and ultimately affect a phenotype
, as the final effect. The process is summarized in the central dogma of molecular biology
first formulated by Francis Crick
Gene expression is the most fundamental level at which a genotype
gives rise to a phenotype
, i.e., observable trait. The genetic information stored in DNA
represents the genotype, whereas the phenotype results from the synthesis of proteins that control an organism's structure and development, or that act as enzymes
catalyzing specific metabolic pathways. A large part of DNA (e.g., >98% in humans) is non-coding
, meaning that these sections do not serve as patterns for protein sequences
. Messenger RNA
(mRNA) strands are created using DNA strands as a template in a process called transcription
, where DNA bases are exchanged for their corresponding bases except in the case of thymine (T), for which RNA substitutes uracil
Under the genetic code
, these mRNA strands specify the sequence of amino acids
within proteins in a process called translation
, which occurs in ribosomes
. This process is used by all life—eukaryotes
(including multicellular organisms
), and utilized by viruses
—to generate the macromolecular
machinery for life. Gene products are often proteins
, but in non-protein-coding genes such as transfer RNA (tRNA)
and small nuclear RNA (snRNA)
, the product is a functional non-coding RNA
All steps in the gene expression process can be regulated, including the transcription
, RNA splicing
, and post-translational modification
of a protein. Regulation of gene expression
gives control over the timing, location, and amount of a given gene product (protein or ncRNA) present in a cell and can have a profound effect on cellular structure and function.
is an organism's complete set of DNA
, including all of its genes.
Sequencing and analysis of genomes can be done using high throughput DNA sequencing
to assemble and analyze the function and structure of entire genomes.
Many genes encode more than one protein, with posttranslational modifications
increasing the diversity of proteins within a cell. A cell's proteome
is its entire set of proteins expressed by its genome.
The genomes of prokaryotes are small, compact, and diverse. In contrast, the genomes of eukaryotes are larger and more complex such as having more regulatory sequences
and much of its genome are made up of non-coding DNA sequences for functional RNA (rRNA
, and mRNA
) or regulatory sequences. The genomes of various model organisms
such as arabidopsis
, fruit fly
, mice, nematodes
, and yeast
have been sequenced. The sequencing of the entire human genome
has yielded practical applications such as DNA fingerprinting
, which can be used for paternity testing
. In medicine
, sequencing of the entire human genome has allowed for the identification mutations
that cause tumors
as well as genes that cause a specific genetic disorder
Construction of recombinant DNA, in which a foreign DNA fragment is inserted into a plasmid vector
is the use of cells or organisms to develop products for humans.
It includes tools such as recombinant DNA
, which are DNA molecules formed by laboratory methods of genetic recombination
such as molecular cloning
, which bring together genetic material from multiple sources, creating sequences
that would otherwise not be found in a genome
. Other tools include the use of genomic libraries
, DNA microarrays
, expression vectors
, synthetic genomics
, and CRISPR gene editing
Many of these tools have wide applications such as creating medically useful proteins, or improving plant cultivation
and animal husbandry
, for example, was the first medicine to be made using recombinant DNA technology. Other approaches such as pharming
can produce large quantities of medically useful products through the use of genetically modified organisms
Genes, development, and evolution Development
is the process by which a multicellular organism
) goes through a series of a changes, starting from a single cell, and taking on various forms that are characteristic of its life cycle.
There are four key processes that underlie development: Determination
, and growth. Determination sets the developmental fate of a cell, which becomes more restrictive during development. Differentiation is the process by which specialized cells from less specialized cells such as stem cells
Stem cells are undifferentiated
or partially differentiated cells
that can differentiate into various types of cells
indefinitely to produce more of the same stem cell.
Cellular differentiation dramatically changes a cell's size, shape, membrane potential
, metabolic activity
, and responsiveness to signals, which are largely due to highly controlled modifications in gene expression
. With a few exceptions, cellular differentiation almost never involves a change in the DNA
Thus, different cells can have very different physical characteristics despite having the same genome
. Morphogenesis, or development of body form, is the result of spatial differences in gene expression.
Specially, the organization of differentiated tissues into specific structures such as arms or wings, which is known as pattern formation
, is governed by morphogens
, signaling molecules that move from one group of cells to surrounding cells, creating a morphogen gradient as described by the French flag model
, or programmed cell death, also occurs during morphogenesis, such as the death of cells between digits in human embryonic development, which frees up individual fingers and toes. Expression of transcription factor
genes can determine organ placement in a plant and a cascade of transcription factors themselves can establish body segmentation in a fruit fly.
A small fraction of the genes in an organism's genome called the developmental-genetic toolkit
control the development of that organism. These toolkit genes are highly conserved among phyla
, meaning that they are ancient and very similar in widely separated groups of animals. Differences in deployment of toolkit genes affect the body plan and the number, identity, and pattern of body parts. Among the most important toolkit genes are the Hox genes
. Hox genes determine where repeating parts, such as the many vertebrae
, will grow in a developing embryo or larva.
Variations in the toolkit may have produced a large part of the morphological evolution of animals. The toolkit can drive evolution in two ways. A toolkit gene can be expressed in a different pattern, as when the beak of Darwin's large ground-finch
was enlarged by the BMP
or when snakes lost their legs as Distal-less (Dlx)
genes became under-expressed or not expressed at all in the places where other reptiles continued to form their limbs.
Or, a toolkit gene can acquire a new function, as seen in the many functions of that same gene, distal-less
, which controls such diverse structures as the mandible in vertebrates,
legs and antennae in the fruit fly,
and eyespot pattern
in butterfly wings
Given that small changes in toolbox genes can cause significant changes in body structures, they have often enabled convergent
or parallel evolution
A central organizing concept in biology is that life changes and develops through evolution
, which is the change in heritable characteristics
over successive generations
Evolution is now used to explain the great variations of life on Earth. The term evolution
was introduced into the scientific lexicon by Jean-Baptiste de Lamarck
and fifty years later Charles Darwin
and Alfred Russel Wallace
formulated the theory of evolution by natural selection.
According to this theory, individuals differ from each other with respect to their heritable traits, resulting in different rates of survival and reproduction. As a results, traits that are better adapted to their environment are more likely to be passed on to subsequent generations.
Darwin was not aware of Mendel's work of inheritance and so the exact mechanism of inheritance that underlie natural selection was not well-understood
until the early 20th century when the modern synthesis
reconciled Darwinian evolution
with classical genetics
, which established a neo-Darwinian
perspective of evolution by natural selection.
This perspective holds that evolution occurs when there are changes in the allele frequencies
within a population of interbreeding organisms. In the absence of any evolutionary process acting on a large random mating population, the allele frequencies will remain constant across generations as described by the Hardy–Weinberg principle
Another process that drives evolution is genetic drift
, which is the random fluctuations of allele frequencies within a population from one generation to the next.
When selective forces are absent or relatively weak, allele frequencies are equally likely to drift
upward or downward at each successive generation because the alleles are subject to sampling error
This drift halts when an allele eventually becomes fixed, either by disappearing from the population or replacing the other alleles entirely. Genetic drift may therefore eliminate some alleles from a population due to chance alone.
A phylogeny is an evolutionary history of a specific group of organisms or their genes.
A phylogeny can be represented using a phylogenetic tree
, which is a diagram showing lines of descent among organisms or their genes. Each line drawn on the time axis of a tree represents a lineage
of descendents of a particular species or population. When a lineage divides into two, it is represented as a node (or split) on the phylogenetic tree. The more splits there are over time, the more branches there will be on the tree, with the common ancestor of all the organisms in that tree being represented by the root of that tree. Phylogenetic trees may portray the evolutionary history of all life forms, a major evolutionary group (e.g., insects
), or an even smaller group of closely related species
. Within a tree, any group of species designated by a name is a taxon
(e.g., humans, primates, mammals, or vertebrates) and a taxon that consists of all its evolutionary descendants is a clade
, otherwise known as a monophyletic
Closely related species are referred to as sister species
and closely related clades are sister clades. In contrast to a monophyletic group, a polyphyletic
group does not include its common ancestor whereas a paraphyletic
group does not include all the descendants of a common ancestor.
Phylogenetic trees are the basis for comparing and grouping different species.
Different species that share a feature inherited from a common ancestor are described as having homologous
features. Homologous features may be any heritable traits
such as DNA sequence
, protein structures, anatomical features, and behavior patterns. A vertebral column
is an example of a homologous feature shared by all vertebrate animals. Traits that have a similar form or function but were not derived from a common ancestor are described as analogous features
. Phylogenies can be reconstructed for a group of organisms of primary interests, which are called the ingroup. A species or group that is closely related to the ingroup but is phylogenetically outside of it is called the outgroup
, which serves a reference point in the tree. The root of the tree is located between the ingroup and the outgroup.
When phylogenetic trees are reconstructed, multiple trees with different evolutionary histories can be generated. Based on the principle of Parsimony (or Occam's razor)
, the tree that is favored is the one with the fewest evolutionary changes needed to be assumed over all traits in all groups. Computational algorithms
can be used to determine how a tree might have evolved given the evidence.
Phylogeny provides the basis of biological classification, which is based on Linnaean taxonomy
that was developed by Carl Linnaeus
in the 18th century.
This classification system is rank-based, with the highest rank being the domain
followed by kingdom
, and species
All organisms can be classified as belonging to one of three domains
(originally Archaebacteria); bacteria
(originally eubacteria), or eukarya
(includes the protist
, and animal
A binomial nomenclature
is used to classify different species. Based on this system, each species is given two names, one for its genus and another for its species.
For example, humans are Homo sapiens
, with Homo
being the genus and sapiens
being the species. By convention, the scientific names of organisms are italicized, with only the first letter of the genus capitalized.
History of life
of coexisting bacteria and archaea were the dominant form of life in the early Archean
Epoch and many of the major steps in early evolution are thought to have taken place in this environment.
The earliest evidence of eukaryotes
dates from 1.85 billion years ago,
and while they may have been present earlier, their diversification accelerated when they started using oxygen in their metabolism
. Later, around 1.7 billion years ago, multicellular organisms
began to appear, with differentiated cells
performing specialised functions.
appear during the Ediacaran
, along with most other modern phyla
originated about 525 million years ago during the Cambrian explosion
During the Permian
, including the ancestors of mammals
, dominated the land,
but most of this group became extinct in the Permian–Triassic extinction event
252 million years ago.
During the recovery from this catastrophe, archosaurs
became the most abundant land vertebrates;
one archosaur group, the dinosaurs
, dominated the Jurassic
After the Cretaceous–Paleogene extinction event
66 million years ago killed off the non-avian dinosaurs,
mammals increased rapidly in size and diversity
Such mass extinctions
may have accelerated evolution by providing opportunities for new groups of organisms to diversify.
Bacteria and Archaea
constitute the other domain of prokaryotic cells and were initially classified
, receiving the name archaebacteria (in the Archaebacteria kingdom
), a term that has fallen out of use.
Archaeal cells have unique properties separating them from the other two domains
. Archaea are further divided into multiple recognized phyla
. Archaea and bacteria are generally similar in size and shape, although a few archaea have very different shapes, such as the flat and square cells of Haloquadratum walsbyi
Despite this morphological
similarity to bacteria, archaea possess genes
and several metabolic pathways
that are more closely related to those of eukaryotes, notably for the enzymes
involved in transcription
. Other aspects of archaeal biochemistry are unique, such as their reliance on ether lipids
in their cell membranes
. Archaea use more energy sources than eukaryotes: these range from organic compounds
, such as sugars, to ammonia
, metal ions
or even hydrogen gas
archaea (the Haloarchaea
) use sunlight as an energy source, and other species of archaea fix carbon
, but unlike plants and cyanobacteria
, no known species of archaea does both. Archaea reproduce asexually
by binary fission
, or budding
; unlike bacteria, no known species of Archaea form endospores
The first observed archaea were extremophiles
, living in extreme environments, such as hot springs
and salt lakes
with no other organisms. Improved molecular detection tools led to the discovery of archaea in almost every habitat
, including soil, oceans, and marshlands
. Archaea are particularly numerous in the oceans, and the archaea in plankton
may be one of the most abundant groups of organisms on the planet.
Archaea are a major part of Earth's life
. They are part of the microbiota
of all organisms. In the human microbiome
, they are important in the gut
, mouth, and on the skin.
Their morphological, metabolic, and geographical diversity permits them to play multiple ecological roles: carbon fixation; nitrogen cycling; organic compound turnover; and maintaining microbial symbiotic and syntrophic
communities, for example.
Diversity of protists
Diversity of plants
Seed plants (or spermatophyte
) comprise five divisions, four of which are grouped as gymnosperms
and one is angiosperms. Gymnosperms includes conifers
, and gnetophytes
. Gymnosperm seeds develop either on the surface of scales or leaves, which are often modified to form cones
, or solitary as in yew
Angiosperms are the most diverse group of land plants
, with 64 orders
, 416 families
, approximately 13,000 known genera
and 300,000 known species
, angiosperms are seed-producing plants
. They are distinguished from gymnosperms by having characteristics such as flowers
within their seeds
, and production of fruits
that contain the seeds.
organisms that include microorganisms such as yeasts
, as well as the more familiar mushrooms
. A characteristic that places fungi in a different kingdom from plants, bacteria, and some protists is chitin
in their cell walls
. Fungi, like animals, are heterotrophs
; they acquire their food by absorbing dissolved molecules, typically by secreting digestive enzymes
into their environment. Fungi do not photosynthesize
. Growth is their means of mobility
, except for spores
(a few of which are flagellated
), which may travel through the air or water. Fungi are the principal decomposers
in ecological systems. These and other differences place fungi in a single group of related organisms, named the Eumycota
), which share a common ancestor
(from a monophyletic group
). This fungal group is distinct from the structurally similar myxomycetes
(slime molds) and oomycetes
Most fungi are inconspicuous because of the small size of their structures, and their cryptic
lifestyles in soil or on dead matter. Fungi include symbionts
of plants, animals, or other fungi and also parasites
. They may become noticeable when fruiting
, either as mushrooms or as molds. Fungi perform an essential role in the decomposition of organic matter and have fundamental roles in nutrient cycling
and exchange in the environment.
The fungus kingdom encompasses an enormous diversity of taxa
with varied ecologies, life cycle
strategies, and morphologies
ranging from unicellular aquatic chytrids
to large mushrooms. However, little is known of the true biodiversity
of Kingdom Fungi, which has been estimated at 2.2 million to 3.8 million species.
Of these, only about 148,000 have been described,
with over 8,000 species known to be detrimental to plants and at least 300 that can be pathogenic to humans.
Diversity of animals. From top to bottom, first column: Echinoderm
, and arachnid
. Second column: Sponge
, and flatworm
. Third column: Cephalopod
, and phoronida
The majority (~97%) of animal species are invertebrates
which are animals
that neither possess nor develop a vertebral column
(commonly known as a backbone
), derived from the notochord
. This includes all animals
apart from the subphylum Vertebrata
. Familiar examples of invertebrates include arthropods
, and myriapods
, and octopuses
), and cnidarians
, sea anemones
, and corals
). Many invertebrate taxa
have a greater number and variety of species than the entire subphylum of Vertebrata.
In contrast, vertebrates
comprise all species of animals
within the subphylum
). Vertebrates represent the overwhelming majority of the phylum Chordata
, with currently about 69,963 species
Vertebrates include such groups as jawless fishes
, jawed vertebrates
such as cartilaginous fishes
, and ratfish), bony fishes
such as amphibians
When infected, a host cell is forced to rapidly produce thousands of identical copies of the original virus. When not inside an infected cell or in the process of infecting a cell, viruses exist in the form of independent particles, or virions
, consisting of the genetic material
), a protein
coat called capsid
, and in some cases an outside envelope
. The shapes of these virus particles range from simple helical
forms to more complex structures. Most virus species have virions too small to be seen with an optical microscope
, as they are one-hundredth the size of most bacteria.
Plant form and function
Root and shoot systems in a eudicot
The plant body is made up of organs
that can be organized into two major organ systems
: a root system
and a shoot system
The root system anchors the plants into place. The roots themselves absorb water and minerals and store photosynthetic products. The shoot system is composed of stem
, and flowers
. The stems hold and orient the leaves to the sun, which allow the leaves to conduct photosynthesis. The flowers are shoots
that have been modified for reproduction
. Shoots are composed of phytomers
, which are functional units
that consist of a node carrying one or more leaves, internode, and one or more buds
A plant body has two basic patterns (apical–basal and radial axes) that been established during embryogenesis
Cells and tissues are arranged along the apical-basal axis from root to shoot whereas the three tissue systems (dermal
, and vascular
) that make up a plant's body are arranged concentrically around its radial axis.
The dermal tissue system forms the epidermis
(or outer covering) of a plant, which is usually a single cell layer that consists of cells that have differentiated into three specialized structures: stomata
for gas exchange in leaves, trichomes
(or leaf hair) for protection against insects
and solar radiation
, and root hairs
for increased surface areas and absorption of water and nutrients. The ground tissue makes up virtually all the tissue that lies between the dermal and vascular tissues in the shoots and roots. It consists of three cell types: Parenchyma
, and sclerenchyma
cells. Finally, the vascular tissues are made up of two constituent tissues: xylem
. The xylem is made up two of conducting cells called tracheids
and vessel elements
whereas the phloem is characterized by the presence of sieve tube elements
and companion cells
Plant nutrition and transport
The xylem (blue) transports water and minerals from the roots upwards whereas the phloem (orange) transports carbohydrates between organs.
Plants need water to conduct photosynthesis
, transport solutes
between organs, cool their leaves by evaporation
, and maintain internal pressures that support their bodies.
Water is able to diffuse
in and out of plant cells
. The direction of water movement across a semipermeable membrane
is determined by the water potential
across that membrane.
Water is able to diffuse across a root cell's membrane through aquaporins
whereas solutes are transported across by the membrane by ion channels
. In vascular plants
, water and solutes are able to enter the xylem
, a vascular tissue
, by way of an apoplast
. Once in the xylem, the water and minerals are distributed upward by transpiration
from the soil to the aerial parts of the plant.
In contrast, the phloem
, another vascular tissue, distributes carbohydrates
) and other solutes such as hormones by translocation
from a source
(e.g., mature leaf
or root) in which they were produced to a sink
(e.g., root, flower
, or developing fruit
) in which they will be used and stored.
Sources and sinks can switch roles, depending on the amount of carbohydrates accumulated or mobilized for the nourishment of other organs.
is regulated by environmental cues and the plant's own receptors
, and genome
Morever, they have several characteristics that allow them to obtain resources for growth and reproduction such as meristems
, post-embryonic organ formation, and differential growth.
Development begins with a seed
, which is an embryonic plant
enclosed in a protective outer covering
. Most plant seeds are usually dormant
, a condition in which the seed's normal activity is suspended.
Seed dormancy may last may last weeks, months, years, and even centuries. Dormancy is broken once conditions are favorable for growth, and the seed will begin to sprout, a process called germination
is the first step in germination, whereby water is absorbed by the seed. Once water is absorbed, the seed undergoes metabolic changes whereby enzymes
are activated and RNA
are synthesized. Once the seed germinates, it obtains carbohydrates
, amino acids
, and small lipids
that serve as building blocks for its development. These monomers
are obtained from the hydrolysis
, and lipids that are stored in either the cotyledons
. Germination is completed once embryonic roots called radicle
have emerged from the seed coat
. At this point, the developing plant is called a seedling
and its growth is regulated by its own photoreceptor proteins
in which growth is determinate, i.e., ceases when the adult state is reached, plant growth is indeterminate as it is an open-ended process that could potentially be lifelong.
Plants grow in two ways: primary
. In primary growth, the shoots and roots are formed and lengthened. The apical meristem
produces the primary plant body, which can be found in all seed plants
. During secondary growth, the thickness of the plant increases as the lateral meristem
produces the secondary plant body, which can be found in woody eudicots
such as trees and shrubs. Monocots
do not go through secondary growth.
The plant body is generated by a hierarchy of meristems
. The apical meristems in the root and shoot systems give rise to primary meristems (protoderm, ground meristem, and procambium
), which in turn, give rise to the three tissue systems (dermal
, and vascular
(or flowering plants) engage in sexual reproduction
are organs that facilitate reproduction
, usually by providing a mechanism for the union of sperm
with eggs. Flowers may facilitate two types of pollination
: self-pollination and cross-pollination. Self-pollination
occurs when the pollen from the anther is deposited on the stigma of the same flower, or another flower on the same plant. Cross-pollination
is the transfer of pollen from the anther of one flower to the stigma of another flower on a different individual of the same species. Self-pollination happened in flowers where the stamen and carpel mature at the same time, and are positioned so that the pollen can land on the flower’s stigma. This pollination does not require an investment from the plant to provide nectar and pollen as food for pollinators.
Like animals, plants produce hormones
in one part of its body to signal cells in another part to respond. The ripening
and loss of leaves in the winter are controlled in part by the production of the gas ethylene
by the plant. Stress from water loss, changes in air chemistry, or crowding by other plants can lead to changes in the way a plant functions. These changes may be affected by genetic, chemical, and physical factors.
To function and survive, plants produce a wide array of chemical compounds not found in other organisms. Because they cannot move, plants must also defend themselves chemically from herbivores
and competition from other plants. They do this by producing toxins
and foul-tasting or smelling chemicals. Other compounds defend plants against disease, permit survival during drought, and prepare plants for dormancy, while other compounds are used to attract pollinators
or herbivores to spread ripe seeds.
Many plant organs contain different types of photoreceptor proteins
, each of which reacts very specifically to certain wavelengths of light.
The photoreceptor proteins relay information such as whether it is day or night, duration of the day, intensity of light available, and the source of light. Shoots generally grow towards light, while roots grow away from it, responses known as phototropism
and skototropism, respectively. They are brought about by light-sensitive pigments like phototropins
and the plant hormone auxin
Many flowering plants
bloom at the appropriate time because of light-sensitive compounds that respond to the length of the night, a phenomenon known as photoperiodism
In addition to light, plants can respond to other types of stimuli. For instance, plants can sense the direction of gravity
to orient themselves correctly. They can respond to mechanical stimulation.
Animal form and function
The cells in each animal body are bathed in interstitial fluid
, which make up the cell's environment. This fluid and all its characteristics (e.g., temperature, ionic composition) can be described as the animal's internal environment
, which is in contrast to the external environment that encompasses the animal's outside world.
Animals can be classified as either regulators or conformers. Animals such as mammals
are regulators as they are able to maintain a constant internal environment such as body temperature despite their environments changing. These animals are also described as homeotherms
as they exhibit thermoregulation
by keeping their internal body temperature constant. In contrast, animals such as fishes
are conformers as they adapt their internal environment (e.g., body temperature) to match their external environments. These animals are also described as poikilotherms
as they allow their body temperatures to match their external environments. In terms of energy, regulation is more costly than conformity as an animal expands more energy to maintain a constant internal environment such as increasing its basal metabolic rate
, which is the rate of energy consumption.
Similarly, homeothermy is more costly than poikilothermy. Homeostasis
is the stability of an animal's internal environment, which is maintained by negative feedback
The body size of terrestrial animals
vary across different species but their use of energy does not scale
linearly according to their size.
Mice, for example, are able to consume three times more food than rabbits in proportion to their weights as the basal metabolic rate per unit weight in mice is greater than in rabbits. Physical activity
can also increase an animal's metabolic rate. When an animal runs, its metabolic rate increases linearly with speed.
However, the relationship is non-linear in animals that swim
. When a fish swims faster, it encounters greater water resistance and so its metabolic rates increases exponential.
Alternatively, the relationship of flight speeds and metabolic rates is U-shaped in birds.
At low flight speeds, a bird must maintain a high metabolic rates to remain airborne. As it speeds up its flight, its metabolic rate decreases with the aid of air rapidly flows over its wings. However, as it increases in its speed even further, its high metabolic rates rises again due to the increased effort associated with rapid flight speeds. Basal metabolic rates can be measured based on an animal's rate of heat
Water and salt balance
An animal's body fluids
have three properties: osmotic pressure
composition, and volume
Osmotic pressures determine the direction of the diffusion
of water (or osmosis
), which moves from a region where osmotic pressure (total solute concentration) is low to a region where osmotic pressure (total solute concentration) is high. Aquatic animals are diverse with respect to their body fluid compositions and their environments. For example, most invertebrate animals in the ocean have body fluids that are isosmotic
with seawater. In contrast, ocean bony fishes
have body fluids that are hyposmotic
to seawater. Finally, freshwater animals have body fluids that are hyperosmotic
to fresh water. Typical ions that can be found in an animal's body fluids are sodium
, and chloride
. The volume of body fluids can be regulated by excretion
animals have kidneys
, which are excretory organs made up of tiny tubular structures called nephrons
, which make urine
from blood plasma. The kidneys' primary function is to regulate the composition and volume of blood plasma by selectively removing material from the blood plasma itself. The ability of xeric
animals such as kangaroo rats
to minimize water loss by producing urine that is 10-20 times concentrated than their blood plasma allows them to adapt in desert
environments that receive very little precipitation
Nutrition and digestion
Different digestive systems in marine fishes
Animals are heterotrophs
as they feed on other organisms to obtain energy and organic compounds
They are able to obtain food in three major ways such as targeting visible food objects, collecting tiny food particles, or depending on microbes for critical food needs. The amount of energy stored in food
can be quantified based on the amount of heat (measured in calories
) emitted when the food is burnt in the presence of oxygen. If an animal were to consume food that contains an excess amount of chemical energy, it will store most of that energy in the form of lipids
for future use and some of that energy as glycogen
for more immediate use (e.g., meeting the brain's energy needs).
The molecules in food are chemical building blocks that are needed for growth and development. These molecules include nutrients such as carbohydrates
, and proteins
(e.g., calcium, magnesium, sodium, and phosphorus) are also essential. The digestive system
, which typically consist of a tubular tract that extends from the mouth to the anus, is involved in the breakdown (or digestion
) of food into small molecules as it travels down peristaltically
through the gut lumen
shortly after it has been ingested
. These small food molecules are then absorbed
into the blood from the lumen, where they are then distributed to the rest of the body as building blocks (e.g., amino acids) or sources of energy (e.g., glucose).
In addition to their digestive tracts, vertebrate animals have accessory glands such as a liver and pancreas as part of their digestive systems.
The processing of food in these animals begins in the foregut
, which includes the mouth, esophagus
, and stomach
. Mechanical digestion of food starts in the mouth with the esophagus serving as a passageway for food to reach the stomach, where it is stored and disintegrated (by the stomach's acid) for further processing. Upon leaving the stomach, food enters into the midgut
, which is the first part of the intestine
(or small intestine
) and is the principal site of digestion and absorption. Food that does not get absorbed are stored as indigestible waste (or feces
) in the hindgut
, which is the second part of the intestine (or large intestine
in mammals). The hindgut then completes the reabsorption of needed water and salt prior to eliminating the feces from the rectum
Respiratory system in a bird
The respiratory system
consists of specific organs
and structures used for gas exchange
. The anatomy and physiology that make this happen varies greatly, depending on the size of the organism, the environment in which it lives and its evolutionary history. In land animals
the respiratory surface is internalized as linings of the lungs
. Gas exchange
in the lungs occurs in millions of small air sacs; in mammals and reptiles these are called alveoli
, and in birds they are known as atria
. These microscopic air sacs have a very rich blood supply, thus bringing the air into close contact with the blood.
These air sacs communicate with the external environment via a system of airways, or hollow tubes, of which the largest is the trachea
, which branches in the middle of the chest into the two main bronchi
. These enter the lungs where they branch into progressively narrower secondary and tertiary bronchi that branch into numerous smaller tubes, the bronchioles
. In birds
the bronchioles are termed parabronchi
. It is the bronchioles, or parabronchi that generally open into the microscopic alveoli
in mammals and atria
in birds. Air has to be pumped from the environment into the alveoli or atria by the process of breathing
, which involves the muscles of respiration
Circulatory systems in arthropods, fish, reptiles, and birds/mammals.
A circulatory system
usually consists of a muscular pump such as a heart
, a fluid (blood
), and system of blood vessels
that deliver it.
Its principal function is to transport blood
and other substances to and from cell (biology)s
. There are two types of circulatory systems: open
. In open circulatory systems, blood exits blood vessels as it circulates throughout the body whereas in closed circulatory system, blood is contained within the blood vessels as it circulates. Open circulatory systems can be observed in invertebrate
animals such as arthropods
, and lobsters
) whereas closed circulatory systems can be found in vertebrate
animals such as fishes
, and mammals
. Circulation in animals occur between two types of tissues: systemic tissues
and breathing (or pulmonary) organs
Systemic tissues are all the tissues and organs that make up an animal's body other than its breathing organs. Systemic tissues take up oxygen but adds carbon dioxide to the blood whereas a breathing organs takes up carbon dioxide but add oxygen to the blood.
In birds and mammals, the systemic and pulmonary systems are connected in series.
In the circulatory system, blood is important because it is the means by which oxygen
, carbon dioxide
, agents of immune system, heat, wastes, and other commodities are transported.
such as earthworms
, blood is propelled by peristaltic waves
of the heart muscles that make up the blood vessels. Other animals such as crustaceans (e.g., crayfish
), have more than one heart to propel blood throughout their bodies. Vertebrate hearts are multichambered
and are able to pump blood when their ventricles
contract at each cardiac cycle
, which propels blood through the blood vessels.
Although vertebrate hearts are myogenic
, their rate of contraction (or heart rate
) can be modulated by neural input from the body's autonomic nervous system
Muscle and movement
Asynchronous muscles power flight in most insects. a: Wings b: Wing joint c: Dorsoventral muscles power upstrokes d: Dorsolongitudinal muscles power downstrokes.
, the muscular system
consists of skeletal
and cardiac muscles
. It permits movement of the body, maintains posture and circulates blood throughout the body.
Together with the skeletal system
, it forms the musculoskeletal system
, which is responsible for the movement of vertebrate animals.
Skeletal muscle contractions are neurogenic
as they require synaptic input
from motor neurons
. A single motor neuron is able to innervate multiple muscle fibers, thereby causing the fibers to contract at the same time. Once innervated, the protein filaments within each skeletal muscle fiber slide past each other to produce a contraction, which is explained by the sliding filament theory
. The contraction produced can be described as a twitch, summation, or tetanus, depending on the frequency of action potentials
. Unlike skeletal muscles, contractions of smooth
and cardiac muscles
as they are initiated by the smooth or heart muscle cells themselves instead of a motor neuron. Nevertheless, the strength of their contractions can be modulated by input from the autonomic nervous system
. The mechanisms of contraction are similar in all three muscle tissues.
In invertebrates such as earthworms
, circular and longitudinal muscles
cells form the body wall of these animals and are responsible for their movement.
In an earthworm that is moving through a soil, for example, contractions of circular and longitudinal muscles occur reciprocally while the coelomic fluid
serves as a hydroskeleton
by maintaining turgidity of the earthworm.
Other animals such as mollusks
, and nematodes
, possess obliquely striated muscles, which contain bands of thick and thin filaments that are arranged helically rather than transversely, like in vertebrate skeletal or cardiac muscles.
such as wasps
, and beetles
possess asynchronous muscles
that constitute the flight muscles in these animals.
These flight muscles are often called fibrillar muscles
because they contain myofibrils that are thick and conspicuous.
Most multicellular animals have nervous systems
that allow them to sense from and respond to their environments. A nervous system is a network of cells that processes sensory
information and generates behaviors
. At the cellular level, the nervous system is defined by the presence of neurons
, which are cells specialized to handle information.
They can transmit or receive information at sites of contacts called synapses
More specifically, neurons can conduct nerve impulses (or action potentials
) that travel along their thin fibers called axons
, which can then be transmitted directly to a neighboring cell through electrical synapses
or cause chemicals called neurotransmitters
to be released at chemical synapses
. According to the sodium theory, these action potentials can be generated by the increased permeability of the neuron's cell membrane
to sodium ions.
Cells such as neurons or muscle cells may be excited or inhibited upon receiving a signal from another neuron. The connections between neurons can form neural pathways
, neural circuits
, and larger networks
that generate an organism's perception of the world and determine its behavior. Along with neurons, the nervous system contains other specialized cells called glia
or glial cells, which provide structural and metabolic support.
In vertebrates, the nervous system consists of the central nervous system
(CNS), which includes the brain
and spinal cord
, and the peripheral nervous system
(PNS), which consists of nerves
that connect the CNS to every other part of the body. Nerves that transmit signals from the CNS are called motor nerves
or efferent nerves
, while those nerves that transmit information from the body to the CNS are called sensory nerves
or afferent nerves
. Spinal nerves
are mixed nerves
that serve both functions. The PNS is divided into three separate subsystems, the somatic
, and enteric
nervous systems. Somatic nerves mediate voluntary movement. The autonomic nervous system is further subdivided into the sympathetic
and the parasympathetic
nervous systems. The sympathetic nervous system is activated in cases of emergencies to mobilize energy, while the parasympathetic nervous system is activated when organisms are in a relaxed state. The enteric nervous system functions to control the gastrointestinal
system. Both autonomic and enteric nervous systems function involuntarily. Nerves that exit directly from the brain are called cranial nerves
while those exiting from the spinal cord are called spinal nerves.
are signaling molecules transported in the blood to distant organs to regulate their function.
Hormones are secreted by internal glands
that are part of an animal
's endocrine system
. In vertebrates
, the hypothalamus
is the neural control center for all endocrine systems. In humans
specifically, the major endocrine glands
are the thyroid gland
and the adrenal glands
. Many other organs that are part of other body systems have secondary endocrine functions, including bone
. For example, kidneys secrete the endocrine hormone erythropoietin
. Hormones can be amino acid complexes, steroids
, or prostaglandins
The endocrine system can be contrasted to both exocrine glands
, which secrete hormones to the outside of the body, and paracrine signaling
between cells over a relatively short distance. Endocrine glands have no ducts
, are vascular, and commonly have intracellular vacuoles or granules that store their hormones. In contrast, exocrine glands, such as salivary glands
, sweat glands
, and glands within the gastrointestinal tract
, tend to be much less vascular and have ducts or a hollow lumen
Animals can reproduce
in one of two ways: asexual
. Nearly all animals engage in some form of sexual reproduction.
They produce haploid gametes
. The smaller, motile gametes are spermatozoa
and the larger, non-motile gametes are ova
These fuse to form zygotes
which develop via mitosis
into a hollow sphere, called a blastula. In sponges, blastula larvae swim to a new location, attach to the seabed, and develop into a new sponge.
In most other groups, the blastula undergoes more complicated rearrangement.
It first invaginates
to form a gastrula
with a digestive chamber and two separate germ layers
, an external ectoderm
and an internal endoderm
In most cases, a third germ layer, the mesoderm
, also develops between them.
These germ layers then differentiate to form tissues and organs.
Some animals are capable of asexual reproduction
, which often results in a genetic clone of the parent. This may take place through fragmentation
, such as in Hydra
and other cnidarians
; or parthenogenesis
, where fertile eggs are produced without mating
, such as in aphids
The end of gastrulation signals the beginning of organogenesis
, whereby the three germ layers
form the internal organs
of the organism.
The cells of each of the three germ layers undergo differentiation
, a process where less-specialized cells become more-specialized through the expression of a specific set of genes. Cellular differentiation is influenced by extracellular signals such as growth factors that are exchanged to adjacent cells, which is called juxtracrine
signaling, or to neighboring cells over short distances, which is called paracrine signaling
Intracellular signals consist of a cell signaling itself (autocrine signaling
), also play a role in organ formation. These signaling pathways allows for cell rearrangement and ensures that organs form at specific sites within the organism.
Processes in the primary immune response
Nearly all organisms have some kind of immune system. Bacteria
have a rudimentary immune system in the form of enzymes
that protect against virus
infections. Other basic immune mechanisms evolved in ancient plants and animals
and remain in their modern descendants. These mechanisms include phagocytosis
, antimicrobial peptides
, and the complement system
. Jawed vertebrates
, including humans, have even more sophisticated defense mechanisms, including the ability to adapt to recognize pathogens more efficiently. Adaptive (or acquired) immunity creates an immunological memory
leading to an enhanced response to subsequent encounters with that same pathogen. This process of acquired immunity is the basis of vaccination
Brood parasites, such as the cuckoo, provide a supernormal stimulus to the parenting species.
play a central a role in animals' interaction with each other and with their environment.
They are able to use their muscles to approach one another, vocalize
, seek shelter, and migrate
. An animal's nervous system
activates and coordinates its behaviors. Fixed action patterns
, for instance, are genetically determined and stereotyped behaviors that occur without learning.
These behaviors are under the control of the nervous system and can be quite elaborate.
Examples include the pecking of kelp gull
chicks at the red dot on their mother's beak. Other behaviors that have emerged as a result of natural selection
, and altruism
In addition to evolved behavior, animals have evolved the ability to learn by modifying their behaviors as a result of early individual experiences.
Terrestrial biomes are shaped by temperature and precipitation.
is the study of the distribution and abundance of life
, the interaction between organisms and their environment
of living (biotic
) organisms in conjunction with the nonliving (abiotic
) components (e.g., water, light, radiation, temperature, humidity
, and soil) of their environment is called an ecosystem
These biotic and abiotic components are linked together through nutrient cycles and energy flows.
Energy from the sun enters the system through photosynthesis
and is incorporated into plant tissue. By feeding on plants and on one another, animals play an important role in the movement of matter
through the system. They also influence the quantity of plant and microbial biomass
present. By breaking down dead organic matter
back to the atmosphere and facilitate nutrient cycling
by converting nutrients stored in dead biomass back to a form that can be readily used by plants and other microbes.
The Earth's physical environment is shaped by solar energy
The amount of solar energy input varies in space and time due to the spherical shape of the Earth and its axial tilt
. Variation in solar energy input drives weather
patterns. Weather is the day-to-day temperature and precipitation
activity, whereas climate is the long-term average of weather, typically averaged over a period of 30 years.
Variation in topography also produces environmental heterogeneity. On the windward
side of a mountain, for example, air rises and cools, with water changing from gaseous to liquid or solid form, resulting in precipitation
such as rain or snow.
As a result, wet environments allow for lush vegetation to grow. In contrast, conditions tend to be dry on the leeward side of a mountain due to the lack of precipitation as air descends and warms, and moisture remains as water vapor in the atmosphere. Temperature
and precipitation are the main factors that shape terrestrial biomes
Reaching carrying capacity through a logistic growth curve
is the number of organisms
of the same species
that occupy an area
from generation to generation.
Its abundance can be measured using population density
, which is the number of individuals per unit area (e.g., land or tree) or volume (e.g., sea or air).
Given that it is usually impractical to count every individual within a large population to determine its size, population size
can be estimated by multiplying population density by the area or volume. Population growth
during short-term intervals can be determined using the population growth rate equation
, which takes into consideration birth
, and immigration rates
. In the longer term, the exponential growth
of a population tends to slow down as it reaches its carrying capacity
, which can be modeled using the logistic equation
The carrying capacity of an environment
is the maximum population size of a species
that can be sustained by that specific environment, given the food, habitat
, and other resources
that are available.
The carrying capacity of a population can be affected by changing environmental conditions such as changes in the availability resources and the cost of maintaining them. In human populations
, new technologies
such as the Green revolution
have helped increase the Earth's carrying capacity for humans over time, which has stymied the attempted predictions of impending population decline, the famous of which was by Thomas Malthus
in the 18th century.
A (a) trophic pyramid and a (b) simplified food web. The trophic pyramid represents the biomass at each level.
is a group of populations
of two or more different species
occupying the same geographical area at the same time. A biological interaction
is the effect that a pair of organisms
living together in a community have on each other. They can be either of the same species (intraspecific interactions), or of different species (interspecific interactions). These effects may be short-term, like pollination
, or long-term; both often strongly influence the evolution
of the species involved. A long-term interaction is called a symbiosis
. Symbioses range from mutualism
, beneficial to both partners, to competition
, harmful to both partners.
Every species participates as a consumer, resource, or both in consumer–resource interactions
, which form the core of food chains
or food webs
There are different trophic levels
within any food web, with the lowest level being the primary producers (or autotrophs
) such as plants and algae that convert energy and inorganic material into organic compounds
, which can then be used by the rest of the community.
At the next level are the heterotrophs
, which are the species that obtain energy by breaking apart organic compounds from other organisms.
Heterotrophs that consume plants are primary consumers (or herbivores
) whereas heterotrophs that consume herbivores are secondary consumers (or carnivores
). And those that eat secondary consumers are tertiary consumers and so on. Omnivorous
heterotrophs are able to consume at multiple levels. Finally, there are decomposers
that feed on the waste products or dead bodies of organisms.
On average, the total amount of energy incorporated into the biomass
of a trophic level per unit of time is about one-tenth of the energy of the trophic level that it consumes. Waste and dead material used by decomposers as well as heat lost from metabolism make up the other ninety percent of energy that is not consumed by the next trophic level.
Fast carbon cycle
showing the movement of carbon between land, atmosphere, and oceans in billions of tons per year. Yellow numbers are natural fluxes, red are human contributions, white are stored carbon. Effects of the slow carbon cycle
, such as volcanic and tectonic activity, are not included.
In the global ecosystem (or biosphere
exist as different interacting compartments, which can be biotic or abiotic as well as accessible or inaccessible, depending on their forms and locations.
For example, matter from terrestrial autotrophs are both biotic and accessible to other organisms whereas the matter in rocks and minerals are abiotic and inaccessible. A biogeochemical cycle
is a pathway by which specific elements
of matter are turned over or moved through the biotic (biosphere
) and the abiotic (lithosphere
, and hydrosphere
) compartments of Earth
. There are biogeochemical cycles for nitrogen
, and water
. In some cycles there are reservoirs
where a substance remains or is sequestered
for a long period of time.
includes both global warming driven by human-induced emissions of greenhouse gases
and the resulting large-scale shifts in weather patterns. Though there have been previous periods of climatic change
, since the mid-20th century humans have had an unprecedented impact on Earth's climate system and caused change on a global scale.
The largest driver of warming is the emission of greenhouse gases
, of which more than 90% are carbon dioxide
. Fossil fuel
, and natural gas
) for energy consumption
is the main source of these emissions, with additional contributions from agriculture, deforestation, and manufacturing
Temperature rise is accelerated or tempered by climate feedbacks
, such as loss of sunlight-reflecting snow and ice cover
, increased water vapor
(a greenhouse gas itself), and changes to land and ocean carbon sinks
Efforts are made to preserve the natural characteristics of Hopetoun Falls
, Australia, without affecting visitors' access.
is the study of the conservation of Earth
with the aim of protecting species
, their habitats
, and ecosystems
from excessive rates of extinction
and the erosion of biotic interactions.
It is concerned with factors that influence the maintenance, loss, and restoration of biodiversity and the science of sustaining evolutionary processes that engender genetic
, and ecosystem diversity.
The concern stems from estimates suggesting that up to 50% of all species on the planet will disappear within the next 50 years,
which has contributed to poverty, starvation, and will reset the course of evolution on this planet.Biodiversity
affects the functioning of ecosystems, which provide a variety of services
upon which people depend.
Conservation biologists research and educate on the trends of biodiversity loss
, species extinctions
, and the negative effect these are having on our capabilities to sustain
the well-being of human society. Organizations and citizens are responding to the current biodiversity crisis
through conservation action plans that direct research, monitoring, and education programs that engage concerns at local through global scales.
The first eukaryotes were "neither plants, animals, nor fungi", hence as defined, protists would include the last eukaryotic common ancestor.
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