What is the smallest oligocelluar organism?

What is the smallest oligocelluar organism?

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What is the smallest oligocelluar organism?

How many cells does it have?


The question is motivated by this [email protected]

EDIT as recommended in comments

I'm looking for an example of an organism made of very few (the fewer, the better) sister cells (obtained by mitosis or aggregation) that are morphologically and functionally different. I would accept examples of species that exist in different "conformation" (unicellular and multicellular).

The classic example, though I am sure there are others that are smaller, is the slime mold Dictyostelium discoideum:

It can have up to 100,000 cells and exists as both single cells and as a multicellular organism (emphasis mine):

Dictyostelium amoebae grow as separate, independent cells but interact to form multicellular structures when challenged by adverse conditions such as starvation. Up to 100,000 cells signal each other by releasing the chemoattractant cAMP and aggregate together by chemotaxis to form a mound that is surrounded by an extracellular matrix. This mechanism for generating a multicellular organism differs radically from the early steps of metazoan embryogenesis. However, subsequent processes depend on cell-cell communication in both Dictyostelium and metazoans. Many of the underlying molecular and cellular processes appear to have arisen in primitive precursor cells and to have remained fundamentally unchanged throughout evolution. Basic processes of development such as differential cell sorting, pattern formation, stimulus-induced gene expression, and cell-type regulation are common to Dictyostelium and metazoans.

Not sure, whether it fits in your requirement (since it was not clear for me with the terdon's answer), but I'd also mention plasmodia and other cases of species with syncytial organization: they have multiple nuclei, sometimes have even macroscopic sizes, but formally there is only one cell boundary within their body.

(If we broaden this definition, we may also include all kinds of protists with multiple nuclei: from diplomonads (which have two equal nuclei) and ciliates (most of which have two functionally differentiated nuclei) to Pelomyxa or giant foraminiferans with huge numbers of nuclei.)

Some examples:

  • Myxosporidia, once thought to be a group of protists and now believed to be highly aberrant metazoans - are parasites of fish (first generation) and some invertebrates (second generation), which form highly differentiated plasmodial bodies in their hosts.

  • Orthonectids, a group of "lower metazoans", have a biphasic life-cycle with sexual generation represented by oligocellular free-living males and females, which produce zygotes developing into parasitic plasmodia.

  • Myxogastrid slime molds are famous in having plasmodial fruit bodies: they are relatives of Dictyostelium and have a similar life-cycle with mono-nuclear amoebae, which conjugate and produce the plasmodium. So, these are still formally "unicellular" even as fruit bodies.

  • Some algae have plasmodial organization with the most famous case of Caulerpa, species of which "are unusual because they consist of only one cell with many nuclei, making them among the biggest single cells in the world.".

  • Cannot recall right now the name of the group (can someone help?), but there are amoebae, which are "normally" unicellular, but able to aggregate on rich food resources to form trophic plasmodium.

  • Lots of other cases.


A microorganism is a living thing that is too small to be seen with the naked eye. Examples of microorganisms include bacteria, archaea, algae, protozoa, and microscopic animals such as the dust mite.

These microorganisms have been often under-appreciated and under-studied. Indeed, until Anton von Leeuwenhoek invented the microscope, we did not know they existed! Until that time, it was thought that phenomena such as illness and food spoilage were caused by “vapors” or “spontaneous generation.”

Leeuwenhoek’s invention of the microscope soon led Louis Pasteur to realize that many diseases were caused by microorganisms – and to the practice of pasteurization, which kills microorganisms and makes our food products safe to eat today.

Now, we know that microorganisms are responsible for many things that happen in the world around us.

Microorganisms are found virtually everywhere, except for environments that have been made artificially sterile by humans. Even these must be constantly sterilized and carefully protected, lest microorganisms be tracked in from the outside world.

Microorganisms live in water, in soil, and on the skin and in the digestive tracts of animals. This is why all living things must have immune systems – while many microorganisms can be helpful to them, some can be harmful and cause disease.

Like all organisms, microorganisms play important roles in the ecosystems they inhabit. Here are a few of their roles.

Smallest unit of Life

Cell is considered as the smallest unit of life. Each living organism is made up of one or more cells. Cell biology or the science of cells is known as cytology. But the two terms cell biology and psychology is not the same meaning. Cytology specially refers study of structure and composition of sales whereas cell biology includes study of both structured and function sales and the relation between them.

As the cells are the smallest unit of life and most of them are too small to be seen with naked eyes, some tools and techniques are required to study their structures. In the field of cytology and most important tool is the microscope.

Different types of microscope: –
A microscope is comparable to human eye. We know that both human eye and the microscope have the systems of lens and in both cases images of the object are formed. The construction and utility of the microscope is based on the

principle of getting a magnified image of the object through the lenses. Microscope used in biological study especially study of smallest unit of life, the cell are mainly of two types, those are light microscope and electron microscope. The light microscope in the biological laboratory of schools and colleges whereas from microscope for research and higher studies.

All living organisms are composter of cells which we called the smallest unit of life that originate from pre-existing cells. An organism may be made up of only one cell or number of many cells. Those organisms which are made up of one single cell only are called unicellular organism. Examples of such unicellular organism are the bacteria, Chlamydomonas, Yeast, Amoeba etc. Otherwise those organisms which are made up of more than one cells, are called multi-cellular organisms. Numbered of sales in multi-cellular organism may be only a few cells to several billions cells. As example of such multi-cellular organism we may say the big trees, human beings and large animals like Tiger, cow, elephant etc. The life of every organism, whether plant or animal, begins as a single cell and so the cell is called the smallest unit of life.

We unicellular organism Complete there are entire life cycle as a single cell, while multi-cellular organisms begin their life form just a single cell which in course of life divides deadly the number of sales for forming the multi-cell body. Thus the cells are considered as the structural units of a living body. Each cell has its own function and in the multi-cellular organism, a number of different types of cell of different functionalities are exit together. So, the activities of such an organism are the sum of total coordinated activities of its component cells. Thus the cells are not only the smallest unit of life but also the functional units of the life.

In a multi-cellular organism, certain cells become specialized to perform some specific functions. The cells which have a common origin and similar specific functionality constitute a tissue. Different types of tissues collectively form an organ and every organ performs some specific functions. A group of organs performing some specific functions together constitute a organ system, such as the digestive system, respiratory system, reproductive system, etc. Each cell exhibits all the characteristics of life such as respiration, metabolism, growth, reproduction, etc. through the components of the cell. So the smallest unit of life is the cell or the cell is the structural and functional unit of life.
Now our discussion is about the types of cell. Click ► ► Types of Cell
Click here for MCQ

Cell Theory

Close your eyes and picture a brick wall. What is the basic building block of that wall? It is a single brick, of course. Like a brick wall, your body is composed of basic building blocks and the building blocks of your body are cells. Your body has many kinds of cells, each specialized for a specific purpose. Just as a home is made from a variety of building materials, the human body is constructed from many cell types. For example, bone cells help to support and protect the body. Cells of the immune system fight invading bacteria. And red blood cells carry oxygen throughout the body. Each of these cell types plays a vital role during the growth, development, and day-to-day maintenance of the body. In spite of their enormous variety, however, all cells share certain fundamental characteristics.

The microscopes we use today are far more complex than those used in the 1600s by Antony van Leeuwenhoek, a Dutch shopkeeper who had great skill in crafting lenses. Despite the limitations of his now-ancient lenses, van Leeuwenhoek observed the movements of single-celled organism and sperm, which he collectively termed &ldquoanimalcules.&rdquo In a 1665 publication called Micrographia, experimental scientist Robert Hooke coined the term &ldquocell&rdquo (from the Latin cella, meaning &ldquosmall room&rdquo) for the box-like structures he observed when viewing cork tissue through a lens. In the 1670s, van Leeuwenhoek discovered bacteria and protozoa. Later advances in lenses and microscope construction enabled other scientists to see different components inside cells.

By the late 1830s, botanist Matthias Schleiden and zoologist Theodor Schwann were studying tissues and proposed the unified cell theory, which states that all living things are composed of one or more cells, that the cell is the basic unit of life, and that all new cells arise from existing cells. These principles still stand today. There are many types of cells, and all are grouped into one of two broad categories: prokaryotic and eukaryotic. Animal, plant, fungal, and protist cells are classified as eukaryotic, whereas bacteria and archaea cells are classified as prokaryotic.

All cells share four common components: 1) a plasma membrane, an outer covering that separates the cell&rsquos interior from its surrounding environment 2) cytoplasm, consisting of a jelly-like region within the cell in which other cellular components are found 3) DNA, the genetic material of the cell and 4) ribosomes, particles that synthesize proteins. However, prokaryotes differ from eukaryotic cells in several ways.

Components of Prokaryotic Cells

A prokaryotic cell is a simple, single-celled (unicellular) organism that lacks a nucleus, or any other membrane-bound organelle. We will shortly come to see that this is significantly different in eukaryotes. Prokaryotic DNA is found in the central part of the cell: a darkened region called the nucleoid (Figure (PageIndex<1>)).

Figure (PageIndex<2>). This figure shows the generalized structure of a prokaryotic cell.

Unlike Archaea and eukaryotes, bacteria have a cell wall made of peptidoglycan (molecules comprised of sugars and amino acids) and many have a polysaccharide capsule. The cell wall acts as an extra layer of protection, helps the cell maintain its shape, and prevents dehydration. The capsule enables the cell to attach to surfaces in its environment. Some prokaryotes have flagella, pili, or fimbriae. Flagella are used for locomotion. Pili are used to exchange genetic material during a type of reproduction called conjugation. Fimbriae are protein appendages used by bacteria to attach to other cells.

Eukaryotic Cells

A eukaryotic cell is a cell that has a membrane-bound nucleus and other membrane-bound compartments called organelles. There are many different types of organelles, each with a highly specialized function (see Figure (PageIndex<3>)). The word eukaryotic means &ldquotrue kernel&rdquo or &ldquotrue nucleus,&rdquo alluding to the presence of the membrane-bound nucleus in these cells. The word &ldquoorganelle&rdquo means &ldquolittle organ,&rdquo and, as already mentioned, organelles have specialized cellular functions, just as the organs of your body have specialized functions.

Cell Size

At 0.1&ndash5.0 µm in diameter, most prokaryotic cells are significantly smaller than eukaryotic cells, which have diameters ranging from 10&ndash100 µm (Figure (PageIndex<3>)). The small size of prokaryotes allows ions and organic molecules that enter them to quickly spread to other parts of the cell. Similarly, any wastes produced within a prokaryotic cell can quickly move out. However, larger eukaryotic cells have evolved different structural adaptations to enhance cellular transport. Indeed, the large size of these cells would not be possible without these adaptations. In general, cell size is limited because volume increases much more quickly than does cell surface area. As a cell becomes larger, it becomes more and more difficult for the cell to acquire sufficient materials to support the processes inside the cell, because the relative size of the surface area through which materials must be transported declines.

Figure (PageIndex<3>). This figure shows the relative sizes of different kinds of cells and cellular components. An adult human is shown for comparison.

Animal Cells versus Plant Cells

Despite their fundamental similarities, there are some striking differences between animal and plant cells (Figure (PageIndex<3>)). Animal cells have centrioles, centrosomes, and lysosomes, whereas plant cells do not. Plant cells have a rigid cell wall that is external to the plasma membrane, chloroplasts, plasmodesmata, and plastids used for storage, and a large central vacuole, whereas animal cells do not.


From an ecological perspective, chloroplasts are a particularly important type of organelle because they perform photosynthesis. Photosynthesis forms the foundation of food chains in most ecosystems. Chloroplasts are only found in eukaryotic cells such as plants and algae. During photosynthesis, carbon dioxide, water, and light energy are used to make glucose and molecular oxygen. One major difference between algae/plants and animals is that plants/algae are able to make their own food, like glucose, whereas animals must obtain food by consuming other organisms.

Figure (PageIndex<6>). This simplified diagram of a chloroplast shows its structure.

Chloroplasts have outer and inner membranes, but within the space enclosed by a chloroplast&rsquos inner membrane is a set of interconnected and stacked, fluid-filled membrane sacs called thylakoids (Figure (PageIndex<4>) below). Each stack of thylakoids is called a granum (plural = grana). The fluid enclosed by the inner membrane and surrounding the grana is called the stroma. Each structure within the chloroplast has an important function, which is enabled by its particular shape. A common theme in biology is that form and function are interrelated. For example, the membrane-rich stacks of the thylakoids provide ample surface area to embed the proteins and pigments that are vital to photosynthesis.

Cell Theory

Scientists once thought that life spontaneously arose from nonliving things. Thanks to experimentation and the invention of the microscope, it is now known that life comes from preexisting life and that cells come from preexisting cells.

Micrographia Cover

English scientist Robert Hooke published Micrographia in 1665. In it, he illustrated the smallest complete parts of an organism, which he called cells.

Photograph by Universal History Archive/Universal Images Group via Getty Images

In 1665, Robert Hooke published Micrographia, a book filled with drawings and descriptions of the organisms he viewed under the recently invented microscope. The invention of the microscope led to the discovery of the cell by Hooke. While looking at cork, Hooke observed box-shaped structures, which he called &ldquocells&rdquo as they reminded him of the cells, or rooms, in monasteries. This discovery led to the development of the classical cell theory.

The classical cell theory was proposed by Theodor Schwann in 1839. There are three parts to this theory. The first part states that all organisms are made of cells. The second part states that cells are the basic units of life. These parts were based on a conclusion made by Schwann and Matthias Schleiden in 1838, after comparing their observations of plant and animal cells. The third part, which asserts that cells come from preexisting cells that have multiplied, was described by Rudolf Virchow in 1858, when he stated omnis cellula e cellula (all cells come from cells).

Since the formation of classical cell theory, technology has improved, allowing for more detailed observations that have led to new discoveries about cells. These findings led to the formation of the modern cell theory, which has three main additions: first, that DNA is passed between cells during cell division second, that the cells of all organisms within a similar species are mostly the same, both structurally and chemically and finally, that energy flow occurs within cells.

English scientist Robert Hooke published Micrographia in 1665. In it, he illustrated the smallest complete parts of an organism, which he called cells.

Photograph by Universal History Archive/Universal Images Group via Getty Images

What is the smallest oligocelluar organism? - Biology

What is the smallest living thing?

This question is controversial and it is also a subject of endless discussion, since we also need to know "What is life?". For this answer, I'll begin with bacteria, which are undoubtedly alive.

What is the smallest free living organism? Most bacteriology textbooks say Mycoplasma genitalium is the smallest known organism capable of independent growth and reproduction. Its size is given as 0.2 to 0.3 µm (micrometers). A µm is one millionth of a meter (or one thousandth of a millimeter). An average bacterium, like E. coli, is about 1 µm by 3 µm (it has a rod shape). A red blood cell is 8 µm in diameter and the average human cell is about 25 µm across.

Although mycoplasma can live in complex media in the laboratory, in nature they are always found living parasitically, attached to other cells. Since they take preformed nutrients from other cells, they have streamlined their metabolism and only have about 470 genes to use to make all the proteins needed for cell division, energy production, and protein synthesis, etc. These are the simplest cells found so far.

Other small bacteria are rickettsia and chlamydia which can be as small as 0.3 µm. But it's a big world, and probably fewer than 1% of the total bacterial strains have been characterized. That means there are many bacteria that have never been seen, or have been poorly studied. For instance, the marine ultramicrobacteria, Shingomonas sp strain RB2256, has been reported to be able to pass through a 0.22 µm ultrafilter. It should be noted that many bacteria, in response to starvation, go to a dormant state of much smaller size.
( It is not clear how many of these "ultramicrobacteria" represent nutrient downsized bacteria.

In 1990, Bob Folk at UT, using an electron microscope, observed 0.05 µm (50 nanometers) "nanobacteria" in rocks from hot springs. Nanobacteria were later found in blood, kidney stones, and in meteorites that came from Mars. Of course, this created quite a stir. But other researchers have not been able to find DNA or protein in nanobacteria, and it may be that the objects seen in the electron microscope are mineral microcrystals. Because of this, many experts doubt the existence of nanobacteria (see Nature (2000) 408, p394). But, supporters continue looking for evidence that nanobacteria are living.

A theoretical discussion of what could be the smallest bacterium possible gives a diameter of 0.17 µm This figure also precludes the possibility of nanobacteria. This nas nanopanel site has an excellent series of articles on the size limits of organisms.

Viruses are DNA or RNA that are unable to "live" without invading another cell and they use the molecular machinery of the host cell for metabolism and replication. "Are viruses alive?" is a philosophical question. They certainly have a life cycle and many would say that they are alive when they are infecting a cell. But whether they are alive or not, they don't have to carry around all the genes needed for an independent existence. Therefore, viruses can be very small, from 0.3 µm to 0.02 µm, or 300 to 20 nanometers (nm) in size. Picorna ("little RNA") viruses, like polio, are only 30 nm in diameter. Other viruses, like Parvoviruses, are just a linear single strand of DNA in a capsid with no extra proteins. Parvovirus DNA can be less than 5000 nucleotides long and the Parvoviruses can be as small as 20 nm in size.

And then there are viroids. Viroids are small circular single strands of RNA that lack a protein coat. To date, they have been shown to only cause plant diseases. Their RNA can be as short as 248 nucleotides long (80,000 molecular weight), which can be less than 10 nm diameter.

What about prions? Prions are thought to be infectious protein particles that cause several diseases in humans and animals, including Mad Cow Disease. The latest theory is that Prions recruit proteins similar to themselves in the membranes of brain cells. By lining up next to these proteins, they cause a conformational change that converts these brain proteins into Prion proteins, which are about 30,000 molecular weight, or about 5 nm. (These proteins can't be broken down by the cell, so they form aggregates that clog the cell.) It might be a stretch to say that prions are alive, but for completeness, here they are. Somewhere from prions to bacteria you can say life starts, and there are the sizes.

A word about methods for determining sizes. Standard methods to determine the sizes of bacteria and viruses are microscopy and ultrafiltration. Methods for determining the sizes of proteins and DNA molecules are ultrafiltration, gel filtration, gel electrophoresis, centrifugation, and direct calculation of molecular weight. Note that DNA is a long linear molecule and proteins are generally globular and RNA is in between, so direct comparison of sizes is complicated and the figures given are approximations.

I hope this has given you an idea of what's involved in answering the question, "What is the smallest living thing?". Mike Conrad.

Try the links in the MadSci Library for more information on General Biology.


By the late 17th and early 18th centuries, the digestion of meat by stomach secretions [7] and the conversion of starch to sugars by plant extracts and saliva were known but the mechanisms by which these occurred had not been identified. [8]

French chemist Anselme Payen was the first to discover an enzyme, diastase, in 1833. [9] A few decades later, when studying the fermentation of sugar to alcohol by yeast, Louis Pasteur concluded that this fermentation was caused by a vital force contained within the yeast cells called "ferments", which were thought to function only within living organisms. He wrote that "alcoholic fermentation is an act correlated with the life and organization of the yeast cells, not with the death or putrefaction of the cells." [10]

In 1877, German physiologist Wilhelm Kühne (1837–1900) first used the term enzyme, which comes from Greek ἔνζυμον, "leavened" or "in yeast", to describe this process. [11] The word enzyme was used later to refer to nonliving substances such as pepsin, and the word ferment was used to refer to chemical activity produced by living organisms. [12]

Eduard Buchner submitted his first paper on the study of yeast extracts in 1897. In a series of experiments at the University of Berlin, he found that sugar was fermented by yeast extracts even when there were no living yeast cells in the mixture. [13] He named the enzyme that brought about the fermentation of sucrose "zymase". [14] In 1907, he received the Nobel Prize in Chemistry for "his discovery of cell-free fermentation". Following Buchner's example, enzymes are usually named according to the reaction they carry out: the suffix -ase is combined with the name of the substrate (e.g., lactase is the enzyme that cleaves lactose) or to the type of reaction (e.g., DNA polymerase forms DNA polymers). [15]

The biochemical identity of enzymes was still unknown in the early 1900s. Many scientists observed that enzymatic activity was associated with proteins, but others (such as Nobel laureate Richard Willstätter) argued that proteins were merely carriers for the true enzymes and that proteins per se were incapable of catalysis. [16] In 1926, James B. Sumner showed that the enzyme urease was a pure protein and crystallized it he did likewise for the enzyme catalase in 1937. The conclusion that pure proteins can be enzymes was definitively demonstrated by John Howard Northrop and Wendell Meredith Stanley, who worked on the digestive enzymes pepsin (1930), trypsin and chymotrypsin. These three scientists were awarded the 1946 Nobel Prize in Chemistry. [17]

The discovery that enzymes could be crystallized eventually allowed their structures to be solved by x-ray crystallography. This was first done for lysozyme, an enzyme found in tears, saliva and egg whites that digests the coating of some bacteria the structure was solved by a group led by David Chilton Phillips and published in 1965. [18] This high-resolution structure of lysozyme marked the beginning of the field of structural biology and the effort to understand how enzymes work at an atomic level of detail. [19]

Enzymes can be classified by two main criteria: either amino acid sequence similarity (and thus evolutionary relationship) or enzymatic activity.

Enzyme activity. An enzyme's name is often derived from its substrate or the chemical reaction it catalyzes, with the word ending in -ase. [1] : 8.1.3 Examples are lactase, alcohol dehydrogenase and DNA polymerase. Different enzymes that catalyze the same chemical reaction are called isozymes. [1] : 10.3

The International Union of Biochemistry and Molecular Biology have developed a nomenclature for enzymes, the EC numbers (for "Enzyme Commission"). Each enzyme is described by "EC" followed by a sequence of four numbers which represent the hierarchy of enzymatic activity (from very general to very specific). That is, the first number broadly classifies the enzyme based on its mechanism while the other digits add more and more specificity. [20]

The top-level classification is:

  • EC 1, Oxidoreductases: catalyze oxidation/reduction reactions
  • EC 2, Transferases: transfer a functional group (e.g. a methyl or phosphate group)
  • EC 3, Hydrolases: catalyze the hydrolysis of various bonds
  • EC 4, Lyases: cleave various bonds by means other than hydrolysis and oxidation
  • EC 5, Isomerases: catalyze isomerization changes within a single molecule
  • EC 6, Ligases: join two molecules with covalent bonds.

These sections are subdivided by other features such as the substrate, products, and chemical mechanism. An enzyme is fully specified by four numerical designations. For example, hexokinase (EC is a transferase (EC 2) that adds a phosphate group (EC 2.7) to a hexose sugar, a molecule containing an alcohol group (EC 2.7.1). [21]

Sequence similarity. EC categories do not reflect sequence similarity. For instance, two ligases of the same EC number that catalyze exactly the same reaction can have completely different sequences. Independent of their function, enzymes, like any other proteins, have been classified by their sequence similarity into numerous families. These families have been documented in dozens of different protein and protein family databases such as Pfam. [22]

Enzymes are generally globular proteins, acting alone or in larger complexes. The sequence of the amino acids specifies the structure which in turn determines the catalytic activity of the enzyme. [23] Although structure determines function, a novel enzymatic activity cannot yet be predicted from structure alone. [24] Enzyme structures unfold (denature) when heated or exposed to chemical denaturants and this disruption to the structure typically causes a loss of activity. [25] Enzyme denaturation is normally linked to temperatures above a species' normal level as a result, enzymes from bacteria living in volcanic environments such as hot springs are prized by industrial users for their ability to function at high temperatures, allowing enzyme-catalysed reactions to be operated at a very high rate.

Enzymes are usually much larger than their substrates. Sizes range from just 62 amino acid residues, for the monomer of 4-oxalocrotonate tautomerase, [26] to over 2,500 residues in the animal fatty acid synthase. [27] Only a small portion of their structure (around 2–4 amino acids) is directly involved in catalysis: the catalytic site. [28] This catalytic site is located next to one or more binding sites where residues orient the substrates. The catalytic site and binding site together compose the enzyme's active site. The remaining majority of the enzyme structure serves to maintain the precise orientation and dynamics of the active site. [29]

In some enzymes, no amino acids are directly involved in catalysis instead, the enzyme contains sites to bind and orient catalytic cofactors. [29] Enzyme structures may also contain allosteric sites where the binding of a small molecule causes a conformational change that increases or decreases activity. [30]

A small number of RNA-based biological catalysts called ribozymes exist, which again can act alone or in complex with proteins. The most common of these is the ribosome which is a complex of protein and catalytic RNA components. [1] : 2.2

Substrate binding

Enzymes must bind their substrates before they can catalyse any chemical reaction. Enzymes are usually very specific as to what substrates they bind and then the chemical reaction catalysed. Specificity is achieved by binding pockets with complementary shape, charge and hydrophilic/hydrophobic characteristics to the substrates. Enzymes can therefore distinguish between very similar substrate molecules to be chemoselective, regioselective and stereospecific. [31]

Some of the enzymes showing the highest specificity and accuracy are involved in the copying and expression of the genome. Some of these enzymes have "proof-reading" mechanisms. Here, an enzyme such as DNA polymerase catalyzes a reaction in a first step and then checks that the product is correct in a second step. [32] This two-step process results in average error rates of less than 1 error in 100 million reactions in high-fidelity mammalian polymerases. [1] : 5.3.1 Similar proofreading mechanisms are also found in RNA polymerase, [33] aminoacyl tRNA synthetases [34] and ribosomes. [35]

Conversely, some enzymes display enzyme promiscuity, having broad specificity and acting on a range of different physiologically relevant substrates. Many enzymes possess small side activities which arose fortuitously (i.e. neutrally), which may be the starting point for the evolutionary selection of a new function. [36] [37]

"Lock and key" model

To explain the observed specificity of enzymes, in 1894 Emil Fischer proposed that both the enzyme and the substrate possess specific complementary geometric shapes that fit exactly into one another. [38] This is often referred to as "the lock and key" model. [1] : 8.3.2 This early model explains enzyme specificity, but fails to explain the stabilization of the transition state that enzymes achieve. [39]

Induced fit model

In 1958, Daniel Koshland suggested a modification to the lock and key model: since enzymes are rather flexible structures, the active site is continuously reshaped by interactions with the substrate as the substrate interacts with the enzyme. [40] As a result, the substrate does not simply bind to a rigid active site the amino acid side-chains that make up the active site are molded into the precise positions that enable the enzyme to perform its catalytic function. In some cases, such as glycosidases, the substrate molecule also changes shape slightly as it enters the active site. [41] The active site continues to change until the substrate is completely bound, at which point the final shape and charge distribution is determined. [42] Induced fit may enhance the fidelity of molecular recognition in the presence of competition and noise via the conformational proofreading mechanism. [43]


Enzymes can accelerate reactions in several ways, all of which lower the activation energy (ΔG ‡ , Gibbs free energy) [44]

  1. By stabilizing the transition state:
    • Creating an environment with a charge distribution complementary to that of the transition state to lower its energy [45]
  2. By providing an alternative reaction pathway:
    • Temporarily reacting with the substrate, forming a covalent intermediate to provide a lower energy transition state [46]
  3. By destabilising the substrate ground state:
    • Distorting bound substrate(s) into their transition state form to reduce the energy required to reach the transition state [47]
    • By orienting the substrates into a productive arrangement to reduce the reaction entropy change [48] (the contribution of this mechanism to catalysis is relatively small) [49]

Enzymes may use several of these mechanisms simultaneously. For example, proteases such as trypsin perform covalent catalysis using a catalytic triad, stabilise charge build-up on the transition states using an oxyanion hole, complete hydrolysis using an oriented water substrate. [50]


Enzymes are not rigid, static structures instead they have complex internal dynamic motions – that is, movements of parts of the enzyme's structure such as individual amino acid residues, groups of residues forming a protein loop or unit of secondary structure, or even an entire protein domain. These motions give rise to a conformational ensemble of slightly different structures that interconvert with one another at equilibrium. Different states within this ensemble may be associated with different aspects of an enzyme's function. For example, different conformations of the enzyme dihydrofolate reductase are associated with the substrate binding, catalysis, cofactor release, and product release steps of the catalytic cycle, [51] consistent with catalytic resonance theory.

Substrate presentation

Substrate presentation is a process where the enzyme is sequestered away from its substrate. Enzymes can be sequestered to the plasma membrane away from a substrate in the nucleus or cytosol. Or within the membrane, an enzyme can be sequestered into lipid rafts away from its substrate in the disordered region. When the enzyme is released it mixes with its substrate. Alternatively, the enzyme can be sequestered near its substrate to activate the enzyme. For example, the enzyme can be soluble and upon activation bind to a lipid in the plasma membrane and then act upon molecules in the plasma membrane.

Allosteric modulation

Allosteric sites are pockets on the enzyme, distinct from the active site, that bind to molecules in the cellular environment. These molecules then cause a change in the conformation or dynamics of the enzyme that is transduced to the active site and thus affects the reaction rate of the enzyme. [52] In this way, allosteric interactions can either inhibit or activate enzymes. Allosteric interactions with metabolites upstream or downstream in an enzyme's metabolic pathway cause feedback regulation, altering the activity of the enzyme according to the flux through the rest of the pathway. [53]

Some enzymes do not need additional components to show full activity. Others require non-protein molecules called cofactors to be bound for activity. [54] Cofactors can be either inorganic (e.g., metal ions and iron-sulfur clusters) or organic compounds (e.g., flavin and heme). These cofactors serve many purposes for instance, metal ions can help in stabilizing nucleophilic species within the active site. [55] Organic cofactors can be either coenzymes, which are released from the enzyme's active site during the reaction, or prosthetic groups, which are tightly bound to an enzyme. Organic prosthetic groups can be covalently bound (e.g., biotin in enzymes such as pyruvate carboxylase). [56]

An example of an enzyme that contains a cofactor is carbonic anhydrase, which uses a zinc cofactor bound as part of its active site. [57] These tightly bound ions or molecules are usually found in the active site and are involved in catalysis. [1] : 8.1.1 For example, flavin and heme cofactors are often involved in redox reactions. [1] : 17

Enzymes that require a cofactor but do not have one bound are called apoenzymes or apoproteins. An enzyme together with the cofactor(s) required for activity is called a holoenzyme (or haloenzyme). The term holoenzyme can also be applied to enzymes that contain multiple protein subunits, such as the DNA polymerases here the holoenzyme is the complete complex containing all the subunits needed for activity. [1] : 8.1.1


Coenzymes are small organic molecules that can be loosely or tightly bound to an enzyme. Coenzymes transport chemical groups from one enzyme to another. [58] Examples include NADH, NADPH and adenosine triphosphate (ATP). Some coenzymes, such as flavin mononucleotide (FMN), flavin adenine dinucleotide (FAD), thiamine pyrophosphate (TPP), and tetrahydrofolate (THF), are derived from vitamins. These coenzymes cannot be synthesized by the body de novo and closely related compounds (vitamins) must be acquired from the diet. The chemical groups carried include:

  • the hydride ion (H − ), carried by NAD or NADP +
  • the phosphate group, carried by adenosine triphosphate
  • the acetyl group, carried by coenzyme A
  • formyl, methenyl or methyl groups, carried by folic acid and
  • the methyl group, carried by S-adenosylmethionine[58]

Since coenzymes are chemically changed as a consequence of enzyme action, it is useful to consider coenzymes to be a special class of substrates, or second substrates, which are common to many different enzymes. For example, about 1000 enzymes are known to use the coenzyme NADH. [59]

Coenzymes are usually continuously regenerated and their concentrations maintained at a steady level inside the cell. For example, NADPH is regenerated through the pentose phosphate pathway and S-adenosylmethionine by methionine adenosyltransferase. This continuous regeneration means that small amounts of coenzymes can be used very intensively. For example, the human body turns over its own weight in ATP each day. [60]

As with all catalysts, enzymes do not alter the position of the chemical equilibrium of the reaction. In the presence of an enzyme, the reaction runs in the same direction as it would without the enzyme, just more quickly. [1] : 8.2.3 For example, carbonic anhydrase catalyzes its reaction in either direction depending on the concentration of its reactants: [61]

The rate of a reaction is dependent on the activation energy needed to form the transition state which then decays into products. Enzymes increase reaction rates by lowering the energy of the transition state. First, binding forms a low energy enzyme-substrate complex (ES). Second, the enzyme stabilises the transition state such that it requires less energy to achieve compared to the uncatalyzed reaction (ES ‡ ). Finally the enzyme-product complex (EP) dissociates to release the products. [1] : 8.3

Enzymes can couple two or more reactions, so that a thermodynamically favorable reaction can be used to "drive" a thermodynamically unfavourable one so that the combined energy of the products is lower than the substrates. For example, the hydrolysis of ATP is often used to drive other chemical reactions. [62]

Enzyme kinetics is the investigation of how enzymes bind substrates and turn them into products. [63] The rate data used in kinetic analyses are commonly obtained from enzyme assays. In 1913 Leonor Michaelis and Maud Leonora Menten proposed a quantitative theory of enzyme kinetics, which is referred to as Michaelis–Menten kinetics. [64] The major contribution of Michaelis and Menten was to think of enzyme reactions in two stages. In the first, the substrate binds reversibly to the enzyme, forming the enzyme-substrate complex. This is sometimes called the Michaelis–Menten complex in their honor. The enzyme then catalyzes the chemical step in the reaction and releases the product. This work was further developed by G. E. Briggs and J. B. S. Haldane, who derived kinetic equations that are still widely used today. [65]

Enzyme rates depend on solution conditions and substrate concentration. To find the maximum speed of an enzymatic reaction, the substrate concentration is increased until a constant rate of product formation is seen. This is shown in the saturation curve on the right. Saturation happens because, as substrate concentration increases, more and more of the free enzyme is converted into the substrate-bound ES complex. At the maximum reaction rate (Vmax) of the enzyme, all the enzyme active sites are bound to substrate, and the amount of ES complex is the same as the total amount of enzyme. [1] : 8.4

Vmax is only one of several important kinetic parameters. The amount of substrate needed to achieve a given rate of reaction is also important. This is given by the Michaelis–Menten constant (Km), which is the substrate concentration required for an enzyme to reach one-half its maximum reaction rate generally, each enzyme has a characteristic KM for a given substrate. Another useful constant is kcat, also called the turnover number, which is the number of substrate molecules handled by one active site per second. [1] : 8.4

The efficiency of an enzyme can be expressed in terms of kcat/Km. This is also called the specificity constant and incorporates the rate constants for all steps in the reaction up to and including the first irreversible step. Because the specificity constant reflects both affinity and catalytic ability, it is useful for comparing different enzymes against each other, or the same enzyme with different substrates. The theoretical maximum for the specificity constant is called the diffusion limit and is about 10 8 to 10 9 (M −1 s −1 ). At this point every collision of the enzyme with its substrate will result in catalysis, and the rate of product formation is not limited by the reaction rate but by the diffusion rate. Enzymes with this property are called catalytically perfect or kinetically perfect. Example of such enzymes are triose-phosphate isomerase, carbonic anhydrase, acetylcholinesterase, catalase, fumarase, β-lactamase, and superoxide dismutase. [1] : 8.4.2 The turnover of such enzymes can reach several million reactions per second. [1] : 9.2 But most enzymes are far from perfect: the average values of k c a t / K m >/K_< m >> and k c a t >> are about 10 5 s − 1 M − 1 < m >^<-1>< m >^<-1>> and 10 s − 1 >^<-1>> , respectively. [66]

Michaelis–Menten kinetics relies on the law of mass action, which is derived from the assumptions of free diffusion and thermodynamically driven random collision. Many biochemical or cellular processes deviate significantly from these conditions, because of macromolecular crowding and constrained molecular movement. [67] More recent, complex extensions of the model attempt to correct for these effects. [68]

Enzyme reaction rates can be decreased by various types of enzyme inhibitors. [69] : 73–74

Types of inhibition


A competitive inhibitor and substrate cannot bind to the enzyme at the same time. [70] Often competitive inhibitors strongly resemble the real substrate of the enzyme. For example, the drug methotrexate is a competitive inhibitor of the enzyme dihydrofolate reductase, which catalyzes the reduction of dihydrofolate to tetrahydrofolate. [71] The similarity between the structures of dihydrofolate and this drug are shown in the accompanying figure. This type of inhibition can be overcome with high substrate concentration. In some cases, the inhibitor can bind to a site other than the binding-site of the usual substrate and exert an allosteric effect to change the shape of the usual binding-site. [72]


A non-competitive inhibitor binds to a site other than where the substrate binds. The substrate still binds with its usual affinity and hence Km remains the same. However the inhibitor reduces the catalytic efficiency of the enzyme so that Vmax is reduced. In contrast to competitive inhibition, non-competitive inhibition cannot be overcome with high substrate concentration. [69] : 76–78


An uncompetitive inhibitor cannot bind to the free enzyme, only to the enzyme-substrate complex hence, these types of inhibitors are most effective at high substrate concentration. In the presence of the inhibitor, the enzyme-substrate complex is inactive. [69] : 78 This type of inhibition is rare. [73]


A mixed inhibitor binds to an allosteric site and the binding of the substrate and the inhibitor affect each other. The enzyme's function is reduced but not eliminated when bound to the inhibitor. This type of inhibitor does not follow the Michaelis–Menten equation. [69] : 76–78


An irreversible inhibitor permanently inactivates the enzyme, usually by forming a covalent bond to the protein. [74] Penicillin [75] and aspirin [76] are common drugs that act in this manner.

Functions of inhibitors

In many organisms, inhibitors may act as part of a feedback mechanism. If an enzyme produces too much of one substance in the organism, that substance may act as an inhibitor for the enzyme at the beginning of the pathway that produces it, causing production of the substance to slow down or stop when there is sufficient amount. This is a form of negative feedback. Major metabolic pathways such as the citric acid cycle make use of this mechanism. [1] : 17.2.2

Since inhibitors modulate the function of enzymes they are often used as drugs. Many such drugs are reversible competitive inhibitors that resemble the enzyme's native substrate, similar to methotrexate above other well-known examples include statins used to treat high cholesterol, [77] and protease inhibitors used to treat retroviral infections such as HIV. [78] A common example of an irreversible inhibitor that is used as a drug is aspirin, which inhibits the COX-1 and COX-2 enzymes that produce the inflammation messenger prostaglandin. [76] Other enzyme inhibitors are poisons. For example, the poison cyanide is an irreversible enzyme inhibitor that combines with the copper and iron in the active site of the enzyme cytochrome c oxidase and blocks cellular respiration. [79]

As enzymes are made up of proteins, their actions are sensitive to change in many physio chemical factors such as pH, temperature, substrate concentration, etc.

The following table shows pH optima for various enzymes. [80]

Enzyme Optimum pH pH description
Pepsin 1.5–1.6 Highly acidic
Invertase 4.5 Acidic
Lipase (stomach) 4.0–5.0 Acidic
Lipase (castor oil) 4.7 Acidic
Lipase (pancreas) 8.0 Alkaline
Amylase (malt) 4.6–5.2 Acidic
Amylase (pancreas) 6.7–7.0 Acidic-neutral
Cellobiase 5.0 Acidic
Maltase 6.1–6.8 Acidic
Sucrase 6.2 Acidic
Catalase 7.0 Neutral
Urease 7.0 Neutral
Cholinesterase 7.0 Neutral
Ribonuclease 7.0–7.5 Neutral
Fumarase 7.8 Alkaline
Trypsin 7.8–8.7 Alkaline
Adenosine triphosphate 9.0 Alkaline
Arginase 10.0 Highly alkaline

Enzymes serve a wide variety of functions inside living organisms. They are indispensable for signal transduction and cell regulation, often via kinases and phosphatases. [81] They also generate movement, with myosin hydrolyzing ATP to generate muscle contraction, and also transport cargo around the cell as part of the cytoskeleton. [82] Other ATPases in the cell membrane are ion pumps involved in active transport. Enzymes are also involved in more exotic functions, such as luciferase generating light in fireflies. [83] Viruses can also contain enzymes for infecting cells, such as the HIV integrase and reverse transcriptase, or for viral release from cells, like the influenza virus neuraminidase. [84]

An important function of enzymes is in the digestive systems of animals. Enzymes such as amylases and proteases break down large molecules (starch or proteins, respectively) into smaller ones, so they can be absorbed by the intestines. Starch molecules, for example, are too large to be absorbed from the intestine, but enzymes hydrolyze the starch chains into smaller molecules such as maltose and eventually glucose, which can then be absorbed. Different enzymes digest different food substances. In ruminants, which have herbivorous diets, microorganisms in the gut produce another enzyme, cellulase, to break down the cellulose cell walls of plant fiber. [85]


Several enzymes can work together in a specific order, creating metabolic pathways. [1] : 30.1 In a metabolic pathway, one enzyme takes the product of another enzyme as a substrate. After the catalytic reaction, the product is then passed on to another enzyme. Sometimes more than one enzyme can catalyze the same reaction in parallel this can allow more complex regulation: with, for example, a low constant activity provided by one enzyme but an inducible high activity from a second enzyme. [86]

Enzymes determine what steps occur in these pathways. Without enzymes, metabolism would neither progress through the same steps and could not be regulated to serve the needs of the cell. Most central metabolic pathways are regulated at a few key steps, typically through enzymes whose activity involves the hydrolysis of ATP. Because this reaction releases so much energy, other reactions that are thermodynamically unfavorable can be coupled to ATP hydrolysis, driving the overall series of linked metabolic reactions. [1] : 30.1

Control of activity

There are five main ways that enzyme activity is controlled in the cell. [1] : 30.1.1


Enzymes can be either activated or inhibited by other molecules. For example, the end product(s) of a metabolic pathway are often inhibitors for one of the first enzymes of the pathway (usually the first irreversible step, called committed step), thus regulating the amount of end product made by the pathways. Such a regulatory mechanism is called a negative feedback mechanism, because the amount of the end product produced is regulated by its own concentration. [87] : 141–48 Negative feedback mechanism can effectively adjust the rate of synthesis of intermediate metabolites according to the demands of the cells. This helps with effective allocations of materials and energy economy, and it prevents the excess manufacture of end products. Like other homeostatic devices, the control of enzymatic action helps to maintain a stable internal environment in living organisms. [87] : 141

Post-translational modification

Examples of post-translational modification include phosphorylation, myristoylation and glycosylation. [87] : 149–69 For example, in the response to insulin, the phosphorylation of multiple enzymes, including glycogen synthase, helps control the synthesis or degradation of glycogen and allows the cell to respond to changes in blood sugar. [88] Another example of post-translational modification is the cleavage of the polypeptide chain. Chymotrypsin, a digestive protease, is produced in inactive form as chymotrypsinogen in the pancreas and transported in this form to the stomach where it is activated. This stops the enzyme from digesting the pancreas or other tissues before it enters the gut. This type of inactive precursor to an enzyme is known as a zymogen [87] : 149–53 or proenzyme.


Enzyme production (transcription and translation of enzyme genes) can be enhanced or diminished by a cell in response to changes in the cell's environment. This form of gene regulation is called enzyme induction. For example, bacteria may become resistant to antibiotics such as penicillin because enzymes called beta-lactamases are induced that hydrolyse the crucial beta-lactam ring within the penicillin molecule. [89] Another example comes from enzymes in the liver called cytochrome P450 oxidases, which are important in drug metabolism. Induction or inhibition of these enzymes can cause drug interactions. [90] Enzyme levels can also be regulated by changing the rate of enzyme degradation. [1] : 30.1.1 The opposite of enzyme induction is enzyme repression.

Subcellular distribution

Enzymes can be compartmentalized, with different metabolic pathways occurring in different cellular compartments. For example, fatty acids are synthesized by one set of enzymes in the cytosol, endoplasmic reticulum and Golgi and used by a different set of enzymes as a source of energy in the mitochondrion, through β-oxidation. [91] In addition, trafficking of the enzyme to different compartments may change the degree of protonation (e.g., the neutral cytoplasm and the acidic lysosome) or oxidative state (e.g., oxidizing periplasm or reducing cytoplasm) which in turn affects enzyme activity. [92] In contrast to partitioning into membrane bound organelles, enzyme subcellular localisation may also be altered through polymerisation of enzymes into macromolecular cytoplasmic filaments. [93] [94]

Organ specialization

In multicellular eukaryotes, cells in different organs and tissues have different patterns of gene expression and therefore have different sets of enzymes (known as isozymes) available for metabolic reactions. This provides a mechanism for regulating the overall metabolism of the organism. For example, hexokinase, the first enzyme in the glycolysis pathway, has a specialized form called glucokinase expressed in the liver and pancreas that has a lower affinity for glucose yet is more sensitive to glucose concentration. [95] This enzyme is involved in sensing blood sugar and regulating insulin production. [96]

Involvement in disease

Since the tight control of enzyme activity is essential for homeostasis, any malfunction (mutation, overproduction, underproduction or deletion) of a single critical enzyme can lead to a genetic disease. The malfunction of just one type of enzyme out of the thousands of types present in the human body can be fatal. An example of a fatal genetic disease due to enzyme insufficiency is Tay–Sachs disease, in which patients lack the enzyme hexosaminidase. [97] [98]

One example of enzyme deficiency is the most common type of phenylketonuria. Many different single amino acid mutations in the enzyme phenylalanine hydroxylase, which catalyzes the first step in the degradation of phenylalanine, result in build-up of phenylalanine and related products. Some mutations are in the active site, directly disrupting binding and catalysis, but many are far from the active site and reduce activity by destabilising the protein structure, or affecting correct oligomerisation. [99] [100] This can lead to intellectual disability if the disease is untreated. [101] Another example is pseudocholinesterase deficiency, in which the body's ability to break down choline ester drugs is impaired. [102] Oral administration of enzymes can be used to treat some functional enzyme deficiencies, such as pancreatic insufficiency [103] and lactose intolerance. [104]

Another way enzyme malfunctions can cause disease comes from germline mutations in genes coding for DNA repair enzymes. Defects in these enzymes cause cancer because cells are less able to repair mutations in their genomes. This causes a slow accumulation of mutations and results in the development of cancers. An example of such a hereditary cancer syndrome is xeroderma pigmentosum, which causes the development of skin cancers in response to even minimal exposure to ultraviolet light. [105] [106]

Similar to any other protein, enzymes change over time through mutations and sequence divergence. Given their central role in metabolism, enzyme evolution plays a critical role in adaptation. A key question is therefore whether and how enzymes can change their enzymatic activities alongside. It is generally accepted that many new enzyme activities have evolved through gene duplication and mutation of the duplicate copies although evolution can also happen without duplication. One example of an enzyme that has changed its activity is the ancestor of methionyl amino peptidase (MAP) and creatine amidinohydrolase (creatinase) which are clearly homologous but catalyze very different reactions (MAP removes the amino-terminal methionine in new proteins while creatinase hydrolyses creatine to sarcosine and urea). In addition, MAP is metal-ion dependent while creatinase is not, hence this property was also lost over time. [107] Small changes of enzymatic activity are extremely common among enzymes. In particular, substrate binding specificity (see above) can easily and quickly change with single amino acid changes in their substrate binding pockets. This is frequently seen in the main enzyme classes such as kinases. [108]

Artificial (in vitro) evolution is now commonly used to modify enzyme activity or specificity for industrial applications (see below).

Enzymes are used in the chemical industry and other industrial applications when extremely specific catalysts are required. Enzymes in general are limited in the number of reactions they have evolved to catalyze and also by their lack of stability in organic solvents and at high temperatures. As a consequence, protein engineering is an active area of research and involves attempts to create new enzymes with novel properties, either through rational design or in vitro evolution. [109] [110] These efforts have begun to be successful, and a few enzymes have now been designed "from scratch" to catalyze reactions that do not occur in nature. [111]

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Levels of Biological Organization

The cell is known to be the basic building block of life. It performs various metabolic functions like providing structure and rigidity to the body, converting food into nutrients and energy, and others. While it is apparently not the smallest particle (organelles, molecules, and atoms are even smaller in size), the cell is called as such because it is the smallest living entity that can function on its own. At cellular level, organisms can be classified into two: single-celled organisms (unicellular) and multiple-celled organisms (multi-cellular).

When similar cells aggregate, they form a tissue. Basically, a tissue is a group of interconnected cells that perform the same function. Like cells, tissues perform metabolic processes that keep the organism alive. In multi cellular organisms, the study of tissue is called histology (from Greek words histos meaning “tissue” and logos meaning “study of”.

Coming from the Latin word organum which means “tool” or “instrument”, an organ is a collection of tissues and similar structures that all function as one. Organs of multi-cellular organisms are in fact very diverse. In plants, their organs include the flowers (if there is) roots, stems, and the leaves. On the other hand, organs of animals include the brain, heart, stomach, eyes, and many more. Further Reading: New World Encyclopedia

Next to the hierarchy is the organ system. By definition, an organ system is an association of different organs and other anatomical structures that perform a certain physiological process. While each organ system in an organism work as a distinct entity, they all function in cooperation with each other in order to help keep the organism alive. In plants, organ systems include the root and shoot system, while animal organ systems include the digestive, nervous, circulatory system, and others.

An organism can be simply defined as any living thing that is composed of various organ systems that function altogether. By far, about 8.7 million organisms are estimated by scientists but only 1.2 million of that have been identified. Hence, various efforts have been continually done in order to discover them. Check out the immortal organisms that defy death.

When similar organisms group together, they form the next level in the organization, a population. By definition, a population is formed when such individuals reside a common environment at a given time. For instance, a population can change over time due to several events like births, mortality, and migration of organisms. Oftentimes, the number of individuals in a population is highly dependent on the abundance of resources and the presence of favorable climate. In addition, predation and competition are also biological factors that control populations.

Next to the hierarchy is the community. A community is defined as the interactions of different populations with each other. Apparently, various interactions can exist such as mutualism, commensalism, predation, parasitism, and competition. Oftentimes, a certain population of organisms tend to dominate the community and hence are relatively more abundant than others. Such is considered as a distinguishable characteristic of a biological community.

A short term for “ecological system”, an ecosystem is the interaction of (organisms, population, and community) to their abiotic or non-living environment. The biotic (living) members of an ecosystem are highly dependent on such abiotic factors which include the weather, sunlight, water depth, salinity, and the availability of nutrients. Hence, the presence or absence of even one factor can greatly affect the ecosystem. In addition to that, one distinct characteristic of an ecosystem is that each organism has a “niche” or role to perform.

Ever heard of tundra, savanna, desert, tropical rain-forest, and grassland? Some of these are quite familiar but if you haven’t heard of them yet, all mentioned are types of the next level in the biological organization, the biome. As described by the different environments, a biome is a very huge geographic area where various ecosystems exist and different organisms adapt to it. In general, a biome is more of the continental grouping of various ecosystems in a particular climate. Further reading: World Biomes.

Coming from the Greek word “bios” meaning “life”, and “sphaira” meaning sphere, biosphere is basically the topmost level in the hierarchy of living organisms. A biosphere is a global system that generally comprises everything where life exist and the abiotic environments they reside in, all blending with each other. It is basically the sum of all ecosystems on Earth, hence, it is also called as the ecosphere.

Our planet is indeed composed of a variety of living things ranging from a simple cell to a massive sphere of life forms. While each organism tend to vary on size and function, still, no one lives alone and can live alone. Each organism somehow depends or affect the life of other living organisms and non-living factors in the environment. Any change in a part of one system can drastically increase or decrease the chances of survival of an organism.

Absolutely, such hierarchy in the biological organization is sufficient enough to show the complexity of life. Doesn’t that make you appreciate life more?

5 Marks Questions

Chapter 13
Organisms and Populations

5 Marks Questions
1. What is altitude sickness? What its causes and symptoms? How does human body try to overcome altitude sickness?
Ans. Breathlessness at high attitudes.Cause :Low atmospheric pressure at high altitudes due to which body does not get enough oxygen. Symptoms :Nausea, fatigue and heart palpitations.
Body adapts by :
(a) increasing red blood cell production
(b) decreasing binding affinity of haemoglobin
(c) by increasing breathing

2. Orchid flower, Ophrys co-evolves to maintain resembelance of its petal to female bee. Explain how and why does it do so?

  • employs ‘Sexual deceit’
  • one petal bears uncanny resemblance to female of the bee.
  • Male bee is attracted to what it perceives as a female ‘pseudocopulates,’during which pollen dusted on male been is body .
  • Male bee transfers pollen to another flower when the same bee pseudocopulates with another flower.
  • Ophrysdoes so because pollination success will be reduced unless it co-evolves with female bee.

3.Describe the exponential growth model of a diagram along with a curve?
Ans. This kind of curve is observed in the case of under population of reindeer growing in apredator free natural environment having plenty of food. In this case, the curve formed is J-curvethe small population first takes time to adjust into new environment so there is no increase in thepopulation. Once they get adapted they multiply exponentially. This growth & multiplicationcontinues so far the food is available. After sometime the food supply becomes less as compared tothe population increases. This causes mass starvation & mortality & results in the formation of Jshaped curve.
The J-shaped growth form is described by equation
Δ N Δ t = rN or Δ N Δ t N ΔNΔt= rN or ΔNΔtN

4.Describe the logistic growth model of population along with a suitable curve. Why is this curve more realistic?
Ans. The logistic growth curve shows a sigmoid or a S-shaped curve. It has three phases:-
(i) Lag-phase :- It is the early phase of little or no growth. Lag phase is one in which under population of cells adapt to or stablises with the growth conditions before embarking up their multiplication.
(ii) Log phase or Exponential phase :- It is the middle phase of rapid or geometric rise, Once stabilized cells starts to multiply rapidly when the small population is stablised, the multiply becomes faster because of the plenty amount of food & other requirements of life.
(iii) Stationary phase or steady phase:- Soon after the amount of food decreases in proportion to the number of cells & this results in the onset of stationary phase. During this phase, the number of new cells produced is roughly equal to the number of cells dead & so there is no net increase in the number of cells.
Sigmoid growth curve is demonstrated by fo Δ N Δ t = rN ( K − N ) N ΔNΔt = rN (K−N)N

Δ N − rate of change inpopulation ΔN− rate of change inpopulation Δ t – change in time . Δt – change in time.
K – carrying capacity
R – biotic potential

5.Give an example to show that completely unrelated species can also compete for same resources?
Ans. Completely unrelated species can also compete for same resources for e.g. In certain shallow lakes of South America the visiting flamingoes & the native fishes compete for the same zooplanktons as their food.

6.What is Age pyramid? What are the different types of age pyramid?
Ans. The geometrical diagrammatic representation of different age groups in a population of any organism is called Age of pyramids. These are of three types:-
i) Expanding pyramid:- It is a broad base, triangular pyramid which represents a population containing large number of young people. It is rapidly expanding population with high birth rate.
ii) Stable pyramid:- It represents a moderate proportion of young to old. As the rate of growth becomes slow & stable i.e.- pre-reproductive & reproductive age groups becomes more or less equal in size.
iii) Declining Pyramid:- The type of pyramid of population decreasing in size is characterised by a narrow base because there are fewer pre-reproductive individuals than in the other two age categories.

7. Differentiate between regulators & conformers? Why do small animals do not show regulations?
Ans. The organisms which maintain homeostasis by physiological or behavioral means & ensures aconstant body temperature & constant osmotic concentration etc. are called regulators e.g. all birds,mammals some lower vertebrates & invertebrates, for example in summer, when outside temp is morethan our body temperature we sweat profusely evaporative cooling brings the body temp – down.Whereas those organisms which cannot maintain a constant internal environment. Their bodytemperature changes with ambient temperature e.g. majority of animals & nearly all plants.

Small organisms does not show regulationbecause thermoregulation is an energy –expensive process. Since small animals havelarge surface area relative to volume, they tendto lose body heat very fast when it is coldoutside they have to expend much energy togenerate body heat through metabolism.