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There are countless functions that proteins fulfill. Listed below are the most common ones.


Table summary:
Created by KM based on text

1) Enzymes. Every process carried out in the body involves, at some point or entirely, a chemical reaction. Chemical reactions proceed according to a physical law known as Gibbs Free Energy. This law dictates that energy must be put into a system in order for a chemical reaction to take place. The amount of energy needed to start a reaction is referred to as "activation energy". This activation energy is not always readily available; this type of reaction is non-spontaneous. This is why enzymes exist. Enzymes catalyze a reaction, meaning that they speed it up and allow it to proceed quicker than it would spontaneously.

a. An enzyme is a specialized protein that lowers activation energy. It does not add energy to the system, it reduces the amount of energy required to begin the reaction. Special emphasis should be taken on the fact that the requirements are lowered, as this is where students frequently experience misconceptions. (Enzymes do not add energy to a reaction).

Enzymes lower the activation energy:

Enzymes lower the activation energy required by a reaction by binding to their "substrate" (the molecule that enzymes assist in a reaction). Substrates typically fit specific enzymes, making enzymes very precise tools.

Note: an enzyme may have more than one substrate.

In chemical reactions, nothing can occur before the molecules are in close proximity to each other. Hence, enzymes lower activation energy by binding to the two compounds that are needed for the chemical reaction - bringing them together. This greatly increases the productivity of the cell, as it eliminates the need to wait for the molecules to "bump" into each other.

Note: if all reactions necessary for life were allowed to proceed without enzymes, not even the simplest bacteria would be capable of survival! Enzymes are absolutely essential.

There are other ways in which an enzyme may assist a reaction. One such mechanism proceeds by binding to a substrate, and subsequently prying the substrate open so that its functional groups are exposed. This allows the reaction, which normally would not proceed at all (due to an occluded reaction site) to occur.

2) Structural Proteins. Enzymes comprise a large portion of protein functionality, but proteins are also useful in many other applications. For example, cells and tissues could not maintain their structure without structural proteins . Collagen is a well-known structural protein. This protein is often found in the extracellular matrix (the space outside of the cell) holding things like tendons and ligaments together.

Another structural protein found in the human body is called actin. This is a vital part of our cells' cytoskeletons, and is, therefore, very important to the shape and conformation that they hold.

3) Transport Proteins. Oxygen, hormones, and many other substances cannot travel throughout the body without assistance. For this, transport proteins come in very handy. Think of them like a taxi. Sometimes, an individual finds himself in an unfamiliar place, and cannot get to his desired location. So, he calls a cab. Transport proteins are the cabs. Oxygen cannot freely float around in human blood, for various reasons, so a protein called hemoglobin binds to it and takes it to its destination.

4) Motor Proteins. Muscles are important because they work together to produce complex motions. These movements would be impossible without the existence of motor proteins. Proteins such as myosin are capable of changing their conformation in response to chemical stimulus, allowing the cells that possess them to change their shape. This is how they accelerate their position in three-dimensional space.

5) Storage Proteins. Certain substances our bodies rely upon for survival are dangerous to the surrounding tissues if left to drift about unhindered. For that, there are storage proteins . For example, iron is stored in the liver by a protein known as ferritin.

6) Signal Proteins. The body's hormonal system functions as a very complex postal system. Signal proteins , often hormones, are specialized compound synthesized to send a message to a specific or broad location. Some signal proteins send a message to every cell in the body, and some are so specific that only one type of cell can recognize them. These proteins carry commands such as nerve growth factor ( NGF), epidermal growth factor ( EGF), and numerous others.

7) Receptor Proteins. If there are signal proteins, there must be someone to receive them. A well-known example is the acetylcholine receptor, found in muscle cells at neuromuscular junctions. These hold specific conformations, capable of recognizing specific signal proteins.

8) Gene Regulatory Proteins. Gene expression is very complex; it is regulated by proteins, edited, damaged occasionally, re-edited, and sometimes silenced. In order for a gene to be properly transcribed by RNA polymerase, some direction is in order. If all the genes were expressed at once, biological organisms would be aggregated messes of proteins indeed!

To rectify this, the cell uses proteins called regulatory proteins . These bind to the DNA molecule and do one of two things: activate gene expression, or inhibit it. Bacteria contain a lactose repressor that prevents an enzyme necessary for the catabolism of lactose from being expressed when no such sugar is available. Similarly, there are proteins that bind to the DNA strand when a certain gene needs to be expressed - this is usually performed by a protein involved in a signal transduction pathway.

Regulatory protein inhibiting or switching off a gene:

9) Miscellaneous. As first outlined above, cells possess far more than just eight categories of proteins. However, beyond the broad eight categories, the proteins that do not fit within boundaries are typically tailor made for the cell/organism that contains them. Some jellyfish, for example, have a protein called green fluorescent protein ( GFP) that gives them mystical, green, glow-in-the-dark properties.

This list referenced a textbook called Essential Cell Biology, Fourth Edition throughout its composition. The bulk of the material was found on page 122. Authors of this book include: Bruce Alberts, Dennis Bray, Karen Hopkin, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, and Peter Walter. For further reading, this textbook may be purchased from Google Books [here]



Well, yes.


Amino acids are the building blocks of proteins, and have an amine group #(-NH_2)#, as well as a carboxyl group #(-COOH)#.

When hundreds and thousands of them combine using peptide bonds, they form proteins, which are nutrients essential for survival.

So, we can say that amino acids are monomers of proteins.

Here is a simple picture to represent how amino acids build up proteins:

To read about peptide bonding, visit:



Sexual Reproduction produces more variance in a species that Asexual Reproduction which is the organism 'cloning' itself.


In Sexual Reproduction, variance occurs, which is also the main driver in evolution and natural selection. If you are constantly cloning yourself, like in Asexual reproduction, no variance or natural selection or evolution occurs.

A good example of this are bananas. Wild Bananas have seeds and reproduce sexually, whereas the Bananas we consume, the Cavendish variety, reproduce asexually, and we take cuttings of underground roots from these trees and plant them elsewhere to produce more Cavendish plants.
However, due to the lack of variety, we have had (pretty much) whole species of bananas wipe out, such as the Gros Michel variety, in which much of the species became extinct due to Panama Disease- which is a fungus which lives off of the plant and eventually kills it.

If the plant was instead sexually reproducing, we would not have lost the species as there would have been a few members of the variety which would probably have had immunity to the disease, or at least could fight back.
However, this would not be so as in this situation, each Gros Michel was in fact an identical clone to one another, all a cuttings from the "original Gros Michel", and were all genetically the same- and were seedless meaning there is pretty much no way they could sexually reproduce.

This is why (I'm going slightly off topic sorry) in Star Wars the Empire fade out the Clone Troopers, and instead have a variety of recruits from around the galaxy in their Storm trooper ranks. This is due to the fact a biological weapon could be developed to infect and target the clones, and due to all having exactly the same genetics, would all die and the Republic/New Empire would lose its entire army.

Anyway, hope this clears it up for you, if you don't sexually reproduce, anything new changes to your environment can potentially kill off your whole species, rather than leaving a few who have the characteristics to adapt and survive this new change.

Hope this helps!
-Charlie Palmer



The trp operon specifically, is repressible.


First, we must understand that there are a lot of operons out there in molecular biology that can do a variety of things.
All operons are either under positive or negative control. Positively controlled operons are ones where gene expression is only stimulated by the presence of a regulatory protein. Negatively controlled operons are ones where gene expression is turned off in the presence of a repressor - this is either repressible or inducible.

The trp operon you are talking about is a repressible system. It has multiple domains (structures that are specifically activated under certain conditions on a molecular scale).
So yes, the trp operon regulates the production of tryptophan. Let's give two situations where it is most obvious.

When levels of tryptophan are very high in the cell, levels of tryptophan RNA are also obviously very high. Therefore, immediately after translation, the mRNA will move quickly through the ribosome complex on domain 1 and the short peptide structure in tryptophan is translated very quickly.As an effect of quick translation, domain 2 in the trp operon becomes chemically associated with the ribosome complex, effectively blocking it, while domain 3 binds to domain 4, stopping translation as a loop feedback occurs. This is known as an attenuation of transcription, and only about 10% of regular mRNA is created.

On the other hand, when cellular levels of tryptophan are low, translation of the short peptide is translated slowly in domain 1. As the mRNA moves slowly, the domain 2 binds to domain 3 and because the ribosome doesn't bind to domain 2, transcription occurs normally and biosynthesis of tryptophan occurs normally.



See Below


ATP is the phosphorylated form of the nucleoside Adenosine.

Adenosine is a ribose sugar and the base Adenine.


In order to make ATP, you just have to add 3 phosporic acid groups (one bond is a phospho-ester, the other bonds are phophoanhydrides).

The result of the phosphorylation is ATP, Adenosine Triphosphate.




Cells with the full set of chromosomes are #"diploid somatic cells."#


Somatic cells are the cells that make up the vast majority of the body.

Somatic cells each have the complete set of chromosomes.

In humans, that means that the somatic cells have #46# chromosomes each #-# #23# pairs, one set of #23# from each parent, for a total of #46.#

In order to maintain the correct number of chromosomes when the egg cell and sperm cell combine, the chromosome number in the gametes is cut in half during #"meiosis"# (the "reduction" division.)

Somatic cells, with the full set of chromosomes are #diploid,"# with the #2n# chromosome number.

Gametes, with one half the full number of chromosomes, are #"haploid,"# with the #1n# chromosome number.

During fertilization, the somatic cells' #2n  "diploid"# chromosome number is restored when both of the #1n  "haploid"# gametes fuse with each other.

Here's an image of this process:
The #"diploid"# #(2n)# cells are #"somatic"# cells. and the #"haploid"  (1n)# cells are the gametes.

enter image source here



Here's what I get.


A digestive system takes in nutrients, energy, and water and gets rid of waste products.

Humans have evolved an efficient internal digestive and circulatory system that extends to every cell in the body,

If we had no internal digestive system, we would need a different way to take in nutrients and get rid of wastes.

1. The plant option

We might develop roots on our feet that would temporarily dip into the soil to obtain water and nutrients.

We would then have to develop a large surface area (leaves?) and a photosynthesis mechanism to obtain energy from sunlight.

Our mobility would be restricted, and we would look completely different.

2. The fungal option

We might develop methods of absorbing nutrients directly through our cell walls.

Then, we might have to lie around in the soil most of the day to absorb the nutrients.

If we depended on diffusion through cell walls as our source of food, we would be much smaller.

As we get bigger, our outer cells will be unable to keep up with needs of the inner cells.

Eventually, we get so big that our inner cells will die because they can't get nutrients quickly enough.

We can follow the diffusion of #"NaOH"# in agar cubes.

The "nutrients" have diffused all the way to the centre of the smallest cube, but the largest cube is mostly "starved" in the centre.

3. Other options

Who knows? Perhaps we might evolve some other type of digestive system.

Personally, I'm happy with what I have.



One method is using a respirometer with woodlice or another small organism inside it, another method is using a spirometer


Respirometer apparatus can be set up as shown: This involves two containers, connected by a capillary tube containing a coloured fluid (a manometer).

A live organism (eg a woodlouse) is placed in one container, along with a #CO_2# absorbent (eg potassium hydroxide solution) to remove any #CO_2# produced by the respiring organism.

A glass bead of equal mass and volume is placed in the other container to ensure both containers are identical in every way other than the presence of the organism, so that the only variable is the amount of #O_2# consumed.

As the organism present respires, it will consume #O_2#, causing the pressure of the gas in the container on the left-hand side to decrease, drawing the coloured fluid in the manometer up into the left side. The distance moved by the coloured fluid can then be used to calculate the volume of #O_2# that was lost from the left container, if the width of the manometer tube is known. This is done using the equation for the volume of a cylinder #V = pir^2h#. The average rate of #O_2# consumption can then be measured by dividing the volume of #O_2# consumed by the time.

Similarly, spirometer apparatus can be set up as shown:

A spirometer works in much the same way as a respirometer. A patient inhales in and out of the mouthpiece. Any #CO_2# they breathe out is absorbed by a #CO_2# absorbant, so the only change in the inside of the spirometer is a gradual decrease in #O_2# present as more and more of it is absorbed by the patient.

An image trace as shown is produced:

As the patient exhales, the volume of the spirometer increases, and vice versa as the patient inhales. This causes the peaks and troughs shown on the graph. The average volume of the spirometer gradually decreases as #O_2# is absorbed by the patient.

The rate at which the graph decreases (ie the gradient of the trend line) can be found - this gives a rate for average #O_2# consumption, in #dm^3 s^-1#.



papain (enzyme)


Papain is an enzyme, present in the leaves, latex, root, and fruit of the papaya plant.

As an enzyme, papain catalyses the breakdown of proteins by hydrolysis (addition of a water molecule).

This means that papain is able to break down tough meat fibers and make meat more tender.

Therefore, meat can cook faster when a papaya is added.




The Fluid Mosaic model of the cell membrane suggests that embedded within the layers are a series of proteins specially designed to channel specific materials.

Some channel a specific material in both directions, some channel a given material only in one direction. Some particles are small enough and uncharged enough that they can pass through the membrane without any help!

A series of contractile "muscle-like" fibers within the cell can cause the membrane to warp and fold to engulf particles or fluid via a process known as endocytosis.
These same fibers can push fluid or particles out of the cell via the process exocytosis.
The cell membrane has a wide variety of strategies it can employ to move specific things in and out of the cell and is an important part of a cell's function (and survival)