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Proteins

Proteins are a group of biological molecules that play many important roles in living things.

They are very large biological molecules which are made by joining together many smaller molecules called amino acids.

Proteins are made by ribosomes. At a ribosome, hundreds or thousands of amino acids are joined together to form a long chain, which then folds up to form a protein.

Diagram showing how a protein is made. On the left there is a label that says "Amino Acids". Under this, there are several circles, in a variety of different colours. These represent the amino acids. An arrow to the right points to a drawing of the same circles but now joined together into a chain. This is labelled, "Chain of Amino Acids". A label over the arrow reads, "Amino acids joined together at a ribosome". A downwards arrow points to the same chain of amino acids but now folded up into a protein. This is labelled, "Protein". This second arrow is labelled, "Chain of amino acids folds into a protein".

At a ribosome, many amino acids join together into a chain. This chain of amino acids then folds into a protein. This diagram shows a relatively small number of amino acids, but there are actually hundreds or thousands of amino acids in most proteins.

Functions of proteins

Proteins carry out a huge variety of functions in living things. Almost every biological process uses proteins in some way. Some complex biological processes, like photosynthesis, require huge numbers of proteins all carrying out different functions.

Some examples of the functions of proteins are listed below. You do not need to memorise this list - it is just to give you an idea of how important proteins are to living things. Some of these types of proteins will be covered in more detail in other parts of the course.

Enzymes

Enzymes are a group of proteins that catalyse (speed up) chemical reactions. Most chemical reactions in living things would happen too slowly without enzymes to speed them up.

Computer generated image of the enzyme rubisco. It is a spacefilling diagram, which means that it is made of up lots of little blobs representing the atoms. Therefore, it has a lumpy appearance. Its overall shape is also a blob. Different parts of the enzyme have been coloured with different colours in the image.

Computer generated image of an enzyme called rubisco, which plays an important role in photosynthesis.

Transport proteins

Transport proteins are proteins that move substances from one place to another. For example, there are transport proteins that help substances move across membranes.

Diagram of a channel protein, which is a type of transport protein that allows substances to diffuse across cell membranes. A section of a cell membrane is shown. Below the membrane on the diagram is the cytoplasm of the cell. Above the membrane on the diagram is the outside of the cell. The membrane has a channel protein spanning across it. The channel protein is providing a channel for a substance (shown as small, green shapes) to diffuse through.

Diagram showing a channel protein - a type of transport protein which allows substances to diffuse across cell membranes. Image: 0306 Facilitated Diffusion Channel Protein.jpg by OpenStax on Wikimedia Commons (CC BY 3.0 - https://creativecommons.org/licenses/by/3.0/deed.en)

There are also transport proteins that carry substances longer distances. For example, haemoglobin is a protein that carries oxygen in the blood of many animals.

Structural proteins

There are many proteins that have structural roles. For example, all cells contain long fibres made of structural proteins. These fibres gives the cells shape and strength. There are also structural proteins that give strength to hair, nails, skin, scales and horns in animals.

Microscope image of heart muscle cells. The cells are on a black background. The outlines of the cells are not visible. All that is visible are green lines and blue ovals. The blue ovals are the nuclei of the cells. The green lines are structural protein filaments inside the cells. Each cell contains many of these filaments, running roughly parallel to each other. The shape of each cell can be inferred from the way the structural protein filaments are distributed.

Heart muscle cells stained to show structural protein fibres in green and cell nuclei in blue. Image: F-actin filaments in cardiomyocytes.jpg by s1415 on Wikimedia Commons (CC BY-SA 4.0 - https://creativecommons.org/licenses/by-sa/4.0/deed.en)

Hormones

A hormone is a biological molecule that stimulates changes in an organism. Some hormones (such as insulin) are proteins.

Diagram of the structure of insulin. Each individual atom is shown as a blob. The overall structure is a roughly round shape with some bits sticking out.

The structure of insulin. Image: 014-insulin-4ins.gif by David Goodsell on Wikimedia Commons (CC BY 3.0 - https://creativecommons.org/licenses/by/3.0/deed.en)

Antibodies

Antibodies are group of proteins that play important roles in the immune systems of animals.

An animated image of the structure of an antibody. The structure is roughly Y shaped and is very lumpy. It is rotating, allowing all sides of the structure to be seen.

The structure of an antibody. Image: Hcp-0.0-CRYSVITA-antibody rotating 03.gif by CRYSVITA on Wikimedia Commons (CC BY-SA 4.0 - https://creativecommons.org/licenses/by-sa/4.0/deed.en)

Receptors

Receptors are structures in an organism that detect stimuli (such as light, sound or touch). Many receptors are proteins.

For example, all cells have receptor proteins in their cell membranes which allow them to detect other cells that they come into contact with (cell recognition).

Computer generated image of a receptor protein embedded in a cell membrane. The cell membrane is shown as a grey strip behind the receptor protein. The receptor protein has a blobby appearance. A molecule of adrenaline is shown bound to it.

A receptor protein that detects the hormone adrenaline. The protein is shown in red. It is embedded in the cell membrane, which is shown as a grey strip. When a molecule of adrenaline - shown in orange - binds to the receptor protein, changes are triggered within the cell. Image: 100-AdrenergicReceptors-2rh1.tif by David Goodsell on Wikimedia Commons (CC BY 3.0 - https://creativecommons.org/licenses/by/3.0/deed.en)

Proteins that generate movement

There are some proteins that generate movement. Examples of movements that are generated by proteins include: muscle contraction, the contraction of cells during cell division, the movement of flagella and cilia, and the movement of substances within cells.

Photograph taken through a microscope of heart muscle tissue.

Muscle tissue contains large amounts of proteins that generate movement.

Storage proteins

Some proteins are used to store useful substances, such as iron or amino acids. The seeds of many plants contain large amounts of storage proteins which store useful substances for the developing plant.

Computer generated image of the structure of a storage protein called ferritin, which stores iron in cells. The protein is roughly spherical in shape.

A storage protein called ferritin, which stores iron inside cells. Image: Ferritin.png by Vossman on Wikimedia Commons.

Toxins

Toxins are biological molecules that organisms use to cause harm to other organisms. Many toxins are proteins. For example, plants produce many toxic proteins to prevent animals from eating them.

Computer generated image of a toxin. It has a blobby shape, like other globular proteins. It is made up of four different subunits, which are shown in different colours.

The structure of a toxin which is produced by the bacterium Shigella dysenteriae.

Flashcards

Flashcards help you memorise information quickly. Copy each question onto its own flashcard and then write the answer on the other side. Testing yourself on these regularly will enable you to learn much more quickly than just reading and making notes.

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What are proteins?

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What are proteins made of and how are they made?

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