Membrane components

Membrane proteins

Membrane proteins have important roles in the various functions of membranes.
There are two types of proteins in the cell-surface membrane - intrinsic and extrinsic proteins.

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Intrinsic proteins, or integral proteins, are transmembrane proteins that are embedded through both layers of a membrane.
They have amino acids with hydrophobic R-groups on their external surfaces, which interact with the hydrophobic core of the membrane, keeping them in place.

Channel and carrier proteins are intrinsic proteins.
They are both involved in transport across the membrane.
- Channel proteins provide a hydrophillic channel that allows the passive movement of polar molecules and ions down a concentration gradient through membranes. They are held in position by interactions between the hydrophobic core of the membrane and the hydrophobic R-groups on the outside of the proteins.
- Carrier proteins have an important role in both passive transport (down a concentration gradient) into cells.
- This often involves the shape of the protein changing.

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Glycoproteins are intrinsic proteins.
They are embedded in the cell-surface membrane with attached carbohydrate (sugar) chains of varying lengths and shapes.
Glycoproteins play a role in cell adhesion (when cells join together to form tight junctions in certain tissues) and as receptors for chemical signals,

When the chemical binds to the receptor, it elicits a response from the cell.
This may cause a direct response or set off a cascade of events inside the cell.
This process is known as cell communication or cell signalling.
Examples include:
- receptors for neurotransmitters such as acetylcholine at nerve cell synapses. The binding of the neurotransmitters triggers or prevents an impulse in the next neurone.
- receptors for peptide hormones, including insulin and glucagon which affect the uptake and storage of glucose by cells.
Some drugs act by binding to cell receptors.
For example, beta blockers are used to reduce the response of the heart to stress.

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Glycolipids are similar to glycoproteins.
They are lipids with attached carbohydrate (sugar) chains.
These molecules are called cell markers or antigens and can be recognised by the cells of the immune system as self (of the organism) or non-self (of cells belonging to another organism).

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Extrinsic proteins or peripheral proteins are present in one side of the bilayer.
They normally have hydrophillic R-groups on their outer surfaces and interact with the polar heads of the phospholipids or with intrinsic proteins.
They can be present in either layers and some move between layers.

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Cholesterol is a lipid with a hydrophillic end and a hydrophobic end, like a phospholipid.
It regulates the fluidity of membranes.

Cholesterol molecules are positioned between phospholipids in a membrane bilayer, with the hydrophilic end interacting with the heads and the hydrophobic end interacting with the tails, pulling them together.
In this way cholesterol adds stability to membranes without making them too ridgid.
The cholesterol molecules prevent the membranes becoming too solid by stopping the phospholipid molecules from grouping too closely and crystallising.

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Like enzymes, proteins in the membranes forming organelles, or present within organelles, have to be in particular positions for chemical reactions to take place.
For example, the electron carriers and the enzyme ATP synthase have to be in the correct positions within the cristae (inner membrane of the mitochondrion) for the production of ATP in respiration.
The enzymes of photosynthesis are found on the membrane stacks within the chloroplasts.

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