The Structure and Role of Proteins in Cell Membranes

The Structure and Role of Proteins in Cell Membranes

Cells are the building blocks of which all living organisms are

composed. There are lots of different types of cells that make up

living organisms but they are all similar in structure. All cells are

surrounded by a cell membrane (or plasma membrane), which controls the

movement of substances into and out of the cell. Cell membranes are

described as partially permeable. It is also structural and keeps the

cell contents together and separate from other cells. Furthermore it

allows communication between cells and allows recognition of other

external substances. Membranes that surround cell organelles are also

very similar to the cell membrane. Membranes are composed of

phospholipids, which form the bulk of the membrane, proteins, which

are scattered around in the membrane. There may also be some molecules

of carbohydrates and cholesterol present.

Phospholipids are a special type of lipid. Phospholipids have two

fatty acid chains; these form a tail, which is hydrophobic. Fatty acid

tails do not have a charge, so they are non-polar and insoluble in

water. The phosphate group (head of the phospholipids) is hydrophilic

because it has a positive charge and is soluble in water. The

phospholipids are arranged in a bilayer, with their polar, hydrophilic

phosphate heads facing outwards, and their non-polar, hydrophobic

fatty acid tails facing each other in the middle of the bilayer. The

hydrophilic heads are in contact with the cytoplasm or extra-cellular

fluid, both of which are watery environments. The hydrophobic tails

are protected from this, by being as far from the cytoplasm and

ex...

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...o two-way pumps, which are called exchange

pumps. Below is an example of one, which brings potassium into the

cell and removes sodium from the cell.

The processes described so far only apply to small molecules. Large

molecules (such as proteins, polysaccharides and nucleotides) are

moved in and out of cells by using membrane vesicles. This process

requires energy as well.

Facilitated diffusion and active transport both use proteins, both are

specific and controllable. Active transport requires energy because it

goes against the concentration gradient, but facilitated diffuse does

not require energy. So proteins have a very important role in cell

membranes, they help substances move into and out of the cell.

Proteins provide stability in the cell membrane and are responsible

for most of the membranes properties.