There are many different sorts of proteins with complex tertiary structures in a membrane. The proteins usually span from one side of the phospholipid bilayer to the other (intrinsic proteins), these are usually large proteins, but they can also sit on one of the surfaces (extrinsic proteins), these are usually smaller proteins. They also change their position and can slide around the membrane very quickly and collide with each other, but can never flip from one side to the other. The proteins have hydrophilic amino acids in contact with the water on the outside of membranes, and hydrophobic amino acids in contact with the fatty chains inside the membrane. Proteins make up about 50% of the mass of membranes, and are responsible for most of the membrane's properties. Proteins that span the membrane are usually involved in transporting substances across the membrane. Proteins on the inside surface of cell membranes are involved in maintaining the cell's shape. They may also be enzymes, catalysing reactions in the cytoplasm. Proteins on the outside surface of cell membranes can act as receptors by having a specific binding site where hormones or other chemicals can bind. This binding then triggers other events in the cell. They may also be involved in cell signalling and cell recognition, or they may be enzymes, such as maltase in the small intestine. Some membrane proteins on the outside of the bilayer are antigens. The protein has carbohydrates attached, so it is a glycoprotein, most membrane antigens are glycoproteins. The bodies defence cells have membrane receptors; any cell with different antigens is attacked and destroyed by the bodies defence cells.
The carbohydrates are found on the outer surface of cell membranes, and are usually attached to the membrane proteins. Proteins with carbohydrates attached are called glycoproteins. Carbohydrates attached to phospholipids are called glycolipids and are also found on the outside of the membrane. The carbohydrates are short polysaccharides composed of a variety of different monosaccharides, and form a cell coat or glycocalyx outside the cell membrane. The glycocalyx is involved in protection and cell recognition.
Cholesterol is also present in the membrane. It maintains the fluidity and increases the stability of the membrane. Without cholesterol the membrane would easily split apart.
The way molecules are arranged in a cell membrane is described as the fluid mosaic model, because the membrane is fluid, and because of the mosaic arrangement of the protein molecules. Davson and Danielli proposed this idea in the 1970’s and 1980’s. Below is a diagram of the fluid mosaic model.
Cell membranes are a barrier to most substances, and this property allows materials to be concentrated inside cells, excluded from cells, or simply separated from the outside environment. There are five main methods by which substances can move across a cell membrane are: Simple Diffusion, Osmosis, Facilitated Diffusion, Active Transport and Vesicles.
Simple diffusion is where a few substances can diffuse directly through the lipid bilayer part of the membrane. The only substances that can do this are lipid-soluble molecules such as steroids, or very small molecules, such as water, oxygen and carbon dioxide. For these molecules the membrane is no barrier at all. Since lipid diffusion is a passive diffusion process, no energy is involved and substances can only move down their concentration gradient.
Osmosis is the diffusion of water across a partially permeable membrane from a high concentration solution to a low concentration solution. Water is simple diffusion but it has its own name – osmosis.
Molecules that cannot diffuse easily between the phospholipids can be helped through the membrane by facilitated diffusion. Facilitated diffusion is the transport of substances across a membrane by a trans-membrane protein molecule. The transport proteins tend to be specific for one molecule (a bit like enzymes), so substances can only cross a membrane if it contains the appropriate protein. This is a passive diffusion process, so no energy is involved and substances can only move down their concentration gradient. There are two kinds of transport protein channel proteins and carrier proteins. Channel proteins form a water-filled pore or channel in the membrane. This allows charged substances (usually ions) to diffuse across membranes. Most channels can be gated (opened or closed), allowing the cell to control the entry and exit of ions. Carrier Proteins have a binding site for a specific solute and constantly flip between two states so that the site is alternately open to opposite sides of the membrane. The substance will bind on the side where it at a high concentration and be released where it is at a low concentration.
Active transport is the pumping of substances across a membrane by a trans-membrane protein pump (protein carrier) molecule. The protein binds a molecule of the substance to be transported on one side of the membrane, changes shape, and releases it on the other side. The proteins are highly specific, so there is a different protein pump for each molecule to be transported. Active transport happens when the concentration of a molecule is higher inside a cell than outside the cell, this goes against the concentration gradient so energy is required. There are also 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.
