Membrane Topic Write Up

Membranes have several functions:



Membranes are partially permeable.

This means that they only allow certain molecules to pass through them.

Certain molecules, if they are small enough, can pass through the membrane by squeesing through the gaps between the phospholipid molecules that make up the membrane.

Other molecules, which are fat soluble, can dissolve on the lipid part of the membrane until they are small enough to pass through the gaps between the phospholipid molecules that make up the membrane.

Other molecules can pass through the membrane by special protein channels, these are hollow proteins that span the entire thickness of the membrane - these are called integral proteins. Peripheral proteins are those that only span part of the membrane.

Molecules can also pass the membrane via carrier proteins,


The lipid bilayer is made up of two layers of a sea of phospholipid molecules. Each phospholipid molecule consists of one phosphate head, and a lipid tail. The phosphate heads are hydrophyllic, meaning that they are attracted to water, so they face outwards on the membrane, because the water is located outside of it. The lipid tails are hydrophobic, meaning that they are not attracted to water, and therefore face inwards on the membrane, toward each other, where there is no water.

Also in the membrane is cholesterol, which maintains the fluidity of the membrane.

As previously mentioned, the membrane also houses proteins.

We use the Fluid Mosaic Model to describe the function and structure of the membrane. The model consists of a phospholipid bilayer with proteins floating in it. The model shows how the membrane is able to flex due to the phospholipid molecules being able to move passed each other. The model was proposed by Singer & Nicholson in 1972.


Temperature can have an effect on the phospholipid bilayer.
Decreasing temperature reduces the membrane fluidity and permeability. It can also cause the lipid tails to compress. Cholesterol helps to reduce the effects of lower temperatures, and stabalise the membrane.

Increasing temperature increases the membrane fluidity by increasing the phospholipid kinetic energy, this also causes the permeability to increase. As with lower temperatures, cholesterol can reduce the effects of higher temperatures, and stabalise the membrane.

The proteins in a membrane are also effected by temperature. Higher temperatures can denature a protein.


Diffusion is the movement of particles from an area of high concentration to an area of low concentration until both concentrations are equal.

There are two types of diffusion: simple and facilitated.

Simple diffusion is a passive process.

Facilitated diffusion uses helper proteins to transport diffusing particles.

Factors that affect the rate of diffusion:



Osmosis is the diffusion of "free" water molecules across a partially permeable membrane.

Water potential is a measure of the tendancy of free water molecules to diffuse from one region to another.

Pure water has the highest water potential, of 0 kPa. This is because all of its water molecules are free to move.

Osmosis across a plasma membrane goes via aquaporins.

Active Transport uses ATP (energy) to move particles against a concentration gradient, from an area of low concentration to an area of high concentration.

Bulk transport refers to the large quantites of particles in movement.

Endocytosis is the movement of substances into a cell. It uses vesicles that break off of the membrane to carry the substance.

Exocytosis is the movement of substances out of a cell. Vesicles carry these substances to the cell's membrane, where the vesicles fuse with the membrane, and then expell the substance.