Cell Membrane Structure

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Membranes Control What Passes Through Them

All cells are surrounded by membranes. In eukaryotic cells, many of the organelles are surrounded by membranes too.

Cell surface membranes surround cells. They are a barrier between the cell and its environment, controlling which substances enter and leave the cell. They're partially permeable - they let some molecules through but not others. Substances can move across the cell-surface membrane by diffusion, osmosis or active transport.

The membranes around organelles divides the cell into different compartments - they act as a barrier between the organelle and the cytoplasm. They are also partially permeable and control what substances enter and leave the cell.

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Cell Membranes have a 'Fluid Mosaic' Structure

The basic structure of all cell membranes is pretty much the same. They're composed of lipids (mainly phospholipids), proteins and carbohydrates (attached to proteins or lipids).

In the model, the phospholipid molecules form a continuous double layer (bilayer).

This bilayer is 'fluid' because the phospholipids are constantly moving.

Cholesterol molecules are present within the bilayer.

Proteins are scattered through the bilayer. These include channel proteins and carrier proteins, which allow large molecules and ions to pass through the membrane. Receptor proteins on the cell-surface membrane allow the cell to detect chemicals released from other cells. The chemicals signal the cell to respond in some way, e.g. the hormone insulin binds to receptor proteins on the liver cells which tells the cells to absorb glucose.

Some proteins are able to move sideways through the bilayer while others are in a fixed position.

Some proteins have a polysaccharide chain attached - these are called glycoproteins.

Some lipids also have a polysaccharide chain attached - these are glycolipids.

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The Different Components of Cell Membranes have Different Roles

PHOSPHOLIPIDS FORM A BARRIER TO DISSOLVED SUBSTANCES

Phospholipid molecules have a 'head' and a 'tail'.

The head is hydrophilic - it attracts water - and the tail is hydrophobic - it repels water.

The molecules automatically arrange themselves into a bilayer - the heads face outwards towards the water on either side of the membrane.

The centre of the bilayer is hydrophobic so the membrane doesn't allow water soluble substances (like ions) through it - it acts as a barrier to these dissolved substances.

CHOLESTEROL GIVES THE MEMBRANE STABILITY

Cholesterol is a type of lipid.

It's present in all cell membranes (except bacterial cell membranes).

Cholesterol molecules fit between the phospholipids. They bind to the hydrophobic tails of the phospholipids, causing them to pack more closely together. This restricts the movement of the phospholipids, making the membrane less fluid and more rigid.

Cholesterol helps to maintain the shape of animal cells (which don't have cell walls). This is particularly important for cells that aren't supported by other cells, e.g. red blood cells, which float free in the blood

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The Permeability of the Cell Membrane can be Investigated in the Lab

The permeability of cell membranes is affected by different conditions, e.g. temperature and solvent concentration. You can investigate how these things affect permeability by doing an experiment using beetroot. Beetroot cells contain a coloured pigment that leaks out - the higher the permeability of the membrane, the more pigment leaks out of the cell. Here's how you could investigate how temperature affects beetroot membrane permeability:

Use a scalpel to cut five equal sized pieces of beetroot. Rinse the pieces to remove any pigment released during cutting.

Add the five pieces to five different test tubes, each containing 5cm^3 of water. Use a measuring cylinder or pipette to measure the water.

Place each test tube in a water batj at a different temperature, e.g. 10, 20, 30 degrees etc for the same length of time.

Remove the pieces of beetroot from the tubes leaving just coloured pigment.

Now you need to use a colorimeter - a machine that passes light through the liquid and measures how much of the light is absorbed. The higher the absorbance, the more pigment released so the higher the permeability of the membrane.

INCREASING THE TEMPERATURE INCREASES MEMBRANE PERMEABILITY

Experiments like the one above have shown that membrane permeability changes with temperature.

Temperature below 0 - the phospholipids don't have much energy, so they can't move very much. They're packed closely together and the membrane is rigid. But channel proteins and carrier proteins in the membrane deform, increasing the permeability of the membrane. Ice crystals may form and pierce the membrane making it highly permeable when it thaws.

Temperatures between 0 and 45 - the phospholipids can move around and aren't packed as tightly together - the membrane is partially permeable. As the temperature increases the phospholipids move more because they have more energy - this increases the permeability of the membrane.

Temperatures above 45 - the phospholipid bilayer starts to melt and the membrane becomes more permeable. Water inside the cell expands, putting pressure on the membrane. Channel proteins and carrier proteins deform so they can't control what enters or leaves the cell - this increases the permeability of the membrane.