Aqa Biology - Transport Across Cell-Membranes

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Plasma membrane

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Plasma membrane: membranes around and within all cells that the same basic structure.

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Cell-surface membrane

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The name specifically given to the membrane which forms the boundary between the cell cytoplasm and the environment.

It allows different conditions to be established inside and outside of the cell.

It also controls the movement of substances in and out of the cell.

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Phospholipids

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The hydrophilic heads of both phospholipid layers point to the outside of the cell-surface membrane attracted by water on both sides (water soluble - polar).

The hydrophobic tails of both phospholipid layers point into the centre of the cell membrane, repelled by the water on both sides (lipid soluble - non-polar).

Lipid soluble material moves through the membrane via the phospholipid portion.

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Functions of phospholipids

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1) Allow lipid-soluble substances to enter and leave the cell

2) Prevent water-soluble substances entering and leaving the cell

3) Make the membrane flexible and self-sealing

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Proteins

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Proteins are embedded in the phospholipid bilayer in two ways:

1) Occurring in the surface of the bilayer and never extend completely across it. They act to give mechanical support to the membrane in conjunction with glycolipids and as cell receptors for molecules such as hormones.

2) Completely spanning the phospholipid bilayer from one side to another. Some are protein channels, which form water-filled tubes to allow water-soluble ions to diffuse across the membrane. Others are carrier proteins that bind to ions or molecules like glucose and amino acids, which change in shape in order to move these molecules across the membrane.

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Functions of proteins

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1) Provide structural support.

2) Act as channels transporting water-soluble substances across the membrane.

3) Allow active transport across the membrane through carrier proteins.

4) Form cell-surface receptors for identifying cells.

5) Help cells adhere together.

6) Act as cell receptors, e.g. hormones.

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Cholesterol

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Cholesterol molecules occur within the phospholipid bilayer of the cell-surface membrane. They add strength to the membrane. Cholesterol molecules are very hydrophobic and play an important role in preventing loss of water and dissolved ions from the cell. They also pull together the fatty acid tails of the phospholipid molecules, which limits their movement and that of other molecules but without making the membrane too rigid.

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Functions of cholesterol

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1) Reduce later movement of other molecules including phospholipids.

2) Make the membrane less fluid at high temperatures.

3) Prevents leakage of water and dissolved ions from the cell.

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Glycolipids

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Glycolipids are made up of a carbohydrate covalently bonded with a lipid. The carbohydrate portion extends from the phospholipid bilayer into the watery environment outside the cell where it acts as a cell-surface receptor for specific chemicals e.g. the human ABO blood system operates as a result of glycolipids on the cell-surface membrane.

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Function of glycolipids

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1) Act as recognition sites

2) Help maintain the stability of the membrane

3) Help cells to attach to one another and so form tissues.

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Glycoproteins

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Carbohydrate chains are attached to many extrinsic proteins on the outer surface of the cell membrane. These glycoproteins also act as cell-surface receptors, more specifically for hormones and neurotransmitters.

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Functions of glycoproteins

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1) Act as recognition sites

2) Help cells to attach to one another and so form tissues

3) Allows cells to recognise one another, e.g. lymphocytes can recognise an organism's own cells.

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Permeability of the cell-surface membrane

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The cell-surface membrane controls the movement of substances into and out of the cell. Most molecules do not freely diffuse across because:

- not soluble in lipids and therefore cannot pass through the phospholipid layer

- too large to pass through the channels in the membrane

- of the same charge as the charge on the protein channels and so even if they are small enough to pass through, they are repelled

- electrically charged (polar) and therefore have difficulty passing through the non-polar hydrophobic tails in the phospholipid bilayer.

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Fluid-mosaic model of the cell-surface membrane

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Fluid because the individual phospholipid molecules can move relative to one another. this gives the membrane a flexible structure that is constantly changing shape.

Mosaic because the proteins that are embedded in the phospholipid bilayer vary in shape, size and pattern.