Revision:Membranes

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Draw a diagram showing the fluid mosaic model of a cell membrane

Draw a diagram showing the fluid mosaic model of a cell membrane including the phospholipid bilayer, cholesterol, glycoproteins and intrinsic and extrinsic proteins.

Explain how the hydrophobic and hydrophilic properties of phospholipids help to maintain the structure of cell membranes

Membranes are made up of a lipid bilayer that includes two sheets of phospholipid molecules. A phospholipid molecule is made up of two hydrophobic fatty acid tails and a phosphate head, which is hydrophilic. The tails are faced inwards, to each other because they are hydrophobic, while the heads face outwards, since they are water loving.

Define Diffusion

The passive movement of particles from a region of higher concentration to a region of lower concentration.

Define Osmosis

The passive movement of water molecules, across a partially permeable membrane from a region of lower solute concentration to a region of higher solute concentration.

Describe passive transport across membranes in terms of diffusion and osmosis

See above.

Describe active transport across membranes

Describe active transport across membranes including the role of protein carriers, ATP and a concentration gradient.

Mention carrier-assisted transport and role of transport proteins in membrane which results in high selectivity.

Also favourable concentration gradients for facilitated diffusion transport.

Mention membrane pumps the diffusing molecule or ion combines with the protein and is carried across the membrane and deposited on the other side. This movement is passive and goes with the concentration gradient. Active transport is selective.

Compare endocytosis (phagocytosis/pinocytosis) and exocytosis

In this process the plasma membrane engulfs portions of the external medium, forms an almost complete sphere around it, and then draws the membrane-bounded vesicle into the cell.

Several types of endocytosis have been distinguished:

• in pinocytosis, the vesicles are small and contain fluid;
• in phagocytosis, the vesicles are larger and contain solid matter;
• in vesicle-mediated endocytosis, material binds to a specific receptor on the external face of the plasma membrane, triggering the process by which it is engulfed. Cholesterol enters cells by the last route.

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