A-Level Biology

3.2.3 Transport Across Membranes

Structure of cell membranes

The Plasma Membrane

The cell surface membrane or plasma membrane controls the entry and exit of substances intoand out of cells. The plasma membrane surrounds the cytoplasm it also surrounds the nucleus andother organelles such as the mitochondria and chloroplast. 

Recap the structure of phospholipids from the lipids unit 

Phospholipids are arranged into a bilayer with their hydrophobic, fatty acid tails (non-polar) on theinside and their hydrophilic, phospho-glycerol heads (polar) to the outside. 

• Protein molecules are embedded in the phospholipid bilayer. 

• Cholesterol decreases permeability and increases the stability of the membrane. Different typesof cells have different amounts of cholesterol. 

• The phospholipid bilayer allows lipid-soluble molecules to pass through but restricts the passageof ions and polar molecules; these can pass through channel proteins (pores) that span themembrane. 

• Carrier proteins aid the transport of ions and polar molecules (e.g. glucose) by facilitateddiffusion and active transport. 

• Other protein molecules act as specific receptors for hormones (e.g. insulin), which attach tothem and so allow a cell to respond. 

• Enzymes can be found within the cell membrane. The shape of the enzymes active site isspecific and complementary to its substrate. Maltase enzyme is found in the cell membrane ofthe small intestine where it hydrolyses maltose into glucose.

• Glycoproteins, composed of carbohydrate and protein, are on the outer surface of themembrane and are important in cell recognition, sometimes acting as antigens 

• Glycolipids are composed of carbohydrates attached to a phospholipid within the membrane.They are also important in cell recognition. 

• Water can move through the phospholipid bilayer, but as they are polar it's not easy! There arealso 'channels' available in the membrane called aquaporin's that let more water through thanthe normal parts of the membrane. A cell that has a lot of aquaporin's is said to be verypermeable to water

Diffusion 

Diffusion is the movement of substances down a concentration gradient. It is a passiveprocess so does not require energy from respiration. The only substances that can dothis are lipid-soluble molecules such as steroids, small, non-polar and not charged such as water, oxygen and carbon dioxide. 

Facilitated diffusion

Diffusion of substances through carrier proteins or channel proteins down a concentrationgradient. Facilitated diffusion is a passive process. 

• Channel Proteins enable charged substances (usually ions) to diffuse across membranes.Most channels are gated, 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 twostates so that the site is alternately open to opposite sides of the membrane. The substancewill bind on the side where it is at a high concentration and be released where it is at a lowconcentration.

Facilitated diffusion has a curved relationship with a maximum rate. This is due to the rate beinglimited by the number of transport proteins. As the concentration of substrate increase the rateincreases up until the point of saturation (where the graph plateaus) where by all carrier proteins areoccupied and so the rate of diffusion reaches its maximum. To overcome this more transport proteins