Active transport

Sometimes substances need to be moved from where there is there is a low concentration to where there is a high concentration - against the concentration gradient. This allows the cell to take up essential molecules even when they are at low concentration outside.

Because molecules are being moved against the concentration gradient, it requires energy. It is thought that active transport uses the same protein carriers as those involved in facilitated diffusion. The energy used is ATP. Cells which carry out active transport have more mitochondria to produce the ATP needed through respiration.

Osmosis

Osmosis is the movement of water molecules from a region of higher water potential to a lower water potential through a partially permeable membrane.

The water potential of pure water is zero. When solutes are added to water potential goes down. I.E becomes a negative value.

A partially permeable membrane is a membrane that only allows small molecules through.

Water potential is measured in kilopascals (kPa)

Diffusion

Molecules move from where there is high concentration to where there is a lower concentration. I.E They diffuse down a concentration gradient. This requires no energy and is a passive process.

Molecules move until they reach a state of equilibrium or DYNAMIC EQUILIBRIUM.

Factors which affect the rate of diffusion.

• Concentration gradient - the greater the difference in the difference in concentration of the molecules or ions on either side of the exchange surface, the faster the rate of diffusion.
• Area over which diffusion takes place - the larger the area of an exchange surface, the faster the rate of diffusion.
• Thickness of the exchange surface - the thinner the exchange surface, the faster the rate of reaction.

Phospholipids

Phosphate - A base group that replaces one fatty acid chain. It makes this part of the molecule soluble in water whilst the fatty acid chain remains insoluble. The phosphate group is HYDROPHILIC.

A phospholipid has a HYDROPHILIC head and a HYDROPHOBIC tail.

The Cell Membrane

Lipids

Lipids contain the elements Carbon, Hydrogen and Oxygen

Triglycerides are the main types of lipids and are usually known as fats and oils.

Triglycerides are formed by the combination of three fatty acid molecules and one molecule of Glycerol.

This is a condensation reaction and for every trigyceride formed, three molecules of water are released.

Triglycerides are insoluble in water but soluble in organic solvents such as alcohol. This is because of the long Hydrogen tails of the fatty acids.

Triglycerides are HYDROPHOBIC.

Roles of lipids

Lipids are an energy source- when oxidised, lipids provide more than twice the energy as the same mass of carbohydrates.

Waterproofing - lipids are insoluble in water and therefore useful as waterproofing.

Insulation - fats are slow conductors of heat and when stored beneath the body surface, they help retain heat.

Protectection - Fat is often stored around delicate organs such as the kidneys.