5.1 the structure and function of membranes

Membrane structure

All the membranes in a cell have the same basic structure. The cell surface membrane which separates the cell from its external environment is known as the plasma membrane. Membranes arc formed from a phospholipid bilayer. The hydrophilic phosphate heads of the phospholipids form both the inner and outer surface of a membrane, sandwiching the fatty acid tails of the phospholipids to form a hydrophobic core inside the membrane. Cells normally exist in aqueous environments. The inside of cells and organelles arc also usually aqueous environments. Phospholipid bilayers arc perfectly suited as membranes because the outer surfaces of the hydrophilic phosphate heads can interact with water.

5.2 Factors affecting membrane structure

Temperature

Phospholipids in a cell membrane are constantly moving. When temperature is increased the phospholipids will have more kinetic energy and will move more. This makes a membrane more fluid and it begins to lose its structure. If temperature continues to increase the cell will eventually break down completely. This loss of structure increases the permeability of the membrane, making it easier for particles to cross it. Carrier and channel protein s in the membrane will be denatured at higher temperatures. These proteins are involved in transport across the membrane so as they denature, membrane permeability will be affected.

5.3 Diffusion

Diffusion

Diffusion is the net. or overall, movement of particles (atoms, molecules or ions) from a region of higher concentration to a region of lower concentration. lt is a passive process and it will continue until there is a concentration equilibrium between the two areas. Equilibrium means a balance or no difference in concentrations.

Diffusion happens because the particles in a gas or liquid have kinetic energy. This movement is random and an unequal distribution of particles will eventually become an equal distribution. Equilibrium doesn't mean the panicles stop moving, just that the movements are equal in both directions. Particles move at high speeds and are constantly colliding, which slows down their overall movement. This means that over short distances diffusion is fast, bm as diffusion distance increases the rate of diffusion slows down because more collisions have taken place. For this reason cells are generally microscopic- the movement of particles within cells depends on diffusion and a large cell would lead to slow rates of diffusion. Reactions would not get the substrates they need quickly enough or ATP would be supplied too slowly to energy- requiring processes.

5.4 Active transport

Active transport

Active transport is the movement o[ molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration. The process requires energy and carrier proteins. Energy is needed as the particles are being moved up a concentration gradient. in the opposite direction to diffusion. Metabolic energy is supplied by ATP.

Carrier proteins span the membranes and act as 'pumps'. The general process of active transport is described below - in this example transport is from outside to inside a cell

5.5 Osmosis

Water potential

A solute is a substance dissolved in a solvent {[or example water) forming a solution. The amount of solute in a certain volume of aqueous solution is the concentration. Water potential is the pressure exerted by water molecules as they collide with a membrane or container. It is measured in units of pressure pascals (Pa) or kilopascals (kPa). The symbol for water potential is the Greek letter psi 'l'.

Pure water is defined as having a water potential of 0 kPa (at standard temperature and atmospheric pressure - 25 oc and I00 kPa). This is the highest possible value for water potential, as the presence of a solute in water lowers the water potential below zero. All solutions have negative water potentials- the more concentrated the solution the more negative the water potential.