THE MEMBRANES OF DIFFERENT TYPES OF CELLS ARE INVOLVED IN MANY DIFFERENT FUNCTIONS – 25 MARKS
Membranes are found in all organisms, around and within cells. They control the movements in and out of cells, as well as protecting cells. They also allow different functions to occur.
They have the same basic structure called the plasma membrane. All plasma membranes, around or inside cells have a same phospholipid bilayer structure. Phospholipids contribute to the flexibility of membranes and transfer of lipid soluble substances across them. Lipids cause the waterproofing of a plasma membrane, as they prevent water-soluble substances from entering or leaving. The hydrophobic tails of the lipids point inwards towards the centre of the bilayer. The hydrophilic heads of one layer points outwards, while the hydrophilic heads of the other layer point inwards. This arrangement prevents the transport of water in and out of the cell.
The phospholipid bilayer also contains proteins (intrinsic) which acts as a carrier to transport water-soluble substances across the membrane. Some proteins such as carrier proteins are also embedded in the bilayer. When a molecule that is specific to the protein, it binds to the protein, which causes the carrier protein to change shape in a way that the molecule is released inside the membrane. This occurs in facilitated diffusion where molecules move from a region of high concentration to a region of low concentration, using no external energy. The membrane size can also affect the rate of diffusion. The thicker the membrane, the slower the rate of diffusion. Osmosis the movement of water movement form a high to low concentration through a semi-permeable membrane. The cell has a semi-permeable plasma membrane which allows certain molecules to enter and leave depending on their charge or size. Water can leave or enter depending on the concentration either side of the membrane. The cell wall of a plant