Cell membranes and transport
Cell membranes and transport covers the fluid mosaic model, diffusion, osmosis, facilitated diffusion, active transport and co-transport. The exam tests membrane structure, permeability, water potential and transport mechanisms.
Membrane structure
- Phospholipids form a bilayer; hydrophilic heads face water and hydrophobic tails face inward.
- Proteins are embedded or peripheral; channel and carrier proteins transport polar or charged substances. Glycoproteins and glycolipids are involved in recognition.
- Cholesterol reduces membrane fluidity and permeability to small water-soluble molecules.
Transport
Diffusion is passive movement down a concentration gradient. Facilitated diffusion uses proteins and is passive. Osmosis is the movement of water from higher water potential to lower water potential through a partially permeable membrane. Active transport uses ATP and carrier proteins against a gradient; co-transport couples movement of one substance down its gradient to another against its gradient.
Worked example
A cell has water potential -150 kPa and is placed in a solution of -200 kPa. The solution has the lower water potential, so water leaves the cell by osmosis. The cell may become flaccid or plasmolysed. If the external solution were -100 kPa, water would enter.
Traps
Water potential is negative or zero; pure water is 0 kPa, not 100 kPa. Osmosis is not active transport. Do not say facilitated diffusion uses ATP. Turgor pressure supports plant cells but does not stop all water movement.