A-LEVEL BIOLOGY AQA NOTES
transport across cell membranes
Cell Membranes
- Cell membranes act as barriers and can control what passes into and out of cells and organelles
- The cell membrane is composed of phospholipids, proteins, glycoproteins, glycolipids and cholesterol.
- Cholesterol has a hydrophilic end and a hydrophobic end & regulates membrane fluidity by intercalating between the phospholipids
Passive Transport
- Passive transport involves exchange of substances without requiring metabolic energy from the cell
- Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration (down their concentration gradient).
- Facilitated diffusion is the net movement of particles down their concentration gradient across a partially permeable cell membrane via carrier or channel proteins
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- Water potential is a measure of the tendency of water molecules to move from one area to another area and describes the pressure created by these water molecules; the more dilute a solution, the higher (less negative) the water potential (Ѱ).
- Osmosis is the net movement of water from an area of higher water potential to an area of lower water potential across a partially permeable membrane.
- The rate of diffusion can be increased by increasing the number of channel & carrier proteins, the surface area of the cell membrane, reducing the diffusion distance and creating a steeper concentration gradient.
Active Transport
Active transport is the movement of particles from an area of low concentration to an area of high concentration (against their concentration gradient) across a cell membrane, using ATP and carrier proteins.
- Co-transport occurs when the transport of one substance is coupled with the transport of another substance across a membrane.
- Glucose & sodium are co-transported in the ileum:
