A-Level Biology OCR Notes
2.1.5 Biological membranes
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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.
- Glycolipids and glycoproteins allow cell adherence, stability and act as recognition sites
- Cholesterol has a hydrophilic end and a hydrophobic end & regulates membrane fluidity by intercalating between the phospholipids.
- At high temperatures the phospholipid bilayer increases its fluidity and permeability, increasing the risk of the proteins becoming denatured. At low temperatures, the phospholipids are compressed, making the membrane stiff
- Organic solvents disrupt the bilayer and dissolve the membrane
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.
- Bulk transport transports large substances across the cell membrane
- Endocytosis is the bulk transport of large molecules into a cell. The membrane invaginates and encloses the material being taken up into a vesicle, which pinches off.
- Exocytosis is the bulk transport of large molecules out of a cell. A vesicle fuses with the cell membrane, releasing its material
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