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CC - Cell membranes (Cell membrane structure (Selective permeability of…
CC - Cell membranes
Cell membrane structure
Phospholipids
- Forms a bilayer
- Hydrophilic heads on both layers interact with water in the cytoplasm and the water surrounding the cell
- Hydrophobic tails are at the centre of the membrane - allows lipid-soluble molecules across by diffusion but not water-soluble molecules
- Phospholipid molecules can move freely within a layer relative to one another (fluid)
Proteins
- Extrinsic: found on either surface of the bilayer; provide structural support; form recognition sites by identifying cells; form receptor sites for hormone attachment
- Intrinsic: extend across both layers of the bilayer; carrier proteins transport water-soluble substances across by facilitated diffusion and active transport; channel proteins transport ions across by facilitated diffusion
- Proteins vary in shape, size and pattern (mosaic)
Other things in the membrane
- Cholesterol: occurs in animal cell membranes and make the membrane more stable
- Glycoproteins and glycolipids:found in the outer layer; carbohydrate layer is called the glycocalyx; some molecules in the glycocalyx act as hormone-receptors or in cell recognition
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Bulk Transport
Endocytosis - material is engulfed by the cell membrane and cytoplasm, making a vesicle. Two types: Phagocytosis and Pinocytosis
Phagocytosis - the uptake of solid material that's too big for diffusion or active transport. Granulocytes engulf bacteria, a lysosome fuses with the vesicle formed and enzymes digest the cells. The products are absorbed into the cytoplasm
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Exocytosis - Substances leave the cell in a vesicle, which fuses with the cell membrane
The cell membrane has to change shape for endocytosis and exocytosis so it needs ATP from respiration
Osmosis
Definitions
Osmosis - The diffusion of water from a region of higher water potential to a region of lower water potential down a water potential gradient through a partially permeable membrane
Water potential - The free energy of water molecules and the tendency for water to move (kPa). It's pressure and solute potential added together.
Solute potential - The tendency for water molecules to move out of a solution
Pressure potential - the pressure exerted by the cell contents (in the vacuole) on the plant cell wall
Hypotonic - When the WP of the external solution is less negative (higher) than the solution inside the cell. Water flows into the cell until it can't take anymore water and becomes turgid, providing support for the plant. An animal cell has no pressure potential and it bursts (haemolysis).
Hypertonic - When the WP of the external solution is more negative (lower) than the solution inside the cell so water flows out of the cell and it becomes plasmolysed/flaccid and can't support the plant. Incipient plasmolysis occurs when the membrane starts to pull away from the cell wall. Animal cells shrink, becoming 'crenated'
Isotonic - The cell has the same WP as the surrounding solution so there's no net water movement