AS LEVEL- Biology- Plasma membranes (1)
- Created by: Ammarah123
- Created on: 02-11-18 12:58
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- Plasma membranes (1)
- Roles: *Partially permeable barriers between the cell + its environment, between organelles + the cytoplasm + within organelles *Site of chemical reactions *Site of cell signalling
- Also called fluid- mosaic model
- Phospholipids
- *Forms a barrier to dissolved substances *Has a 'head' + 'tail' *Head = hydrophilic (attracts water) *Tail = hydrophobic (repels water) *Membarne doesn't allow water-soluble substances eg. ions through it
- Cholesterol
- *Gives the membrane stability *Type of lipid *Cholesterol molecules fit between the phospholipids. They bind to the hydrophobic tails of the phospholipids, causing them to pack more closely together- makes the membrane less fluid + more rigid
- Proteins
- *Control what enters + leaves the cell *Channel- allows small or charged particles through *Carrier- transporst molecules + ions across the membrane by active transport + facilitated diffusion
- Glycolipids + Glycoproteins
- *Act as receptors for messenger molecules *Stabilises the membrane by forming hydrogen bonds with surrounding water molecules *Also where drugs, hormones + antibodies bind *Also antigens
- Phospholipids
- Cell signalling
- Receptor proteins have specific shapes- only messenger molecules with a complementary shape can bind to them
- Drugs work by binding to receptors in cell membrane. They trigger a response in the cell
- Hormones- Chemical messengers released by the cell
- Factors affecting membrane structure + permeability
- Temp
- Below 0°C
- 1) The phospholipids don't have much energy, so they can't move very much
- 2) They're packed close together + membrane is rigid
- 3) Channel + carrier proteins in the membrane deform, increasing the permeability of the membrane
- 4) Ice crystals may form + pierce the membrane making it highly permeable when it thaws
- Between 0-45°C
- 1) The phospholipids can move around + aren't as tightly packed together- membrane is partially permeable
- 2) As the temp increases, the phospholipids move around more as they have more energy- increases permeability of the membrane
- Above 45°C
- 1) The phospholipid bilayer starts to melt + the membrane becomes more permeable
- 2) Water inside the cell expands, putting pressure on the membrane
- 3) Channel + carrier proteins deform so they can't control what enters or leaves the cell- increases permeability of the membrane
- Below 0°C
- Solvents-
- 1) Surrounding cells in a solvent increases the permeability of their cell membranes
- 2) This is because solvents dissolve the lipids in a cell membrane, so the membrane loses its structure
- 3) Some solvents increase cell permeability more than others eg. ethanol increases cell permeability than methanol
- 4) Increasing the concentration of the solvent will also increase membrane permeability
- Temp
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