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Cell Membrane & Transport of materials. Section 6. Cell/Plasma Membrane. Cell membrane surrounds all cells Cell membrane consists of phospholipid & protein molecules This forms the fluid mosaic model. Phospholipid Bilayer. Phospholipids arranged into a double layer (bi-layer)
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Cell Membrane & Transport of materials Section 6
Cell/Plasma Membrane • Cell membrane surrounds all cells • Cell membrane consists of phospholipid & protein molecules • This forms the fluid mosaic model
Phospholipid Bilayer • Phospholipids arranged into a double layer (bi-layer) • Hydrophilic heads form hydrogen bonds with water on either side of the cell • Hydrophobic tails face inwards • Cholesterol is also a component of the bilayer • Structure is fluid, yet is a stable boundary • Tiny molecules can pass through easily
Proteins • Proteins are spread throughout the phospholipid bilayer. • FUNCTIONS: • Support • Channels :allowing small molecules through the membrane • Carriers: actively pump molecules across • Enzymes: catalysing reactions at the membrane • Receptors: for hormones to attach to • Markers: Identify cell’s blood/tissue type
Other membranes • Many organelles have membranes • Nucleus – double membrane • Mitochondrion – folded inner membrane • Endoplasmic Reticulum & Golgi – membranes can pinch off = vesicles • All membranes are selective barriers • Entry/exit of materials is regulated
Modes of transport - diffusion • Molecules/ions move along a concentration gradient • Process is passive (no energy) • Small molecules diffuse easily through tiny pores • Larger molecules cannot diffuse • Membrane is selectively permeable
Osmosis • Movement of water from HWC to LWC through a selectively permeable membrane • In red blood cells: • In hypotonic solution – burst • In isotonic solution – no change • In hypertonic solution – shrink (plasmolysed)
Active Transport • Movement of molecules against a concentration gradient • Requires ATP energy • Protein molecules are carriers – recognise specific ions/molecules • Sodium/Potassium pump exchanges ions back and forth • Important in nerve cells • Affected by temperature, oxygen, & glucose availability
Endocytosis/Exocytosis • Endocytosis: a cell engulfing large particles • A ‘pouch’ is formed – forms an intracellular vesicle • 1) Phagocytosis • – engulfing of solid particles • (e.g. against bacteria) • 2) Pinocytosis – engulfing of liquid • Exocytosis: Vesicles from inside, fuse with membrane, and contents expelled • e.g. enzymes, hormones secreted