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Cell Membrane and Transport

Cell Membrane and Transport. Cell membrane structure. Made of Phospholipids Proteins Cholesterol Carbohydrate chains (glycolipids and glycoproteins). Fluid Mosaic model. Phospholipid bilayer has been seen under electron microscope

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Cell Membrane and Transport

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  1. Cell MembraneandTransport

  2. Cell membrane structure • Made of • Phospholipids • Proteins • Cholesterol • Carbohydrate chains (glycolipids and glycoproteins)

  3. Fluid Mosaic model • Phospholipid bilayer has been seen under electron microscope • The rest is what scientists believe to be true so it is called a model • Fluid – phospholipids and proteins move within their area • Mosaic – means pattern which is how the proteins lie between the phospholipids

  4. Features • Phospholipid bilayer – about 7nm • Hydrophobic tails point inwards • Non-polar hydrophobic interior • Unsaturated tails mean membrane more fluid • If temperature decreases become less fluid • Hydrophilic heads point outwards

  5. Proteins • Float in phospholipid bilayer • Extrinsic on outside of membrane • Intrinsic on inside • Transmembrane span membrane • Hydrophilic outer regions • Hydrophobic inner regions • Sit between phospholipids

  6. Glycolipids • Branched carbohydrate attached to phospholipid • Glycoproteins • Branched carbohydrate attached to protein • Cholesterol

  7. Function • Phospholipids • Barrier to most water-soluble substances • Non-polar region • Proteins • Transport • Hydrophilic channels. Good for polar molecules. • Specific to certain substances • Enzymes • Specific organelle functions(eg involved in respiration in mitochondrial membranes)

  8. Glycolipids and glycoproteins • Form hydrogen bonds with surrounding fluid • Stabilises membrane structure(restricts movement) • Act as receptor molecules • Bind with particular substances eg. Hormones • Specific • Antigens – for like-cell recognition • Cholesterol • Hydrophilic and hydrophobic regions • Regulate fluidity and give mechanical stability.

  9. Transport across the membrane • 4 types: • Diffusion • Osmosis • Active transport • Bulk transport

  10. Diffusion • All molecules have kinetic energy (ie they move) ≡ net movement of molecules from a region of their higher concentration to a region of their lower concentration down a concentration gradient.

  11. Diffusion in a liquid

  12. Molecules that diffuse through cell membranes • Oxygen – Non-polar so diffuses very quickly. • Carbon dioxide – Polar but very small so diffuses quickly. • Water – Polar but also very small so diffuses quickly.

  13. Diffusion Rate • Dependant on: • Steepness of concentration gradient • Temperature • Surface area • Type of molecule • Large and polar molecules slower

  14. Facilitated (to make possible) diffusion • Large polar molecules can’t go through the bilayer, so therefore…….. • Use hydrophilic channels in proteins • Eg. Glucose, amino acids, Na+ and Cl- • Specific (allow only one thing to pass) • Passive (no energy involved) • Rate depends on number of channels available

  15. Osmosis • Diffusion of water ≡ the movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane. Although water is a polar molecule and shouldn’t pass through non-polar membranes it is small enough to go through

  16. Water Potential • The tendency of water molecules to move from one place to another • ψ

  17. Effect of water potential on plant cells • If water moves into the plant cell the vacuole expands and puts pressure on the cell wall. • Turgid • If water moves out the vacuole shrinks and it becomes flaccid

  18. If this occurs too much the cell membrane pulls away from the cell wall. • This is called plasmolysis. • The spaces will be filled with whatever solution the plant cell is sitting in.

  19. Animal cells and water potential • In a solution with lower water potential the cell shrinks • In a solution with higher water potential the cell swells until it bursts.

  20. Pressure Potential • The tendency of water to move in response to pressure • Symbol Ψp. • In turgid plant cells it usually has a positive value as the entry of water causes the protoplast to push against the cell wall.

  21. Solute Potential • Solute potential is a measure of the change in water potential of a system due to the presence of solute molecules. • The more solute molecules are present, the lower (and more negative) is ψs. • Solute potential is always negative.

  22. Active Transport • The energy consuming transport of molecules against a concentration gradient.

  23. Bulk Transport • Endocytosis • Phagocytosis • Pinocytosis • Exocytosis

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