Cell Membrane and Transport

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

5. 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

6. 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

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

8. 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)

9. 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.

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

11. 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.

12. Diffusion in a liquid

14. 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.

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

16. 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

18. 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

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

21. 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

22. 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.

23. 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.

24. 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.

25. 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.

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

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