Cell Membrane and Transport

Cell membrane structure Made of Made of –Phospholipids –Proteins –Cholesterol –Carbohydrate chains (glycolipids and glycoproteins)

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

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

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

Glycolipids Glycolipids –Branched carbohydrate attached to phospholipid Glycoproteins Glycoproteins –Branched carbohydrate attached to protein Cholesterol Cholesterol

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

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

Transport across the membrane 4 types: 4 types: – Diffusion – Osmosis – Active transport – Bulk transport

Diffusion All molecules have kinetic energy (ie they move) 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.

Diffusion in a liquid

Molecules that diffuse through cell membranes 1. Oxygen – Non-polar so diffuses very quickly. 2. Carbon dioxide – Polar but very small so diffuses quickly. 3. Water – Polar but also very small so diffuses quickly.

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

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

Osmosis Diffusion of water 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

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

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

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

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

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

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

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

Bulk Transport Endocytosis Endocytosis –Phagocytosis –Pinocytosis Exocytosis Exocytosis