Structure and Function of Cell Membranes Aims: Aims: Must be able to describe the structure of the cell membrane. Must be able to describe the structure of the cell membrane. Should be able to outline the function of the cell membrane. Should be able to outline the function of the cell membrane. Could be able to outline the processes of movement across a cell membrane. Could be able to outline the processes of movement across a cell membrane.

Structure Composed of phospholipids. Composed of phospholipids. Phospholipids base choline = LECITHIN – Most abundant phospholipid in cell membranes. Phospholipids base choline = LECITHIN – Most abundant phospholipid in cell membranes. Usually drawn as: Usually drawn as: When shaken with water they form a phospholipid bi- layer: When shaken with water they form a phospholipid bi- layer: Also has proteins imbedded in phospholipid bi-layer. Also has proteins imbedded in phospholipid bi-layer. Model is known as the ‘fluid mosaic model’ (Singer and Nicholson 1972). Model is known as the ‘fluid mosaic model’ (Singer and Nicholson 1972).

Functioning Phospholipids provide a continuous boundary. Phospholipids provide a continuous boundary. Creates a selectively permeable layer: Creates a selectively permeable layer: Non-polar compounds e.g. alcohol can be absorbed – can diffuse through. Non-polar compounds e.g. alcohol can be absorbed – can diffuse through. Water and urea are very small – can squeeze between the phospholipid molecules. Water and urea are very small – can squeeze between the phospholipid molecules. Minerals cannot cross the bilayer. Minerals cannot cross the bilayer. Proteins give the membrane it’s function: Proteins give the membrane it’s function:

Channel Proteins Control the movement of molecules through the membrane. Control the movement of molecules through the membrane. E.g. Sodium gates in nerve cells. E.g. Sodium gates in nerve cells. Pump molecules into cells against the concentration gradient. Pump molecules into cells against the concentration gradient. Allows the cell to concentrate molecules inside or outside the cell. E.g. Nitrate pumps in root hair cells. Allows the cell to concentrate molecules inside or outside the cell. E.g. Nitrate pumps in root hair cells. Require energy to work. Require energy to work. Membrane Pumps

Hormone Receptors Allow cells to respond to a hormonal signal. Allow cells to respond to a hormonal signal. E.g. Liver and Muscle cells have receptors for Insulin. E.g. Liver and Muscle cells have receptors for Insulin. Brain cells lack these receptors – don’t respond to insulin. Brain cells lack these receptors – don’t respond to insulin. Protein with a carbohydrate attached. Protein with a carbohydrate attached. Involved in cell recognition. Involved in cell recognition. Act as ANTIGENIC MARKERS which play a role in tissue rejection. Act as ANTIGENIC MARKERS which play a role in tissue rejection. Blood group antigens are also glycoproteins. Blood group antigens are also glycoproteins. Also involved in CELL ADHESION – carbohydrate acts as glue. Also involved in CELL ADHESION – carbohydrate acts as glue. Glycoproteins

Peripheral Proteins Bound to one surface only. Bound to one surface only. Some act as ENZYMES. Some act as ENZYMES. E.g. Liver lipase breaks down fats in the blood stream. E.g. Liver lipase breaks down fats in the blood stream. When the correct compound binds to the protein, ENDOCYTOSIS takes place. When the correct compound binds to the protein, ENDOCYTOSIS takes place. 1Compound binds to receptor. 2-3 Membrane invaginating 4Compound taken into cell in a vesicle Transport

Transport Across the Membrane Methods include: Methods include: OSMOSIS – movement of water from an area of high water content to an area of low water content. OSMOSIS – movement of water from an area of high water content to an area of low water content. DIFFUSION – movement of a substance from a high concentration to low concentration DIFFUSION – movement of a substance from a high concentration to low concentration FACILITATED TRANSPORT – Glucose, Minerals, Amino Acids, Vitamins FACILITATED TRANSPORT – Glucose, Minerals, Amino Acids, Vitamins ACTIVE TRANSPORT – E.g. Sodium ions ACTIVE TRANSPORT – E.g. Sodium ions ENDO / EXOCYTOSIS – E.g. Secretion of hormones. ENDO / EXOCYTOSIS – E.g. Secretion of hormones.

Facilitated Transport Diagrams: Diagrams: Features: Features: Selective – Pseudosubstrates either not transported or transported very slowly. Selective – Pseudosubstrates either not transported or transported very slowly. Works with the concentration gradient, only transports form high to low concentration. Works with the concentration gradient, only transports form high to low concentration. Doesn’t require metabolic energy (ATP). Doesn’t require metabolic energy (ATP).

Active Transport Diagrams: Diagrams: Features: Features: Selective – Pseudosubstrates either not transported or transported very slowly. Selective – Pseudosubstrates either not transported or transported very slowly. Can work against concentration gradient – can be concentrated in or outside cells Can work against concentration gradient – can be concentrated in or outside cells Requires metabolic energy (ATP). Requires metabolic energy (ATP). Inhibited by metabolic poisons (e.g. cyanide). Inhibited by metabolic poisons (e.g. cyanide). Metabolic poisons will stop active transport but not effect facilitated transport. Metabolic poisons will stop active transport but not effect facilitated transport.

Pinocytosis Diagrams: Diagrams: Allows transport of extracellular fluid. Allows transport of extracellular fluid. Method for transporting large molecules across the membrane. Method for transporting large molecules across the membrane. E.g. transport of maternal antibodies across the placenta. E.g. transport of maternal antibodies across the placenta. During later stages of pregnancy fetal placental cells feed by pinocytosis. During later stages of pregnancy fetal placental cells feed by pinocytosis.

Endo and Exocytosis Transport large macromolecules and micro-organisms into (endocytosis) and out of (exocytosis) the cell. Transport large macromolecules and micro-organisms into (endocytosis) and out of (exocytosis) the cell. Activated by receptor proteins embedded in membrane. Activated by receptor proteins embedded in membrane. In endocytosis membrane vesicle formed often fuses with the lysosome – intracellular digestion. In endocytosis membrane vesicle formed often fuses with the lysosome – intracellular digestion. In exocytosis vesicles bud off the golgi body. Vesicles contain products of protein synthesis. In exocytosis vesicles bud off the golgi body. Vesicles contain products of protein synthesis. Vesicles will be transported to the plasma membrane, where it will fuse and release it’s contents into the extracellular fluid. Vesicles will be transported to the plasma membrane, where it will fuse and release it’s contents into the extracellular fluid.

Activity Complete the ‘Structure of Membranes’ sheet. Complete the ‘Structure of Membranes’ sheet.