Cell membranes

• Cell membranes at the surface provide a barrier between extra and intercellular fluid, controlling what enters the cell. It is also partially permeable allowing diffusion, facilitated diffusion, active transport and osmosis. They also provide cell communication.
• Intercellular membranes divide cells into different compartments and stop cytoplasm mixing with the organelle, allowing efficient function.
• Intracellular membranes also provide transport in the cell, such as vesicles. They also provide chemical reactions, some membranes are folded increasing surface area.
• Cell membranes are made up of phospholipids, a group of them are called micelle when they group together to form a ball.
• Phospholipids can move laterally at 10 7 times per second, and flip flop movement occurs once per month (switch with the lipid above them.)
• The cell membrane consists of carbohydrate chains attached to glycoproteins, and glycolipids.
• Carbohydrates on the outside of the cell act as receptors for hormones or antibodies.
• The fluidity of a membrane is determined by the amount of cholesterol and unsaturated hydrocarbon tails. These have kinks which prevent the membrane from packing together.
• The average thickness of a membrane is 7-8nm
• Cholesterol makes up about 25% of the membrane.
• A phospholipid bilayer has two layers of phospholipids arranged so that their polar heads face outwards and their nonpolar tails face inward.
• Protein molecules embedded within the membrane include ion channels, carrier proteins, enzymes, and structural proteins.
• Carrier proteins transport substances across the membrane using energy from ATP.
• Enzymes catalyze chemical reactions at the surface of the membrane.
• Ion channels are protein pores that allow specific ions to pass through the membrane.
• The fluid mosaic model describes the structure of the plasma membrane as a dynamic and constantly changing arrangement of lipids and proteins.
• Glycoproteins have carbohydrate groups attached to them and can be used as markers on cell surfaces.
• Viscous membranes have high levels of cholesterol and saturated hydrocarbon tails, which allow the membrane to pack together.
• Glycoproteins (carb chains attached to proteins) and glycolipids (lipids with carbs attached) stabilise the membrane by forming hydrogen bonds with surrounding water molecules. They can also act as receptors for messenger molecules and in cell signalling for drugs + hormones to bind to.
• Cholesterol fits between phospholipids and binds to the hydrocarbon tails causing them to pack more closely.
• Proteins control what enters and leaves the cell. Channel proteins allow small charged proteins through. Carrier proteins transport larger, charged molecules.
• Ethanol can dissolve the lipids in the membrane so that it loses it's structure. Other solvents can increase permeability more than others. Increasing conc. also increases concentration.
• At low temps, phospholipids have little energy so don't move much so pack closely. Proteins denature so can't control what enters and leaves the cell. Ice crystals can also pierce the membrane, so when thawed holes are left.
• Above 45 degrees C, the membrane begins to break down. Water inside the cell expands and puts pressure on the membrane. Proteins denature, so can't control what enters and leaves the cell.
• Diffusion is the net movement of molecules from a region of high concentration to a region of low concentration.
• Facilitated diffusion is diffusion assisted by channel and carrier proteins.
• Osmosis is the net movement of water molecules from a high-water potential to a low water potential, across a partially permeable membrane.
• The greater the water potential, the more free water particles. All solutions have a negative water potential. Pure water= 0
• Plant cells can become turgid when water enters the cell. The cell wall prevents it from bursting.
• Plant cells become plasmolyised when water moves out of the cell. The cell membrane moves away from the cell wall, solute can enter these gaps.
• Animal cells can haemolyse when the cell gains water and it bursts.
• Animal cells can crenate when they lose water and shrivel up.
• Active transport is the movement of ions/molecules from a low conc. to a high conc. using ATP released by respiration.
• Active transport requires carrier proteins and ATP.
• 2 types of bulk transport include endocytosis and exocytosis.
• Endocytosis occurs when some particles are too large to enter the cell by carrier proteins. The substance becomes invaginated by being surrounded by the plasma membrane which pinches off to from a vesicle. These then fuse with lysosomes to be digested.
• Exocytosis occurs when substances need to leave the cell. Vesicles containing the substance fuse with the plasma membrane and release their contents. Some are inserted straight into the membrane. These require ATP.
• Phagocytosis involves solids, and pinocytosis involves liquids.
• Cell signalling occurs to control processes inside to body and respond to changes in the environment.
• Receptors are made of proteins and have a specific shapes on specific target cells so the messenger molecule is complimentary to that type of cell.