B2.1 - Membranes and Transport

• The lipid of membranes is the phospholipid, together with other lipid types.
• Monolayer - forms in contact with water, heads dissolve, tails stick out
• When mixed in water, phospholipids rearrange into a bilayer, hydrophobic tails are attracted
• Lipid bilayer attractions between tails on the inside and heads + H2O on the outside, make a stable strong barrier (the membrane)
• Hydrocarbon tails make chains which forms the core of the membrane.
• The core of the membrane has low permeability to large and hydrophilic molecules, therefore functions as an effective barrier
• Cells may secrete substances such as hormones and enzymes (these molecules can go into or out of our cells), and water, respiratory gases, nutrients, essential ions, and excretory products
• Pants secrete the chemicals that make up their walls through their membranes and assemble and maintain the wall outside the membrane
• Some mammalian cells secrete structural proteins such as collagen, in a form that can be assembled outside the cells. All these molecules need to move across the lipid bilayer to get into or out of our cells.
• Diffusion: the movement from high to low concentrations - energy source: kinetic energy of molecules
• Osmosis: diffusion of water molecules across a membrane
• Active Transport: Selective movement of substances against a concentration gradient - energy source: energy from metabolism
• Bulk Transport: Transport of solids and/or liquids by vesicles at the plasma membrane - energy source: energy from metabolism
• The term 'Membrane Diffusion' is used to describe the movement of molecules through the phospholipid bilayer of membranes.
• O2 and CO2 are very small molecules and can diffuse through the phospholipid bilayer
• Oxygen diffused into cells from a higher concentration in the blood plasma into tissue fluid, then into cells where it is used for aerobic respiration. CO2, produced by respiration, travels in the opposite direction.
• Diffusion across the cell membrane occurs where the plasma membrane is fully permeable to the solute or the lipid bilayer of the plasma membrane is permeable to non-polar substances
• Two main types of proteins in the cell membrane: Integral and Peripheral proteins
• Peripheral Protein: A protein that is attached to the surface of the bilayer and faces the cytoplasm
• Peripheral proteins may shuffle between integral proteins on the surface of the membrane, be scaffold proteins to hold shape, or be receptors for extracellular signals
• Integrated proteins are proteins that are embedded in one of both of the lipid layers of the membrane. They may act as channels for transport of metabolites, or be enzymes and carries. Some may be receptors or antigens.
• Too much or too little water in the blood creates osmotic effects, where water will either move into cells in the blood (too much water), causing them to burst, or move out of the cells (too little water). This can be damaging to the body.
• There are protein lined pores in the plasma membrane called aquaporins, which allow water to pass through. It occurs easily where the plasma membrane contains phospholipids with unsaturated hydrocarbon tails, because these tails are spaced more widely
• Facilitated Diffusion: Particles of a substance that cannot diffuse across the plasma membrane through the phospholipid bilayer are helpex across the membrane by integral proteins
• Channel Proteins (made up of globular proteins) for pores large enough to allow diffusion. They provide hydrophilic channels for polar and charged molecules to pass through.
• Polar and charged molecules such as carbohydrates, amino acids, and ions, cross the plasma membrane through facilitated diffusion
• In active transport, metabolic energy is produced by the cell, stored as ATP, is transferred to drive the transport of molecules and ions across cell membranes
• Pump proteins (and some carrier proteins) use energy transferred from ATP to move specific particles across membranes and therefore these particles can be moved against the concentration gradient
• Most membrane pumps are specific to particular ions or molecules, bring about selective transport. If the pump molecule for a particular substance is not present, the substance will not be transported.
• Pump molecules pick up certain molecules or ions and transfer them to the other side of the membrane, where they are then released
• Cytoplasm of the cell normally holds reserves of valuable molecules - cell membranes keep them inside
• Energy released during respiration is used to form ATP from ADP and P. ATP can be regarded as a short term form of stored chemical energy in cells.
• Glycoproteins and glycolipids are on the outside of the membrane
• Glycolipids and glycoproteins fall under the category of glycocalyx
• The functions of glycocalyx include cell to cell recognition and the binding into tissues (cell adhesion)
• The glycocalyx is highly hydrophilic and attracted large amounts of water to the cells surface, which helps the cells interactions with its watery environment and with the cells ability to obtain substances dissolved in water
• Fluid Mosaic Model: Fluid because the components (lipids and proteins are on the move) and mosaic because the proteins are scattered in a pattern
• Unsaturated fatty acids in lipid bilayers have lower melting points, so membranes are fluid and more flexible at temperatures experienced by a cell.
• Unsaturated fatty acids melt at a lower temp because their unsaturated hydrocarbon tails do not pack so closely together, the same way saturated ones do.
• An excess of unsaturated fatty acid tails makes the membrane more fluid.