Biological Membranes

• Cell membranes are all partially permeable, the site of chemical reactions and have a role in cell communication.
• Proteins within the phospholipid bilayer can either be extrinsic or intrinsic, whereby extrinsic is where they are just on one side of the membrane, and intrinsic proteins span across the entire membrane.
• Extrinsic proteins provide mechanical support or are the glycoproteins and glycolipids for cell recognition as receptors.
• Intrinsic proteins can be carrier proteins or protein channels.
• Protein channels form tubes filled with water to allow the diffusion of water-soluble ions.
• Carrier proteins bind with other ions and large molecules and change their shape to transport them across the membrane.
• Cholesterol restricts the lateral movement of other molecules, which makes the membrane less fluid at higher temperatures and stops leaking of water and oils.
• If the temperature increases, there is more kinetic energy, so more movement within the membrane. This means there is more fluidity, and a greater permeability. This makes it easier for particles to travel across the membrane. Also, carrier and protein channels will denature at a higher temperature.
• Organic solvents, like alcohols, dissolve the phospholipid bilayer, because they dissolve lipids, and so they increase fluidity, making it more permeable.
• Simple diffusion is the net movement of particles from an area of high concentration to an area of low concentration. Particles must be small and lipid soluble to move across the membrane via diffusion.
• Facilitated diffusion is when particles are too big or not lipid soluble to cross the membrane by simple diffusion, but can use carrier proteins or channel proteins to pass through the membrane.
• Osmosis is the net movement of water from an area of high water potential to an area of low water potential across a partially permeable membrane.
• An isotonic solution is a solution that has the same concentration of solutes as the cell inside of it.
• A hypertonic solution is a solution with a more positive water potential than the cell. This means the cell swells up and eventually bursts.
• A hypotonic solution has a more negative water potential than the cell, with animal cells going through cremation, and plant cells being plasmolysed. They both just mean the cell will shrivel.
• Active transport is the active movement of molecules and ions from an area of low concentration to an area of high concentration using ARP and carrier proteins.
• Active transport is selective because only some molecules bind to the receptors on the carrier proteins.
• Carrier proteins change shape because when ATP binds to it, it is hydrolysed to form ADP and Pi.
• Once the molecule has been released, the Pi group is released, reverting the carrier protein back to its original shape.
• Endocytosis is the bulk transport if molecules into a cell. In it, the cell surface membrane bends inwards to surround the molecule, forming a vesicle around it. The vesicle pinches off and moves around the cytoplasm.
• Phagocytosis refers to when the molecule that the vesicles is formed around is a solid, and pinocytosis is when that molecule is a liquid.
• Endocytosis is a type of active transport and requires energy for the cell to change shape around the molecule.
• Exocytosis is the bulk transport of materials outside of a cell, whereby vesicle fuse with the cell-surface membrane and release its contents outside of the cell.
• Exocytosis is a form of active transport and requires energy as the vesicles need to move along the cytoskeleton.
• Water potential is the pressure exerted by water molecules as they collide with a membrane or container.
• Turgor is pressure pushed against a cell wall.