Cell Membranes and transport across membranes

• A cell membrane is a phospholipid bilayer
• Phospholipids arrange themselves so that the hydrophobic fatty acid tails can face inwards to reduce contact with water and so that the hydrophilic phosphate heads face outwards to maximise contact with water
• Within cell membranes, there is:

  Glycoproteins & Glycolipids - (Utilised as receptors and signallers)

  Cholesterol - (To strengthen the cell membrane)

  Channel Proteins - (To let through large/charged ions)

  Carrier proteins - (To move large/charged ions)
• The fluid mosaic model describes how the lipid molecules are able to rotate freely within the plane of the membrane.
• Diffusion is the movement of particles from a high to low concentration across a plasma membrane - this is a passive process
• Facillitated Diffusion is when substances cross the membrane using carrier or channel proteins - This is a passive process
• Active transport is the movement of particles from a low to high concentration using ATP
• Osmosis is the net movement of water molecules from a high water potential to a low water potential across a selectively permeable membrane
• Isotonic solutions have equal solute concentrations on both sides of the membrane so no water moves into or out of the cell
• Hypertonic solutions have higher solute concentrations outside the cell than inside causing water to leave the cell by osmosis
• Hypotonic solution has lower solute concentration outside than inside, causing water to move into the cell by osmosis
• Plasmolysis occurs when plant cells lose water due to hypertonic solution, the cell wall pulls away from the cytoplasm