3.2.3 Transport across cell membranes

• Cell membrane- phospholipid bilayer, separates cytoplasm from outside of cell, controls movement of substances in and out of cell
• Phospholipid bilayer- permeable to water molecules and small polar molecules, impermeable to ions and large molecules
• The phosphate head is found on the outside of the phospholipid bilayer
• The fatty acid tails are found on the inside of the phospholipid bilayer
• Proteins in cell membrane- transports molecules, enzymes, hormones across
• Cholesterol in cell membrane- stabilises membrane, prevents solidification or fluidity
• Glycoproteins/glycolipids- cell signalling receptors, recognition site for antigens, increases cell adhesion
• Functions of cell membrane- limits what enters and exits the cell, increases flexibility, acts as a recognition site
• Membranes in the cell- increases surface area for organelle reactions, contains digestive enzymes
• Temperatures below 0°- channel proteins slow, ice crystals pierce membrane, less fluidity, permeability increases
• Temperatures 0-45°- more movement of phospholipids, fluidity and permeability increases
• Temperatures above 45°- phospholipid bilayer melts, channel proteins denature, fluidity increases, permeability increases
• Diffusion- random movement of particles from a high concentration to a lower concentration until they are evenly spread
• Rate of diffusion- affected by temperature, size of molecules, concentration gradient, surface area
• Carrier proteins- binds to specific molecules and transports them across the membrane, can be passive or active
• Facilitated diffusion- a type of passive transport that requires specific carrier proteins to transport large molecules and ions across a membrane, down the concentration gradient
• Osmosis- the net movement of water molecules from a higher water potential to a region of lower water potential, through a selectively permeable membrane
• Water potential - tendency of water molecules to move from one place to another
• Isotonic- equal concentration of particles in the solution and in the cell
• Hypertonic- more particles in the solution than the cell so the cell has a higher water potential, water moves out of the cell, cell shrinks
• Hypotonic- more particles in the cell than the solution, so the cell has a lower water potential, water enters the cell, cell swells
• Active transport- movement of particles from a low concentration to a high concentration, against the concentration gradient, using ATP energy, using carrier proteins
• Co-transport- the movement of molecules using carrier proteins, in both directions, at the same time
• How active transport works:

  • ATP binds to carrier proteins
  • ATP is hydrolysed into ADP and a phosphate group
  • Carrier protein changes shape, enabling it to bind to the particle and carry it across the phospholipid bilayer, against the concentration gradient
• Name and describe 5 ways substances can move across the cell-surface membrane into a cell (5 marks):

  Simple diffusion is where small molecules move down the concentration gradient. (1) Facilitated diffusion is where molecules move down the concentration gradient using channel proteins. (2) Osmosis is where water molecules move down the water potential gradient. (3) Active transport is where molecules move against the concentration gradient using ATP energy. (4) Co-transport of two different molecules using carrier proteins. (5)
• ATP hydrolase- an enzyme which hydrolyses ATP into ADP and a phosphate group to release energy, which is used to move ions against the concentration gradient
• Describe how the movement of substances across membranes is affected by membrane structure (5 marks):

  Carrier proteins allow active transport. (1) The number of carrier proteins determines how much movement of substances occurs. (2) The shape of carrier proteins determines which substances are moved. (3) Cholesterol affects membrane fluidity. (4) Phospholipid bilayer allows the movement of small, non-polar molecules. (5)