2.4 Transport Across Cell Membranes

• The cholesterol in the cell membrane prevents leakage of water and dissolved ions from the cell.
• The glycoprotein in the cell membrane is a protein attached to a carbohydrate that acts as recognition sites for cell signalling.
• The glycolipid in the cell membrane is a lipid attached to a carbohydrate that acts as a recognition site for cell signalling.
• Channel proteins are proteins that allow large and charged substances into the cell. It is made up of water-filled hydrophilic channels which let water-soluble ions through. If a particular ion is not present it remains closed.
• Carrier proteins are proteins that allow charged and large substances into the cell. When a molecule specific to the protein is present it binds with the protein. This causes a change in shape which pushes the molecule into the cell.
• The phospholipid bilayer allows lipid-soluble substances to enter and leave the cell. Charged molecules cannot pass through as they are repelled by the hydrophobic fatty acid tails of the phospholipids.
• The cell membrane is fluid -> individual phospholipid molecules can move relative to one another.
• The cell membrane is a mosaic -> proteins embedded ion the membrane vary in shape, size, and the pattern is similar to that of tiles of a mosaic.
• Most molecules cannot cross the cell membrane because they're not soluble in lipids, they're too large, they're polar molecules, or they are of the same charge as the charge on the protein.
• Cell membranes can adapt for more rapid transport by increasing the surface area or increasing the number of protein channels or carrier proteins.
• Diffusion is the net movement of particles from an area of high to lower concentration down a concentration gradient, due to the random movement of particles. It is a passive process, meaning no energy (ATP) is required.
• Simple diffusion --> small, non-polar can diffuse across the phospholipid bilayer from a higher to lower concentration.
• Facilitated diffusion is also a passive process (does not require ATP) and particles still move from an area of higher to lower concentration down a concentration gradient. It is different because charged and polar molecules cannot pass through the phospholipid bilayer, therefore protein channels and carrier proteins are needed.
• Active transport --> the ATP requiring movement of molecules from an area of lower concentration to an area of higher concentration through a carrier protein.
• Diffusion and facilitated diffusion are passive transports.
• A concentration gradient is the difference in concentration between two areas of a cell or tissue.
• Facilitated diffusion requires the use of carrier proteins and protein channels.
• In a protein channel, molecules are pushed through rather than connected and carried like in a carrier protein.
• Which molecule provides the energy needed for active transport?

  ATP.
• How is active transport different than diffusion and facilitated diffusion?

  It is an active process (requires ATP) whereas diffusion and facilitated diffusion are passive (don't require ATP).
• Osmosis is the net movement of water molecules from a higher water potential to a lower water potential through a semi-permeable membrane, using protein channels called aquaporins to increase the rate at which it happens.
• Water potential is a measure of water pressure.
• The water potential of pure water is 0kPa.
• An animal cell in a hypotonic solution would swell and may burst (lysis) as water would enter.
• An animal cell in an isotonic solution would have no net movement of water.
• An animal cell in a hypertonic solution would shrink and crenate as water would leave because the water potential in the cell is higher than the outside solution.
• A plant cell in a hypotonic solution would swell a bit and become turgid as water enters.
• A plant cell in an isotonic solution would have no net movement of water into or out of the cell as the water potential of the solution equals the water potential of the cell.
• A plant cell in a hypertonic solution will undergo plasmolysis as the cytoplasm will shrink due to water leaving because the water potential of the solution is lower than that of the cell.
• When animal cells are placed in a solution that is hypertonic they undergo lysis, why does this not happen to plant cells?

  Plant cells have a cell wall.
• Give two similarities in the movement of substances by diffusion and by osmosis.

  -They both move molecules from an area of higher concentration/water potential to and area of lower concentration/water potential.

  -They are both passive processes.
• Endocytosis - requires ATP:

  1. molecule binds to a receptor on cell membrane.
  2. this triggers plasma membrane to engulf molecules.
  3. the plasma membrane fuses with itself and pinches to form a vesicle.
  4. vesicle moves through the cell, transporting molecules to where they are needed in the cell.
  5. vesicle takes molecules to endoplasmic reticulum or golgi apparatus.
• Exocytosis - requires ATP:

  1. molecules are packaged into vesicles by RER, SER, or golgi apparatus.
  2. vesicle moves to plasma membrane, carrying molecules.
  3. vesicle fuses with plasma membrane, releasing molecules out of the cell.