Cell membranes and transport

• A protein that goes through one layer of the membrane is extrinsic.
• A protein that goes through both layers of the membrane is intrinsic.
• When detergents were used to remove the proteins from the cell membrane, this shows proteins weren’t easily removed so the bilayer had to be dissolved.
• The freeze-fracture technique showed that proteins were embedded in the bilayer by leaving an impression.
• Simple diffusion is the passive movement of particles from a high concentration to a low concentration.
• Examples of molecules that can cross a membrane by simple diffusion are small molecules such as oxygen and non-polar molecules.
• Facilitated diffusion is the movement of molecules across a membrane with the help of a protein like a carrier protein from a high concentration to a low concentration.
• Examples of molecules that can cross the membrane by facilitated diffusion are polar molecules like sugars, ions or salts.
• Active transport is the movement of molecules from an area of low concentration to an area of high concentration against a concentration gradient which requires energy in the form of ATP.
• Exocytosis is a form of bulk transport by which the cell moves materials from within the cell to the extracellular fluid. They do this using vesicles which fuse with the membrane.
• Examples of molecules that can be transported using exocytosis are hormones, enzymes or proteins.
• Endocytosis is a form of bulk transport that involves an area of the cell membrane surrounding the material being brought into the cell, which then buds off inside the cell to form a vesicle which contains the material.
• Simple diffusion, facilitated diffusion and osmosis are all passive processes.
• Active transport, exocytosis and endocytosis are active processes.
• Pinocytosis involves engulfing liquids.
• Phagocytosis involves engulfing solids.
• Osmosis is the movement of water from an area of high water potential to an area of lower water potential across a partially permeable membrane.
• Water potential is the potential energy of water in a system compared to pure water.
• Pure water has more potential energy and has a water potential of 0 which is the highest value possible.
• The unit for water potential is kilopascals.
• Water potential = solute potential + pressure potential
• Solute potential measure how easily water molecules move out of a solution.
• The more solute that is present, the more water molecules are tightly held so water is less likely to move out.
• The more concentrated the solution, the more negative the solute potential.
• Pressure potential is the force of water in the cytoplasm pushing against the cell wall (plant cells only). This force opposes the solute potential.
• Pressure potential increase the tendency of water to move out.
• In a hypotonic solution, the water potential is higher than the solution inside the cell. Water flows into the cell and the cell becomes turgid as the vacuole absorbs water and pushes the cytoplasm against the cell wall.
• In an isotonic solution, the water potential is the same as the solution inside the cell so there is no net movement of water. The cell is at incipient plasmolysis as the cell membrane is just starting to pull away from the cell wall. The cell wall does mot exert any pressure on the cytoplasm.
• In a hypertonic solution, the water potential is lower than he solution inside the cell and water moves out of the cell. The cell becomes plasmolysed as the vacuole shrinks and the cytoplasm is pulled away from the cell wall.
• In animal cells, in hypertonic solutions, the cell will crenate and water will move out.
• In animal cells, in hypotonic solutions, the cell will swell and burst because there is no cell wall to prevent bursting.
• Lysis means to destroy by bursting.
• The purpose of cholesterol is to maintain integrity and fluidity of the membrane.
• Channel proteins transport ions and water molecules through the membrane.
• The cell membrane is made of a phospholipid bilayer.
• A phospholipid has a hydrophilic head and a hydrophobic tail.