Topic 5

• Roles of membranes

  Partially permeable barriers between the cell and its environment, partially permeable barriers between organelles and the cytoplasm, partially permeable membranes within organelles, sites of chemical reactions, sites of cell signalling
• Other term for cell communication

  Cell signalling
• Fluid Mosaic Model of Membrane Structure

  Phospholipids are free to move within the phospholipid bilayer relative to each other, making them fluid and giving the membrane flexibility. Proteins are embedded in the bilayer and they vary in shape, size and position, like tiles do in a mosaic.
• Components of cell membrane structure

  Phospholipids, cholesterol, glycolipids, extrinsic proteins, glycoproteins, intrinsic proteins
• Role of phospholipids in a cell membrane

  To form the lipid bilayer which is the basic structure of the cell membrane.
• Role of cholesterol in a cell membrane

  Provides stability and flexibility
• Role of glycoplipids in a cell membrane

  Recognition site (Antigens)
• Role of intrinsic proteins

  Channel and carrier proteins which are important for passive and active transport into and out of cells
• Role of glycoproteins

  Recognition site, cell adhesion
• Channel proteins

  Hydrophilic channel that allows the passive movement of polar molecules and ions down a concentration gradient through membranes,
• Uses of carrier proteins

  Passive transport, active transport
• Role of membrane-bound receptors

  Sites where hormones and drugs can bind
• Factors affecting membrane structure and permeability

  Temperature, solvents
• Effect of temperature on membrane structure and permeability

  Increased temperature will increase the kinetic energy of phospholipids so they will move more, making the membrane more fluid and causing it to lose its structure, increasing the permeability of a cell membrane
• Effect of solvents on membrane structure and permeability

  Organic solvents dissolve cell membranes, non-polar solvent molecules get between the phospholipids and disrupt the membrane so it is more fluid and permeable
• Passive methods of movement across membranes

  Diffusion, facilitated diffusion
• Diffusion

  Net movement of particles from a region of higher concentration to a region of lower concentration
• Facilitated Diffusion

  Net movement of particles from a region of higher concentration to a region of lower concentration through protein channels
• Methods of movement across membranes

  Diffusion, facilitated diffusion, active transport, endocytosis, exocytosis
• Methods of movement across membranes that require ATP


  Active transport, endocytosis, exocytosis
• Factors that can affect diffusion rates

  Surface area, thickness of membrane
• Practical investigation into how surface area affects diffusion rates

  Use blocks of different surface areas containing phenolphthalein which turns pink when in the presence of an alkali. Immerse them in a sodium hydroxide solution for ten minutes. Remove the blocks and use a ruler to see the distance that the sodium hydroxide has diffused
• How to make a model cell

  Tying a piece of dialysis tubing at one end, filling it with a solution and then tying the other end before placing the 'cell' into a solution
• How to use model cells to estimate diffusion rates

  Changes in concentration of solute molecules inside and outside the model cell can be measured over time. Rates of diffusion can then be calculated from this.
• How to use model cells to see how concentration can change diffusion rates


  Place the model cells in solutions with different solute concentrations.
• Osmosis


  The net movement of water molecules from a region of a higher water potential down a water potential region to a region of lower water potential through a partially permeable membrane.
• Effect on a plant cell of being in a more concentrated solution

  Water will leave the cell, causing it to plasmolyse and the contents to shrink.
• Effect on a plant cell of being in a less concentrated solution

  Water will enter the cell, causing it to swell and become turgid.
• Effect on an animal cell of being in a more concentrated solution

  Water will leave the cell, causing it to shrink and shrivel and be crenated
• How active transport works

  Molecule binds to receptors in the channel of the carrier protein on the outside of the cell, ATP binds to the carrier protein on the inside of the cell and is hydrolysed into ADP and phosphate, binding of phosphate molecule to the carrier protein causes the protein to change shape which opens up the inside of the cell, molecule is released to inside of the cell, phosphate molecule is released from the carrier protein and recombines with ADP to form ATP, carrier protein returns to original shape
• Types of bulk transport

  Endocytosis, exocytosis
• Endocytosis


  Bulk transport of material into cells
• Types of endocytosis


  Phagocytosis, pinocytosis
• Phagocytosis


  Bulk transport of solid materials into cells
• Pinocytosis


  Bulk transport of liquid materials into cells
• How endocytosis works


  Cell surface membrane invaginates when it comes into contact with the material to be transported. The membrane enfolds the material until the membrane fuses to form a vesicle. Vesicle pinches off and moves into the cytoplasm to transfer the material for further processing within the cell.