Cell Membrane and Transport

• Plasma membrane function


  controls what comes in and out of the cell

  
• Structure of plasma membrane


  Phospholipid bilayer with embedded proteins

  
• Selectively permeable


  A property of cell membranes that allows some substances to pass through, while others cannot
• Passive transport


  The movement of molecules from high to low concentration, along the concentration gradient. Requires no energy.

  
• Diffusion


  The net movement of molecules from an area of high concentration to an area of low concentration through a semi-permeable membrane.

  
• Facilitated diffusion


  Movement of specific molecules across cell membranes from high concentrations to low concentrations through protein channels.

  
• Osmosis


  The net movement of water from an area of high concentration to an area of low concentration through a semi-permeable membrane

  
• Hypotonic


  Solutions with a lower solute concentration and a higher water concentration

  
• Hypertonic


  Solution with a higher solute concentration and a lower water concentration

  
• Isotonic


  Solutions of equal solute concentration

  
• Surface area to volume ratio


  Ratio of a cell's outside area to its internal volume. A large SA: V means that diffusion of materials in and out of the cells is more efficient.

  
• Active transport


  Movement of molecules from an area of low concentration to an area of high concentration. This is against a concentration gradient and requires energy.

  
• Protein pump


  Type of cell membrane protein used to move small things against the concentration gradient.

  
• Endocytosis


  The process by which a cell takes material into the cell by infolding of the cell membrane

  
• Exocytosis


  The release of substances out a cell by the fusion of a vesicle with the membrane.

  
• Active transport requires energy input and involves the use of ATP.
• Passive transport does not require energy input and occurs by diffusion or osmosis.
• The cell membrane is selectively permeable, allowing some substances to pass through while preventing others from doing so.
• Diffusion is the net movement of particles down their concentration gradient until equilibrium is reached.
• Facilitated diffusion is the passive movement of particles through channels formed by proteins embedded in the plasma membrane.
• Tonicity is the measure of concentration of solutes on either side of the membrane
• Isotonic solutions have equal concentrations on both sides of the membrane and do not affect cell volume.
• When a cell is placed in a hypertonic solution, cells shrink and shrivel, pulling away from the cell wall as water moves from an area of high water concentration inside the cell (low solute %) to an area of low water concentration outside the cell (high solute %)
• When a cell is placed in a hypotonic solution, cells swell up and can potentially burst as water moves from an area of high water concentration outside the cell (low solute %) to an area of low water concentration inside the cell (high solute %)
• ATP


  Adenosine triphosphate - the energy currency of the cell
• Mitosis


  Cell division for growth and repair. Creates two daughter cells.
• Osmosis


  The net movement of water across a semipermeable membrane from an area of high concentration to an area of low concentration
• turgid


  firm or hard
• flaccid


  shrunken and shrivelled
• Factors affecting the rate of diffusion

  temperature, concentration gradient, size of particles
• Diffusion factors: Temperature


  Molecules will diffuse quicker at higher temperatures as increasing temperatures causes molecules to move faster.
• Diffusion factors: Concentration gradient


  If the concentration gradient is steep, the rate of diffusion will be faster. Gas molecule diffuse faster than liquid molecules.
• Enzymes


  Biological catalysts made up of proteins, which means they speed up reactions.
• Structure of enzymes


  Enzymes are proteins, so made of amino acids folded into a specific 3D shape. They have an active site, which joins or breaks substrates.
• How enzymes work


  Enzymes increase the rate of biological reactions by lowering the activation energy. They are specific to a reaction and are not used up in the reaction
• Induced fit model


  When a substrate combines with an enzyme it induces a change in the shape of the enzyme. The active site is then moulded precisely to fit the substrate

  
• Effect of temperature on enzyme activity

  As temperature increases, so does the rate of reaction up until the optimal temperature. After the optimal temperature, the rate of reaction decreases because the enzyme is denatured.

  
• Effect of pH on enzyme activity

  All enzymes work best at optimal pH. Above and below the optimal pH the rate of reaction decreases. Extreme pH changes can denature the enzyme.

  
• Denature


  The bonds holding the enzyme break (due to high temperatures or pH extremes) causing the enzyme to change shape and alter the active site so that substrates can no long fit.

  
• Effect of substrate concentration on enzyme activity

  an increase in substrate concentration means more can combine with the enzymes' active site and therefore increase the rate of reaction until the available enzyme molecules become 'saturated' and the rate of reaction levels off