Bio chapter 2

• Diffusion

  Net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient
• Concentration gradient - the difference in concentration between 2 regions. The steeper the concentration gradient, the faster the rate of diffusion for that substance
• Factors affecting rate of diffusion

  concentration gradient

  diffusion distance

  surface area to volume ratio
• The importance of the surface area to volume ratio in limiting cell size

  As cell size increases, surface area : volume decreases. The rate of diffusion of materials across membrane will decrease.

  Furthermore, as cell size increases, metabolic wastes accumulate, overheating might occur, hence cell size must be limited.
• Particles of fluids possess movement (kinetic) energy. They are continuously moving about. As their movement is random, the particles will move (diffuse) down the concentration gradient and become evenly spaced out after some time
• Diffusion distance

  The distance through which a substance diffuses.

  The shorter the distance, the less time is needed for substance to travel, and so rate of diffusion is higher
• Passive processes

  Diffusion and osmosis
• Oxygen and carbon dioxide can pass through the cell membrane but proteins cannot. Diffusion is an important way which oxygen and carbon dioxide move in and out of the cell
• Surface area to volume ratio

  • The rate of movement of a substance across a cell membrane depends on how large the cell membrane is.
  • The cell with the larger surface area allows for higher rate of diffusion.
  • The greater the SA:V ratio, the higher the rate at which substances move in and out of the cell
• High solute concentration = low water potential
• Osmosis

  the net movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane
• Water potential is a measure of the tendency of water molecules to move from one place to another
• When a partially permeable membrane separates 2 solutions of different water potentials, a water potential gradient is established
• Starch is too big to pass trough visking tubing, but iodine is small enough
• Rate of osmosis is affected by

  Water potential gradient

  Distance over which water molecules need to move

  Temperature

  SA:V ratio
• As temperature rises, molecules gain kinetic energy. Increased kinetic energy leads to more collisions between water molecules and the selectively permeable membrane, thus quicker diffusion/osmosis rate
• Hypotonic solution

  When external solution has a low solute concentration/high water potential, water will enter the cell.

  Lysed/burst or turgid
• Isotonic solution

  When external solution has same solute concentration/water potential
• Hypertonic solution

  When external solution has higher solute concentration/lower water potential, water will move out of the cell.

  Crenated/shriveled or plasmolysed
• Animal cell after hypertonic solution

  Cell will shrink in size as there is lower solute concentration in the cell than the solution. As the cell is placed in hypertonic solution, water molecules move out of the cell by osmosis.
• Active transport

  the net movement of molecules and ions across a cell membrane from a region of lower concentration to a region of higher concentration, against the concentration gradient
• Plants cells in hypotonic solution

  Cell will swell as there is higher solute concentration in the cell than the solution. As the cell is placed in hypotonic solution, water molecules enter the vacuole by osmosis