2 - movement of substances

• diffusion is the net movement of particles from a region where they are of higher concentration to a region where they are of lower concentration, down a concentration gradient
• net movement of particles: the overall movement of particles when the movement of particles in one direction is greater than the movement of particles in the opposite direction
• concentration gradient is the difference in concentration between two regions
• • concentration is the quantity or amount of a substance in a fixed volume
  • concentrations can be expressed as mass over volume (g/cm3 or g/ml)
• diffusion is a passive process, which does not require additional energy input
• cells in living organisms posses a partially permeable membrane.

  • gas exchange - oxygen and carbon dioxide move into and out of cells
  • animals/ humans: lungs
  • plants: root hair cells
  • absorbing nutrients
  • (humans) dissolved amino acids/ glucose are absorbed by diffusion into the blood capillaries in the small intestine
• factors affecting the rate of diffusion:

  1. concentration gradient: the steeper the concentration gradient, the faster the rate of diffusion
  2. diffusion distance: the shorter the diffusion distance, the less time needed for the substance to travel, hence the faster the rate of diffusion
  3. temperature: as the temperature increases, the particles gain more energy and move faster, hence the rate of diffusion increases
  4. surface area - to - volume ratio: the greater the surface area - to - volume ratio, the higher the rate of diffusion
• osmosis is 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
• dilute solution = higher water potential

  concentrated solution = lower water potential

  lower solute concentration = higher water potential

  higher solute concentration = lower water potential
• factors affecting the rate of osmosis:

  1. water potential gradient: the steeper the water potential gradient, the faster the rate of osmosis
  2. distance over which molecules need to move: the shorter the distance over which molecules need to move, the faster the rate of osmosis
  3. temperature: as the temperature increases, particles gain more energy and moves faster, hence increasing the rate of osmosis
  4. surface area - to - volume ratio: the greater the surface area to volume ratio, the higher the rate of osmosis
• cytoplasm is a mixture of dissolved substances, which is also an aqueous solution

  hence, the net movement of water molecules will occur between the cytoplasm of a cell and its external environment across the plasma membrane

  • in plants, it will be between the cell sap in the vacuole and the external solution
• HYPOTONIC SOLUTIONS (water enters the cell by osmosis)

  -> less concentrated

  -> higher water potential

  animal cells: cell swells and may even burst, or lyse, as it does not have a cell wall to protect it

  plant cells: the vacuole increases in size and pushes the cytoplasm against the cell wall. the cell swells, but does not burst as the cell wall is strong and relatively inelastic. it prevents over-expansion of the cell by exerting turgor pressure. when the cell wall is eventually stretched to the maximum, it is described as being fully turgid
• HYPERTONIC SOLUTIONS (water leaves the cell by osmosis)

  -> less concentrated

  -> higher water potential

  animal cells: will shrink, or crenate, becoming dehydrated and eventually dies

  plant cells: the vacuole and cytoplasm decrease in size. the plasma membrane starts to pull away from the cell wall, leaving a visible gap. this is known as plasmolysis. when the plasma membrane has fully plasmolysed, it is now flaccid
• turgor in plants

  • how turgid the cells are maintains the shape of soft tissues in plants
  • stems and leaves are able to remain firm and errect