Chapter 2: Movement of substances

• Diffusion is the net movement of particles (atoms,

  molecules or ions) from a region where they are of

  higher concentration to a region where they are of

  lower concentration; that is, down a concentration

  gradient.
• Concentration gradient is the difference in

  concentration between two regions.
• How is a concentration gradient related to diffusion?

  • The movement of particles is random
  • Particles will diffuse down their concentration gradient, till both region are of equal concentration (equilibrium)
• Diffusion across a membrane:

  • Diffusion can occur with or without a partially permeable membrane
  • Both the solvent and solutes can pass through a permeable membrane
• how the concentration gradient affects rate of diffusion:

  The steeper the concentration gradient, the faster the rate of diffusion
• how the diffusion distance affects rate of diffusion:

  The shorter the diffusion distance, the less time is needed for the substance to travel, and hence the rate of diffusion is higher
• in living organisms, diffusion distance is kept short by having a one-cell thick area for exchange of substances.

  for example:

  • gaseous exchange in the lungs
  • gaseous exchange in plants
  • absorption of nutrients in small intestine
• alveolus means air sac
• Describe how the alveolus is adapted for the exchange of gases?

  the walls of the alveolus is one cell thick

  There is a short diffusion distance for the oxygen to diffuse from the air spaces in the lungs across the one-cell thick alveolus and capillary walls into the red blood cell.

  This results in a faster rate of diffusion
• The rate if food and oxygen intake is slower as the cell grows larger.

  Hence, it is not beneficial for the cell to grow too big
• Describe the purpose of microvilli in the small intestine.?

  this is to Increase surface area-to-volume ratio for the absorption of digested food into the blood capillaries

  This results in a faster rate of diffusion
• describe how the villi is adapted for the absorption of digested food?

  The walls of the villi is one-cell thick. there is a short diffusion distance for the digested food to be absorb into the blood stream. This results in a faster rate of diffusion
• osmosis is the net movement of water molecules down a water potential gradient from a region of higher water potential to a region of lower water potential through a partially permeable membrane
• the partially permeable membrane allows some substances to pass through it but not others.

  sucrose is too large to pass through the partially permeable membrane
• both arms a and b have the same solute concentration

  solute concentration refers to the number of solute molecules per unit volume
• dilute solution has less solute -> higher water potential

  concentrated solution has more solute -> lower water potential
• Factors affecting rate of osmosis:

  • Water potential gradient
  • Distance that water molecules need to move
  • Surface area-to-volume ratio
• how does osmosis affect living organisms:

  cells are living osmotic systems.

  • A living cell is enclosed by a partially permeable membrane.
  • A plant cell behaves differently from an animal cell when placed in solutions of differing water potentials due to its cell wall.
• what happens to a plant cell in a solution with higher water potential?

  cell sap has a lower water potential that the solution. Water enters the plant cell down a water potential gradient through a partially permeable membrane via osmosis. This results in the plant cells to expand and becomes turgid.
• why is turgor important in plants:

  • maintain the shape of soft tissues in plants
  • plants are able to remain firm and erect
  • high rate of water loss from the cells, they lose their turgidity and the plant wilts
  • changes in turgor also cause the movements of some plant parts, e.g. opening and closing of stomata
• what happens to an animal cell in a solution with higher water potential?

  Animal cell has a lower water potential than the solution. Water enters the animal cell down a water potential gradient through a partially permeable membrane via osmosis. This results in the plant cells to expand and eventually lysed
• What happens to a plant cell in a solution with lower water potential?

  Cell sap has a higher water potential than the solution. Water leaves the plant cell down a water potential gradient through a partially permeable membrane via osmosis. The cytoplasm shrinks away from the cell wall and is plasmolysed
• What happens to an animal cell in a solution with lower water potential?

  Animal cell has a higher water potential than the solution. Water leaves the animal cell down a water potential gradient through a partially permeable membrane via osmosis. This results in the animal cell to crenate
• Describe why the plant cell becomes turgid when placed into solution X??

  Solution X has a higher water potential than the cell sap.

  Water enter the cell through the partially permeable membrane down the water potential gradient via osmosis.

  This results in the cell sap to increase in size and the plant cell to be turgid.
• Describe why the red blood cell Crenates when placed into solution Y?

  The solution has a lower water potential than the red blood cells.

  Water leaves the cell through the partially permeable membrane down the water potential gradient via osmosis.

  This results in the crenation of the cell.
• Active transport:

  Active transport is the process in which energy is used to move particles of a substance across a partially permeable membrane against its concentration gradient, that is, from a region where the particles are of lower concentration to a region where they are of higher concentration
• difference between concentration gradient and diffusion:

  diffusion is down a concentration gradient whereas active transport is against a concentration gradient and requires energy
• Active transport occur:

  • Active transport requires energy obtained through respiration. Thus, active transport occurs only in living cells.
  • Dissolved mineral salts are taken up by root hair cells via active transport.
• Differences between diffusion, osmosis, active transport (pt 1):

  both diffusion and active transport are net movement of particles (ions, molecules, atoms) whereas osmosis is the net movement of water molecules
• Differences between diffusion, osmosis, active transport (pt 2):

  Diffusion: movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient

  Osmosis: movement of water molecules from a region of higher water potential to a region of lower water potential, down a water potential gradient

  active transport: movement of particles from a region of lower concentration to a region of higher concentration, against a concentration gradient
• Differences between diffusion, osmosis, active transport (pt 3):

  Energy is not required in diffusion and osmosis whereas active transport requires energy
• Differences between diffusion, osmosis, active transport (pt 4):

  diffusion does not require a partially permeable membrane whereas osmosis and active transport require a partially permeable membrane
• there are 3 ways in which substances move into and out of cells in living organisms. Compare these three types of movement of substances. (?)

  Diffusion and active transport involve the net movement of particles, while osmosis involve only the net movement of water molecules.

  Both diffusion and osmosis involve movement of substances down a concentration gradient and water potential gradient respectively, while active transport involves movement of substances against the concentration gradient.

  No energy is required for diffusion and osmosis, while energy is required for active transport.
• 1. name the process by which water passes from the blood to the dialysis fluid.
  2. describe the process you have named in 1 (?

  1.Osmosis

  2. Net movement of water molecules from the blood with higher water potential to the dialysis fluid of lower water potential, through a partially permeable membrane.
• Explain why water passes out of the blood but proteins do not?

  water molecule is small enough to pass through the partially permeable membrane while proteins are too large to pass through.
• i) Explain the changes that have occurred in cell b and c:

  Cell B: The water potential in Cell b is lower the sugar solution. Water enters the cell through the partially permeable membrane via osmosis, causing the cell to be turgid

  Cell C: The water potential in Cell C is higher the sugar solution. Water leaves the cell through the partially permeable membrane via osmosis, causing the cell undergo plasmolysis

  
• Root hair cells have a high ion concentration. explain how this is maintained and its importance pt1:

  • High number of mitochondria in root hair cell providing sufficient energy via aerobic respiration for active transport of ions from the soil
• Root hair cells have a high ion concentration. explain how this is maintained and its importance pt2:

  • Creates a concentration gradient with neighbouring cells closer to the xylem, allowing ions to diffuse across the cells
  • Results in lower water potential in the cell compared to the surrounding soil
  • Water from the soil enters the root hair cell via osmosis, down the water potential gradient, resulting in the intake of water
• What happens to a cell in a solution with same water potential?

  A cell immersed in a solution with the same water potential as its cytoplasm will not change its size or shape.
• Explain how the table shown in table 7.1 in terms of water potential (pt1):

  As the concentration of sucrose solution increases from 0.0 moldm-3 to 0.4 moldm-3, there is a positive percentage change in length of potato of +7.0, +5.0 and +2.0 respectively.

  Water potential in the potato cell sap is lower than the sucrose solution. Water enters the plant cell down a water potential gradient through a partially permeable membrane via osmosis.

  This results in the positive percentage change in length of potato.