movement of substances

diffusion

• net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient until an equilibrium is reached

• e.g. diffusion in amoeba: partially permeable cell membrane allows small molecules like oxygen and carbon dioxide to enter but not large molecules like protein

• e.g. diffusion in lungs: blood passing through lungs have more carbon dioxide than oxygen

  • blood has lower concentration of oxygen than carbon dioxide so, net movement of oxygen molecules from higher conc in air space in lungs to lower conc in blood, down a conc gradient

  • net movement of carbon dioxide molecules from a region of higher conc in blood to lower conc in air spaces in lung, down a conc gradient

osmosis

• net movement of water molecules from a region of higher water potential to a region of lower potential, down a concentration gradient, through a partially permeable membrane

osmosis in plant & animal cells

• dilute solution: solution has a higher water potential than cell, net movement of water molecules from solution to cell down a water potential gradient, across a partially permeable membrane, plant cell will swell and become turgid but not burst due to cell wall being able to withstand turgor pressure but animal cell will lyse

• concentrated solution: cell has a higher water potential than solution, net movement of water molecules from cell to solution down a water potential gradient, across a partially permeable membrane, plant cell will become plasmolysed while animal cell becomes crenated

• isotonic solution: cell and solution has same water potential so no net movement of water molecules hence, no change in size and turgidity of cell

active transport

• net movement of molecules from a region of lower concentration to a region of higher concentration against a concentration gradient

• energy released via respiraton is needed for active transport to take place, hence only living things that respire undergoes active transport

factors that affect movement of molecules

• travel distance - the shorter the travel distance between molecules, the higher the rate of movement of molecules

• steepness of gradient - the steeper the gradient, the higher the rate of movement of molecules

• surface area to volume ratio - the larger the surface area to volume ratio, the higher the rate of movement of molecules

• temperature of particles - the higher the temperature, molecules gain kinetic energy and move faster