2.12 – Understand definitions of diffusion, osmosis and active transport.
– Diffusion: The passive movement of particles down a concentration gradient.
– Osmosis: The movement of water molecules down a water-potential gradient through
a selectively-permeable membrane.
– Active transport: The movement of particles against a concentration gradient using
energy in the form of ATP from respiration.
2.13 – Understand that movement of substances into and out of cells can be by diffusion,
osmosis and active transport.
– Substances can move into and out of cells by diffusion, osmosis or active
transport.
• This generally applies to small molecules only (e.g. water) or gases as bigger
molecules cannot pass through membranes.
2.14 – Understand the importance in plants of turgid cells as a means of support.
– Turgidity (Turgor) is the water pressure in plant cells that act as support.
– Hypotonic: Higher concentration of solution inside of cell. The cell experiences net
water gain and swells up.
• Cytolysis: The bursting of a cell due to excessive water gain.
– Hypertonic: Higher concentration of solution outside of cell. The cell experiences net
water loss and shrinks.
• Plasmolysis: The shrinking of a cell due to excessive water loss.
– Isotonic: Equal concentration both inside and outside of cell. The cell experiences no
net water gain or loss.
2.15 – Understand the factors that affect the rate of movement of substances into and
out of cells, to include the effects of surface area to volume ratio, temperature and
concentration gradient.
– SA:Vol. – the bigger the ratio, the faster the rate of movement.
– Concentration gradient – the steeper the gradient (the bigger the concentration
difference), the faster the movement.
– Temperature – the higher the temperature, the faster the movement as particles have
higher kinetic energy at higher temperatures.
2.16 – Describe experiments to investigate diffusion and osmosis using living and nonliving systems.
– AGAR JELLY
• When hydrochloric acid reacts with potassium permanganate, the purple colour
disappears.
• A Petri dish is prepared with a layer of agar jelly, dyed purple with potassium
permanganate.
– Three cubes are cut out of the jelly with side lengths of 2cm, 1cm and 0.5cm.
• The three cubes are dropped into a beaker of dilute hydrochloric acid.
• The time taken for each cube to turn colourless is recorded using a stopwatch.
• The smaller cube will turn colourless first as it has the largest SA:Vol. out of the
three cubes.
• The larger cube will be the last one to turn colourless because it has the lowest
SA:Vol. ratio.
– Small cube with dimensions of 0.5cm:
• (0.5・0.5)・6 = 1.5; 0.53 = 0.125 → SA:Vol = 12:1
– Large cube with dimensions of 2cm:
• (2・2)・6 = 24; 23 = 8 → SA:Vol = 3:1
– POTATO CUBES
• Cut three different sized cubes of potato, with side lengths of 2cm, 1cm and 0.5cm.
• Weigh the potato cubes and record their masses.
• Place each cube into a beaker of glucose solution with a different
concentration and leave them for 12 hours.
• Weigh the potato cubes again and record their new masses.
• The potato cubes in a dilute solution will experience net water gain (hypotonicity),
and thus increase in mass.
• The potato cubes in a concentrated solution will experience net water loss
(hypertonicity), and thus a decrease in mass.