Biology AS · Cell membranes and transport. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
Substances cross membranes in two ways. Passive processes need no energy from the cell: particles move down a concentration gradient, from where they are more concentrated to where they are less concentrated. Active processes use ATP and can move substances against a gradient.
Osmosis is the passive movement of water. Water always moves from a higher water potential to a lower one through a partially permeable membrane. Pure water has the highest possible value, 0 kPa. Dissolved solutes make the value negative.
Size matters for exchange. As a cell or organism gets larger, its volume grows faster than its surface area, so diffusion alone becomes too slow.
Treating −700 kPa as higher than −400 kPa because 700 is the bigger number. −400 kPa is the higher water potential, so water moves from −400 kPa to −700 kPa.
Answer: The ratio doubles from 3 : 1 to 6 : 1, so a substance diffuses to the centre of the agar more quickly.
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