Movement of substances, at a glance

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient.

• At equal concentrations, particles still move in both directions. There is no net movement.
• Diffusion supports nutrient absorption and gas exchange in humans and plants. It does not require energy supplied by respiration.

Keep in mind: Particles continue moving randomly in both directions. Equal rates of movement mean no net movement; they do not mean no movement.

Osmosis is the net movement of water molecules from higher to lower water potential through a partially permeable membrane.

• For the same solute at equal pressure, adding more dissolved solute lowers water potential.
• Only water movement is called osmosis. Water still crosses both ways when there is no net movement.

Keep in mind: For these solutions at the same pressure, more dissolved sugar means lower water potential. Be precise: solute concentration and water potential change in opposite directions.

Both cell types gain or lose water by osmosis. A plant cell has a rigid cell wall that limits expansion; an animal cell does not.

• Water enters: an animal cell swells and may burst; a plant cell becomes turgid as its vacuole expands and its wall resists further expansion.
• Water leaves: an animal cell shrinks; a plant cell loses turgor and becomes flaccid. After sufficient loss, its membrane pulls away from its wall: plasmolysis.
• The cell membrane is partially permeable. The plant cell wall allows water through and provides support.

Keep in mind: Water can pass through the freely permeable cell wall. The partially permeable cell membrane is the relevant barrier for osmosis; the wall provides mechanical support.

A gain in potato mass suggests net water entry; a loss suggests net water loss. Use percentage change to compare pieces with different starting masses.

• Percentage mass change = (final mass - initial mass) / initial mass x 100.
• Positive: net water entry. Negative: net water loss. Approximately zero: no net water movement, not no molecular movement.
• Control the source and dimensions of tissue, time, temperature, solution volume and blotting method. Repeat measurements and calculate means.

Keep in mind: Water can still move both ways. Zero change indicates approximately equal movement in and out overall, within the limits of the measurements.

A full description gives the direction, quotes the first and last values with units, and quotes the value at any turning point or level-off. Percentage change uses the original value as the denominator and keeps its sign.

• Describe: direction, then first and last values with units, then any turning point or level-off with its values.
• Percentage change = (new value - original value) / original value x 100. Keep the + or - sign.
• Three significant figures: -13.469% becomes -13.5%; -10% becomes -10.0%.
• An anomalous result does not fit the pattern of the other results. Leave it out of the mean and repeat that reading.

Keep in mind: Mark schemes reject figures without units. Write the unit every time you quote a value, for example "-8.8% at 0.6 mol/dm3".