3. Movement into and out of cells

Particles move randomly in every direction. Where concentration is higher, more particles leave that region per unit time than return, so the overall—or net—movement is towards the lower concentration.

As the difference in concentration becomes smaller, net diffusion slows. At equal concentration, particles still move randomly in both directions, but there is no net movement.

Kinetic energy is the energy of movement. Molecules and ions already possess it, so their continuous random motion produces diffusion when a concentration gradient exists.

At a higher temperature, particles have more kinetic energy and move faster. This can increase the rate of diffusion; it does not change the energy source into respiration or ATP.

For each substance, compare its concentration on the two sides of the membrane. If the membrane allows that substance through, its net movement is from the side of higher concentration to the side of lower concentration.

A membrane can allow one substance through while preventing another. Therefore, both the concentration gradient and the membrane's permeability to that substance must be considered.

A complete explanation names the substance, identifies the two regions, states which has the higher concentration and gives the biological benefit of the transfer.

Blood flow, ventilation and the use or production of substances by cells can maintain concentration differences, allowing diffusion to continue rapidly.

Change one factor across a suitable range, measure a diffusion outcome over a fixed time—or the time to reach a fixed endpoint—and keep the other three factors constant. Repeat each condition and compare mean rates.

For coloured dye diffusing through agar jelly, change temperature while keeping the dye concentration, agar dimensions and observation time constant. Measure diffusion distance, repeat, calculate a mean, and plot mean distance or rate against temperature.

Choose an outcome that measures diffusion, identify the independent and dependent variables, control relevant variables, and use the trend to state how the tested factor affects rate.

The useful sequence is: a substance dissolves in water → it can move in a fluid or contact reacting molecules → it can be absorbed, transported or excreted.

A cell surface membrane is partially permeable. A cell wall is freely permeable and is not the membrane that controls osmosis.

Osmosis is passive: it can continue without energy supplied by respiration.

Compare the cell contents with the surrounding solution, identify which is more dilute, then trace the net movement of water through the cell membrane from the more dilute side towards the more concentrated side.

Animal cells have no cell wall: excessive water entry can make them swell and burst, while water loss makes them shrink.

Water moves through the tubing into the more concentrated solution, so the bag gains mass or the liquid level rises. Solute movement must be considered separately according to whether that solute can cross the membrane.

Rinse and blot each bag consistently before weighing, check for leaks, repeat each concentration and compare mean percentage change: (final − initial) ÷ initial × 100.

Cut pieces from the same tissue, use equal initial dimensions and solution volumes, control temperature and time, blot identically, repeat, calculate means and express change as a percentage of the initial value.

Plot mean percentage change against solution concentration. The concentration where the curve crosses 0% estimates the concentration producing no net water movement.

When water enters a plant cell, the vacuole and cell contents press the cell membrane against the strong cell wall. The wall resists further expansion, so the firm cell contributes to support of the tissue.

Many firm, water-filled cells together support non-woody stems and leaves without bones. Loss of water lowers this pressure across the tissue and the plant droops.

When water potentials are negative, the value closer to zero is higher. For example, water moves from −200 kPa to −450 kPa, provided a partially permeable membrane separates the regions.

Water molecules move randomly both ways. A water-potential difference makes more move from higher to lower water potential than in the reverse direction; at equal water potential there is no net movement.

Turgor pressure is the pressure of the cell contents pressing outwards on the cell wall. Plasmolysis occurs when water loss causes the cell membrane and cytoplasm to pull away from the cell wall.

Turgid cells make plant tissue firm. Flaccid or plasmolysed cells provide little support, so leaves and non-woody stems wilt.

Uptake continues only while a gradient exists. Solute uptake can lower water potential inside root cells, while water transport away from the root helps maintain the gradient from soil to root.

A complete explanation names the two regions, compares their water potentials, states that water crosses a partially permeable cell membrane by osmosis, and gives the organism-level consequence.

Moving particles against their gradient does not occur by their net random spreading. The cell must supply energy released by respiration to drive the movement.

Because energy comes from respiration, cells performing much active transport often contain many mitochondria. Reduced oxygen can reduce aerobic respiration and therefore reduce active uptake.

A complete root-hair explanation states: mineral-ion concentration is lower in soil than in the root cell, ions cross the cell membrane against the gradient, and respiration supplies energy.

A carrier's binding region fits particular molecules or ions, so different substances may require different carrier proteins.

If uptake reaches a maximum even as external concentration rises, a possible explanation is that all available carriers are working; the number of carriers limits the rate.