Cells and organisation, Year 10: diffusion, osmosis and active transport

The concentration gradient

HintThe second word is also used for the steepness of a hill.

The whyDiffusion happens down a concentration gradient, from the higher concentration to the lower. The bigger the difference, the steeper the gradient and the faster the net movement of particles.

The particles have more energy and move faster

HintThink about what heating does to the motion of the tiny bits that make up a liquid or gas.

The whyDiffusion is caused by the random movement of particles. At a higher temperature the particles move more quickly, so they spread out, and cross membranes, in less time.

More particles can cross it at the same time

HintCompare a crowd leaving a hall through one door and through ten doors.

The whyEach particle crosses at the same speed whatever the size of the membrane, but a bigger membrane gives more room for particles to cross at once. This is why exchange surfaces such as the lungs are folded into an enormous area.

urea

HintIt gives its name to the liquid the kidneys produce.

The whyUrea is made when the body breaks down proteins it does not need. It is at a higher concentration in the cells that make it than in the blood, so it diffuses into the plasma, which carries it to the kidneys.

More particles move out of the crowded area than move into it

HintCount how many are available to set off from each side.

The whyMovement happens both ways all the time. Where there are more particles, more of them happen to move away, so the overall, or net, movement is towards the emptier area. It carries on until the concentrations are equal.

3

HintA cube has six square faces; find their total area, then the space inside.

The whySurface area = 6 × (2 × 2) = 24 cm². Volume = 2 × 2 × 2 = 8 cm³. The ratio 24 : 8 simplifies by dividing both numbers by 8.

It gets smaller

HintCompare a cube of side 1 cm with a cube of side 2 cm.

The whyA cube of side 1 cm has a ratio of 6 : 1; a cube of side 2 cm has 3 : 1. Volume grows faster than surface area, so a larger body has less surface for each unit of volume.

Its surface area is too small compared with its volume

HintThink how far the cells in the middle of a large body are from the outside.

The whyA single cell has a large surface area to volume ratio, so enough substances cross its surface to supply all of it. In a large animal the ratio is small and most cells are far from the outside, so it needs exchange surfaces and a transport system.

thin; blood

HintOne describes how far a particle has to travel across; the other is what carries substances away afterwards.

The whyA large area lets many particles cross at once. A short path means each particle crosses quickly. In animals, an efficient flow of transport fluid carries the substance away, which keeps the concentration gradient steep.

It keeps bringing in fresh air, so the concentration gradient stays steep

HintAsk what would happen to the oxygen level in the lungs if nothing in them were ever replaced.

The whyOxygen diffuses into the blood because it is more concentrated in the air sacs than in the blood. Ventilation replaces the used air, keeping the oxygen concentration high on the air side, so diffusion stays fast.

A partially permeable membrane

HintThink of a sieve with holes of one particular size.

The whyA cell membrane has tiny gaps that small molecules such as water can pass through, while larger dissolved molecules such as sugar cannot. Osmosis is the diffusion of water across a membrane of this kind.

Water moves out of its cells by osmosis, into the more concentrated solution

HintCompare the liquid inside the potato with the liquid around it.

The whyThe solution outside is more concentrated than the solution inside the cells. Water moves by osmosis from the dilute solution to the concentrated one, so it leaves the cells and the cylinder gets lighter and softer.

12

HintFind the change first, then compare it with where the cylinder started.

The whyThe change in mass is 2.80 − 2.50 = 0.30 g. Percentage change = change ÷ original mass × 100 = 0.30 ÷ 2.50 × 100. Always divide by the starting mass, not the final one.

The cylinders start with different masses, so percentages allow a fair comparison

HintNo two pieces cut from a potato weigh exactly alike.

The whyA large cylinder gains or loses more grams than a small one in the same solution. Dividing each change by the starting mass removes the effect of size, so only the effect of the solution is compared.

To remove liquid on its surface, which would add to the measured mass

HintThe balance cannot tell what is inside the potato from what is clinging to its outside.

The whyOnly water that has entered or left the cells should be measured. Drops of solution on the outside would make every cylinder seem heavier. Blotting must be gentle and the same each time, because squeezing would force water out of the cells.

It equals the concentration of the solution inside the potato cells

HintAt that point the tissue neither gains nor loses water.

The whyAbove the axis the cylinders gained water; below it they lost water. Where the line crosses, there is no net movement of water, so the solutions inside and outside the cells must be equally concentrated. The graph is used to estimate a value that cannot be measured directly.

Active transport

HintThe first word is the opposite of "passive".

The whyDiffusion can only move substances down a concentration gradient. When a cell needs to take in a substance that is already more concentrated inside, it has to move it against the gradient, and that needs energy.

The ions are more concentrated inside the root cells than in the soil

HintCompare the amount dissolved on each side of the membrane.

The whySoil water is a very dilute solution of mineral ions. Diffusion would carry ions out of the root, not in. Root hair cells therefore use active transport to move ions in against the concentration gradient.

respiration

HintIt is the process that takes place in mitochondria.

The whyMoving a substance against its concentration gradient is like pushing something uphill: it will not happen by itself. Cells that do a lot of active transport, such as root hair cells, contain many mitochondria.

Active transport goes from low to high concentration; diffusion goes from high to low

HintOne follows the gradient and the other works against it.

The whyDiffusion and osmosis follow the concentration gradient and need no energy from the cell. Active transport moves substances the other way, against the gradient, which is why it needs energy from respiration.

It stops, because active transport needs energy from respiration

HintAsk which process brings those ions in, and what that process depends on.

The whyMineral ions enter roots against a concentration gradient. With no respiration there is no energy to drive this, so uptake stops. Water uptake by osmosis would not be stopped in the same way.