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

The concentration gradient

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

为什么Diffusion 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

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

为什么Diffusion 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

提示Compare a crowd leaving a hall through one door and through ten doors.

为什么Each 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

提示It gives its name to the liquid the kidneys produce.

为什么Urea 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

提示Count how many are available to set off from each side.

为什么Movement 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

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

为什么Surface 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

提示Compare a cube of side 1 cm with a cube of side 2 cm.

为什么A 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

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

为什么A 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

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

为什么A 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

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

为什么Oxygen 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

提示Think of a sieve with holes of one particular size.

为什么A 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

提示Compare the liquid inside the potato with the liquid around it.

为什么The 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

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

为什么The 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

提示No two pieces cut from a potato weigh exactly alike.

为什么A 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

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

为什么Only 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

提示At that point the tissue neither gains nor loses water.

为什么Above 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

提示The first word is the opposite of "passive".

为什么Diffusion 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

提示Compare the amount dissolved on each side of the membrane.

为什么Soil 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

提示It is the process that takes place in mitochondria.

为什么Moving 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

提示One follows the gradient and the other works against it.

为什么Diffusion 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

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

为什么Mineral 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.