Transport in Cells

The spreading out of particles of a gas, or of a substance in solution, resulting in a net movement from a higher concentration to a lower concentration.

The overall movement in one direction. Particles move both ways, but more move one way.

No. The particles move on their own.

The difference in concentration between two places.

From a higher concentration to a lower concentration, down the concentration gradient.

Oxygen and carbon dioxide.

Urea.

The kidneys remove it so it can be excreted.

Into the cell, because oxygen is used up there, so its concentration is lower inside.

Respiration makes it, so its concentration is higher inside the cell.

The concentration gradient, the temperature, and the surface area of the membrane.

It increases.

The particles have more energy and move faster.

More particles can cross the membrane at the same time.

Never for the particles themselves, but there is no net movement once they are evenly spread.

Divide the amount that changed by the time it took.

5 units per minute.

Breathing brings fresh air in, and flowing blood carries oxygen away.

Saying particles move only from high to low. They move both ways, but the net movement is from high to low.

Active transport, which needs energy.

What does the arrow marked X show?

The net movement of particles across the membrane, from the higher concentration to the lower concentration. This is diffusion.

Which of these three changes make diffusion faster?

All three: a bigger concentration gradient, a higher temperature and a larger surface area of membrane.

The surface area of an object divided by its volume.

It gets smaller.

24 cm², because \(6 \times 2 \times 2\).

27 cm³, because \(3 \times 3 \times 3\).

2 : 1, from 54 ÷ 27.

It has a large surface area to volume ratio, so substances reach the whole cell fast enough.

Its surface area to volume ratio is small, so substances would take too long to reach the cells in the middle.

Exchange surfaces and a transport system.

A large surface area, a thin membrane, an efficient blood supply (animals) and ventilation (animals, for gas exchange).

It gives a short diffusion path.

It carries substances away, keeping the concentration gradient steep.

It keeps the concentration of the gas high on the outside, keeping the gradient steep.

Finger-like folds in the lining of the small intestine that give a large surface area.

The tiny air sacs in the lungs where oxygen and carbon dioxide are exchanged.

By breathing.

Water flows over them all the time.

They have a long thin extension, which gives a large surface area.

They are flat and thin, with air spaces inside and stomata in the surface.

Tiny pores in the leaf surface that let carbon dioxide in and oxygen out.

Those of plants: the roots and the leaves.

Work out the surface area to volume ratio of each cube.

1 cm: 6 cm² ÷ 1 cm³ = 6 : 1. 2 cm: 24 ÷ 8 = 3 : 1. 3 cm: 54 ÷ 27 = 2 : 1. The bigger the cube, the smaller the ratio.

Why do the middle cells of a large organism need an exchange surface and a transport system?

Its surface area to volume ratio is small, so diffusion from the outside would be too slow to reach them. Exchange surfaces and a transport system get substances to every cell.

The diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.

A membrane that lets small molecules such as water through, but not larger dissolved molecules.

From the more dilute solution to the more concentrated solution.

One with a low concentration of dissolved substance.

One with a high concentration of dissolved substance.

There is no net movement of water.

Yes. It is the diffusion of water through a partially permeable membrane.

Water enters by osmosis and the cell becomes turgid.

Firm and swollen with water.

Its strong cell wall.

Water leaves by osmosis and the cell becomes flaccid.

Soft and limp, because the cell has lost water.

When so much water leaves that the membrane pulls away from the cell wall.

Its cells go flaccid, so they cannot hold the stems and leaves up.

Water enters by osmosis, the cell swells and may burst.

An animal cell has no cell wall to resist the swelling.

Water leaves by osmosis and the cell shrinks and shrivels.

Into the cell.

Saying water moves towards more water. It moves from more water (dilute) to less water (concentrated).

One with the same concentration of dissolved substances as the body's fluids.

What is the structure marked X, and what does it let through?

A partially permeable membrane. It lets small molecules such as water through, but not larger molecules such as sugar.

Dilute solution: water enters by osmosis, so the cell swells and may burst. Same concentration: no net movement of water, so it stays normal. Concentrated solution: water leaves by osmosis, so the cell shrinks.

To investigate the effect of different concentrations of salt or sugar solution on the mass of plant tissue.

It is cheap, easy to cut into identical pieces, and made of cells with partially permeable membranes.

A cork borer.

The concentration of the solution.

The percentage change in mass of the potato.

The size of the cylinders, the volume of solution, the time in solution (also the temperature and the type of potato).

To remove liquid from the surface, which would add extra mass.

(final mass − start mass) ÷ start mass × 100.

+15%.

The potato lost mass, because water left it by osmosis.

The cylinders start with slightly different masses, and percentages make the results fair to compare.

It is more dilute than the potato cell sap.

It is more concentrated than the potato cell sap.

There is no change in mass, so the solution has the same concentration as the cell sap.

Read the concentration where the line crosses 0% change.

It is the most dilute solution, and shows the most water entering the potato.

Repeat the test with several cylinders and calculate a mean.

A result that does not fit the pattern of the others.

Use extra solutions of concentrations close to where the line crosses zero.

Cut on a tile, with the blade pointing away from your hands.

Read the concentration of the potato cell sap from this graph.

About 0.33 mol/dm³: where the line crosses 0% change, so there is no net movement of water.

−15%. The change is 2.72 − 3.20 = −0.48 g, then −0.48 ÷ 3.20 × 100 = −15%.

The movement of substances from a more dilute to a more concentrated solution, against a concentration gradient, using energy from respiration.

Energy from respiration.

Up it: from low concentration to high concentration.

Mitochondria.

They release the energy from respiration that active transport needs.

A protein in the cell membrane that picks up a particular substance and moves it across.

Mineral ions being taken into root hair cells from the dilute soil solution.

For healthy growth. For example nitrate for proteins and magnesium for chlorophyll.

The ions are at a lower concentration in the soil than in the cell.

Sugar being absorbed from the gut into the blood.

Sometimes the concentration of sugar in the gut is lower than in the blood.

Sugar molecules are used for cell respiration.

It stops, because there is no energy.

Less oxygen means less respiration, so less energy for active transport.

It goes against the concentration gradient and needs energy. Diffusion goes down the gradient and needs none.

Diffusion and osmosis.

Osmosis.

From a lower concentration to a higher concentration.

Saying it is the same as diffusion, or forgetting that it needs energy.

Active transport: it takes energy to push it up.

What is the structure marked X, and what does it do?

A carrier protein. It picks up a substance and moves it across the cell membrane, against the concentration gradient, using energy from respiration.

Why can't a root hair cell take in mineral ions by diffusion?

The mineral ions are at a lower concentration in the soil than in the root hair cell, so they would diffuse out. Active transport moves them in against the gradient, using energy from respiration.