GCSE · Biology · AQA · Spec 8461

Active transport

Soil water is so dilute that a root hair cell already holds more mineral ions than the water around it. Yet the plant keeps taking them in.

Three ways in and out of a cell

What do they share, and what is theirs alone?

Diffusion, osmosis and active transport all move substances into and out of cells, but not in the same way. Tick every process each statement is true for, then check your diagram.

A Diffusion
B Osmosis
C Active transport

Moves substances into and out of cells
Described as movement from a more dilute solution to a more concentrated solution
Needs energy from respiration
Only water moves
Net movement goes from where the moving substance is more concentrated to where it is less concentrated
Can move dissolved particles such as sugar molecules
Moves a substance up its concentration gradient without using any energy
Moves a substance against its concentration gradient
Can move the particles of a gas
Happens without energy from respiration

Predict, then check

Think only about diffusion here. What would the ions do on their own?

Soil water is very dilute. A root hair cell holds a higher concentration of mineral ions than the water around it. If diffusion were the only way the ions could move, where would their net movement go?

Why it costs energy

Reason it through

Why does active transport need energy from respiration when diffusion and osmosis do not?

Link 1 of 4

First link · your turn

Left to themselves, which way do the particles of a substance end up moving overall?

Locked — reveal the link above first

Same words, different process

Is osmosis just active transport?

Osmosis moves water 'from a dilute solution to a concentrated solution'. Active transport moves substances 'from a more dilute solution to a more concentrated solution'. A classmate reads the two side by side and says: 'So they are really the same process.'

Which is closest to what you think right now?

They are the same. Both go from dilute to concentrated, so both must need energy.They are the same, except active transport is faster because energy speeds it up.They are both different from diffusion, because moving anything into a cell takes energy.They are different. In osmosis water moves; in active transport a dissolved substance is moved up its gradient.

How sure are you?

I'm sureI think soI'm guessing

They are the same. Both go from dilute to concentrated, so both must need energy.

This is a common misconception

The words match, but the thing moving does not. Osmosis moves water, and a dilute solution holds more water than a concentrated one. So water heading into the concentrated solution is going down its own concentration gradient, and that is free. In active transport it is the dissolved substance that heads into the concentrated side, which is up its gradient.

They are the same, except active transport is faster because energy speeds it up.

This is a common misconception

Energy does not hurry anything along. It changes which way is possible. Without it, ions or sugar only drift one way, down their gradient. With it, the cell can push them into the side that already holds more.

They are both different from diffusion, because moving anything into a cell takes energy.

This is a common misconception

A lot of what enters a cell costs it nothing. Water arrives by osmosis, and particles spreading out arrive by diffusion, without the cell spending anything. The cell pays only when something has to be pushed up its concentration gradient.

They are different. In osmosis water moves; in active transport a dissolved substance is moved up its gradient.

This matches the evidence

Yes. Ask what is moving, then ask which way it is going compared with its own concentration. Water in osmosis is going from more water to less. A substance in active transport is going from less of it to more.

Same words, opposite directions. Before you name the process, ask: is it water, or a dissolved substance?

Put it in your own words

Explain the differences

Explain the differences between diffusion, osmosis and active transport. [4 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

Diffusion and osmosis give net movement down a concentration gradient (from higher to lower concentration). (1 mark)
Active transport moves a substance against its concentration gradient (from a more dilute to a more concentrated solution). (1 mark)
Active transport needs energy from respiration, but diffusion and osmosis do not. (1 mark)
Osmosis is the movement of water only, through a partially permeable membrane, while diffusion can involve particles of any substance in solution or of a gas. (1 mark)

Model answer

Diffusion and osmosis both give a net movement down a concentration gradient, from higher to lower concentration, and neither needs energy. Osmosis is different from diffusion in general because only water moves, through a partially permeable membrane, whereas diffusion can move the particles of any substance in solution or of a gas. Active transport is different from both: it moves a substance against its concentration gradient, from a more dilute solution into a more concentrated one, and it needs energy from respiration to do so.

A difference has two sides. For each point, say what one process does and what the other does instead.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Diffusion and osmosis both run down a concentration gradient for free. Active transport runs the other way, and the cell pays for it with energy from respiration.

What you need to know

Diffusion is the spreading out of the particles of any substance in solution, or particles of a gas, resulting in a net movement from an area of higher concentration to an area of lower concentration.
Osmosis is the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.
Active transport moves substances from a more dilute solution to a more concentrated solution, against a concentration gradient, and requires energy from respiration.
Root hairs use active transport to absorb mineral ions from very dilute solutions in the soil. Plants need these ions for healthy growth.
Active transport also absorbs sugar molecules from lower concentrations in the gut into the blood, which has a higher sugar concentration. Cells use the sugar for respiration.

The big picture

Diffusion, osmosis and active transport all move substances into and out of cells. Diffusion is the net movement of particles from a higher to a lower concentration, and osmosis is that same spreading out happening to water, through a partially permeable membrane. Both run down a concentration gradient and cost the cell no energy. Active transport runs the other way: from a more dilute solution into a more concentrated one, paid for with energy from respiration. It is how root hairs take in mineral ions from very dilute soil water, and how sugar reaches blood that already holds more.

Key points

1Diffusion and osmosis go down a concentration gradient for free. Active transport goes against one, and that is why it needs energy.

2To tell osmosis from active transport, ask what is moving. In osmosis it is water, going down its own gradient. In active transport it is a dissolved substance, going up its gradient.

3Where diffusion would carry a substance the wrong way, moving ions out of a root hair or sugar from the blood into the gut, active transport is how the cell still takes it in.

4The energy comes from respiration, so a cell has to keep respiring to keep active transport going.

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Worked example

Problem

The blood has a higher concentration of sugar than the gut does. Explain how sugar molecules can still be absorbed from the gut into the blood.

⚠ Watch out

Calling any movement 'from a dilute to a concentrated solution' osmosis. Active transport is described with the same words. Check what moves: water means osmosis, with no energy needed; a dissolved substance such as mineral ions or sugar means active transport, powered by respiration.

🧠

Memory hook

Down is free, up costs. Diffusion and osmosis roll downhill on their own. Active transport is carried uphill, and the cell pays the fare with energy from respiration.

✓

Check yourself

Cover the page. Which two of diffusion, osmosis and active transport need no energy? Which one moves only water? Which one goes against the concentration gradient, and where does its energy come from?

Flashcards

(12)

What is diffusion?

The spreading out of the particles of any substance in solution, or particles of a gas, giving a net movement from an area of higher concentration to an area of lower concentration.

What is osmosis?

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

What is active transport?

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

Where does the energy for active transport come from?

Respiration.

Which of the three processes need no energy from respiration?

Diffusion and osmosis. Both go down a concentration gradient.

What does 'against the concentration gradient' mean?

Moving from where a substance is less concentrated to where it is more concentrated. This is the opposite of the way diffusion carries it.

Why can't root hairs rely on diffusion to take in mineral ions?

Soil water is very dilute, so the root hair cell holds more ions than the soil does. Diffusion would move ions out of the root, not in.

Why do plants need mineral ions?

For healthy growth.

Name the two examples of active transport in this topic.

Mineral ions taken into plant root hairs from the soil, and sugar taken into the blood from the gut.

What do cells use the sugar absorbed from the gut for?

Respiration.

Osmosis and active transport both go 'from dilute to concentrated'. What is different?

What moves. Osmosis moves water, down its own gradient, with no energy. Active transport moves a dissolved substance against its gradient, using energy from respiration.

Which of the three processes can move the particles of a gas?

Diffusion.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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What do they share, and what is theirs alone?

Is osmosis just active transport?

Explain the differences

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