GCSE · Biology · Edexcel · Spec 1BI0

Diffusion, osmosis and active transport

Your cells are busy border crossings: oxygen in, waste out, water both ways. Two questions tell you how anything got across.

Biology · Transport across membranes

Two questions decide every case

Pick a route. Ask the questions in order, and follow the answer that fits the substance in front of you.

Question 1: is it water? → Question 2: which way, compared with the gradient?

Start here every time. Before you name a process, ask what is moving and which way it is going.

3 possible routes.

On A substance is crossing a cell membrane. 2 branches to choose from.

Picture the concentration gradient as a slope. Diffusion and osmosis roll downhill by themselves, so they cost the cell nothing. Active transport carries a particle back uphill, so the cell has to pay with energy.

Watch out: Only one route is ever taken. A substance moving against its concentration gradient can't be 'diffusing': the direction alone rules diffusion out.

Osmosis: what's really moving?

Which way does the water go?

Some tissue cells sit right next to a capillary. Right now the tissue cells have a higher water concentration than the blood. A partially permeable membrane separates them.

Which is closest to what you think happens?

The dissolved substances in the blood move into the cells. That's what osmosis is.Water moves into the tissue cells, because water goes to where there's already more water.There's a net movement of water out of the tissue cells and into the blood, by osmosis, with no energy needed.Water can only cross if the cells use energy to push it through.

How sure are you?

I'm sureI think soI'm guessing

The dissolved substances in the blood move into the cells. That's what osmosis is.

This is a common misconception

Osmosis never means the dissolved substances moving. It is the movement of water, and only water. When a dissolved substance such as urea crosses a membrane down its gradient, that's diffusion. So in this situation, look at the water: the cells have more of it, so the net movement of water is out of the cells and into the blood.

Water moves into the tissue cells, because water goes to where there's already more water.

This is a common misconception

That's the direction backwards. Osmosis moves water from the higher water concentration to the lower one, downhill like any diffusion. The cells have the higher water concentration here, so the net movement is out of the cells and into the blood, not further into the cells.

There's a net movement of water out of the tissue cells and into the blood, by osmosis, with no energy needed.

This matches the evidence

Yes. The cells have the higher water concentration and the blood has the lower one, so overall water moves from the cells into the blood through the partially permeable membrane. It happens by itself, because osmosis is passive. If the blood had the higher water concentration instead, the water would go the other way, into the cells.

Water can only cross if the cells use energy to push it through.

This is a common misconception

No energy is needed. Osmosis is a type of diffusion, and diffusion is passive: the substance moves down its concentration gradient by itself. Energy only comes into it when something is moved against its gradient, and that's active transport.

Osmosis moves water from the side with more water to the side with less, and never the dissolved substances. Find the higher water concentration first and the direction follows.

Exam line: Always say where the water concentration is higher and where it is lower: 'water moves from the tissue cells, where the water concentration is higher, into the blood, where it is lower, by osmosis.'

Active transport: carrying a particle uphill

Step through what happens at the membrane. Look for the moment the energy gets used.

Predict: blood meets tissue

Use Question 2 from the decision tree. Where is each substance more concentrated?

Blood in a capillary has a higher oxygen concentration than the tissue cells next to it. The tissue cells have higher concentrations of carbon dioxide and urea than the blood. What happens?

Predict: a kidney dialysis machine

New situation, same questions. Is it water? Which side has the higher concentration?

In kidney dialysis, a patient's blood flows past dialysis fluid, with a partially permeable membrane between them. The blood has a higher concentration of urea than the fluid, and a higher concentration of water. What happens?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Three ways across a cell membrane, and two questions that tell you which one you're looking at.

What you need to know

A cell membrane is partially permeable (selectively permeable): it lets some substances pass through but not others.
Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration, down a concentration gradient. It is passive: no energy from the cell.
Osmosis is the net movement of water from an area of higher water concentration to an area of lower water concentration through a partially permeable membrane. It is a type of diffusion that applies only to water, and it is passive.
Active transport moves particles from an area of lower concentration to an area of higher concentration, against the concentration gradient. It needs energy (ATP) and uses carrier proteins in the membrane.

The big picture

Substances cross cell membranes in three ways. The membrane is partially permeable, so it lets some substances through but not others. Diffusion is the net movement of particles from higher to lower concentration, down the concentration gradient, and needs no energy. Osmosis is the same idea for water only: the net movement of water from higher to lower water concentration through a partially permeable membrane. Active transport moves particles against the concentration gradient, from lower to higher concentration, using energy and carrier proteins.

Key points

1Small particles such as oxygen, carbon dioxide and urea pass straight through cell membranes by diffusion.

2Oxygen diffuses from the blood in the capillaries into the tissue cells; carbon dioxide and urea diffuse from the tissue cells into the blood.

3The direction of osmosis depends on which side has the higher water concentration, so water can move from tissue cells into the blood or from the blood into tissue cells.

4In active transport, the particle binds to a carrier protein, the carrier uses energy to change shape, and the particle is released on the other side of the membrane.

5Substances such as glucose and amino acids can be moved into or out of cells by carrier proteins using active transport.

6For an unfamiliar situation, find the side with the higher concentration of the substance (or of water), then name the process: in kidney dialysis, water leaves the blood by osmosis and urea leaves by diffusion.

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Worked example

Problem

A cell already has a higher concentration of glucose inside it than outside. Glucose keeps moving into the cell. Name the process and explain how it happens.

⚠ Watch out

Saying osmosis is the movement of the dissolved substance, or that water moves towards more water. Osmosis is only ever water, and it goes from higher water concentration to lower water concentration.

🧠

Memory hook

Downhill is free, uphill costs. Water on its own downhill route is osmosis; anything else going downhill is diffusion; anything going uphill is active transport, paid for with energy.

✓

Check yourself

Cover the page. A substance moves across a membrane from where it is less concentrated to where it is more concentrated. Name the process, say what it needs, and explain why it can't be diffusion.

Flashcards

(13)

What does 'partially permeable' mean for a cell membrane?

It lets some substances pass through but not others. 'Selectively permeable' means the same thing.

Define diffusion.

The net movement of particles from an area of higher concentration to an area of lower concentration, down a concentration gradient.

Does diffusion need energy from the cell?

No. Diffusion is a passive process.

Name three small particles that pass through cell membranes by diffusion.

Oxygen, carbon dioxide and urea.

Which way does oxygen diffuse between capillary blood and tissue cells?

From the blood in the capillaries into the tissue cells.

Define osmosis.

The net movement of water from an area of higher water concentration to an area of lower water concentration through a partially permeable membrane.

How is osmosis related to diffusion?

Osmosis is a type of diffusion that applies only to water.

Can water move from the blood into tissue cells, as well as the other way?

Yes. It moves towards whichever side has the lower water concentration, so it can go either way.

Define active transport.

The movement of particles from an area of lower concentration to an area of higher concentration, against the concentration gradient.

What two things does active transport need that diffusion doesn't?

Energy (in the form of ATP) and carrier proteins in the membrane.

What does the carrier protein use energy for in active transport?

To change its shape, which releases the bound particle on the other side of the membrane.

Name two substances that can be moved by carrier proteins using active transport.

Glucose and amino acids.

In kidney dialysis, which process moves water out of the blood, and which moves urea out?

Water moves out by osmosis; urea moves out by diffusion. Both are more concentrated in the blood.

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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Diffusion, osmosis and active transport

Two questions decide every case

Which way does the water go?

Three processes, four yes-or-no questions

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