GCSE · Biology · AQA · Spec 8461

Osmosis

Water is always crossing your cell membranes. One simple question tells you which way it will go.

Biology · Osmosis

Which way will the water go?

Something is crossing the membrane between the blood and some tissue cells. Choose what it is, then where the water is more concentrated, and follow the route to its end.

What is crossing? → Where is water more concentrated?

A selectively permeable (also called partially permeable) membrane lets some substances through but not others. Water can cross it. So can some dissolved substances, such as urea.

3 possible routes.

On A particle crosses a selectively permeable membrane. 2 branches to choose from.

Osmosis is the net movement of water particles from a higher concentration of water particles to a lower one, through a selectively permeable membrane.

Watch out: Look at the water, not the dissolved stuff. Water moves from HIGH water concentration to LOW water concentration.

What do you really think?

What does osmosis actually move?

Blood is flowing past some tissue cells. Water and dissolved substances are both on the move across the membrane between them.

Which of these is closest to what you think right now?

Osmosis is water moving, overall, from where there's more water to where there's less, across a membrane that lets only some things through.Osmosis is how dissolved substances like urea get across the membrane.Water moves from where it is less concentrated to where it is more concentrated.The membrane is just a thin barrier that lets everything through.

How sure are you?

I'm sureI think soI'm guessing

Osmosis is water moving, overall, from where there's more water to where there's less, across a membrane that lets only some things through.

This matches the evidence

Spot on. Water, from high water concentration to low, across a selectively permeable membrane. Nothing else counts as osmosis.

Osmosis is how dissolved substances like urea get across the membrane.

This is a common misconception

Urea does cross membranes, but by diffusion, not osmosis. Osmosis is only to do with water. If the thing moving isn't water, it isn't osmosis.

Water moves from where it is less concentrated to where it is more concentrated.

This is a common misconception

That's backwards. Water moves from HIGH water concentration to LOW. The mix-up usually comes from thinking about the dissolved stuff: more dissolved substance means LESS water. So water does move towards a concentrated solution, but in water terms that's still high to low.

The membrane is just a thin barrier that lets everything through.

This is a common misconception

If it let everything through, it wouldn't be selectively permeable. It lets some substances pass but not others, and osmosis needs exactly that kind of membrane.

If it isn't water, it isn't osmosis. And for water, always ask where there's MORE water before you say which way it goes.

Why it's free

Reason it through

Why doesn't osmosis need any energy from the cell?

Link 1 of 4

First link · your turn

What are the water particles doing all the time, on both sides of the membrane?

Locked — reveal the link above first

Shared or special?

Where does osmosis sit?

For each statement, tick every process it is true of. Some are true of both.

A Osmosis
B Diffusion

A net movement from a higher concentration to a lower concentration
Can happen across a selectively permeable membrane
Water moving from a high to a low water concentration across a selectively permeable membrane
Only ever involves water
How urea crosses a membrane
Moves particles from a lower concentration to a higher one

Predict, then check

Use the tree's two questions: is it water? Where is water more concentrated?

In kidney dialysis, a patient's blood flows past a selectively permeable membrane with fluid from the dialysis machine on the other side. Water is more concentrated in the blood than in the machine. What happens to the water and to the urea?

Your turn to explain

Put it in your own words

The fluid inside some tissue cells contains a lot of dissolved substances. The blood flowing past them contains fewer dissolved substances. Explain which way water will move between the blood and the tissue cells, and whether the cells need to supply energy for this. [4 marks]

Your answer

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

The mark scheme

Identifies that the blood has the higher water concentration and the tissue cells the lower water concentration. (1 mark)
States that water moves from the blood into the tissue cells. (1 mark)
Names the process as osmosis, across a selectively (partially) permeable membrane. (1 mark)
States that this is a net movement down the water concentration gradient, which is passive / needs no energy. (1 mark)

Model answer

The blood contains fewer dissolved substances, so it has a higher concentration of water than the tissue cells. There is a net movement of water from the blood into the tissue cells by osmosis, across a selectively permeable membrane. The water moves down its concentration gradient, so the process is passive and needs no energy.

Water concentration first, then direction, then the process. Get those three down and the explanation writes itself.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Follow the water: find where it is more concentrated, and you know which way it moves.

What you need to know

Osmosis is the net movement of water particles from a high concentration of water particles to a low concentration of water particles, through a selectively permeable membrane.
Osmosis is a type of diffusion that only involves water: no other molecules move by osmosis. Urea, for example, crosses a membrane by diffusion.
A selectively permeable membrane lets some substances pass through but not others. Selectively permeable and partially permeable mean the same thing.
Osmosis is passive: water moves down its concentration gradient, so no energy is needed.
The direction depends on which side has the higher water concentration. More water in the tissue cells: water moves from the cells into the blood. More water in the blood: water moves from the blood into the cells.

The big picture

Osmosis is the net movement of water particles from a higher concentration of water to a lower concentration of water, through a selectively permeable membrane. It only ever involves water: other substances, such as urea, cross membranes by diffusion. It is passive, because water moves down its own concentration gradient and no energy is needed. To predict any osmosis question, first find where the water is more concentrated, then move it from there to where it is less concentrated.

Key points

1If it isn't water, it isn't osmosis.

2Find the water first: which side has more water particles?

3Water goes from high water concentration to low water concentration.

4More dissolved substance means less water, so a concentrated solution has a lower water concentration.

5No energy needed: the particles' own random movement does the work.

6Dialysis: water leaves the blood by osmosis, urea leaves by diffusion.

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Worked example

Problem

A selectively permeable membrane separates two solutions. Solution P is 90% water. Solution Q is 75% water. Which way will water move overall, and by what process?

⚠ Watch out

Writing 'water moves from high concentration to low concentration' without saying what is concentrated. It is a high concentration of WATER to a low concentration of water, which means towards the side with more dissolved substance.

🧠

Memory hook

Follow the water, not the stuff dissolved in it. Water spreads out from where it's crowded to where there's less of it, like people drifting out of a packed room into an emptier one.

✓

Check yourself

Urea and water are both crossing the same membrane. Which one could be moving by osmosis, and why only that one?

Flashcards

(12)

What is osmosis?

The net movement of water particles from a high concentration of water particles to a low concentration of water particles, through a selectively permeable membrane.

Which substance can move by osmosis?

Only water. No other molecules move by osmosis.

Osmosis is a type of which process?

Diffusion, but only for water. Other substances, such as urea, cross membranes by ordinary diffusion instead.

What does 'selectively permeable' mean?

The membrane lets some substances pass through but not others.

What is another name for a selectively permeable membrane?

A partially permeable membrane. The two terms mean the same thing.

Is osmosis active or passive?

Passive. Water moves down its concentration gradient and no energy is needed.

Why does osmosis need no energy from the cell?

Water particles move randomly on their own. More of them cross from the higher water concentration than cross back, so the net movement happens by itself.

What does 'net movement' mean?

The overall direction: particles cross both ways, but more cross one way than the other.

What is the first thing to do in any osmosis question?

Identify where the water concentration is higher and where it is lower.

Water is more concentrated in the blood than in the tissue cells. Which way does it move?

From the blood into the tissue cells, by osmosis.

Which has the lower water concentration: a concentrated solution or a dilute one?

The concentrated solution, because more dissolved substance means less water.

In kidney dialysis, why does water move out of the blood into the machine?

Water is more concentrated in the blood than in the dialysis machine, so it moves out of the blood by osmosis.

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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Which way will the water go?

What does osmosis actually move?

Where does osmosis sit?

Put it in your own words

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