Osmosis (incl. RP3) — AQA GCSE Biology

SPEC 4.1.3.3 • CELL BIOLOGY • PAPER 1 (HT)

Active Transport

The third transport mechanism — and the only one that needs energy. The mark-scheme wording, the two spec-named examples, and the energy chain that wins those tricky 5-mark explain questions.

Why This Matters

Active transport is the topic where AQA examiner reports flag the same two errors year after year. Students lose marks even when they understand the biology — because they don't use the right words.

The Two Recurring Errors

First: writing that mitochondria produce energy (or make, or create). This is never credited. Second: naming mitochondria but failing to link to where the energy comes from — respiration. Both errors lose marks even when the student clearly knows how active transport works.

In The Next 10 Minutes

You'll know the full active-transport chain that wins those marks — the spec definition, the two named examples, and the energy language that gets credited.

Learning Objectives

By the end of this lesson, you'll be able to:

Objective 1

Define active transport using the spec wording — "more dilute to more concentrated, against the gradient, requires energy from respiration".

Objective 2

Compare active transport with diffusion and osmosis — direction, energy, and what moves.

Objective 3

Explain the two named examples: mineral ion uptake in plant root hair cells, and glucose absorption in the small intestine.

The Spec Definition

This is the wording AQA uses in spec 4.1.3.3. The mark scheme uses the phrases "more dilute" and "more concentrated" — these are the credited terms.

Active Transport

The movement of substances from a more dilute solution to a more concentrated solution — that is, against the concentration gradient. This process requires energy from respiration.

Memorise Verbatim

"More dilute → more concentrated" is the spec phrasing. "Low to high concentration" is also accepted, but learn the spec wording first — it's safest.

Two Things Make It Different

Compared to diffusion and osmosis, only two features make active transport stand out. Both have to be in the answer.

Feature 1

Direction — moves substances against the concentration gradient (low → high). Diffusion and osmosis go the other way.

Feature 2

Energy — requires energy released by respiration. Diffusion and osmosis are passive (no energy).

Both Features Must Appear

If you only mention "against the gradient" but skip the energy, the answer is incomplete. If you mention "needs energy" but not the direction, same problem. Always pair them.

The Three Mechanisms — Visualised

Before the comparison table, here's what each mechanism actually looks like at the cell membrane. Notice what's the same and what's different.

What's The Same, What's Different

All three involve a membrane and a concentration difference. Diffusion and osmosis follow the gradient (passive). Active transport fights the gradient — and only it needs the mitochondrion's energy supply.

Diffusion vs Osmosis vs Active Transport

The three transport mechanisms in the spec, side by side. Confusing them is the most common error in this topic.

ProcessDirectionEnergy?What Moves

DiffusionHigh → low (down gradient)No — passiveAny small particle

OsmosisLess concentrated → more concentrated solution (water down its own gradient, through partially permeable membrane)No — passiveWater only

Active transportMore dilute → more concentrated ( against gradient)Yes — from respirationSpecific ions and molecules

The Tell

Cells doing lots of active transport — root hair cells, intestine cells — have many mitochondria. That's the visual signature.

Synoptic Link — Cell Specialisation (4.1.2.1)

The spec explicitly links active transport to cell specialisation. When you see "many mitochondria" in a cell description, it's telling you the cell does lots of energy-demanding work — usually active transport. Root hair cells and villus cells are textbook examples.

Example 1 — Root Hair Cells

The spec wording: active transport allows mineral ions to be absorbed into plant root hairs from very dilute solutions in the soil. Plants require ions for healthy growth — for example, nitrate (used to make amino acids and proteins) and magnesium (used to make chlorophyll).

The Concentration Problem

Soil mineral ion concentration is very dilute.

The root hair cell already contains a higher concentration of mineral ions.

Diffusion would move ions out of the root — the wrong way.

The Solution

The root hair cell uses active transport to move mineral ions into the cell, against the gradient. Energy from respiration in the cell's many mitochondria drives this.

Mark-Scheme-Aligned Sentence

"Mineral ions move from a more dilute solution (soil) to a more concentrated solution (cell) by active transport, which requires energy from respiration."

Example 2 — Sugar Absorption In The Gut

The spec wording: active transport allows sugar molecules (glucose) to be absorbed from lower concentrations in the gut into the blood which has a higher sugar concentration. Sugar molecules are used for cell respiration — that's why the body works hard to absorb every last bit.

When Diffusion Works

Just after eating, gut sugar is higher than blood sugar. Sugar molecules diffuse down the gradient from gut to blood. Passive — no energy needed.

When Active Transport Takes Over

Most sugar has been absorbed. Now gut sugar is lower than blood sugar. Diffusion would reverse — sending sugar back. The body uses active transport to keep absorbing sugar against the gradient.

Why It Matters Biologically

Active transport ensures all the sugar can be absorbed — even when concentrations would otherwise stop diffusion. The absorbed sugar is then used by cells for respiration, releasing the energy the body needs.

The Energy Chain

This is the chain the mark scheme wants. Memorise it. Every step matters — skipping one costs a mark.

Step 1

Mitochondria

Step 2

Respiration (aerobic)

Step 3

Energy released

Energy Is Always "Released"

Mark schemes are clear: do not accept "produced", "made", or "created". Use "released" or "transferred". This is a universal rule across every AQA biology paper that mentions energy.

Why The Chain Matters

"Mitochondria release energy" is incomplete. The mark scheme wants the link to respiration — that's where the energy actually comes from. Saying "respiration" alone scores; saying "aerobic respiration" is the safer fuller answer.

Worked Example

SPEC-ALIGNED PRACTICE QUESTION 5 marks

A scientist examines a plant root hair cell under a microscope and observes a large number of mitochondria. The concentration of mineral ions inside the root hair cell is higher than the concentration in the surrounding soil.

Explain how the mineral ions are transported from the soil into the root hair cell.

Plan The Answer

Five marks → five points. Build the chain in order: name the process → state the direction → state the energy need → name the source of energy → name where the energy is released. The stem already gives you "many mitochondria" and "ions are higher inside" — use them.

P1

The mineral ions are transported by active transport.

P2

This moves the ions from a more dilute solution (soil) to a more concentrated solution (cell) — against the concentration gradient.

P3

Active transport requires energy.

P4

The energy is released by respiration (not produced or made).

P5

Respiration takes place inside the mitochondria, which is why the cell contains so many.

Mark Scheme Match — 5/5

Here are the five creditable points. Each one in the answer ticks one of these boxes.

Active transport named as the process

Movement is from more dilute → more concentrated (or "against the concentration gradient")

Active transport requires energy

Energy is released by respiration — not produced/made/created

Mitochondria named as the site of respiration

Stay Focused On The Chain

Examiner reports flag a recurring problem: students try to list every organelle they can name (ribosomes, nucleus, vacuole) instead of focusing on the structure the question is actually asking about. Stick to the chain. Don't pad with irrelevant organelles.

Examiner Alert 1 — Energy Language

The single most-cited error in AQA biology. Universal across every paper that mentions energy.

✗ Wrong — 0 marks

"Mitochondria produce energy."

"Mitochondria make energy."

"Mitochondria create energy."

✓ Correct

"Mitochondria release energy by respiration."

"Energy is transferred from glucose by respiration."

Why The Strict Rule

Energy can't be created — that's a fundamental physics principle. Mark schemes enforce this rigorously. Recent examiner reports note that a substantial number of students still write "produced", "made", or "created" — and lose the mark every time, even when the rest of their answer is correct.

Examiner Alert 2 — Link Mitochondria → Respiration

Naming mitochondria alone is half the answer. The other half is naming the process happening inside them.

✗ Incomplete — 1 mark

"The cell has mitochondria which release energy for active transport."

Mitochondria named ✓ — but no link to where the energy comes from. Missed marks.

✓ Full chain — 2 marks

"The cell has mitochondria which carry out respiration, releasing energy for active transport."

Both marks: structure named, process named.

Auto-pilot Sentence

Whenever you write "mitochondria" in an exam answer, your hand should automatically continue: "… which carry out respiration to release energy". Build the muscle memory. Adding "aerobic" is the safer, fuller version.

Examiner Alert 3 — Against vs Down The Gradient

Mark schemes keep "against" and "down" strictly distinct. Confusing them voids the answer.

PhraseBelongs ToEnergy?

Down the gradient (high → low)Diffusion, osmosisNo

Against the gradient (low → high)Active transportYes

Common Voiding Error

Writing "active transport moves substances down the gradient" — even if the rest of the answer is correct — contradicts the definition and loses marks. Always check direction language before submitting.

Common Errors

Four error patterns that show up across every paper. Pause and screenshot.

✗ Wrong

"Mitochondria produce energy"

✓ Right

"Mitochondria release energy by aerobic respiration"

✗ Wrong

"Active transport is the same as diffusion"

✓ Right

"Active transport is against the gradient and requires energy; diffusion is down the gradient and is passive"

✗ Wrong

"Active transport moves anything anywhere"

✓ Right

"Active transport moves substances against the concentration gradient"

✗ Wrong

"Plants do active transport for water"

✓ Right

"Water moves by osmosis; active transport is for mineral ions"

Recap — Three Takeaways

Three things. You're set for any active transport question on Paper 1.

One — The Definition

Active transport moves substances from more dilute to more concentrated — against the gradient. It requires energy from respiration. Both halves matter.

Two — The Two Spec Examples

Root hair cells absorb mineral ions from very dilute soil — plants need ions for healthy growth. Gut absorbs sugar molecules into the blood when gut sugar is lower — sugar is used for cell respiration. Both link to many mitochondria.

Three — The Energy Chain

Mitochondria → respiration → energy released (never produced/made/created). Use mark-scheme energy language. Always.

Up Next

You've covered all three transport mechanisms in the spec. The next lesson zooms out to the principle behind why exchange surfaces exist at all.

Lesson 1.09

Surface Area to Volume Ratio & Exchange Surfaces

Why a small fish absorbs oxygen through skin while a whale needs lungs. Why an ant doesn't need a circulatory system but a horse does. The principle behind every adaptation in cell biology — captured in one number.

See You There

Module 1 is almost done. After 1.09 comes the Module 1 Masterclass — synoptic 6-markers that combine everything we've covered.