2.1.2Scaling Up

Cellular Transport: Active Transport

5 min read 9 parts Lesson 3 of 16 1 quiz

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Section 1 of 9What is active transport?

Active Transport Across the Cell Membrane

Active transport: movement against concentration gradient (ATP)

Active transport is the movement of substances across a cell membrane from a region of lower concentration to a region of higher concentration, that is, against the concentration gradient.

Energy

Because the particles are being pushed uphill, the process needs energy, which comes from respiration.

Carrier proteins in the membrane use this energy to pick up a specific particle on one side and release it on the other.

Diffusion and osmosis are passive and need no energy from the cell. Active transport is the opposite: it can build up a concentration inside a cell that is higher than outside.

Section 2 of 9Comparing diffusion and active transport

Facilitated Diffusion of Glucose — 3D-Drawn Step Simulation

Feature	Diffusion	Active transport
Direction	Down the concentration gradient	Against the concentration gradient
Energy from respiration	Not needed	Needed
Carrier proteins	Not needed for simple diffusion	Needed
Effect of oxygen or respiration poison	None	Slows or stops
Example	Oxygen into blood at alveoli	Mineral ions into root hair cells

Section 3 of 9Active transport in plants: root hair cells

Structure and movement of water from soil to root, stem and leaf

The soil water around roots contains very low concentrations of mineral ions such as nitrate, phosphate and magnesium.

Active transport in plants: root hair cells

The cytoplasm of a root hair cell contains a much higher concentration.

If the cell relied on diffusion, ions would leak out rather than come in.

Instead, carrier proteins in the root hair membrane actively transport the ions into the cell, using energy from respiration.

Root hair cells are adapted for this: the long, thin hair extension gives a large surface area, and the cytoplasm contains many mitochondria to supply the energy.

Active transport in plants: root hair cells

Nitrate is needed to make amino acids and proteins; magnesium is needed to make chlorophyll.

A plant in waterlogged soil can struggle to absorb ions because the roots cannot respire aerobically, so active transport slows.

Section 4 of 9Active transport in animals

In the small intestine, digested glucose is absorbed into the blood by diffusion when its concentration in the gut is higher than in the blood. After the meal, the gut concentration can fall below the blood concentration. Glucose is then absorbed by active transport so that none is wasted. Epithelial cells lining the villi contain many mitochondria for this reason.

Active transport is also important in the kidney, where glucose is reabsorbed from the filtrate back into the blood, and in nerve cells, which pump sodium and potassium ions across their membranes.

Section 5 of 9Investigating and interpreting

Investigating Photosynthesis — 2D Simulation

Typical data question

ion uptake by roots is measured at different oxygen levels, or with and without a respiration poison such as cyanide.
If uptake falls when oxygen is removed or a poison is added, this supports the idea that ion uptake needs energy from respiration, so it is active transport.
If uptake continues unchanged, diffusion is more likely.

Link to other topicsthe energy comes from aerobic respiration, the ions are used in photosynthesis and protein synthesis, and the transpiration stream then carries the ions up the xylem.

Section 6 of 9Why diffusion alone is not enough

A useful way to think about this topic is to ask what would happen if a cell had only passive transport.

Why diffusion alone is not enough

A root hair cell would never accumulate nitrate at a concentration above that of the soil,

and a plant on poor soil would be unable to make enough protein to grow.

In the same way, the gut would lose glucose in the faeces when the gut concentration fell below that of the blood.

Active transport lets the organism be selective: carrier proteins are specific, so the cell can take in the ions it needs and ignore others.

Because the process is driven by respiration, anything that reduces respiration reduces active transport.

Why diffusion alone is not enough

Low temperature slows the enzymes involved in respiration, lack of oxygen limits aerobic respiration,

and poisons such as cyanide stop the electron transport chain.

Graph questions usually show uptake levelling off as every carrier protein is working at its maximum rate; this is a limit set by the number of carriers, not by the concentration outside.

When describing active transport in an answer, give three linked points: against the concentration gradient, uses energy from respiration, and uses carrier proteins.

Worked reasoningif a graph shows ion uptake rising steadily with oxygen concentration and then plateauing, the rise shows that more oxygen allows more respiration and therefore more energy for the carriers, and the plateau shows that another factor, such as the number of carrier proteins, has become limiting.

Section 7 of 9Exam-style questions

Labelled root hair cell showing organelles

Mineral Ions — Full 2D Simulation (Mg²⁺ & NO₃⁻)

Explain why root hair cells have many mitochondria. [3]3 marks
The concentration of nitrate ions is 0.02 units in soil water and 0.5 units in a root hair cell. State how nitrate enters the cell and give evidence from the data. [3]3 marks
Give two differences between diffusion and active transport. [2]2 marks

Section 8 of 9Key terms

Term	Meaning
Active transport	Movement of particles against a concentration gradient using energy from respiration.
Carrier protein	A membrane protein that binds a specific particle and moves it across the membrane.
Mitochondria	Organelles where aerobic respiration releases energy.
Root hair cell	A plant cell with a long extension that absorbs water and mineral ions from soil.
Mineral ions	Dissolved charged particles such as nitrate and magnesium that plants need.
Concentration gradient	A difference in concentration between two regions.

Section 9 of 9Exam technique

Saying active transport moves substances from low to high concentration "by diffusion"; it is a different process and needs carrier proteins.

Forgetting to state that the energy comes from respiration.

Claiming that active transport only happens in plants; it also happens in the gut and kidney tubules of animals.

Saying that waterlogged soil stops water entering the plant but not ions; ion uptake also fails because roots lack oxygen for respiration.

Cambridge 9700 AS Biology Core Formula

Magnification Triangle & Unit Conversion

Formula: M = I / A ⇔ A = I / M ⇔ I = M × A

Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.