Active Transport

This lesson covers active transport — the process by which cells move substances against a concentration gradient — as required by the AQA GCSE Combined Science Trilogy specification (8464). You need to understand the definition, how it differs from diffusion and osmosis, and be able to describe examples of active transport in living organisms.

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What Is Active Transport?

Active transport is the movement of particles from an area of lower concentration to an area of higher concentration — i.e. against the concentration gradient.

Key features:

• Active transport moves substances up (against) the concentration gradient — from low to high concentration.
• It requires energy from cellular respiration (unlike diffusion and osmosis, which are passive).
• The energy is used to power carrier proteins (transport proteins) in the cell membrane.
• Active transport allows cells to absorb substances even when the concentration inside the cell is already higher than outside.

$\text{Active transport: LOW concentration} \xrightarrow{\text{energy required}} \text{HIGH concentration}$Active transport: LOW concentrationenergy required​HIGH concentration

Exam Tip: Active transport is the opposite of diffusion in terms of direction. Diffusion goes from high to low (passive, no energy); active transport goes from low to high (active, requires energy).

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How Active Transport Works

Active transport uses carrier proteins (also called transport proteins) embedded in the cell membrane:

1. A particle binds to the carrier protein on one side of the membrane (the low-concentration side).
2. Energy from respiration (ATP) causes the carrier protein to change shape.
3. The particle is moved to the other side of the membrane (the high-concentration side) and released.
4. The carrier protein returns to its original shape, ready to transport another particle.

graph LR
    A["LOW concentration<br/>(outside cell)"] -->|"Particle binds to<br/>carrier protein"| B["Carrier protein<br/>changes shape<br/>(energy from ATP)"]
    B -->|"Particle released<br/>inside cell"| C["HIGH concentration<br/>(inside cell)"]

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Comparing Transport Methods

Feature	Diffusion	Osmosis	Active Transport
Direction	High → Low concentration	High → Low water potential	Low → High concentration
Energy required?	No (passive)	No (passive)	Yes (from respiration)
Particles moved	Any small molecules/ions	Water only	Specific molecules/ions
Membrane required?	Not necessarily	Yes (partially permeable)	Yes (carrier proteins)
Concentration gradient	Down (with)	Down (with)	Up (against)

Exam Tip: If an exam question says substances are moving "against the concentration gradient" or from low to high concentration, the answer is always active transport. This is the defining feature.

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Examples of Active Transport

1. Absorption of Mineral Ions by Root Hair Cells

Root hair cells absorb mineral ions (e.g. nitrate ions, magnesium ions) from the soil.

• The concentration of mineral ions is usually higher inside the root hair cell than in the surrounding soil solution.
• Therefore, mineral ions cannot enter the cell by diffusion (which only goes from high to low).
• The root hair cell uses active transport to absorb mineral ions against the concentration gradient.
• This requires energy, which is provided by the many mitochondria in root hair cells.

Exam Tip: This is why root hair cells have many mitochondria — they need a large supply of energy for active transport of mineral ions. This is a common "explain the adaptation" question.

2. Absorption of Glucose in the Small Intestine

After digestion, glucose is absorbed from the small intestine (gut lumen) into the blood.

• Initially, glucose moves by diffusion (the concentration in the gut is higher than in the blood).
• As glucose is absorbed, the concentration in the gut decreases.
• Eventually, the concentration of glucose in the blood may become higher than in the gut lumen.
• At this point, active transport is used to continue absorbing the remaining glucose against the concentration gradient.
• This ensures that all available glucose is absorbed — none is wasted.

The epithelial cells lining the villi of the small intestine have:

Adaptation	Purpose
Microvilli (tiny projections) on their surface	Increase the surface area for absorption
Many mitochondria	Provide energy for active transport
Good blood supply (dense network of capillaries)	Carries absorbed glucose away, maintaining a concentration gradient

3. Absorption of Sugar in Plants (Phloem Loading)

In plants, sugars produced by photosynthesis in the leaves are loaded into the phloem for transport to other parts of the plant. This loading process uses active transport, with energy provided by companion cells that are rich in mitochondria.

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Why Is Active Transport Important?

Active transport is essential because:

1. Plants could not survive without it — the concentration of mineral ions in the soil is often very low (lower than in the root cells). Without active transport, plants could not absorb the minerals they need for growth (e.g. nitrates for making proteins, magnesium for making chlorophyll).

2. Animals could not absorb all nutrients — without active transport, glucose and other nutrients remaining in the gut after the concentration gradient has equalised would be lost in faeces. Active transport ensures complete absorption.

3. Maintaining ion concentrations — cells need to maintain specific concentrations of ions (e.g. potassium, sodium) inside them, which are often different from the concentrations outside. Active transport maintains these differences.

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The Role of Mitochondria

Active transport requires energy. This energy comes from aerobic respiration, which takes place in the mitochondria:

$\text{glucose} + \text{oxygen} \rightarrow \text{carbon dioxide} + \text{water} + \text{energy (ATP)}$glucose+oxygen→carbon dioxide+water+energy (ATP)

Cells that carry out a lot of active transport contain many mitochondria to provide the necessary energy. Examples include:

• Root hair cells (active transport of mineral ions)
• Epithelial cells of the small intestine (active transport of glucose)
• Phloem companion cells (loading sugars into the phloem)

Exam Tip: If a question asks why a particular cell type has many mitochondria, consider whether the cell carries out active transport. If it does, the mitochondria provide the energy needed.

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Worked Example

Question: A student observes that plant root cells continue to absorb nitrate ions from the soil even when the concentration of nitrate ions inside the root cell is higher than in the soil. Explain how this is possible.

Solution:

The root hair cell absorbs nitrate ions by active transport. Active transport moves substances from an area of lower concentration (the soil) to an area of higher concentration (inside the root cell) — i.e. against the concentration gradient. This process requires energy, which is released by aerobic respiration in the cell's mitochondria. Carrier proteins in the cell membrane use this energy to transport the nitrate ions into the cell.

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Worked Example — Linking to Respiration

Question: A plant is placed in conditions where it cannot carry out respiration (e.g. in the absence of oxygen and glucose). Predict what would happen to the uptake of mineral ions by the roots. Explain your answer.

Solution: