Active Transport

This lesson covers active transport — the third transport mechanism across cell membranes — as required by the Edexcel GCSE Biology specification (1BI0), Topic 1: Key Concepts in Biology. You need to understand how active transport differs from diffusion and osmosis, know key examples, and be able to compare all three processes.

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What Is Active Transport?

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

Key Characteristics

• Active transport moves substances against the concentration gradient (from low → high concentration).
• It requires energy from cellular respiration (the energy currency is ATP — adenosine triphosphate).
• It uses carrier proteins (specific transport proteins) embedded in the cell membrane.
• It is an active process — the cell must expend metabolic energy.

Exam Tip: The key difference from diffusion is the direction. Diffusion goes down the concentration gradient (high → low) passively. Active transport goes against the concentration gradient (low → high) and requires energy. If a question says substances move "against the concentration gradient", the answer is always active transport.

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How Active Transport Works

1. A substance (e.g. a mineral ion) on the low concentration side of the membrane binds to a specific carrier protein in the membrane.
2. Energy from ATP (produced by respiration in mitochondria) causes the carrier protein to change shape.
3. The substance is transported across the membrane to the other side — the high concentration side.
4. The substance is released on the other side.
5. The carrier protein returns to its original shape, ready to transport another molecule.

This process allows cells to accumulate substances that they need, even when the external concentration is lower than the internal concentration.

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Why Is Active Transport Necessary?

Sometimes, cells need to absorb substances that are already at a higher concentration inside the cell than outside. In these situations, diffusion cannot work because it can only move substances down the concentration gradient.

Active transport is essential when the concentration gradient is unfavourable — when the cell needs to absorb substances that are at a very low concentration in the surroundings.

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Examples of Active Transport

1. Mineral Ion Absorption by Root Hair Cells

• Plants need mineral ions (e.g. nitrate ions, magnesium ions, potassium ions) from the soil for healthy growth.
• The concentration of mineral ions is usually higher inside the root hair cell than in the dilute soil solution.
• Therefore, mineral ions cannot enter by diffusion (they would need to move against the concentration gradient).
• Root hair cells use active transport to absorb mineral ions from the soil against the concentration gradient.
• This requires energy from respiration — root hair cells have many mitochondria to provide this energy.

2. Glucose Absorption in the Small Intestine

• After digestion, glucose molecules are absorbed from the gut lumen into the blood through the epithelial cells of the villi.
• Initially, glucose concentration in the gut is higher than in the blood, so glucose is absorbed by diffusion.
• As absorption continues, the glucose concentration in the gut drops below the concentration in the blood.
• At this point, active transport is used to continue absorbing the remaining glucose against the concentration gradient.
• This ensures that all glucose is absorbed from digested food — none is wasted.
• The epithelial cells of the villi have many mitochondria to supply the energy needed for active transport.

Exam Tip: In the small intestine, glucose absorption uses BOTH diffusion (initially, when gut concentration is high) AND active transport (later, when gut concentration drops below blood concentration). If the exam asks specifically about absorbing glucose against the concentration gradient, the answer is active transport.

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Root Hair Cells — Adaptations for Absorption

Root hair cells are specialised cells on the surface of plant roots. They are adapted for the efficient absorption of water (by osmosis) and mineral ions (by active transport).

Adaptation	How It Helps
Long, thin root hair extension	Greatly increases the surface area for absorption of water and mineral ions
Thin cell wall	Short diffusion distance — water and ions can enter more quickly
Large surface area to volume ratio	Maximises the rate of absorption
Many mitochondria	Provide the energy (ATP) needed for active transport of mineral ions
Large permanent vacuole	Contains dilute cell sap, maintaining a concentration gradient for osmosis (water enters by osmosis)

Exam Tip: When describing root hair cell adaptations, always link the feature to its function. For example: "Root hair cells have many mitochondria because they carry out active transport of mineral ions, which requires energy from respiration."

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The Role of Mitochondria in Active Transport

Active transport requires energy, which is supplied by ATP produced during aerobic respiration in mitochondria.

The equation for aerobic respiration:

glucose + oxygen → carbon dioxide + water (+ energy transferred)

Cells that carry out a lot of active transport have many mitochondria to meet their energy demands. Examples include:

• Root hair cells (absorbing mineral ions from soil)
• Epithelial cells of the villi (absorbing glucose in the small intestine)
• Kidney tubule cells (reabsorbing glucose and other useful substances)

Exam Tip: If a question describes a cell with "many mitochondria", this is a strong hint that the cell carries out active transport (or has a high energy demand for another reason). Link mitochondria → respiration → ATP → energy for active transport.

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Comparison: Diffusion, Osmosis, and Active Transport

This comparison table is one of the most frequently tested topics in the exam.

Feature	Diffusion	Osmosis	Active Transport
Definition	Net movement of particles from high to low concentration	Movement of water from dilute to concentrated solution through a partially permeable membrane	Movement of substances from low to high concentration (against the gradient)
Direction	Down the concentration gradient (high → low)	Down the water potential gradient (high water potential → low water potential)	Against the concentration gradient (low → high)
Energy required?	No (passive)	No (passive)	Yes (from respiration / ATP)
Particles moved	Any dissolved substance (molecules or ions)	Water molecules only	Specific substances (e.g. mineral ions, glucose)
Membrane needed?	Not necessarily (can occur in air or liquid)	Yes — partially permeable membrane required	Yes — carrier proteins in the membrane required
Carrier proteins?	No (for simple diffusion)	No	Yes
Examples	O₂ into cells, CO₂ out of cells, glucose absorption	Water into root hair cells, water into/out of potato cells	Mineral ions into root hair cells, glucose absorption in gut

Exam Tip: This comparison table is essential. You are very likely to be asked to compare two or all three of these processes. Practise writing out this table from memory. The key distinguishing features are: direction of movement, whether energy is needed, and what is being moved.

graph LR
    subgraph Diffusion
        D1["Particles move HIGH to LOW concentration"] --> D2["No energy needed"]
        D2 --> D3["Any dissolved substance"]
    end
    subgraph Osmosis
        O1["Water moves HIGH to LOW water potential"] --> O2["No energy needed"]
        O2 --> O3["Water molecules only"]
    end
    subgraph Active Transport
        A1["Particles move LOW to HIGH concentration"] --> A2["Energy from ATP required"]
        A2 --> A3["Specific substances via carrier proteins"]
    end

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Worked Example: Identifying the Transport Process