Active Transport

This lesson covers active transport as required by the Edexcel GCSE Combined Science specification (1SC0). You need to define active transport, explain why it requires energy, compare it to diffusion and osmosis, and describe key biological examples including root hair cells and the gut lining.

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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 — against the concentration gradient. This requires energy from cellular respiration.

Key features:

• Moves substances against the concentration gradient (from low to high concentration).
• Requires energy from respiration (ATP).
• Uses specific carrier proteins (transport proteins) in the cell membrane.
• Allows cells to absorb substances even when the concentration inside the cell is already higher than outside.

Exam Tip: The key distinction is the direction of movement. Diffusion and osmosis are passive (high to low concentration). Active transport moves substances from low to high concentration and requires energy.

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Why Is Energy Needed?

In diffusion and osmosis, particles move naturally down their concentration gradient — from high to low concentration. This happens spontaneously and does not require energy.

In active transport, particles move against the concentration gradient — from a low concentration to a higher concentration. This is not a natural direction of movement, so the cell must use energy from respiration to power the process.

The energy comes from ATP (adenosine triphosphate), which is produced during aerobic respiration in the mitochondria:

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

This is why cells that carry out a lot of active transport (e.g. root hair cells, cells lining the small intestine) contain many mitochondria — they need to produce large amounts of ATP.

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How Active Transport Works

1. A substance (e.g. mineral ion) binds to a specific carrier protein on one side of the cell membrane.
2. Energy from ATP causes the carrier protein to change shape.
3. The substance is moved through the membrane to the other side — against the concentration gradient.
4. The substance is released, and the carrier protein returns to its original shape.

graph TD
    A[Substance binds to carrier protein on LOW concentration side] --> B[Energy from ATP changes protein shape]
    B --> C[Substance transported across membrane]
    C --> D[Released on HIGH concentration side]
    D --> E[Carrier protein resets]

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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 / ATP)
Membrane needed?	Not always	Yes (partially permeable)	Yes (carrier proteins)
What moves?	Any dissolved substance or gas	Water only	Specific substances (e.g. minerals, glucose)
Examples	O₂ into blood, CO₂ out of cells	Water into root hair cells, water into/out of potato cells	Minerals into root hair cells, glucose from gut into blood

Exam Tip: If a question states that a substance moves "against the concentration gradient" or from "low to high concentration", you should immediately think of active transport. If it says "down the concentration gradient" or "from high to low", it is diffusion (or osmosis if water is involved).

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Example 1: Root Hair Cells

Root hair cells absorb water and mineral ions from the soil.

Water Absorption

• Water enters root hair cells by osmosis.
• The soil solution is more dilute than the cell sap, so water moves from the soil (high water potential) into the root hair cell (lower water potential).

Mineral Ion Absorption

• Mineral ions (e.g. nitrate ions, magnesium ions) are present in the soil at a lower concentration than inside the root hair cell.
• They cannot enter by diffusion because they would need to move from low to high concentration.
• Instead, they are absorbed by active transport, using energy from respiration.
• Root hair cells have many mitochondria to provide the ATP needed for active transport.

Adaptations of Root Hair Cells

Adaptation	Purpose
Long, thin extension (the "hair")	Increases surface area for absorption of water and minerals
Thin cell wall	Short diffusion distance
Large permanent vacuole	Maintains a low water potential inside the cell to promote osmosis
Many mitochondria	Provide energy (ATP) for active transport of mineral ions

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Example 2: The Small Intestine (Gut Lining)

After digestion, dissolved food molecules (e.g. glucose and amino acids) are absorbed into the blood from the small intestine.

How Glucose Is Absorbed

• Initially, glucose concentration in the gut is higher than in the blood, so glucose moves into the blood by diffusion.
• As more glucose is absorbed, the concentration in the gut decreases. Eventually, the concentration in the gut may become lower than in the blood.
• To continue absorbing the remaining glucose, the cells lining the small intestine use active transport to move glucose from the gut (low concentration) into the blood (higher concentration).

Adaptations of the Small Intestine

Adaptation	Purpose
Villi (finger-like projections)	Increase the surface area for absorption
Each villus has a thin wall (one cell thick)	Short diffusion distance
Dense network of blood capillaries	Maintains a steep concentration gradient by carrying absorbed molecules away
Many mitochondria in epithelial cells	Provide energy for active transport

Exam Tip: The small intestine uses both diffusion and active transport to absorb glucose. Diffusion occurs first when the concentration gradient is favourable. Active transport takes over when the gut glucose concentration falls below the blood glucose concentration.

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Why Active Transport Matters

Without active transport, organisms would not be able to:

• Absorb mineral ions from dilute soil solutions.
• Fully absorb all available glucose from the gut.
• Maintain the correct internal concentrations of substances needed for cell function.

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Summary

• Active transport moves substances from low concentration to high concentration (against the concentration gradient).
• It requires energy from respiration (ATP) and uses carrier proteins in the membrane.
• Root hair cells use active transport to absorb mineral ions from the soil. They have many mitochondria.
• Cells lining the small intestine use active transport to absorb glucose when its concentration in the gut falls below that of the blood.
• Diffusion and osmosis are passive (no energy needed); active transport is active (energy needed).
• Cells performing active transport typically have many mitochondria.

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Deeper Look: When and Why Cells Use Active Transport

Active transport allows cells to maintain concentrations very different from their surroundings. Without it, ions such as potassium, sodium and nitrate could only be absorbed to the same concentration as the surrounding environment — which is often far below what the cell needs. By using energy from respiration, cells can selectively accumulate specific substances.

Energy from Respiration — The ATP Currency