Active TransportCambridge IGCSE Biology: Revision notes
Section 1
What is Active Transport?
Active transport is the movement of particles (molecules or ions) across a cell membrane against the concentration gradient — that is, from an area of lower concentration to an area of higher concentration. This process requires energy from respiration (ATP) because it moves substances in the opposite direction to which they would naturally diffuse.
Unlike passive transport (diffusion and osmosis), active transport is an active process that demands metabolic energy. This makes it essential for cells to accumulate substances they need, even when those substances are already present in higher concentrations elsewhere.
Examiners specifically test whether you can explain that active transport moves substances against the concentration gradient and that it requires energy from respiration. Always mention both points together for full marks.
Think of active transport as pushing a shopping trolley up a hill against gravity — you need energy (effort) to move it upwards, whereas it would naturally roll down without any work on your part.
Section 2
How Do Protein Carriers Enable Active Transport?
Active transport relies on carrier proteins (also called transport proteins) embedded in the cell membrane. These proteins work by:
- Binding to the specific molecule or ion that needs to be transported
- Using energy from ATP (released during respiration) to change shape
- Changing shape in a way that moves the particle across the membrane
- Releasing the particle on the other side of the membrane
Each carrier protein is specific — meaning one type of protein will only transport one particular molecule or ion. This specificity ensures that the cell can control exactly which substances enter or leave.
The carrier protein essentially acts as a pump or shuttle, actively moving the substance across the membrane rather than allowing it to pass through a channel.
When answering questions about active transport, always mention that protein carriers/transport proteins are involved and explain they use energy from ATP/respiration to move particles. Examiners look for this mechanism explanation.
Many students describe active transport correctly but forget to mention protein carriers — without naming this, you will lose marks even if your explanation of the energy requirement is correct.
Section 3
Why Is Active Transport Important for Root Hair Cells?
Root hair cells are a key example of active transport in plants. These cells actively uptake mineral ions (such as nitrate, phosphate, and potassium) from the soil solution against a concentration gradient.
Why mineral ions need active transport:
- Mineral ions are usually in lower concentration in the soil than inside the root hair cell
- Plants must accumulate these ions to build proteins, nucleic acids, and other essential compounds
- Diffusion alone would move ions out of the root cell, not in
- Active transport allows the plant to selectively absorb the ions it needs
The process in root hair cells:
- Root hair cells have many mitochondria to provide ATP for active transport
- Carrier proteins in the root hair cell membrane actively pump mineral ions inwards
- This creates a high concentration of ions in the root cell
- Water then enters the root cell by osmosis, generating root pressure that helps water move up the plant
This is an example of how active transport is essential for plant nutrition and growth.
A root hair cell takes in nitrate ions from soil even though the soil solution has a low concentration of nitrate. This happens because the root hair uses ATP from mitochondrial respiration to power carrier proteins that pump nitrate ions in — moving them against their concentration gradient.
Exam questions often ask you to explain why plants need active transport in roots. Always connect this to the fact that mineral ions are in lower concentration outside the cell, so diffusion would move them out, not in.
Section 4
What Is the Difference Between Active Transport and Diffusion?
Understanding the contrast between active transport and diffusion is crucial for exam success:
| Feature | Active Transport | Diffusion |
|---|---|---|
| Direction of movement | Against concentration gradient (low to high) | With concentration gradient (high to low) |
| Energy required | Yes — uses ATP from respiration | No — passive process |
| Protein carriers needed | Yes — carrier proteins essential | No — no proteins needed |
| Rate of transport | Can be controlled and regulated | Depends on concentration difference only |
| Examples | Mineral ion uptake in roots; glucose uptake in small intestine | Oxygen diffusion into cells; spread of smell in air |
Key point: Active transport is the only transport mechanism that allows cells to move substances uphill against a concentration gradient. Diffusion always moves substances downhill in the direction of lower concentration.
Exam questions often ask you to compare active transport and diffusion. Always structure your answer to include direction of movement, energy requirement, and the role of proteins — these are the points that distinguish them.
Section 5
How Does Respiration Link to Active Transport?
Active transport is directly dependent on aerobic respiration, which releases energy in the form of ATP. This link is fundamental:
The respiration-active transport connection:
- Mitochondria in the cell perform aerobic respiration
- Respiration breaks down glucose (or other organic molecules) in the presence of oxygen
- Energy is released and captured in ATP molecules
- ATP diffuses through the cell and reaches the carrier proteins in the cell membrane
- The ATP breaks down, releasing energy that powers the carrier protein to change shape
- The carrier protein pumps the molecule or ion across the membrane against the gradient
Why this matters for cells:
- Cells with high metabolic rates (like root hair cells or intestinal epithelial cells) have many mitochondria to supply enough ATP
- If respiration is blocked or oxygen is depleted, active transport stops immediately
- This is why cells cannot actively transport substances in anaerobic conditions (without oxygen)
This is why it is scientifically accurate to say active transport uses energy from respiration — not just energy in general, but specifically the ATP generated by cellular respiration.
If you place a root hair cell in an environment without oxygen, active transport of mineral ions will quickly stop because mitochondria cannot perform aerobic respiration without oxygen, so no ATP is produced. This shows the direct dependency on respiration.
Must Know
- Active transport moves particles against a concentration gradient from lower to higher concentration, which is the opposite of diffusion
- Active transport requires energy from aerobic respiration in the form of ATP; it stops without oxygen
- Carrier proteins (transport proteins) in the cell membrane are essential — they specifically recognise and bind to the substance being transported, then use energy from ATP to change shape and pump the particle across the membrane
- Root hair cells actively transport mineral ions (nitrate, phosphate, potassium) from the soil against a concentration gradient; these cells have many mitochondria to provide the ATP needed
- Active transport is the only transport mechanism that allows cells to accumulate substances in higher concentrations than exist outside the cell
- Always explain active transport using three key elements: against the gradient, requires energy/ATP, and uses carrier proteins
That's the notes covered.
Carry on to the next subtopic.