Active transport
What is Active Transport?
Active transport is the movement of particles (such as ions or molecules) through a cell membrane from a region of lower concentration to a region of higher concentration. This means the particles are moving against the concentration gradient.
Unlike diffusion, which happens naturally without energy input, active transport requires energy. This energy is supplied by respiration within the cell. Because it requires metabolic energy, active transport can only occur in living cells.
Active Transport
Active transport: The movement of substances from a region of lower concentration to a region of higher concentration (against the concentration gradient) across a cell membrane, requiring energy released from respiration.
The Role of Protein Carriers
Particles cannot simply pass through the lipid bilayer of the cell membrane during active transport. Instead, they are moved by protein carriers embedded in the membrane.
These carrier proteins bind to specific ions or molecules on one side of the membrane. Using energy from respiration (often in the form of ATP), the protein changes shape to transport the particle across the membrane and release it on the other side.
Ion Uptake by Root Hair Cells
A primary example of active transport is the uptake of mineral ions (such as nitrate or magnesium) by root hair cells in plants.
In many soils, the concentration of mineral ions is lower than the concentration inside the root hair cells. Therefore, ions must move from the soil (low concentration) into the cell (high concentration). This is against the concentration gradient and requires active transport.
Because this process relies on respiration for energy, it depends on oxygen. If oxygen levels drop (e.g., in waterlogged soil), respiration decreases, less energy is produced, and ion uptake slows down.
⚠︎ Confusing Active Transport with Diffusion
Mistake: Stating that active transport moves particles from high to low concentration.
Correction: Active transport always moves particles against the gradient (low to high). Movement from high to low is diffusion.
Correction: Active transport always moves particles against the gradient (low to high). Movement from high to low is diffusion.
Mistake: Forgetting that active transport requires a cell membrane and energy.
Correction: You must mention both the cell membrane (as the site of transport) and energy from respiration (as the driver). Without either, it is not active transport.
Correction: You must mention both the cell membrane (as the site of transport) and energy from respiration (as the driver). Without either, it is not active transport.
Describing Active Transport in Exams
When to use: When asked to 'describe' or 'define' active transport.
Why examiners accept this: The markscheme specifically looks for three key components: direction of movement, gradient relationship, and energy source.
Why examiners accept this: The markscheme specifically looks for three key components: direction of movement, gradient relationship, and energy source.
Correct phrasing example: "Active transport is the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration, using energy from respiration."
Key phrase to include: Ensure you explicitly state "against the concentration gradient" or "from low to high concentration". Also, always specify "energy from respiration" rather than just "energy," as this links the process to cellular metabolism.
Explaining the Importance of Active Transport in Roots
When to use: When asked to 'explain' why active transport is important for root hair cells or how it aids absorption.
Why examiners accept this: Examiners want to see the link between ion concentration, the gradient, and the necessity of energy.
Why examiners accept this: Examiners want to see the link between ion concentration, the gradient, and the necessity of energy.
Correct phrasing example: "Active transport is important because mineral ions are often more concentrated in the root hair cell than in the soil. Therefore, they must be moved against the concentration gradient using energy from respiration to allow uptake."
Key phrase to include: Mention that ion concentration is lower in the soil than in the cell. State that this creates a need for protein carriers and energy. Avoid saying 'to get nutrients'; be specific about moving ions against the gradient.
Past Paper Style Questions
Q:
Describe active transport. [3 marks]
A:
- Movement of particles through a cell membrane.
- From a region of lower concentration to a region of higher concentration (or against the concentration gradient).
- Using energy from respiration.
Q:
Explain why root hair cells need active transport to absorb nitrate ions from the soil. [3 marks]
A:
- Nitrate ion concentration is lower in the soil than in the root hair cell.
- Therefore, ions must move against the concentration gradient.
- This requires energy from respiration (provided by mitochondria).
Q:
State two features that distinguish active transport from diffusion. [2 marks]
A:
- Active transport moves particles against the concentration gradient (low to high); diffusion moves down the gradient (high to low).
- Active transport requires energy from respiration; diffusion does not.
Q:
Explain the role of protein carriers in active transport. [2 marks]
A:
- Protein carriers are located in the cell membrane.
- They move specific molecules or ions across the membrane (often by changing shape) using energy.