Active transport | IGCSE Biology Revision Notes

1. Overview

Active transport is the process used by cells to move substances "uphill" against their natural direction of flow. Unlike diffusion or osmosis, this process requires the cell to spend energy to capture specific molecules or ions that are in low supply in the environment but are needed in high concentrations inside the cell.

Key Definitions

Active Transport: The movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration (i.e., against a concentration gradient), using energy from respiration.
Concentration Gradient: The difference in the concentration of a substance between two regions. Moving "against" the gradient means moving toward the area where there is already more of that substance.
Carrier Protein: Specialized proteins embedded in the cell membrane that "pump" specific molecules across the membrane using energy.

Core Content

Active transport is essential for maintaining the correct concentrations of molecules within a cell.

Key Characteristics:

Direction: Particles move from Low → High concentration.
Energy: It is an "active" process because it requires energy. This energy is provided by respiration in the form of ATP.
Location: It occurs across a cell membrane.

Step-by-Step Process:

The specific molecule or ion (e.g., a nitrate ion) approaches the cell membrane.
The molecule binds to a specific site on a protein that spans the membrane.
The cell uses energy from respiration to change the shape of the protein.
The change in shape "pushes" or carries the molecule through the membrane and releases it on the other side.

📊A cross-section of a phospholipid bilayer. On the outside (low concentration), three circular particles are shown. One particle is lodged inside a "V-shaped" carrier protein. An arrow labeled "Energy from Respiration" points to the protein. On the inside (high concentration), ten circular particles are already present, and the protein is shown changing shape to release the new particle into this high-density area.

Extended Content (Extended curriculum only)

In the extended curriculum, you must understand the specific mechanism and real-world biological applications of active transport.

The Role of Protein Carriers

Active transport does not happen through the lipid bilayer itself. It is carried out by protein carriers (sometimes called "pumps"). These proteins are highly specific; a protein that transports glucose will not transport sodium ions.

Real-World Examples

Ion Uptake by Root Hair Cells:
- Context: Plants need mineral ions (like nitrates and magnesium) for growth. Often, the concentration of these ions in the soil is much lower than the concentration inside the root hair cell.
- Function: Root hair cells use active transport to "pump" these ions from the soil into the cytoplasm against the concentration gradient.
- Adaptation: Root hair cells have a large surface area and many mitochondria to provide the energy (via respiration) needed for this transport.
Glucose Uptake in the Small Intestine:
- Context: After a meal, glucose is absorbed into the blood via diffusion. However, once the concentration in the blood is higher than in the intestine, diffusion stops.
- Function: To ensure no food is wasted, the cells of the small intestine (villi) use active transport to absorb the remaining glucose molecules.

Key Equations

While there is no single "Active Transport Equation," exams often require you to calculate the Percentage Change in concentration or mass to determine if active transport has occurred.

Percentage Change Formula: $$\text{Percentage Change} = \frac{\text{Final Value} - \text{Initial Value}}{\text{Initial Value}} \times 100$$

Final Value: Concentration/Mass at the end of the experiment.
Initial Value: Concentration/Mass at the start.
Units: Expressed as a percentage (%).

Common Mistakes to Avoid

❌ Wrong: Saying active transport goes from "high to low" concentration.
- ✓ Right: It always moves from low to high (against the gradient).
❌ Wrong: Stating that energy comes from "the sun" or "the gut."
- ✓ Right: Energy specifically comes from respiration within the cell.
❌ Wrong: Confusing active transport with osmosis.
- ✓ Right: Osmosis is specifically the movement of water; active transport involves particles like ions and glucose.

Exam Tips

Command Word - "Describe": If asked to describe active transport, focus on the definition: Low to high concentration, against a gradient, using energy.
Command Word - "Explain": If asked to explain the importance in root hairs, mention that mineral concentration is lower in the soil and the plant needs energy to pump them in using protein carriers.
Calculation Questions: You may be given values like a soil concentration of 1.0 mmol/dm³ and a root concentration of 30.0 mmol/dm³. If the plant is still taking in minerals, you must identify this as active transport because it is moving against the gradient.
Look for Mitochondria: In "Label the Diagram" questions, if a cell (like a root hair or intestinal cell) has an unusually high number of mitochondria, it is likely involved in active transport.