Active transport
Section: Biology | Syllabus: Cambridge IGCSE Combined Science (0653)
What is Active Transport?
Unlike diffusion and osmosis, active transport moves particles against the concentration gradient - from a region of lower concentration to a region of higher concentration. This requires energy.
- Active transport is 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
- Energy for active transport comes from respiration (ATP produced in the mitochondria)
- Cells that carry out a lot of active transport have many mitochondria to supply this energy
- Protein carrier molecules in the cell membrane move the particles across - each carrier is specific to the substance being transported
- If respiration is inhibited (e.g. by lack of oxygen or a metabolic poison), active transport stops - this shows that it requires energy
A protein carrier uses energy from ATP to move a particle against its concentration gradient
Role of Protein Carriers (Supplement)
Protein carrier molecules (also called carrier proteins) are embedded in the cell membrane. During active transport:
- A specific molecule or ion binds to the carrier protein on one side of the membrane
- The carrier protein uses energy from ATP to change shape
- The molecule or ion is released on the other side of the membrane - against the concentration gradient
- Each carrier is specific - it only transports particular molecules or ions
Importance of Active Transport (Supplement)
Active transport allows cells to absorb substances that they need even when those substances are at a lower concentration outside the cell - something diffusion cannot achieve.
Worked Example: Mineral Ion Uptake by Root Hair Cells
- Question: mineral ions (e.g. nitrate, magnesium) are often at a higher concentration inside root cells than in the soil water around them. Explain how root hair cells take up these ions.
- Answer: diffusion alone would move ions out of the roots, not in, since the ions are already more concentrated inside. Root hair cells instead use active transport, powered by energy from respiration, to pump mineral ions from the dilute soil water into the cell against the concentration gradient. This is why root hair cells contain many mitochondria - they need large amounts of ATP to power this process
Worked Example: Glucose Absorption in the Small Intestine
- Question: after digestion, glucose concentration in the small intestine drops below the concentration in the blood. Explain how glucose continues to be absorbed.
- Answer: once the glucose concentration in the intestine is lower than in the blood, diffusion can no longer move glucose into the blood. Active transport continues to absorb glucose against the concentration gradient, ensuring all available glucose is absorbed
Comparing Diffusion, Osmosis and Active Transport
| Feature | Diffusion | Osmosis | Active transport |
|---|---|---|---|
| Substance moved | Any dissolved molecule or gas | Water molecules only | Specific molecules or ions |
| Direction | High → low concentration (down gradient) | High → low water potential (down gradient) | Low → high concentration (against gradient) |
| Energy required? | No (uses kinetic energy of particles) | No (uses kinetic energy of water molecules) | Yes (ATP from respiration) |
| Membrane required? | Not always (also occurs in air or water) | Yes - partially permeable membrane | Yes - cell membrane with carrier proteins |
| Carrier proteins? | Not required | Not required | Yes - essential |
Common Mistakes
- ❌ Assuming any movement of particles across a membrane must be diffusion
- ✓ the key distinguishing features of active transport are: movement is against the concentration gradient, energy from respiration is required, and protein carriers are involved. If a question states that a lack of oxygen stops the uptake of a substance, this confirms active transport - not diffusion - is involved
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