What you'll learn
Active transport is a crucial process that allows cells to absorb substances against their concentration gradient, requiring energy from respiration. This topic is essential for understanding how plants take up mineral ions from soil and how your intestines absorb glucose and nutrients. You'll learn exactly how active transport differs from diffusion and osmosis, where it occurs in organisms, and why it's vital for survival.
Key terms and definitions
Active transport — the movement of substances from a more dilute solution to a more concentrated solution (against a concentration gradient) across a partially permeable membrane, requiring energy from respiration
Concentration gradient — the difference in concentration of a substance between two areas; substances move down a concentration gradient by diffusion (high to low concentration) but move against it in active transport (low to high concentration)
Carrier proteins — protein molecules in the cell membrane that bind to specific substances and transport them across the membrane using energy from respiration
Mitochondria — organelles in cells where aerobic respiration occurs, releasing energy that can be used for active transport
Partially permeable membrane — a membrane that allows some molecules to pass through but not others, such as the cell membrane
Root hair cells — specialized plant cells with extensions that increase surface area for absorption of water and mineral ions from soil
Villi — finger-like projections in the small intestine that increase surface area for absorption of nutrients into the bloodstream
Core concepts
How active transport works
Active transport moves substances against their concentration gradient, meaning from where they are in low concentration to where they are already in high concentration. This is the opposite direction to diffusion and requires energy.
The process works through these steps:
- Specific carrier proteins in the cell membrane recognize and bind to particular molecules or ions on one side of the membrane
- Energy from respiration (in the form of ATP) changes the shape of the carrier protein
- The change in shape moves the substance across the membrane to the other side
- The substance is released into the area of higher concentration
- The carrier protein returns to its original shape, ready to transport another molecule
This process is selective because each carrier protein only transports specific substances. For example, carrier proteins for glucose will only transport glucose molecules, not amino acids or mineral ions.
Why cells need active transport
Diffusion alone cannot always provide cells with the substances they need because:
- Sometimes cells need to absorb substances that are already at a higher concentration inside the cell than outside
- Cells may need to maintain much higher concentrations of certain substances than their surroundings
- Organisms need to absorb nutrients completely from their gut or soil, even when concentrations become very low
Active transport allows cells to:
- Accumulate substances to concentrations much higher than in their environment
- Absorb all available nutrients, not just until concentrations are equal
- Control exactly which substances enter and leave
- Maintain different internal and external environments
Active transport in plants
Root hair cells in plants use active transport extensively to absorb mineral ions from soil water.
Mineral ion uptake:
- Soil water contains very dilute mineral ions (such as nitrate ions and magnesium ions)
- Root hair cells often contain higher concentrations of these mineral ions than the surrounding soil
- Diffusion alone would not allow the plant to absorb enough minerals
- Active transport moves mineral ions from the dilute soil solution into the more concentrated root hair cell cytoplasm
- This requires energy from respiration in the root hair cell mitochondria
Root hair cells are adapted for this function:
- Large surface area through hair-like extensions increases contact with soil water
- Many mitochondria provide energy for active transport
- Carrier proteins in the cell membrane transport specific mineral ions
Plants need mineral ions for various functions:
- Nitrate ions for making amino acids and proteins
- Magnesium ions for making chlorophyll
- Phosphate ions for making DNA and cell membranes
Without active transport, plants would be unable to obtain sufficient minerals, leading to deficiency symptoms and poor growth.
Active transport in animals
The small intestine uses active transport to ensure maximum absorption of nutrients from digested food.
Glucose absorption in the small intestine:
- After a meal, glucose concentration in the small intestine may be higher than in the blood, so glucose diffuses into the blood
- As digestion continues and glucose is absorbed, its concentration in the intestine decreases
- Eventually, glucose concentration in the intestine becomes lower than in the blood
- To absorb all remaining glucose, cells lining the small intestine (epithelial cells) use active transport
- Carrier proteins move glucose from the low concentration in the intestine to the higher concentration in the blood
Epithelial cells in the small intestine are adapted for absorption:
- Villi increase the surface area of the intestine lining
- Microvilli (tiny projections on epithelial cells) further increase surface area
- Many mitochondria in epithelial cells provide energy for active transport
- Rich blood supply maintains concentration gradient for diffusion and removes absorbed nutrients
Other examples in animals:
- Kidney tubules use active transport to reabsorb glucose and useful ions from filtered blood back into the bloodstream
- Nerve cells use active transport to maintain different concentrations of sodium and potassium ions, essential for transmitting electrical impulses
Differences between active transport and diffusion
Understanding the distinctions between these processes is crucial for exam success:
| Feature | Diffusion | Active Transport |
|---|---|---|
| Direction | Down concentration gradient (high to low) | Against concentration gradient (low to high) |
| Energy requirement | No energy from respiration needed | Requires energy from respiration |
| Proteins needed | May occur through cell membrane or through protein channels | Always requires specific carrier proteins |
| Speed | Can be fast if steep gradient | Limited by number of carrier proteins available |
| Selectivity | Non-selective or limited selectivity | Highly selective for specific substances |
Both processes move substances across partially permeable membranes, but active transport is the only process that can work against a concentration gradient.
Factors affecting the rate of active transport
Several factors influence how quickly active transport occurs:
Temperature:
- Increased temperature speeds up respiration, providing more energy
- Higher temperatures increase kinetic energy, so carrier proteins work faster
- Above optimum temperature, carrier proteins denature and active transport stops
- At very low temperatures, respiration slows and less energy is available
Oxygen concentration:
- More oxygen available increases rate of aerobic respiration
- More respiration releases more energy for active transport
- Without oxygen, cells must use anaerobic respiration, which releases much less energy
- Insufficient oxygen significantly reduces active transport rate
Glucose concentration:
- Glucose is needed for respiration to release energy
- More glucose available can increase respiration rate
- Very high glucose concentrations may not further increase the rate as other factors become limiting
Number of carrier proteins:
- More carrier proteins allow more molecules to be transported simultaneously
- This is determined by the cell's genetic makeup and cannot easily be changed
- Carrier proteins can become saturated when all are in use
Worked examples
Example 1: Root hair cells and mineral uptake (3 marks)
Question: A student measured the concentration of nitrate ions in soil water and in root hair cells of a plant. The concentration in the root hair cells was 40 times higher than in the soil water. Explain how the root hair cells absorbed the nitrate ions. (3 marks)
Mark scheme answer:
- Nitrate ions moved by active transport (1 mark)
- Against the concentration gradient / from low to high concentration (1 mark)
- Using energy from respiration (in mitochondria) / using ATP (1 mark)
Examiner note: Students must mention active transport by name, explain the direction of movement, and reference energy. Simply stating "using energy" without linking it to respiration may not receive full marks.
Example 2: Comparing transport processes (4 marks)
Question: A student investigated the uptake of glucose by cells in the small intestine. Describe and explain how glucose is absorbed when its concentration in the intestine is: (a) higher than in the blood (2 marks) (b) lower than in the blood (2 marks)
Mark scheme answer: (a)
- Glucose moves by diffusion (1 mark)
- Down the concentration gradient / from high to low concentration / no energy needed (1 mark)
(b)
- Glucose moves by active transport (1 mark)
- Against the concentration gradient using energy from respiration / using carrier proteins and ATP (1 mark)
Examiner note: The question tests understanding that both processes can occur in the same cells depending on conditions. Students must correctly identify which process occurs in each scenario.
Example 3: Explaining adaptations (5 marks)
Question: Root hair cells are adapted for absorbing mineral ions from soil. Explain how the following features help root hair cells carry out this function:
- Hair-like extension
- Many mitochondria
- Carrier proteins in cell membrane
Mark scheme answer:
- Hair-like extension increases surface area (1 mark)
- (Greater surface area) for more absorption / more contact with soil water (1 mark)
- Many mitochondria carry out (aerobic) respiration (1 mark)
- (Respiration) releases energy for active transport (1 mark)
- Carrier proteins transport specific mineral ions across the membrane / against concentration gradient (1 mark)
Examiner note: Each adaptation must be linked to its specific function. Simply listing features without explaining their purpose will not earn marks.
Common mistakes and how to avoid them
Confusing active transport with diffusion — Always check the direction of movement. If substances move from low to high concentration, it must be active transport. If moving from high to low, it's diffusion. Look for keywords in questions about concentration gradients.
Forgetting to mention energy from respiration — Active transport always requires energy. Write "energy from respiration" rather than just "energy" to show you understand where the energy comes from. Never say active transport "uses ATP from photosynthesis" — photosynthesis makes glucose, not ATP for transport.
Stating that active transport needs oxygen — Active transport needs energy from respiration, not oxygen directly. Aerobic respiration requires oxygen and releases more energy, but anaerobic respiration can also provide energy for active transport, just less efficiently.
Saying carrier proteins "use energy" — Be precise: carrier proteins facilitate the transport, but the energy from respiration changes their shape. Write "carrier proteins use energy from respiration to transport substances" rather than "carrier proteins make energy."
Confusing mineral ions with water uptake in plants — Water moves into root hair cells by osmosis, not active transport. Only mineral ions (and some other specific substances) are actively transported. Don't mix these processes up in exam answers.
Thinking all absorption requires active transport — Diffusion and osmosis are sufficient when concentration gradients are favorable. Active transport only becomes necessary when substances need to move against their concentration gradient or when cells need concentrations higher than diffusion can provide.
Exam technique for "Active transport"
"Explain" questions require mechanisms — When asked to explain active transport, you must state it moves substances against a concentration gradient, uses energy from respiration, and involves carrier proteins. One-word answers or simple descriptions won't earn full marks. Aim for 2-3 marks worth of detail.
Use compare command words carefully — "Compare" means give similarities AND differences. If asked to compare active transport and diffusion, you must explicitly state how they're similar (both move substances across membranes) and different (direction, energy requirement, protein use). Make direct comparisons rather than describing each separately.
Link structure to function in adaptation questions — When describing how cells are adapted for active transport, always explain HOW each feature helps. "Many mitochondria" alone earns no marks; "many mitochondria provide energy from respiration for active transport" earns marks. The link is essential.
Calculate marks per point — A 3-mark question typically requires three distinct points. Don't write everything you know; write precise points that answer the specific question. Check your answer has the right number of separate points for the marks available.
Quick revision summary
Active transport moves substances against their concentration gradient (from low to high concentration) using energy from respiration and specific carrier proteins in the cell membrane. Plants use active transport in root hair cells to absorb mineral ions from dilute soil water. Animals use it in the small intestine to absorb all available glucose and in kidneys to reabsorb useful substances. Unlike diffusion, active transport requires energy, works against concentration gradients, and allows cells to accumulate substances to much higher concentrations than their surroundings.