Active Transport
Laura Armstrong
Teacher
Contents
Recall Questions
This topic requires prior knowledge of membrane structure, energy transfer in cells (ATP), and diffusion mechanisms. You can test your knowledge on these below.
What is the structure of a phospholipid bilayer, and how does it function in the membrane?
The bilayer consists of phospholipids with hydrophilic heads facing outward and hydrophobic tails facing inward. This arrangement allows lipid-soluble molecules to pass through while preventing water-soluble substances from diffusing freely.
Describe the role of ATP in cellular processes.
ATP provides immediate energy for cellular activities. It releases energy by breaking the terminal phosphate bond in a hydrolysis reaction, forming ADP and Pi.
What is facilitated diffusion and how does it differ from simple diffusion?
Facilitated diffusion is the passive movement of molecules down their concentration gradient via channel or carrier proteins in the membrane. It differs from simple diffusion as it requires transport proteins and is used for large, polar or charged molecules.
Topic Explainer Videos
Check out this @LauraDoesBiology video that explains active transport or read the full notes below. Once you've gone through the whole note, try out the practice questions!
Active Transport
- Active transport is the movement of molecules or ions across a membrane against their concentration gradient (from low to high concentration).
- This process requires energy from ATP and the use of specific carrier proteins in the plasma membrane.
How Active Transport Works
1. Carrier Protein Binding:
- A molecule or ion binds to a specific carrier protein on one side of the membrane.
2. ATP Hydrolysis:
- ATP is hydrolysed into ADP and Pi (inorganic phosphate), releasing energy.
3. Conformational Change:
- The energy from ATP changes the shape of the carrier protein, allowing the molecule to be transported across the membrane.
4. Molecule Release & Reset:
- The transported molecule is released on the other side of the membrane.
- The carrier protein returns to its original shape, ready for another transport cycle.
Example of Active Transport: The Sodium-Potassium Pump
- Found in neurone cell membranes.
- Actively transports 3 Na⁺ ions out and 2 K⁺ ions in, maintaining an electrochemical gradient.
- Essential for nerve impulse transmission.
Active Transport Adaptations
Cells that carry out a lot of active transport have adaptations to provide the energy and structures needed for this process. These adaptations include:
1. More Mitochondria
- Mitochondria produce ATP through aerobic respiration, which is essential for active transport.
- Cells that transport a lot of substances (e.g., kidney tubule cells, root hair cells) have a high density of mitochondria to meet their energy demands.
2. More Carrier Proteins in the Membrane
- Active transport requires specific carrier proteins to move molecules against their concentration gradient.
- These cells have a higher number of carrier proteins in their cell membrane to increase transport efficiency.
3. Large Surface Area
- A larger membrane surface area allows more carrier proteins to be embedded, increasing the rate of active transport.
- Example: Root hair cells in plants have long extensions to increase surface area for nutrient uptake.
4. More Folds in the Membrane (Microvilli)
- Some cells (e.g., epithelial cells in the small intestine) have microvilli, which further increase the surface area for transport.
Example Cells That Use a Lot of Active Transport:
- Root Hair Cells: Absorb mineral ions from the soil.
- Epithelial Cells of The Small Intestine: Absorb glucose and amino acids.
- Neurones: Use the sodium-potassium pump to maintain nerve impulses.
- Kidney Tubule Cells: Reabsorb useful substances from urine.
Key Terms
- Active transport: Movement of substances against their concentration gradient, using ATP and carrier proteins.
- ATP (adenosine triphosphate): A molecule that provides energy for active transport.
- Carrier proteins: Proteins in the membrane that change shape to transport molecules across.
- Sodium-potassium pump: A carrier protein that moves Na⁺ out and K⁺ into the cell, using ATP.
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Exam Tips
When explaining active transport in exams, always mention ATP and carrier proteins.
Make sure you can compare and contrast different methods of transport across the cell membrane.
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Two samples of plant roots were placed in solutions containing potassium ions. An inhibitor to prevent respiration was added to one solution. The concentrations of potassium ions in the two solutions were measured at regular intervals. The graph shows the results.
Explain the decrease in the concentrations of potassium ions in the two solutions between 0 and 30 minutes.
a) With inhibitor (2 marks)
b) Without inhibitor (2 marks)
a) With inhibitor: potassium ions absorbed by facilitated diffusion (only).
Equilibrium is reached / potassium ion concentrations in solution and in cells is the same so uptake of potassium ions stops.
b) Without inhibitor: potassium ions absorbed by active transport.
As ATP available from respiration/ active transport requires energy/ ATP from respiration
Common misconception
Would you make this mistake?