3.2 Active Transport - Cambridge O Level Biology Revision Notes

Definition: Active transport is defined as the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration, using energy released by respiration.
Movement Direction: Particles move against the concentration gradient, which is why external energy is required.
Selectivity: The process is highly selective, utilizing specific carrier proteins embedded within the cell membrane to facilitate movement.
Key Distinguishing Feature: Active transport is the only membrane transport process identified in this context that requires energy from respiration.

Binding: A specific particle or ion binds to a matching carrier protein located within the cell membrane.
Energy Supply: Energy derived from cellular respiration is supplied to the carrier protein.
Conformational Change: The carrier protein undergoes a change in shape, physically moving the particle across the membrane.
Release: The particle is released on the opposite side of the membrane, where its concentration is already higher.
Recovery: The carrier protein returns to its original shape and configuration, making it available to be used again for another transport cycle.

Energy Source: The energy required for active transport comes from respiration in the form of $ATP$ .
Mitochondria Density: Cells that perform significant amounts of active transport typically contain a high density of mitochondria to produce the necessary energy.
Respiration Dependency: If respiration is inhibited or lacks necessary substrates, active transport will slow down or stop entirely.

Root Hair Cells: These cells absorb mineral ions from dilute soil water. Even when the internal ion concentration is higher than the concentration in the surrounding soil, active transport allows for continued uptake.
Small-Intestine Villi: Epithelial cells in the villi take up glucose from digested food. This process ensures that glucose enters the body effectively for use in respiration.
Kidney Tubules: Useful glucose is reabsorbed from the filtrate back into the bloodstream to ensure it is not lost through urine excretion.

When describing active transport in root hairs, the following elements should be integrated into a single explanation: Mineral ions move from the lower ion concentration in the soil to the higher ion concentration inside the root hair cell, against the concentration gradient, through carrier proteins, using energy released by respiration.

Process	Direction of Movement	Requirements and Membrane Involvement
Diffusion	High concentration to low concentration	No energy required; particles move down the concentration gradient.
Osmosis	High water potential to low water potential	Involves water only; requires a partially permeable membrane; no energy required.
Active Transport	Low concentration to high concentration	Requires energy from respiration ( $ATP$ ); involves specific carrier proteins; moves against the gradient.

Identifying the Question: If a question describes the movement of substances from a lower concentration to a higher concentration, the answer is Active Transport.
Essential Phrases for Full Marks:
- Mention that movement occurs "against the concentration gradient."
- State that the process requires "energy released by respiration."
- Explicitly mention "carrier proteins" when describing the physical movement across the membrane.
Important Distinction: Never suggest that particles move "naturally" in the direction of active transport via diffusion.

Substrate Availability: A reduction in oxygen or glucose leads to less respiration, resulting in less available energy and a slower rate of active transport.
Temperature Effects:
- Low temperatures slow down enzyme-controlled respiration, reducing the energy supply.
- Very high temperatures can damage or denature the carrier proteins and the cell membranes themselves.
Common Misconceptions:
- Not Osmosis: Active transport moves solutes and ions, not just water. Osmosis is specific to water movement.
- Independence from Diffusion: Active transport can continue even when the concentration gradient would cause diffusion to move particles in the opposite direction.