How does the direction of movement in active transport differ from diffusion?

Diffusion involves the movement of particles down a concentration gradient (high to low), whereas active transport moves particles against a concentration gradient (low to high). This allows cells to accumulate substances even when they are in low supply externally.

What is the primary difference between the energy requirements of osmosis and active transport?

Osmosis is a passive process that relies on the random kinetic energy of water molecules and requires no cellular energy. Active transport is an active process that requires energy released from aerobic respiration to power protein carriers.

How does the role of the cell membrane differ between simple diffusion and active transport?

In simple diffusion, small molecules pass directly through the lipid bilayer or through pores. In active transport, the membrane must contain specific protein carrier molecules that use energy to pump substances across.

Why is it scientifically incorrect to say that active transport 'produces energy'?

Energy cannot be created or destroyed. Active transport *uses* or *requires* energy that has been *released* from glucose during the process of respiration.

What happens to the rate of active transport if a cell is exposed to a poison that inhibits mitochondria?

The rate of active transport will drop to zero. This is because mitochondria are the site of aerobic respiration, and without respiration, no energy is released to power the protein carrier molecules.

Why is it a mistake to describe active transport as 'random movement'?

Unlike diffusion, which is the result of random particle collisions, active transport is a directed, controlled process where protein carriers specifically move molecules in one direction across the membrane.

Define active transport.

Active transport is the movement of substances from a more dilute solution to a more concentrated solution (against a concentration gradient) using energy from respiration.

What are protein carrier molecules?

These are specialized proteins embedded in the cell membrane that bind to specific substances and use energy to move them across the membrane against a concentration gradient.

What is the role of magnesium ions in plants, and how are they obtained?

Magnesium ions are used to make chlorophyll for photosynthesis. They are obtained from the soil via active transport because their concentration in the soil is usually lower than inside the root hair cells.

Why do root hair cells have a large number of mitochondria?

Active transport requires significant energy released from respiration. Mitochondria are the site of aerobic respiration, so a high density of them ensures a sufficient energy supply for absorbing mineral ions.

Active transport | AQA GCSE Science

Combined Science Trilogy / Biology

Cell Biology

Active transport

Summary

Active transport is a vital cellular process that moves substances against their concentration gradient, from an area of lower concentration to an area of higher concentration. Unlike passive processes, it requires energy released from aerobic respiration and utilizes specialized protein carrier molecules within the cell membrane.

1. Definition & Core Concepts

Active transport is the movement of particles across a cell membrane from a region of lower concentration to a region of higher concentration.
This process is described as moving 'against' or 'up' the concentration gradient, which is the opposite direction of natural diffusion.
Because it moves substances against the natural flow, it is an active process that requires a constant supply of metabolic energy.
The energy required for this process is provided by respiration occurring within the cell's mitochondria.

Diagram showing a protein carrier in a cell membrane moving molecules from a low concentration area to a high concentration area using energy.

2. Underlying Principles

The fundamental mechanism involves protein carrier molecules embedded in the lipid bilayer of the cell membrane.
These proteins act as highly specific 'pumps' that recognize and bind to specific molecules or ions on one side of the membrane.
Once the molecule is bound, the protein changes shape to release the molecule on the opposite side; this conformational change is what requires the expenditure of ATP (energy).
Because it relies on cellular metabolism, any factor that limits respiration (such as lack of oxygen or low temperature) will also limit the rate of active transport.

3. Biological Applications in Plants

Root Hair Cells: These specialized cells use active transport to absorb mineral ions from the soil water, where minerals are usually found in very dilute concentrations.
Magnesium Ions: These are actively transported into the plant to produce chlorophyll, which is essential for capturing light energy during photosynthesis.
Nitrate Ions: These are required for the synthesis of amino acids and proteins, supporting the plant's growth and repair mechanisms.
Without active transport, plants would be unable to sustain growth in nutrient-poor soils where the internal concentration of minerals already exceeds the external environment.

4. Biological Applications in Animals

5. Key Distinctions

6. Exam Strategy & Tips