3.3 Active transport

2026 Syllabus Objectives

  1. Describe active transport as 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
  2. Explain the importance of active transport as a process for movement of molecules or ions across membranes, including ion uptake by root hairs
  3. State that protein carriers move molecules or ions across a membrane during active transport

What is Active Transport? 🔑

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Active transport is a cellular process that moves substances against their concentration gradient - from a region of lower concentration to a region of higher concentration. This is the opposite direction to diffusion and osmosis.

Key Characteristics

  • Direction: Against the concentration gradient (low → high concentration)
  • Energy requirement: Uses energy from aerobic respiration
  • Location of energy production: Mitochondria release energy that powers active transport
  • Mechanism: Involves carrier proteins (also called protein carriers) in the cell membrane

Active transport is essential when cells need to take in substances that are only present in small quantities around them, even when the concentration inside the cell is already higher than outside.

Why is Active Transport Necessary? 💡

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Many situations arise where cells need to accumulate substances that would naturally diffuse out of the cell rather than into it. Consider this scenario:

  • A substance has a higher concentration inside the cell than outside
  • According to diffusion principles, this substance should move out of the cell
  • However, if the cell needs this substance, it must work against the natural diffusion gradient
  • The cell uses energy to force the substance to move into the cell

This is where active transport becomes essential - it allows cells to override natural diffusion and take in needed substances even when the concentration gradient opposes this movement.

How Active Transport Works ⚡

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The Role of Carrier Proteins

Carrier proteins are special protein molecules embedded in cell membranes that facilitate active transport. These proteins have a remarkable ability:

  • They can pick up specific molecules or ions from one side of the membrane
  • They use energy from respiration to change shape
  • This shape change pushes the substance through the membrane to the other side

The Active Transport Process

The mechanism involves three main steps:

Step 1: The carrier protein recognizes and binds to the specific molecule or ion on the side of the membrane where concentration is lower

Step 2: Energy from aerobic respiration (provided by mitochondria) causes the carrier protein to change its shape

Step 3: The shape change moves the molecule or ion through the membrane to the side where concentration is higher, releasing it into that region

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