Movement in Cells

Movement in Cells

Why Do Substances Move?

Cells need to exchange substances to stay alive:

  • Take in oxygen and glucose
  • Remove waste like carbon dioxide and urea
  • Keep water levels balanced
 

This happens across the cell membrane using three key processes: 

✔ Diffusion
✔ Osmosis
✔ Active Transport

 

Diffusion

Diffusion is the spreading out of particles from an area of high concentration to low concentration.

 

💡 It’s a passive process – no energy required.

 

Examples in the body:

  • Oxygen moving into blood in the lungs
  • Carbon dioxide moving out of cells
 

Osmosis

Osmosis is the diffusion of water molecules across a partially permeable membrane, from high water concentration to low water concentration.

 

💡 Also passive – no energy needed.

 

Key concept:

  • Water moves to balance concentrations across membranes
  • Important in plant cells (vacuole, turgor pressure)
 

Active Transport

Active transport moves substances against the concentration gradient (from low to high concentration).


💥 Requires energy (from respiration).

 

Examples:

  • Root hair cells absorbing mineral ions
  • Glucose reabsorption in the small intestine

 

 

Comparing Diffusion, Osmosis, and Active Transport

Feature

Diffusion

Osmosis

Active Transport

What moves?

Any particles (e.g. O₂)

Water only

Ions or molecules

Direction

High → Low

High water conc. → Low conc.

Low → High

Energy required?

❌ No

❌ No

✅ Yes (ATP from respiration)

Uses membrane?

Sometimes

✅ Yes (partially permeable)

✅ Yes (carrier proteins)

Examples

Oxygen into blood

Water into root hair cells

Nitrates into plant roots
Diffusion diagram showing particles moving from a high-concentration side to a low-concentration side with arrows indicating the gradient; passive, no energy
GCSE osmosis diagram with blue water molecules moving through a partially permeable membrane from dilute solution to concentrated solution; red circles represent glucose solute.
GCSE active transport diagram showing a carrier protein in the membrane using ATP to move molecules from low to high concentration—against the gradient.

Questions 

  1. What is diffusion?
  2. Does osmosis need energy?
  3. What type of transport requires ATP?
  4. Which process moves water only?
  5. Why is active transport important in plants?
 

Summary 

  • Diffusion and osmosis are passive – they don’t need energy.
  • Active transport is active – it moves substances against the gradient and uses energy.
  • Osmosis is the diffusion of water.
  • All three are essential for cell function, transport, and homeostasis in organisms.