Diffusion: Concentration Gradients and Gas Exchange

Diffusion is the passive movement of molecules from an area of high concentration to an area of low concentration until equilibrium is reached. This process requires no energy from the cell and is vital for:

• Respiration: Oxygen diffuses into cells; carbon dioxide diffuses out
• Photosynthesis: Carbon dioxide diffuses into leaf cells; oxygen diffuses out
• Waste removal: Urea and other metabolic wastes diffuse out of cells into the blood for excretion

Without diffusion, cells could not obtain nutrients or eliminate wastes—life would not be possible!

Diffusion is the net movement of particles (atoms, ions, or molecules) from a region of higher concentration to a region of lower concentration, driven by the particles’ kinetic energy.

Everyday Examples

• Perfume spreading through a room
• Tea diffusing from a tea bag into hot water
• Food coloring dispersing in still water

A concentration gradient is the difference in concentration of a substance between two regions.

• Diffusion always occurs down the concentration gradient (high → low)
• The steeper the gradient, the faster the rate of diffusion
• When concentrations are equal, equilibrium is reached and net movement stops

• Temperature: Higher temperature → more kinetic energy → faster diffusion
• Surface area: Larger surface area (e.g., alveoli, villi) → faster diffusion
• Thickness of membrane: Thinner barriers → faster diffusion
• Size of molecules: Smaller molecules (e.g., O₂, CO₂) diffuse faster than large ones
• Steepness of concentration gradient: Greater difference → faster diffusion

These factors are directly tested in CSEC structured questions—memorize them with examples!

Cell membranes are selectively permeable. Small, non-polar molecules like oxygen and carbon dioxide pass directly through the phospholipid bilayer via simple diffusion.

Larger or charged molecules (e.g., glucose, ions) require protein channels—this may involve facilitated diffusion (still passive, no energy).

Key point: Diffusion is passive—no ATP required!

In the Lungs (Alveoli)

• Oxygen diffuses from alveolar air (high O₂) into blood capillaries (low O₂)
• Carbon dioxide diffuses from blood (high CO₂) into alveoli (low CO₂) to be exhaled

Adaptations for Efficient Diffusion

• Large surface area: Millions of alveoli
• Thin walls: One-cell-thick alveolar and capillary walls
• Moist surface: Gases dissolve easily
• Good blood supply: Maintains steep concentration gradient

Plants exchange gases through stomata (pores mainly on the lower epidermis of leaves).

• During photosynthesis: CO₂ diffuses in; O₂ diffuses out
• During respiration: O₂ diffuses in; CO₂ diffuses out

Leaf Adaptations

• Stomata: Allow gas entry/exit
• Intercellular air spaces: Increase surface area for diffusion
• Thin mesophyll cells: Short diffusion distance

How Diffusion Appears in Exams

• Multiple-choice: “Which process moves oxygen into cells?”
• Structured questions: “Explain how the alveolus is adapted for gas exchange.”
• Diagrams: Label diffusion directions in lungs or leaves

Command Words to Know

• Define: “Diffusion is the net movement of particles from high to low concentration.”
• Describe: List features (e.g., thin walls, large surface area)
• Explain: Link structure to function using cause-and-effect (e.g., “Because the alveolus has a thin wall, the diffusion distance is short, so gases move quickly.”)

Diffusion is a fundamental life process that enables gas exchange, nutrient uptake, and waste removal. Remember:

• Always moves down the concentration gradient
• Passive—no energy required
• Rate depends on temperature, surface area, membrane thickness, molecule size, and gradient steepness
• Alveoli and leaves are perfectly adapted for rapid diffusion

Now you’re ready to tackle any CSEC question on diffusion!