Transport in Cells: Diffusion

This lesson covers diffusion as required by AQA GCSE Biology specification 4.1.3. You need to understand the process of diffusion, the factors that affect the rate of diffusion, and how diffusion is involved in the movement of substances into and out of cells, including examples in the human body and in plants.

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What Is Diffusion?

Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration, down a concentration gradient. It occurs in liquids and gases because the particles are free to move randomly.

Diffusion is a passive process — it does not require energy from the cell (no ATP is needed). It occurs naturally because of the random movement of particles.

Key Definitions

Term	Definition
Diffusion	The net movement of particles from a region of higher concentration to a region of lower concentration.
Concentration gradient	The difference in concentration between two areas. Particles move down the concentration gradient (from high to low).
Net movement	The overall direction of movement. Individual particles move randomly in all directions, but there is a net (overall) movement from high to low concentration.
Equilibrium	When particles are evenly distributed and there is no net movement in any direction. Diffusion continues but is balanced.
Passive process	A process that does not require energy (ATP) from the cell.

flowchart LR
    A["HIGH Concentration<br/>Many particles"] -->|"Net movement of particles"| B["LOW Concentration<br/>Few particles"]
    B -->|"Eventually reaches"| C["EQUILIBRIUM<br/>Even distribution"]

    style A fill:#e74c3c,color:#fff
    style B fill:#3498db,color:#fff
    style C fill:#27ae60,color:#fff

Exam Tip: Always describe diffusion as the "net movement" of particles, not simply the "movement." Particles move randomly in all directions, but there is a NET movement from high to low concentration. Using "net movement" is essential for full marks.

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Diffusion Across Cell Membranes

The cell membrane is selectively permeable (also called partially permeable). This means it allows some substances to pass through but not others.

Substances that can diffuse across the cell membrane include:

Can Diffuse Through	Cannot Diffuse Through
Oxygen (small, non-polar)	Starch (too large)
Carbon dioxide (small, non-polar)	Proteins (too large)
Water (small, polar — also moves by osmosis)	Glycogen (too large)
Urea (small molecule)	Most ions (charged — need transport proteins)
Fatty acids and glycerol
Amino acids (with help of transport proteins)
Glucose (with help of transport proteins)

Small, non-polar molecules (like oxygen and carbon dioxide) can pass directly through the phospholipid bilayer. Larger or charged molecules may require transport proteins (channel proteins or carrier proteins) in the membrane — this is called facilitated diffusion.

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Factors Affecting the Rate of Diffusion

Several factors influence how quickly diffusion occurs:

Factor	Effect on Rate of Diffusion
Concentration gradient	The greater the difference in concentration, the faster the rate of diffusion. A steeper gradient means more net movement per unit time.
Temperature	Higher temperature increases the kinetic energy of particles, so they move faster and diffuse more quickly.
Surface area	A larger surface area allows more particles to cross at the same time, increasing the rate of diffusion.
Distance (thickness of membrane or barrier)	A shorter diffusion distance means particles reach the other side more quickly. Thin barriers speed up diffusion.
Size of molecule	Smaller molecules diffuse faster than larger ones because they move more easily through gaps in the membrane.

Exam Tip: When asked about factors affecting diffusion rate, always explain HOW and WHY the factor has its effect. For example, "Increasing temperature increases the kinetic energy of particles, causing them to move faster and collide with the membrane more frequently, increasing the rate of diffusion."

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Examples of Diffusion in the Body

1. Gas Exchange in the Lungs

Oxygen diffuses from the alveoli into the blood (from high concentration in the alveoli to low concentration in the blood). Carbon dioxide diffuses from the blood into the alveoli (from high concentration in the blood to low concentration in the alveoli).

The alveoli are adapted for efficient gas exchange:

Adaptation	How It Increases Rate of Diffusion
Large surface area	Millions of alveoli provide a huge total surface area (~70 m squared) for gas exchange.
Thin walls (one cell thick)	Reduces the diffusion distance so gases cross more quickly.
Rich blood supply	Blood continuously carries oxygen away and brings carbon dioxide to the alveoli, maintaining a steep concentration gradient.
Moist lining	Gases dissolve in the moisture, which aids diffusion across the membrane.
Good ventilation	Breathing constantly refreshes the air in the alveoli, maintaining a steep concentration gradient for both oxygen and carbon dioxide.

2. Gas Exchange in Body Cells

At the body cells (e.g. muscle cells during exercise):

• Oxygen diffuses from the blood into the cells — cells are constantly using oxygen for aerobic respiration, keeping the concentration low inside the cell.
• Carbon dioxide diffuses from the cells into the blood — respiration produces carbon dioxide, so its concentration is higher inside the cell than in the blood.

3. Absorption of Digested Food in the Small Intestine

Digested food molecules (e.g. glucose, amino acids, fatty acids, and glycerol) are absorbed from the small intestine into the blood by diffusion (and active transport for some molecules).

The small intestine is adapted for efficient absorption:

Adaptation	How It Helps
Villi (finger-like projections)	Massively increase the surface area for absorption.
Microvilli (tiny projections on villus cells)	Further increase surface area.
Thin walls (one cell thick)	Short diffusion distance.
Rich blood supply	Carries absorbed molecules away, maintaining concentration gradient.
Lacteal (lymph vessel in each villus)	Absorbs fatty acids and glycerol.

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Diffusion in Plants

Gas Exchange in Leaves

• During photosynthesis (day): Carbon dioxide diffuses into the leaf through stomata (small pores on the underside of the leaf), and oxygen diffuses out.
• During respiration (all the time): Oxygen diffuses into the leaf and carbon dioxide diffuses out through stomata.
• During the day, the rate of photosynthesis usually exceeds the rate of respiration, so the NET exchange is carbon dioxide in and oxygen out.

Guard cells control the opening and closing of stomata to regulate gas exchange and water loss.