Diffusion

Diffusion is an important process that allows substances to move from one place to another without the cell having to provide energy.

It occurs constantly in living organisms and is responsible for processes such as:

• oxygen moving from the lungs into the blood
• carbon dioxide moving from cells into the blood
• oxygen entering cells
• carbon dioxide entering plant leaves
• dissolved nutrients moving between cells and their surroundings

Diffusion can be explained using the particle model of matter.

What Is Diffusion?

Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration.

The word net is important.

Particles actually move randomly in all directions. However, when there are more particles in one area than another, more particles will move away from the high-concentration area than move back into it.

The overall result is movement:

higher concentration → lower concentration

Diffusion is a passive process, meaning that it does not require energy from the cell.

Understanding Diffusion with the Particle Model

According to the particle model:

• matter is made of tiny particles
• particles are constantly moving
• particles move randomly
• particles collide with each other
• particles spread through the available space

Imagine a container divided into two regions.

One side contains many oxygen molecules.

The other side contains relatively few.

Because the molecules are constantly moving randomly, oxygen molecules gradually spread throughout the container.

Eventually, the oxygen molecules become approximately evenly distributed.

Importantly, the particles do not stop moving when they become evenly distributed. They continue moving randomly, but there is no longer a net movement in one direction.

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Concentration

Concentration describes how much of a substance is present in a particular volume.

An area containing many particles of a substance has a high concentration.

An area containing fewer particles has a low concentration.

For example:

High concentration:

● ● ● ● ● ● ● ● ● ●

Low concentration:

● ● ●

If the two areas are connected, there will be a net movement of particles from the high-concentration region toward the low-concentration region.

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The Concentration Gradient

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

For example:

Side A: high oxygen concentration

Side B: low oxygen concentration

There is therefore a concentration gradient between Side A and Side B.

Oxygen will show a net movement:

Side A → Side B

We say that oxygen moves down its concentration gradient.

Predicting the Direction of Diffusion

To predict the direction of diffusion, compare the concentration of the substance in the two locations.

Ask:

Where is the concentration higher?

Then:

Where is the concentration lower?

The net movement will be:

higher concentration → lower concentration

For example:

Outside cell: high oxygen concentration

Inside cell: low oxygen concentration

Net oxygen movement:

outside → inside

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Diffusion Across Cell Membranes

Cell membranes are selectively permeable, meaning some substances can cross more easily than others.

Small molecules such as oxygen and carbon dioxide can diffuse across cell membranes.

For example, if a cell is using oxygen during cellular respiration, the oxygen concentration inside the cell may become lower than the concentration outside.

Oxygen can therefore diffuse:

outside the cell → inside the cell

At the same time, carbon dioxide produced by cellular respiration may build up inside the cell.

Carbon dioxide can diffuse:

inside the cell → outside the cell

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Diffusion Continues Until Equilibrium

As diffusion occurs, the difference in concentration becomes smaller.

Eventually, the substance may become approximately evenly distributed.

This condition is called dynamic equilibrium.

At equilibrium:

• particles continue moving
• particles still cross between regions
• movement remains random
• approximately equal numbers move in each direction

Therefore, there is no net movement of the substance.

This does not mean that the particles have stopped.

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Example: Perfume in a Room

Diffusion is not limited to cells.

Imagine someone sprays perfume at one side of a room.

At first, perfume molecules are highly concentrated near the person.

The molecules move randomly through the air.

Over time, they spread throughout the room.

Someone on the opposite side of the room may eventually smell the perfume.

This happens because perfume molecules diffuse from areas of higher concentration toward areas of lower concentration.

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Example: Food Colouring in Water

A drop of food colouring placed in water initially forms a concentrated region.

Over time, the colouring spreads through the water.

Eventually, the colour becomes much more evenly distributed.

The particles do not spread because they "want" to become evenly distributed.

They spread because particles are constantly undergoing random motion.

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Diffusion in the Lungs

One of the most important examples of diffusion in the human body occurs in the lungs.

The lungs contain millions of tiny air sacs called alveoli.

When we breathe in, air inside the alveoli contains a relatively high concentration of oxygen.

Blood arriving at the lungs contains less oxygen.

Therefore, oxygen diffuses:

alveoli → blood

At the same time, blood arriving at the lungs contains a higher concentration of carbon dioxide than the air inside the alveoli.

Carbon dioxide therefore diffuses:

blood → alveoli

The carbon dioxide can then be breathed out.

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Why Are Alveoli Good for Diffusion?

Alveoli have several adaptations that make diffusion efficient.

They have:

• very thin walls
• a large surface area
• a good blood supply
• a moist surface

Their thin walls create a short diffusion distance.

Their large surface area provides more space for diffusion to occur.

The blood supply continuously brings carbon dioxide to the lungs and removes oxygen, helping maintain concentration gradients.

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Diffusion Between Blood and Body Cells

Diffusion also occurs between the blood and cells throughout the body.

Body cells constantly use oxygen during cellular respiration.

Therefore:

Blood: higher oxygen concentration

Cells: lower oxygen concentration

Oxygen diffuses:

blood → cells

Cells also produce carbon dioxide.

Therefore:

Cells: higher carbon dioxide concentration

Blood: lower carbon dioxide concentration

Carbon dioxide diffuses:

cells → blood

The blood then transports carbon dioxide toward the lungs.

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Diffusion in Plants

Plants also rely heavily on diffusion.

Leaves contain small openings called stomata.

Carbon dioxide from the air enters leaves through the stomata.

Because carbon dioxide is being used during photosynthesis, its concentration inside photosynthesizing leaf tissues may be lower than in the surrounding air.

Carbon dioxide therefore diffuses:

air → leaf

Oxygen produced during photosynthesis can diffuse in the opposite direction:

leaf → air

Plants therefore depend on diffusion for gas exchange.

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Diffusion in the Small Intestine

Digested food molecules are absorbed from the small intestine into the blood.

Some substances can move by diffusion when a suitable concentration gradient exists.

The small intestine has many finger-like structures called villi.

Villi provide a very large surface area for absorption.

This helps substances move efficiently between the intestine and blood.

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Factors Affecting the Rate of Diffusion

Diffusion does not always occur at the same rate.

Several factors affect how quickly particles diffuse.

Important factors include:

• concentration gradient
• temperature
• surface area
• diffusion distance
• size of the particles

Concentration Gradient

A larger concentration gradient generally produces faster net diffusion.

Compare:

Situation A:

100 particles → 10 particles

Situation B:

60 particles → 50 particles

Situation A has the larger concentration difference.

Therefore, net diffusion will generally occur more rapidly in Situation A.

As diffusion continues and the concentrations become more similar, the rate of net diffusion decreases.

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Temperature

Increasing temperature gives particles more kinetic energy.

The particles move faster.

As a result, diffusion generally occurs more rapidly at higher temperatures.

This can be demonstrated by adding food colouring to hot and cold water.

The colouring normally spreads faster through the warmer water.

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Surface Area

A larger surface area allows more particles to cross at the same time.

This is why many biological exchange surfaces have large surface areas.

Examples include:

• alveoli in the lungs
• villi in the small intestine
• root hair cells in plants
• gills in fish

A large surface area makes exchange more efficient.

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Diffusion Distance

Diffusion occurs more quickly across a short distance than across a long distance.

Biological exchange surfaces are therefore often extremely thin.

For example, alveolar walls and capillary walls are each only about one cell thick.

This creates a short distance for oxygen and carbon dioxide to diffuse.

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Particle Size

Smaller particles generally diffuse more easily and rapidly than larger particles.

The properties of the cell membrane also determine whether a substance can cross directly or requires a transport protein.

For example, small molecules such as oxygen can diffuse directly through the membrane much more readily than large biological molecules.

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Predicting Diffusion: Example 1

A cell contains a carbon dioxide concentration of 8 units.

The surrounding fluid contains a carbon dioxide concentration of 3 units.

Where will carbon dioxide show a net movement?

Inside cell: 8

Outside cell: 3

Carbon dioxide will diffuse:

inside cell → outside cell

because diffusion occurs from higher concentration to lower concentration.

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Predicting Diffusion: Example 2

The oxygen concentration outside a cell is 12 units.

The oxygen concentration inside the cell is 5 units.

Oxygen will diffuse:

outside cell → inside cell

because:

12 > 5

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Predicting Diffusion: Example 3

Suppose the concentration of a substance is:

Inside cell: 6 units

Outside cell: 6 units

There is no concentration difference.

Therefore, there is no concentration gradient.

Particles will still move randomly in both directions, but there will be:

no net movement

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Predicting Diffusion: Example 4

A student observes the following concentrations:

For oxygen:

8 outside > 2 inside

Net movement:

outside → inside

For carbon dioxide:

7 inside > 3 outside

Net movement:

inside → outside

This is similar to what happens in many actively respiring cells.

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Diffusion Does Not Require Cellular Energy

Diffusion is described as a passive transport process.

The cell does not need to use ATP to make diffusion happen.

The particles already possess kinetic energy and are constantly moving.

This is different from active transport, in which the cell uses energy to move substances against a concentration gradient.

A useful comparison is:

Diffusion:

high concentration → low concentration

Active transport:

low concentration → high concentration

and active transport requires cellular energy.

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Why Diffusion Is Important to Life

Cells need a constant exchange of substances with their surroundings.

Diffusion contributes to:

• oxygen uptake
• carbon dioxide removal
• gas exchange in lungs
• gas exchange in leaves
• movement of substances between blood and tissues
• absorption of some digested substances

Without diffusion, many essential biological processes would occur too slowly or would not occur effectively.

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Common Misconceptions

Diffusion means particles only move from high concentration to low concentration.

Individual particles move randomly in all directions. The net movement is from high concentration to low concentration.

Particles stop moving when equilibrium is reached.

Incorrect. Particles continue moving randomly, but there is no net movement in one direction.

Cells use energy to make diffusion happen.

Diffusion is passive and does not require cellular energy.

Diffusion only happens in liquids.

Diffusion can occur in gases and liquids and across suitable membranes.

A concentration gradient is the movement of particles.

A concentration gradient is the difference in concentration between two regions. Diffusion is the movement that can result from that difference.

Particles diffuse because they are trying to spread out.

Particles have no intention or goal. Diffusion results from their constant random motion.

Did You Know?

Your lungs provide an enormous surface for diffusion.

Although they fit inside your chest, the millions of alveoli create a very large total surface area for gas exchange.

Their combination of large surface area, thin walls, and a continuous blood supply makes oxygen and carbon dioxide exchange extremely efficient.

Key Terms

Diffusion – The net movement of particles from an area of higher concentration to an area of lower concentration.

Concentration – The amount of a substance present in a particular volume.

Concentration gradient – A difference in concentration between two regions.

Net movement – The overall movement resulting from particles moving in different directions.

Dynamic equilibrium – A condition in which particles continue moving but there is no net movement between regions.

Passive transport – Movement across a membrane that does not require cellular energy.

Particle model – A model explaining matter in terms of constantly moving particles.

Alveoli – Tiny air sacs in the lungs where gas exchange occurs.

Stomata – Small openings in leaves that allow gases to enter and leave.

Surface area – The total area available for exchange.

Key Takeaways

• Diffusion is the net movement of particles from higher concentration to lower concentration.
• Particles are constantly moving randomly.
• A concentration gradient is a difference in concentration between two regions.
• Diffusion occurs down a concentration gradient.
• At equilibrium, particles continue moving but there is no net movement.
• Diffusion does not require cellular energy.
• Oxygen and carbon dioxide can diffuse across cell membranes.
• In the lungs, oxygen diffuses from the alveoli into the blood while carbon dioxide diffuses from the blood into the alveoli.
• In body tissues, oxygen can diffuse from blood into cells and carbon dioxide from cells into blood.
• Carbon dioxide enters plant leaves by diffusion through the stomata.
• A larger concentration gradient generally increases the rate of diffusion.
• Higher temperature generally increases diffusion because particles move faster.
• Large surface area and short diffusion distance make biological exchange more efficient.
• To predict the direction of diffusion, identify where the substance has the higher concentration and where it has the lower concentration.