# Explain Diffusion

In this worksheet, students will explain the process of diffusion.

### QUESTION 1 of 10

Why can you smell someone's stinky PE socks from all the way across the classroom? Yuck! Normally it's because sweat and other molecules are moving away from the socks and spread out in the air. This is called diffusion!

Diffusion is the spreading out of the particles of any substance in solution, or particles of a gas, resulting in movement from an area of higher concentration (where there are more particles in a certain solution or area) to an area of lower concentration (where there are fewer particles).

Living organisms need different substances to survive and function. These substances need to be transported into and out of their cells through diffusion (as well as osmosis and active transport-you will learn more about these later!). During diffusion, molecules move from an area of high concentration to an area of low concentration. They are said to move down a concentration gradient. Particles diffuse until they are evenly spaced apart. Diffusion is a passive process which means that no energy is needed and it happens naturally.

Surface area to volume ratio of an organism means how much area an organism has in comparison to its volume. As organisms get larger their surface area to volume ratio gets smaller, for example, an elephant has a smaller surface area to volume ratio than a mouse!

Diffusion in organisms

In multicellular organisms, surfaces and organ systems are specialised for exchanging materials. This is to allow sufficient molecules to be transported into and out of cells for the organism’s needs. Diffusion is the main way substances move over short distances in organisms.

Breathing involves exchanging gases in the lungs, this requires diffusion. When you breathe in, oxygen in the inhaled air diffuses through the tiny alveoli (air sacs) in your lungs into your bloodstream. The oxygen is transported around your body. Carbon dioxide is the waste gas produced by respiration. Carbon dioxide diffuses from cells into the bloodstream and is exhaled by the lungs.

The alveoli have a few adaptations that make gas exchange very efficient. They are only one cell thick making them very thin allowing gases to pass through easily and quickly. They also have a large combined surface area, allowing large amounts of gases to be exchanged with each breath.

Another example of diffusion is in the small intestine. Digested food is broken down into small molecules such as glucose and amino acids. These important molecules need to be transported around the body via the blood. The small intestine is lined with many finger-like projections called villi. The molecules diffuse through the villi of the small intestine into the blood to be transported around the body. The villi are adapted by being very thin and having finger-like projections that increase the surface area. Villi also have partially-permeable membranes. This means it has pores that allow smaller molecules through but not larger molecules.

Diffusion also occurs in plants. Plants take in carbon dioxide for photosynthesis and produce oxygen. These enter and leave the plant through the process of diffusion. The structure of the leaf is adapted for gas exchange. There are tiny pores, called stomata, in the surface of the leaf. There are usually more stomata on the underside of a leaf than on the topside. The stomata allow carbon dioxide to enter the leaf for photosynthesis. Molecules of carbon dioxide diffuse from a region of higher concentration (the atmosphere) to a lower concentrated area (leaf). The stomata also allow oxygen to diffuse out.

Factors affecting diffusion

Different factors can affect diffusion and how quickly it happens. Some of these factors are:

- the difference in concentrations (concentration gradient) - having a large difference in concentrations means that diffusion can occur at a quicker rate as particles will naturally move from a high to low concentration.
- the temperature - the higher the temperature the more energy the particles will have to move and spread out.
- the surface area of the membrane - the larger the surface area, the faster the rate of diffusion. This is because more particles can pass through the membrane as there is more area, like in the alveoli in the lungs.
- a thin membrane- to provide a short diffusion path

In the following activity, you will explain the process of diffusion.

Diffusion is the main way in which substances move over short distances in organisms. What type of substances needs to move?

Waste Products

Oxygen

Cells

Food

Describe the process of diffusion.

Waste Products

Oxygen

Cells

Food

Diffusion allows many important processes to occur in different organisms.

Oxygen entering the blood from the lungs

Carbon dioxide entering the heart

Carbon dioxide entering the leaf of a plant for photosynthesis

Glucose entering the blood from the small intestine

 O O O O O O O O O Cell A O O O Cell B

The diagram above shows two cells. Cell A has nine particles and Cell B has three particles. Which direction will the particles diffuse?

From Cell A to Cell B

From Cell B to Cell A

No movement

Red blood cells are small and really thin and have a biconcave shape. Red blood cells have a large surface area. Why is this useful?

More space for water to pass through

Less space for oxygen to move through, resulting in a slower rate of diffusion

More space for oxygen to move through, resulting in a faster rate of diffusion

The rate of diffusion is affected by different factors. Match up the sentences describing how these factors affect the rate of diffusion.

## Column B

The greater the difference in concentration...
...the more space for particles to move through, r...
The greater the temperature...
...the faster the rate of diffusion because more p...
The greater the surface area...
...the greater the movement of particles, resultin...

In which of the following does diffusion not occur?

Solids

Liquids

Gases

Water vapour

Diffusion is the main way in which substances move over short distances in organisms. Oxygen, food and waste products are some of the substances that move by diffusion.

In animals, how do  these vital substances get to where they are needed?

Solids

Liquids

Gases

Water vapour

Breathing involves the exchange of gases in the lungs; a process that occurs by diffusion. What happens when you breathe in?

## Column B

Oxygen in inhaled air diffuses through...
...the alveoli in the lungs and into the bloodstre...
The oxygen is then transported...
...produced by respiration.
Carbon dioxide is the waste gas...
... throughout the body.
Carbon dioxide diffuses from body tissues ...
...into the bloodstream and is exhaled via the lun...

The wall of the small intestine is lined with many projections called villi. What special features do villi have that increases the speed of diffusion?

Very thick membranes

Finger-like projections

Continuous membrane

Very thin

• Question 1

Diffusion is the main way in which substances move over short distances in organisms. What type of substances needs to move?

Waste Products
Oxygen
Food
EDDIE SAYS
The main substances like oxygen, food substances and waste products are crucial for survival so diffusion is an ideal way to obtain these (or get rid of for waste).
• Question 2

Describe the process of diffusion.

EDDIE SAYS
In diffusion particles will move from more concentrated areas to low concentrated areas down a concentration gradient. This is a pretty standard answer when describing diffusion so it's worth remembering for your exams.
• Question 3

Diffusion allows many important processes to occur in different organisms.

Oxygen entering the blood from the lungs
Carbon dioxide entering the leaf of a plant for photosynthesis
Glucose entering the blood from the small intestine
EDDIE SAYS
Diffusion allows gas exchange to occur in the lungs of humans-there is a higher concentration of oxygen in the lungs compared to the blood, so oxygen moves into the blood. Carbon dioxide does the opposite and moves from the blood to the lungs to be exhaled. In the small intestine, glucose and other important molecules diffuse into the blood to be transported around the body to where it's needed. Carbon dioxide, oxygen and water vapour also diffuse into and out of the leaves of a plant.
• Question 4
 O O O O O O O O O Cell A O O O Cell B

The diagram above shows two cells. Cell A has nine particles and Cell B has three particles. Which direction will the particles diffuse?

From Cell A to Cell B
EDDIE SAYS
Particles will move from a higher concentration to a lower concentration. Cell A has more particles than Cell B so particles will move out of Cell A and into Cell B until there are equal numbers of particles in each cell.
• Question 5

Red blood cells are small and really thin and have a biconcave shape. Red blood cells have a large surface area. Why is this useful?

More space for oxygen to move through, resulting in a faster rate of diffusion
EDDIE SAYS
Red blood cells deliver oxygen around the body. Having a large surface area means that there is more area for the oxygen particles to pass through allowing more oxygen to diffuse into the red blood cell.
• Question 6

The rate of diffusion is affected by different factors. Match up the sentences describing how these factors affect the rate of diffusion.

## Column B

The greater the difference in con...
...the faster the rate of diffusi...
The greater the temperature...
...the greater the movement of pa...
The greater the surface area...
...the more space for particles t...
EDDIE SAYS
Take the time to remember these key factors as you are often asked about them in exams. Maybe you can try this question a few times to consolidate your knowledge.
• Question 7

In which of the following does diffusion not occur?

Solids
EDDIE SAYS
In order for diffusion to occur particles need to be able to move. In solids, particles are vibrating around a fixed position but are not free to move like in liquids and gases.
• Question 8

Diffusion is the main way in which substances move over short distances in organisms. Oxygen, food and waste products are some of the substances that move by diffusion.

In animals, how do  these vital substances get to where they are needed?

EDDIE SAYS
In animals, the bloodstream is vital to transport the key substances around the body to the different parts that need it. Diffusion allows a quick exchange of substances, making it an efficient way for cells to get what they need quickly.
• Question 9

Breathing involves the exchange of gases in the lungs; a process that occurs by diffusion. What happens when you breathe in?

## Column B

Oxygen in inhaled air diffuses th...
...the alveoli in the lungs and i...
The oxygen is then transported...
... throughout the body.
Carbon dioxide is the waste gas.....
...produced by respiration.
Carbon dioxide diffuses from body...
...into the bloodstream and is ex...
EDDIE SAYS
Oxygen and carbon dioxide have the opposite pathways, so if you take the time to remember the pathway of one gas, the other gas should be easier to remember in your exams.!
• Question 10

The wall of the small intestine is lined with many projections called villi. What special features do villi have that increases the speed of diffusion?

Finger-like projections
Very thin
EDDIE SAYS
Villi in the small intestine are very thin allowing substances to pass through really quickly. The finger-like projections increase the surface area of the small intestine which increases the speed of diffusion.
---- OR ----

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