Why big organisms need exchange surfaces and a transport system, how to work out surface area to volume ratio, and how alveoli, gills, villi and roots are adapted.
Objectives
Explain why multicellular organisms need exchange surfaces and a transport system.
Calculate and compare surface area to volume ratios.
Explain how the alveoli are adapted for gas exchange by diffusion.
Explain how the small intestine, gills and plant roots are adapted for exchanging materials.
Exchange surfaces and gas exchange in the lungs explained
Start here
Every cell needs oxygen and food, and has to get rid of waste. A single-celled organism can do this across its surface. A human cannot, because most of our cells are deep inside the body, far from the air.
This matters because the lungs, the small intestine, the gills of a fish and the roots of a plant all solve the same problem in the same way: a big surface, a thin barrier and a good supply of whatever is being carried away.
By the end you will be able to calculate a surface area to volume ratio, and explain how the alveoli and other exchange surfaces are adapted.
Why big organisms need exchange surfaces
Substances have to be moved into and out of organisms. They include oxygen, carbon dioxide, water, dissolved food molecules, mineral ions and urea. In a small organism diffusion over the whole surface is enough.
As an organism gets bigger, its volume grows faster than its surface area. The surface area to volume ratio goes down. For a large animal the skin is too small compared with the number of cells to supply them all by diffusion, and the distance is too far.
So multicellular organisms need exchange surfaces with a large area and a transport system, such as blood in animals, to carry the substances to every cell.
Calculating surface area to volume ratio
Use a cube as a model. For a cube with a side of 1 cm, the surface area is 6 faces × 1 cm × 1 cm = 6 cm² and the volume is 1 cm³. The ratio is 6 : 1.
For a cube with a side of 2 cm, the surface area is 6 × 2 × 2 = 24 cm² and the volume is 2 × 2 × 2 = 8 cm³. The ratio is 24 : 8, which is 3 : 1. For a cube with a side of 3 cm, the surface area is 54 cm² and the volume is 27 cm³, so the ratio is 2 : 1.
As the cube gets larger the ratio falls. To divide the ratio down, divide both numbers by the volume. Organisms with a low ratio need special exchange surfaces.
Alveoli
The lungs are the gas exchange organ in humans. The air passes down the windpipe and the bronchi to the alveoli, which are tiny air sacs at the ends of the airways. There are millions of them. Each is surrounded by a network of capillaries.
Oxygen diffuses from the air in the alveoli, across the thin alveolar wall and capillary wall, into the blood, where it joins haemoglobin in red blood cells. Carbon dioxide diffuses the other way, from the blood into the air in the alveoli, and is breathed out.
The alveoli are adapted for fast diffusion. There are very many alveoli, which gives a large surface area. The walls are one cell thick, so the diffusion distance is short. A rich blood supply takes oxygen away and brings carbon dioxide, which keeps a steep concentration gradient. The lining is moist so gases can dissolve. Breathing brings in fresh air to keep the gradient steep.
Other exchange surfaces
The small intestine has villi, tiny finger-like projections that give a large surface area for absorbing digested food. Each villus has a thin wall and a network of capillaries to carry the food molecules away, keeping the concentration gradient steep.
Fish have gills. Each gill has many thin gill filaments. They have a large surface area, thin walls and a good blood supply, so oxygen from the water diffuses into the blood.
In plants, root hair cells have a long projection that increases the surface area to take up water and mineral ions. Leaves are flat and thin, with air spaces in the spongy layer and stomata, so carbon dioxide can diffuse in quickly.
Recap
Large organisms have a small surface area to volume ratio, so they need exchange surfaces and a transport system.
Surface area to volume ratio = surface area ÷ volume. It falls as an object gets bigger.
Good exchange surfaces have a large surface area, thin walls, a good blood supply and, for gases, ventilation. The alveoli, villi, gills and root hairs all follow this pattern.
Key points
Large organisms have a small surface area to volume ratio, so they need exchange surfaces and a transport system.
Surface area : volume. A 1 cm cube is 6 : 1, a 2 cm cube is 3 : 1, a 3 cm cube is 2 : 1.
Alveoli: large surface area, thin walls, rich blood supply, ventilation. Oxygen goes in, carbon dioxide goes out.
Villi (small intestine), gills (fish) and root hairs (plants) are adapted in the same way.
Worked examples
Example 1
Calculate the surface area to volume ratio of a cube with a side of 2 cm. Give the ratio in the form n : 1.
Surface area = 6 × 2 × 2 = 24 cm².
Volume = 2 × 2 × 2 = 8 cm³.
24 ÷ 8 = 3, so the ratio is 3 : 1.
Answer: 3 : 1
Common slips
Slip: Surface area 6 cm², volume 8 cm³, ratio 6 : 8 A cube has 6 faces of area 2 × 2. The surface area is 24 cm².
Slip: 24 : 1 Divide the surface area by the volume to get the ratio.
Slip: 8 : 24 You have written the ratio the wrong way round. It is surface area to volume.
Example 2
Name the part of the lung where gas exchange happens and describe the direction that oxygen and carbon dioxide diffuse.
Gas exchange happens in the alveoli.
Oxygen diffuses from the air in the alveoli into the blood in the capillaries.
Carbon dioxide diffuses from the blood into the air in the alveoli.
Answer: The alveoli. Oxygen diffuses into the blood and carbon dioxide diffuses out of the blood into the alveoli
Common slips
Slip: Gas exchange happens in the bronchi. The bronchi are the airways. The alveoli are the exchange surface.
Slip: Oxygen diffuses from the blood into the alveoli. Oxygen moves into the blood. Carbon dioxide moves out.
Slip: Both gases diffuse into the blood. Carbon dioxide goes the other way, out of the blood.
Example 3
Explain how the structure of the alveoli makes gas exchange efficient.
There are millions of alveoli, which gives a large surface area for diffusion.
The walls are one cell thick, so the diffusion distance is short.
Each alveolus is surrounded by capillaries, so the rich blood supply carries oxygen away and brings carbon dioxide, keeping a steep concentration gradient.
Breathing moves fresh air into the alveoli, which also keeps the gradient steep.
Answer: Large surface area from millions of alveoli, thin walls for a short distance, a rich blood supply and ventilation to keep a steep concentration gradient
Common slips
Slip: The alveoli have thick walls. Thin walls give a short diffusion distance.
Slip: There are only a few alveoli. There are millions, which gives a large surface area.
Slip: Blood flow stops oxygen diffusing. Blood flow takes oxygen away and keeps the gradient steep.
Slip: Gases move into the alveoli by active transport. Gas exchange in the alveoli is by diffusion.
Questions and answers
Exchange surfaces and gas exchange in the lungs practice questions for GCSE Combined Science Biology. Try each one, then open the answer. Easy questions come first.
Question 1 Easy
What are the tiny air sacs in the lungs called?
Show answer
Answer: alveoli
They are where gases are exchanged.
They are the alveoli.
Question 2 Easy
Which gas diffuses from the alveoli into the blood?
Show answer
Answer: oxygen
The body needs oxygen for respiration.
Oxygen diffuses into the blood.
Question 3 Easy
Which gas diffuses from the blood into the alveoli?
Show answer
Answer: carbon dioxide
This is the waste gas from respiration.
Carbon dioxide diffuses out of the blood.
Question 4 Easy
What is the name of the finger-like projections in the small intestine?
Show answer
Answer: villi
They give a large surface area for absorption.
They are villi.
Question 5 Easy
What is the ratio that falls as an organism gets bigger called?
Show answer
Answer: surface area to volume ratio
Big organisms have a smaller surface compared with their volume.
This is the surface area to volume ratio.
Question 6 Medium
Calculate the surface area of a cube with a side of 2 cm. Give your answer in cm².
Show answer
Answer: 24 cm²
A cube has 6 faces.
Each face is 2 × 2 = 4 cm², so 6 × 4 = 24 cm².
Question 7 Medium
Calculate the volume of a cube with a side of 3 cm. Give your answer in cm³.
Show answer
Answer: 27 cm³
Volume = side × side × side.
3 × 3 × 3 = 27 cm³.
Question 8 Medium
A cube has a surface area of 54 cm² and a volume of 27 cm³. What is its surface area to volume ratio in the form n : 1?
Show answer
Answer: 2 : 1
Divide the surface area by the volume.
54 ÷ 27 = 2, so 2 : 1.
Exam-style questions
Longer questions in the style of a written paper, with the marking points. These are Yumio practice questions.
Exam-style question 14 marks
(a) Calculate the surface area to volume ratio of a cube with a side of 2 cm. [2 marks] (b) Explain why a large animal needs a transport system. [2 marks]
Show answer and marking points
Answer: (a) 3 : 1 (b) a large animal has a small surface area to volume ratio, so diffusion across its surface is too slow to reach every cell, so a transport system such as blood carries substances to all the cells
(a) 24 ÷ 8 (surface area 24, volume 8) (1)
(a) 3 : 1 (1)
(b) small surface area to volume ratio or long diffusion distance (1)
(b) diffusion is too slow so a transport system carries substances to all cells (1)
Exam-style question 24 marks
Explain how the alveoli are adapted for gas exchange. [4 marks]
Show answer and marking points
Answer: there are millions of alveoli which gives a large surface area, the walls are one cell thick so the diffusion distance is short, they have a rich blood supply which keeps a steep concentration gradient, and breathing brings in fresh air
large surface area because of many alveoli (1)
thin walls give a short diffusion distance (1)
rich blood supply from capillaries (1)
keeps a steep concentration gradient, or ventilation brings fresh air (1)
Common mistakes
Saying gas exchange happens in the bronchi or windpipe
It happens in the alveoli, which are the tiny air sacs.
Saying oxygen goes out of the blood into the alveoli
Oxygen diffuses into the blood. Carbon dioxide diffuses out.
Writing the ratio as volume to surface area
The ratio is surface area to volume.
Saying the surface area to volume ratio goes up as an organism gets bigger
It goes down as an organism gets bigger.
Saying the alveoli have thick walls
The walls are one cell thick.
Questions students ask
Is Exchange surfaces and gas exchange in the lungs on the Foundation or Higher tier?
Exchange surfaces and gas exchange in the lungs is on both Foundation and Higher papers for GCSE Combined Science Biology, aimed at around grade 5.
What is a common mistake on Exchange surfaces and gas exchange in the lungs?
Saying gas exchange happens in the bronchi or windpipe. It happens in the alveoli, which are the tiny air sacs.
Where are the answers to the Exchange surfaces and gas exchange in the lungs questions?
Open Show answer under each question on this page. The questions are Yumio practice questions, not past exam questions.