Rate of diffusion ∝ (surface area × concentration difference) ÷ thickness of the exchange surface.

Calculate the SA:V of a cube of side 2 cm.

SA = 24 cm², V = 8 cm³, so the ratio is 3 : 1.

Why does an Amoeba not need a gas exchange system?

It has a large SA:V ratio and a short diffusion distance, so diffusion across its surface is enough.

Name the three properties of an efficient gas exchange surface.

Large surface area, thin (short diffusion distance), steep concentration gradient.

How is a steep concentration gradient maintained in the lung?

Ventilation brings fresh air; blood flow removes oxygen and brings carbon dioxide.

What type of cell forms the alveolar wall?

Squamous epithelial cells, one cell thick.

Approximately how many alveoli are in an adult lung?

About 300 million, giving a surface area of about 70 m².

What happens to the rate of diffusion if the thickness of the surface doubles?

It halves, if the other factors stay constant.

Why does pulmonary fibrosis reduce gas exchange?

Scar tissue thickens the alveolar wall, increasing the diffusion distance.

Why does emphysema reduce gas exchange?

Alveolar walls break down, giving fewer, larger air spaces and a smaller surface area.

Why does fluid in the alveoli reduce gas exchange?

It increases the diffusion distance and reduces the area in contact with air.

Why is the lung surface moist?

So gases can dissolve and cross the membrane.

Gas exchange surfacesEdexcel A-Level Biology A: Flashcards

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State Fick's law.

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State Fick's law.: Rate of diffusion ∝ (surface area × concentration difference) ÷ thickness of the exchange surface.
What happens to the surface area to volume ratio as an organism gets bigger?: It decreases.
Calculate the SA:V of a cube of side 2 cm.: SA = 24 cm², V = 8 cm³, so the ratio is 3 : 1.
Why does an Amoeba not need a gas exchange system?: It has a large SA:V ratio and a short diffusion distance, so diffusion across its surface is enough.
Name the three properties of an efficient gas exchange surface.: Large surface area, thin (short diffusion distance), steep concentration gradient.
How is a steep concentration gradient maintained in the lung?: Ventilation brings fresh air; blood flow removes oxygen and brings carbon dioxide.
What type of cell forms the alveolar wall?: Squamous epithelial cells, one cell thick.
Approximately how many alveoli are in an adult lung?: About 300 million, giving a surface area of about 70 m².
What happens to the rate of diffusion if the thickness of the surface doubles?: It halves, if the other factors stay constant.
Why does pulmonary fibrosis reduce gas exchange?: Scar tissue thickens the alveolar wall, increasing the diffusion distance.
Why does emphysema reduce gas exchange?: Alveolar walls break down, giving fewer, larger air spaces and a smaller surface area.
Why does fluid in the alveoli reduce gas exchange?: It increases the diffusion distance and reduces the area in contact with air.
Why is the lung surface moist?: So gases can dissolve and cross the membrane.

Exam questions on Gas exchange surfaces

Single-celled organisms such as Amoeba obtain oxygen by diffusion across their cell surface membrane, but large animals have specialised gas exchange surfaces such as lungs. Students use agar cubes to model how the surface area to volume ratio changes as an organism gets larger.
Explain why a large mammal needs a specialised gas exchange surface, but an Amoeba does not.2 marks
Fick's law states that the rate of diffusion is proportional to (surface area × concentration difference) ÷ thickness of the exchange surface. In a healthy adult the alveolar walls have a thickness of 0.5 μm. In a disease called pulmonary fibrosis, scar tissue thickens the alveolar walls.
During vigorous exercise, a person's breathing rate and heart rate both increase. Explain how this increases the rate of diffusion of oxygen into the blood.2 marks
A student models diffusion across an exchange surface using Fick's law, where rate of diffusion is proportional to (surface area × concentration difference) ÷ thickness. Surface A has an area of 12 cm², a concentration difference of 8 arbitrary units and a thickness of 0.2 mm. Surface B has an area of 12 cm², a concentration difference of 6 arbitrary units and a thickness of 0.1 mm.
Calculate the relative rate of diffusion for surface A and for surface B, and state how many times faster diffusion is across B than across A.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).