4.1 - Surface area to volume ratio

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Last updated 4:27 PM on 8/24/26
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15 Terms

1
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What is the relationship between an organism’s size & their SA:V ratio?

  • As organisms get bigger, their surface area : volume ratio gets smaller

  • Meaning, it becomes more difficult for them to exchange materials with their surroundings


2
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How does SA:V ratio affect transport of molecules?

  • The smaller the SA:V ratio, the further the distance molecules need to travel to reach all parts of the organism

  • So diffusion alone is not sufficient


3
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Why do larger organisms need a mass transport system but smaller organisms do not?

  • Larger organisms have a smaller SA:V ratio, so some cells are far from the surface

  • Diffusion alone cannot deliver oxygen, glucose & nutrients to cells


4
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What 3 features do larger organisms require to survive because they can’t rely on diffusion?

  1. Multicellular: their bodies are composed of many small cells, rather than one big cell

  2. Mass transport system: uses energy (from ATP) to pump nutrient solutions quickly around large bodies

  3. Specialised exchange system: with a large surface area


5
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What is Fick’s law?

Rate of diffusion is proportional to (surface area x concentration difference) / distance

<p>Rate of diffusion is proportional to (surface area x concentration difference) / distance </p>
6
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What does Fick’s law tell us exchange surfaces must have in order to support a fast rate of diffusion?

  • A large surface area

  • A small distance between the source & the destination

  • A mechanism to maintain a high concentration gradient across the gas exchange surface


7
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Where does gas exchange take place in humans & how does this surface meet the 3 requirements of Fick’s law?

On the alveoli inside the lungs:

  • Large surface area - although each alveolus is tiny, an average adult has about 600 million alveoli

  • Small distance between the source & the destination - walls of the alveoli are one cell thick, minimising diffusion distance

  • A mechanism to maintain a high concentration gradient - oxygen always diffuses down its concentration gradient (from air → blood), while simultaneously carbon dioxide can diffuse down its concentration gradient (from blood → air)


8
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How to work out the surface area of a cube/cuboid?

  • Work out the area of each face

  • Add the six areas together

  • Include the units


<ul><li><p>Work out the area of each face</p></li><li><p>Add the six areas together</p></li><li><p>Include the units</p></li></ul><p></p>
9
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What is the equation to calculate the surface area of a sphere?

SA = 4πr²

<p>SA = 4πr²</p>
10
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What is the equation to calculate the surface area of a cylinder?

SA = 2πr² + 2πrh

<p>SA = 2πr² + 2πrh</p>
11
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What is the equation to calculate the volume of a cube/cuboid?

Volume = length x width x height

<p>Volume = length x width x height</p>
12
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What is the equation to calculate the volume of a cylinder?

Volume = πr²h

<p>Volume = πr²h</p>
13
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What is the equation to calculate the volume of a sphere?

Volume = (4/3)πr³

<p>Volume = (4/3)πr³</p>
14
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What is the equation to calculate the circumference of a circle?

C = 2 x π x r

or

C = π × d

<p>C = 2 x π x r</p><p>or</p><p>C = π × d</p>
15
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How is the surface area to volume ratio calculated?

By dividing the surface area by the volume of the object (does not have any units)