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Badge icon "Arrow (6100)" provided by Cengiz SARI, from The Noun Project under Creative Commons - Attribution (CC BY 3.0)

If liquid or gaseous chemicals are left alone they have a tendency to spread out over time. You notice this most if you spray perfume in one corner of a room. Even though, at first, the scent is very strong in that corner the scent will slowly diffuse until the whole room is evenly scented. In cells diffusion works to move chemicals across cell membranes. To unlock this badge you must carry out these 2 sections.

  1. explain how. Complete all of the following to do this.
  • Define diffusion using the terms, concentration and net movement
  • Describe the process of diffusion in cells and include examples of some chemicals which move this way
  • Explain that there is a limit on the size of a cell due to the need for diffusion using the term, surface area to volume ratio.
  • Explain how concentration, temperature and diffusion distance can affect the rate of diffusion

2. carry out a ‘thought experiment’ on graph paper into Cell Growth: Surface area and how it restricts diffusion and therefore how big a cell can get.

The rules:

  1. Each cm2 of the cell needs 1 uL/min of glucose
  2. Each cm of the membrane can allow 2 uL/min of glucose to diffuse into the cell.

Question: What is the maximum possible size of the cell?

E.G. 1cm2  needs 1uL and gets 2uL therefore is a viable cell! Now try 2cm2 and so on.

(UL = microlitre)

Example of how to tackle the problem:





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