Edexcel IGCSE Biology 复习笔记 2.4.2 Practical: Diffusion

Edexcel IGCSE Biology 复习笔记 2.4.2 Practical: Diffusion

Practical: Factors that Influence Diffusion

 

  • Diffusion is the movement of molecules from a region of higher concentration to a region of lower concentration
  • The rate of diffusion is influenced by several factors:
    • Temperature
    • Surface area
    • Concentration gradient
    • Diffusion distance

     

  • You can investigate how temperature affects diffusion using beetroot
    • Beetroot cells contain a dark purple-red pigment
    • Heating above 45℃ can damage the cell membrane meaning that the pigment can leak out
    • The speed at which this pigment leaks out of the cell tells us about the rate of diffusion

     

 

 

 

Investigating the effect of temperature on diffusion

 

Apparatus

  • Beetroot
  • Knife
  • Cork borer (optional)
  • Cutting board
  • Ruler
  • Test tubes
  • Water baths
  • Stopwatch

 

 

 

Method

  • Using a knife, cut 2 equally-sized cubes of beetroot
    • The pieces must have the same dimensions so that they all have equal surface areas and volumes, as these factors could affect the rate at which the pigment leaks out
    • A cork borer can also be used, as long as the cores are cut to the same length

     

  • Rinse the beetroot pieces
    • To remove any pigment released during cutting

     

  • Put 5 cm3 of water into 2 test tubes labelled A and B
  • Keep test tube A at room temperature and transfer test tube B to a hot water bath at 90℃
  • Leave the test tubes for 2 minutes, then add a piece of beetroot into each test tube
  • After 10 minutes, observe the colour of the liquid in both test tubes

 

 

 

Results and Analysis

  • You should notice that at the higher temperaturemore of the pigment has leaked out of the beetroot
  • This is because:
    • The cell membrane of the beetroot cells has become damaged so more pigment can leak out
    • At higher temperatures, particles have more kinetic energy, this results in the faster movement of particles compared to when they have less energy

     

 

 

 

Beetroot-and-diffusion-1Beetroot-and-diffusion-2

 

Investigating the effect of temperature on diffusion in beetroot

 

 

 

Limitations

  • The beetroot pieces may not be identical in size and shape, meaning one test tube could contain slightly more beetroot tissue than the other
    • Solution: cut the beetroot as accurately as possible using a knife and ruler, and repeat each investigation several times to find a mean

     

  • Some parts of beetroot tissue could have more pigment in their cells than others
    • Solution: conduct several repeats, using different parts of the beetroot and find a mean

     

  • Our results would be more reliable if we tested a range of temperatures rather than just testing 2
    • Solution: Set up 5 test tubes in water baths at different temperatures (e.g. 10℃, 20℃, 30℃, 40℃, 50℃)

     

  • Observing the colour is a subjective measure which means it is difficult to really compare the differences in diffusion between the test tubes
    • Solution: use a colorimeter to measure how much light is absorbed as it passes through each of the five samples of coloured liquid

     

 

 

Applying CORMS to practical work

  • When working with practical investigations, remember to consider your CORMS evaluation

 

CORMS-evaluation

 

CORMS evaluation

 

 

 

  • In this investigation, your evaluation should look something like this:
    • C - We are changing the temperature of the environment
    • O - The beetroot cubes will all be taken from the same beetroot or beetroot of the same age
    • R - We will repeat the investigation several times to ensure our results are reliable
    • M1 - We will observe the colour change of the liquid
    • M2 - ...after 10 minutes
    • S - We will control the volume of water used, the dimensions of the beetroot cubes and each cube must be blotted before it is weighed each time

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