ZIMSEC O Level Combined Science: The test for starch

The requirements of photosynthesis

  • As already mentioned the food making process in plants is called photosynthesis.
  • In order for a plant to make food (photosynthesise) it needs to absorb raw materials from the environment.
  • These raw materials are water from the soil and carbon dioxide from the air.
  • In turn, the plant produces outputs/by-products during the photosynthesis process.
  • These are soluble carbohydrates in the form of sugars or starch and oxygen.
  • The plant also needs a source of energy so as to be able to convert these raw materials into outputs.
  • This energy comes in the form of light energy from the sun also known as solar energy.
  • Chlorophyll aids this process by trapping sunlight.
  • Without one of the raw materials, or chlorophyll, or a source of light, the process of photosynthesis cannot take place.
  • The presence of starch in a leaf during the starch test can be used to prove that a plant is photosynthesising.
  • To show that a plant is photosynthesizing a starch test is carried out and if one of its leaves has starch then it is photosynthesising.
  • If none of its leaves has starch then the plant is not photosynthesising.
  • There are several reasons for this:
  • It could be that the plant does not have any one or more of the inputs required to photosynthesise as mentioned above or,
  • It could be producing another compound instead of starch, for example, grasses make sugars.
  • Or it could be diseased.

The control

  • In experiments to test for factors necessary for photosynthesis, one of the plants will have all the conditions necessary for photosynthesis.
  • This plant is known as the control.
  • A lot of experiments make use of the control concept.
  • With a control the experiment is done in almost the same way, but without the variable that you are testing for, so you can compare the results.
  • For example, if you are testing for the effect of light on photosynthesis one plant is kept in the dark while the other is left in the light. The one left in the light is the control as we are not changing any of its variables.

To access more topics go to the Combined Science Notes page.