A small sprig of plant material suitable for viewing chloroplasts. A variety of plants are suitable – most aquatic plants with fine leaves or the edges of a leaf of Elodea. The more fleshy moss species e.g. Mnium spp. also give good results.
Students should select a single leaf from the Elodea plant and place it on a microscope slide. Place a drop of water on the leaf and then gently lower a cover slip onto the slide
“Light energy is absorbed by the green pigment in chloroplasts”
Notes: Light energy is trapped by chlorophyll; a green pigment found in small organelles called chloroplasts. Parts of a plant that contain these chloroplasts can carry out photosynthesis because they can absorb the light energy for the reaction.
There is often confusion between chlorophyll and chloroplast. Simple models can clarify this e.g. a soft centred sweet with the sweet representing the chloroplast and the soft centre representing the chlorophyll (the “phyll”ing for the sweet).
Pupils can look at a thin leaf of either Elodea or a “leafy” moss such as Mnium under the microscope. This will allow them to see chloroplasts. If a bright light is directed from one side the chloroplasts may begin to move around within the cell. They can compare this with tissue e.g epidermal tissue in which there are no chloroplasts.
The Powerpoint includes some images and a link to a video clip for presentation if it is not possible for pupils observe chloroplasts moving themselves. You can show some images of the ultrastructure of the chloroplast and in particular point out the starch grains present inside the chloroplast. This should provide some evidence that the chloroplasts have a vital role in photosynthesis. Pupils can also take a thin section from a potato tuber and stain it with iodine to examine the starch grains. It is important to emphasise that the starch is stored in the tuber – perhaps once again making reference to the amazing capability of plants – taking carbon dioxide and water and eventually producing potato tissue – encourage them to think about this when they next eat their chips!
Show the pupils plants with a variegated leaves – explaining that the white areas of the leaf do not contain chloroplasts.
Ask pupils to make a prediction about which kinds of plants will grow faster – those with variegated or completely green leaves – and ask them to think about why this is.
The discussion needs to bring out the point that the non-green areas of the leaf will not make starch. This leads nicely into the next suggested activity, in which students look at a variegated geranium to investigate which areas of the leaf make starch.
If wished, you can challenge students to think about how a parasitic plant with very little chlorophyll can survive and grow by robbing a host plant of its nutrients. It appears that dodder is able to ‘smell out’ suitable host plants by detecting volatile chemicals in the air and growing towards them. This may well challenge many of students’ preconceptions about plants. There is a good video clip from The Private Life of Plants showing dodder finding a host plant available on the BBC’s website:
Links and references
This video clip shows the movement of chloroplasts in Elodea
This video clip (from the BBC series Botany: A Blooming History) introduces chloroplasts and the accumulation of starch grains, putting them in the context of their discovery by pioneering scientist Julius von Sachs in the 19th century. It includes some excellent footage of chloroplasts moving under the microscope.