Photosynthesis III - Leaf Adaptations & Uses of Glucose
By Joanna George-Johnson, Contributor

Students of Central High School, in Clarendon, tour The Gleaner's North Street offices recently. - Winston Sill/Freelance Photographer

In the previous lesson on photosynthesis, we saw that the leaf has to obtain certain materials at optimum levels in order to produce glucose at optimum levels. We also know that glucose is the organic product of photosynthesis. In the final instalment of our series on this topic, we will look at how the leaf is adapted to fulfil its requirement of raw materials for glucose production, as well as the fate of this glucose after it is produced.

Adaptations of the leaf

As a biology student, you will notice that on all levels of organisation, form follows function. What does this mean? It simply says that all biological systems are designed in such a way that their jobs are done as efficiently as possible. Plant leaves must be adapted to ensure that they make best use of limited resources. For example, atmospheric concentration of CO2 is as low as 0.03 per cent. CO2 is a requirement for photosynthesis, thus its absorption must be extremely efficient (See table below)

The Fate of Glucose after Photosynthesis

Glucose is produced in plants at a very high rate. However, testing a plant leaf for glucose usually reveals that there is little, if any, glucose present in plant leaves.

Several things may happen to the glucose after it is produced:

• The glucose may undergo respiration to release energy. We must realise that even the cells within the leaf require ATP. This is available by breaking the bonds within the glucose molecule, which releases energy for ATP production.
  
  
• It may be converted to starch for storage within the leaf. Glucose isn't stored for many reasons. It is very reactive, and might interfere with the cell metabolism. It is also 'osmotically active', that is, it dissolves in intracellular solution, changing its concentration. This might cause water to enter the cell, causing osmotic bursting.
  
  
• It may be used to make other organic substances. Glucose is not the only organic substances required by plants. Plants also require proteins for cell repair and cellulose for building cell walls. Some plants may even use lipids for storage. In a series of chemical reactions, glucose may be converted to these organics, often with the use of minerals obtained from soil water.
  
  
• It may be transported to other parts of the plant, either for use of storage; glucose cannot be transported directly. It must, therefore, be converted to a less reactive, but just as soluble substance. This carbohydrate is sucrose. The sucrose is loaded on to the phloem by active transport. It then travels by mass flow down the stem to wherever it is required. There, it is either converted to starch for storage or back to glucose for respiration.

Questions

• It is observed that plants in dim light have a deeper green coloration than others. Briefly explain why this is so.
  
  
• In some plants, the palisade cells are turned sideways. Explain why this is a necessary adaptation for plants that do not get much sunlight.

Joanna George-Johnson teaches at Ardenne High School masterbio@gmail.com.