Nutrition – Cont’d

“A comfort zone is a beautiful place, but nothing ever grows there.” – Unknown

GOOD DAY, students. How are you this week? Have you been good, revising, studying, not spending too much time on the phone? I hope so! It means that you are prepared to leave your comfort zone!

This week we continue our study of plant nutrition. Do you remember the name that is given to plants based on their type of nutrition? You do, don’t you? Plants are known as autotrophs because they have the ability to manufacture their own food. You must also remember that the name of the process by which they manufacture their own food is photosynthesis, don’t you? This week, we will take a look at the process by which green plants manufacture their own food using carbon dioxide from the air, water from the soil, along with the sun’s energy to produce carbohydrates in the chloroplasts of leaves.

THE PROCESS

First, let us remind ourselves of what is required. These are:

1. The two raw materials which are two inorganic substances – carbon dioxide, CO2 and water, H2O.

2. The two conditions – chlorophyll and light.

The process occurs in a series of steps or reactions which take place in the chloroplasts of the palisade and spongy mesophyll cells. The reactions also occur in the guard cells, the only epidermal cells which contain chloroplasts. The reactions occur in two distinct stages:

1. A stage that is dependent on light energy supplied by the sun. This is known as the lightdependent stage or the light reaction.

2. A stage that can occur whether or not light is present and is, therefore, not dependent on light. This is known as the light-independent stage.

LIGHT-DEPENDENT STAGE/LIGHT REACTION

In this stage, the following happens:

1. Water (H2O) is split by light to produce oxygen (O2) and hydrogen (H+).

2. Energy is produced.

LIGHT-INDEPENDENT STAGE

In this stage, the following happens:

The H+ produced from the splitting of water is used in a series of reactions, resulting in the reduction of carbon dioxide to carbohydrate. The energy that is produced from the light stage provides the energy needed for the reactions that are carried out in this process.

Both the light-dependent and the light-independent reactions take place in the chloroplasts of the leaf. The process is summarised in the following equation that you are required to learn.

The photosynthesis equation in words and symbols:

WHAT HAPPENS TO THE PRODUCTS THAT ARE FORMED?

The first product formed is glucose (C6H12O6); you can see that from the equations above. The glucose produced may be:

1. Used as the substrate for respiration to produce energy for the plant’s activities.

2. Converted to starch and stored – Why is starch stored and not glucose? Glucose is soluble in water, so it would remain in solution in the cell sap within the cell’s vacuole. This would cause an osmotic gradient to develop between cells, causing some cells to lose water and some to gain. Starch is insoluble, so it would not form a solution and, hence, interfere with the concentration of the cell sap. Additionally, starch can form granules within the cells.

3. Used to make other organic substances, e.g., sucrose, cellulose, and as a starting point for the formation of amino acids.

4. Can be converted to sucrose to be transported to other areas of the plant for use or converted to starch for storage in the storage organs.

5. The oxygen that is formed goes to replace the oxygen that is used in both animal and plant respiration, thus maintaining the oxygen concentration in the atmosphere.

The above covers as much of the process that you will need to know for your CSEC examination. Is that all for photosynthesis? Definitely not! We cannot complete the study without looking at the photosynthesis experiments. Many of the examination questions that are set on the topic are set on these experiments. This is especially true for the multiple-choice exam.

The experiments are designed so that the conditions and the raw materials can be determined. There are certain things that are common to all of them. You will need to know:

1. How to destarch a plant?

2. How to prepare and test a leaf for the presence of starch?

Why do we test for starch? Remember, the end product of photosynthesis is glucose, and this glucose is converted to starch for storage. What do we use to test for starch? We use iodine because iodine is the test reagent for starch. If iodine is added to starch, the colour changes from straw yellow to blue-black.

Next week, we will look at the process used to destarch a plant so that its leaf can be tested for starch formed in photosynthesis. See you then!