Carry out investigations related to photosynthesis (conditions necessary for photosynthesis)

Before setting up investigations, you need to know what conditions affect photosynthesis:

• Sunlight – provides the energy needed
• Carbon dioxide – provides carbon for making glucose
• Water – provides hydrogen for making glucose
• Chlorophyll – the green pigment that traps light energy

These are the four main factors that scientists investigate in photosynthesis experiments.

A good investigation follows the scientific method:

1. Identify the problem – Ask a question, for example: "Does light affect photosynthesis?"
2. Form a hypothesis – Make a prediction, for example: "If a plant receives no light, it will not perform photosynthesis."
3. Design the experiment – Set up a fair test with a control group.
4. Collect data – Observe and record what happens.
5. Analyze results – Look for patterns in your data.
6. Draw a conclusion – State whether your hypothesis was supported or rejected.

Variables in an Experiment

To conduct a fair test, you must identify three types of variables:

• Independent variable – the factor you change (e.g., amount of light)
• Dependent variable – the factor you measure (e.g., amount of oxygen produced)
• Controlled variables – factors you keep the same (e.g., amount of water, temperature, carbon dioxide)

Problem

Does a plant need light to carry out photosynthesis?

Hypothesis

If a leaf is kept in darkness, it will not produce starch (food), so photosynthesis will not occur.

Method

1. Select two similar leaves from the same healthy plant.
2. Boil one leaf in water for 2 minutes to stop enzyme activity (this is called killing the leaf).
3. Boil the same leaf in ethanol (or place in hot water bath with ethanol) to remove chlorophyll – this is called destarching.
4. Place the destarched leaf in a beaker with water and expose it to sunlight for 4–6 hours.
5. Keep a second leaf from the same plant in a dark cupboard for the same time period (this is your control).
6. After the experiment, test both leaves for starch using iodine solution.

Expected Results

Conclusion

The hypothesis is accepted. Light is necessary for photosynthesis because starch was only produced in the leaf exposed to sunlight.

1. Testing for Carbon Dioxide Requirement

• Set up two identical aquatic plants (like Cabomba or Hydrilla) in test tubes with water.
• Add sodium bicarbonate (a source of CO₂) to one tube but not the other.
• Place both in bright light.
• Collect and measure the oxygen bubbles produced.
• The tube with CO₂ produces more oxygen, showing CO₂ is needed.

2. Testing for Chlorophyll Requirement

• Use a variegated leaf (a leaf with green and white parts, like in Coleus).
• Test for starch in both the green and white areas using iodine.
• Only the green areas contain starch, showing chlorophyll is necessary.

3. Testing for Water Requirement

• Keep a potted plant well-watered and another without water for several days.
• Test leaves from both plants for starch.
• The watered plant shows starch; the dry plant does not.

• Always use a control experiment for comparison.
• Record your observations carefully in a table.
• Repeat your experiment to ensure reliable results.
• Handle chemicals like iodine and ethanol with care – they can cause burns.
• Never eat or drink any materials used in experiments.

In Tanzania, local farmers can use the knowledge of photosynthesis to improve their sukuma wiki (kale) or tomato farming. By understanding that plants need sunlight, water, and carbon dioxide, a farmer in Arusha or Dar es Salaam can decide where to place their nursery beds – choosing open areas that receive full sunlight rather than shaded areas near buildings or trees – to ensure their vegetables grow healthy and produce good yields for sale at the masoko (market).