The rate of transpiration, which is the loss of water from the plant through the stomata, is directly affected by the opening and closing of the stomata. When stomata are open, transpiration is high, and when they are closed, transpiration is significantly reduced.

Stomatal Opening and Closing:

• Opening: Stomata open when guard cells take up water, increasing their turgor pressure and causing them to curve outwards, creating an opening. This is typically triggered by:

  • Light: Light stimulates the production of blue light receptors in guard cells, which promotes stomatal opening.
  • CO₂ Concentration: A low concentration of CO₂ inside the leaf stimulates stomatal opening.
  • Water Availability: When water is plentiful, the plant can afford to lose water through transpiration, so stomata are more likely to open.

• Closing: Stomata close when guard cells lose water, decreasing their turgor pressure and causing them to become flaccid, closing the opening. This is typically triggered by:

  • Lack of Light: In the absence of light, blue light receptors are inactive, and stomata close.
  • High CO₂ Concentration: A high concentration of CO₂ inside the leaf stimulates stomatal closure.
  • Water Stress: When water is scarce, the plant closes its stomata to reduce water loss. This is mediated by the hormone abscisic acid (ABA).

Therefore, the opening and closing of stomata are a crucial mechanism for balancing the need for CO₂ for photosynthesis with the need to conserve water.

Chlorophyll is the primary pigment in plants responsible for capturing light energy, which is the first crucial step in photosynthesis. It is located within the chloroplasts of plant cells, primarily in the palisade mesophyll layer of leaves.

When chlorophyll absorbs light energy, electrons within the chlorophyll molecule become energized. These energized electrons are then passed along an electron transport chain, a series of protein complexes embedded in the thylakoid membranes within the chloroplast. As electrons move down the chain, energy is released. This released energy is used to generate ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate), which are energy-carrying molecules.

ATP and NADPH are then used in the Calvin cycle (the 'dark reactions' of photosynthesis) to convert carbon dioxide into glucose. Therefore, chlorophyll's role is to capture the light energy that drives the entire photosynthetic process, ultimately enabling the plant to produce its own food.

Effect of Light Intensity: The rate of photosynthesis increases with increasing light intensity, up to a certain point. Beyond this point, the rate plateaus as other factors become limiting. This is because light energy is required for the light-dependent reactions of photosynthesis. More light energy allows for a faster rate of electron transport and ATP production, leading to increased carbon dioxide fixation.

Experimental Conditions:

• Use a plant (e.g., *Elodea*) in a test tube filled with a dilute sodium bicarbonate solution (to provide CO2).
• Vary the distance between a lamp (acting as the light source) and the test tube. This will control the light intensity. Use a ruler to ensure consistent distances.
• Maintain a constant temperature (e.g., using a water bath).
• Ensure the concentration of carbon dioxide in the solution is constant.
• Measure the rate of photosynthesis by counting the number of oxygen bubbles produced per minute.

Measuring the Rate of Photosynthesis: The rate of photosynthesis can be measured by counting the number of oxygen bubbles produced per minute. Oxygen is a byproduct of the light-dependent reactions. The more oxygen bubbles produced, the faster the rate of photosynthesis.