Photosynthesis in Plants: Process and Factors

Posted on Mar 2, 2025 in Biology

Plants use water from the soil, carbon dioxide from the air, and energy from sunlight to make carbohydrates & oxygen.

Green plants absorb light energy using chlorophyll in their leaves. They use it to react carbon dioxide with water to make a sugar called glucose. The glucose is used in respiration, or converted into starch and stored. Oxygen is produced as a by-product.

Balanced Equation for Photosynthesis

6CO₂ + 6H₂O ^light/_{(chlorophyll)} C₆H₁₂O₆ + 6O₂

Investigating the Necessity for Chlorophyll, Light, and Carbon Dioxide for Photosynthesis

Without enough light, a plant cannot photosynthesize very quickly, even if there is plenty of water and carbon dioxide.

Chlorophyll

Photosynthesis occurs in chloroplasts, which are located in the palisade mesophyll tissue of leaves. Chlorophyll is the pigment that absorbs light energy that is used for photosynthesis. Chloroplasts can move around inside cells. Chloroplasts need a good supply of water, carbon dioxide, and sunlight in order to carry out photosynthesis efficiently.

• A destarched variegated plant was left in the sunlight for four hours.
• A leaf is removed and tested for starch.
• The green parts of the leaf contain starch; the white do not.
• This suggests that chlorophyll is needed for starch production.

Light

A leaf usually has a large surface area, so that it can absorb a lot of light. Its top surface is protected from water loss, disease, and weather damage by a waxy layer. The upper part of the leaf is where the light falls, and it contains a type of cell called a palisade cell. This is adapted to absorb a lot of light.

It has lots of chloroplasts and is shaped like a tall box.

Testing a Leaf for Starch

If you want to see if starch has been produced in a plant, you have to start with a plant that does not have starch.

1. Test the plant for starch.
2. Place the plant in the dark for 2 or 3 days.
3. Test again for starch.

Using Iodine to Test for the Presence of Starch

Iodine solution (an amber-colored liquid) turns dark (blue/black) in the presence of starch. If leaves are to be tested for the presence of starch, the cell walls in the leaf must be broken, and the chlorophyll removed. The cell walls must be broken to make it easier for substances to leave or enter the cell. The chlorophyll must be removed so that the color change in the iodine can be seen.

Light

Showing How Light is Needed to Make Starch

1. Two leaves of a de-starched plant are to be tested for the presence of starch.
2. One is completely covered with aluminum foil, the other with clear plastic wrap.
3. The plant is put in the sun for 4 hours, and both leaves are tested for starch.

The leaf in the plastic contains starch.

Roots

The water absorbed by the root hair cells passes through the plant in xylem tubes, and eventually reaches the leaves. If a plant does not absorb enough water, it will wilt or go floppy. Without water, it may also not photosynthesize quickly enough, and it may die.

Factors Affecting the Rate of Photosynthesis:

• Light
• Temperature
• CO₂

Factors Affecting the Rate of Transpiration:

• Temperature
• Humidity
• Air movement
• Light intensity

Xylem vessels carry water and minerals from the roots to the leaves.

Phloem tubes carry sugar & other organic nutrients made by the plant from the leaves to the rest of the plant.