Notes on Photosynthesis - How Plants Make Food

1. Food Requirement in Plants

- Like animals, plants need food for energy to carry out various metabolic activities.

- Unlike animals, plants do not move in search of food. Instead, they make their own food.

2. Autotrophic Nutrition

- Green plants are autotrophs-they can synthesize their own food.

- This process is called photosynthesis.

3. Meaning of Photosynthesis

- The word 'Photosynthesis' comes from:

- Photo = Light

- Synthesis = To build

- So, it means building up by light.

4. Raw Materials for Photosynthesis

- Carbon dioxide (CO) - absorbed from the air through tiny pores in leaves called stomata.

- Water (HO) - absorbed from the soil by roots and transported to leaves via stem.

- Sunlight - provides the energy needed for the reaction.

- Chlorophyll - a green pigment in chloroplasts of leaf cells that captures sunlight.

5. Location of Photosynthesis

- Takes place mainly in the green leaves of plants.

- Green leaves contain chloroplasts, which have chlorophyll.

6. Photosynthesis Equation

- Carbon dioxide + Water + Sunlight energy Glucose + Oxygen (In the presence of chlorophyll)

7. Products of Photosynthesis

- Glucose - a simple sugar used by the plant for energy and growth.

- Oxygen - released as a by-product into the air.

8. Fate of Glucose

- Glucose may be:

- Used immediately by the plant.

- Converted into starch and stored in leaves and other parts.

- Glucose belongs to a group of food called carbohydrates.

- The carbohydrates contain chemical energy stored from sunlight.

9. Importance of Photosynthesis

- Photosynthesis helps plants grow.

- It converts sunlight energy into chemical energy.

- The food prepared by plants also provides energy to animals and humans who consume plants.

- During respiration, the stored energy in food is released in our body.

 Modes of Nutrition in Plants

Plants generally have two types of modes of nutrition:

1. Autotrophic Nutrition: In this type of nutrition, plants prepare their own food using simple raw

materials like carbon dioxide (CO2) from the air and water (H2O) from the soil. The process through

which plants synthesize their own food is called photosynthesis. In this process, plants convert solar

energy into chemical energy, stored in the form of glucose.

2. Heterotrophic Nutrition: Some plants cannot prepare their own food and depend on other

organisms for nutrition. This mode of nutrition is called heterotrophic nutrition. Examples include

carnivorous plants (like the Venus flytrap), which obtain nutrients by capturing and digesting insects.
Steps Involved in Photosynthesis
Photosynthesis consists of two main phases: the light-dependent reactions and the

light-independent reactions (Calvin Cycle).

A. Light-Dependent Reactions

These reactions occur in the thylakoid membranes of the chloroplasts. They require sunlight to

proceed. The steps involved are:

1. Absorption of light: Chlorophyll absorbs sunlight and becomes excited, which leads to the splitting

of water molecules (photolysis) into oxygen (O2), protons (H+), and electrons (e-).

2. Formation of ATP and NADPH: The energy from excited electrons is used to form ATP

(adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate), which are

energy carriers for the next stage of photosynthesis.

B. Light-Independent Reactions (Calvin Cycle)

These reactions occur in the stroma of the chloroplasts, and do not require light. The steps involved

are:

1. Carbon fixation: The enzyme RuBisCO helps incorporate carbon dioxide (CO2) from the air into a

5-carbon sugar called ribulose bisphosphate (RuBP), forming an unstable 6-carbon compound that

immediately breaks into two molecules of 3-phosphoglycerate (3-PGA).

2. Reduction: ATP and NADPH produced in the light-dependent reactions are used to reduce

3-PGA into glyceraldehyde-3-phosphate (G3P), a 3-carbon sugar.

3. Regeneration of RuBP: Some of the G3P molecules are used to regenerate RuBP, enabling the

cycle to continue. The remaining G3P molecules are used to form glucose and other organic

molecules.
 Conditions Necessary for Photosynthesis

1. Sunlight

Provides the energy required for the process. The chlorophyll absorbs sunlight, which is crucial for

the light-dependent reactions.

2. Water

Water is necessary as it provides hydrogen ions (H+) and electrons during the light-dependent

reactions. It also helps in the transport of nutrients within the plant.

3. Carbon Dioxide (CO2)

CO2 is absorbed from the air through the stomata. It is required for the Calvin Cycle to fix carbon

and produce glucose.

4. Chlorophyll

This green pigment absorbs sunlight and plays a key role in the energy conversion process during

photosynthesis.

5. Appropriate Temperature

Photosynthesis typically occurs most efficiently at moderate temperatures (around 25°C), although it

can occur at a wide range of temperatures. Extremely high temperatures can reduce the efficiency

of the process.
 Experiments to Show the Necessity of Factors for Photosynthesis

1. Experiment: To Show that Sunlight is Necessary for Photosynthesis

Objective:

To prove that sunlight is necessary for the process of photosynthesis.

Materials Required:

- A healthy potted plant (e.g., money plant or china rose)

- Black paper or foil

- Clips or cellotape

- Beaker

- Alcohol (ethanol)

- Iodine solution

- Water

- Burner or hot water bath

- Forceps or tongs

Procedure:

1. Keep the plant in complete darkness for 2-3 days to destarch it.

2. Cover part of a leaf with black paper on both sides.

3. Place the plant in sunlight for 4-6 hours.

4. Pluck the leaf, boil it in water to kill it.

5. Boil the leaf in alcohol in a hot water bath to remove chlorophyll.

6. Rinse the leaf in warm water and pour iodine solution on it.

Observation:

- Exposed part turns blue-black (starch present).

- Covered part remains brown (no starch).

Conclusion:

Sunlight is necessary for photosynthesis.

 Experiments to Show the Necessity of Factors for Photosynthesis

2. Experiment: To Show that Carbon Dioxide is Necessary for Photosynthesis

Objective:

To prove that carbon dioxide is necessary for photosynthesis.

Materials Required:

- Two healthy potted plants

- Two bell jars

- Potassium hydroxide (KOH) solution

- Corks with test tube holders

- Vaseline

- Alcohol, iodine solution, water, burner

Procedure:

1. Destarch both plants by keeping them in darkness for 2-3 days.

2. Place each in a separate bell jar. Put KOH in one to absorb CO2; water in the other.

3. Seal jars with vaseline. Place both in sunlight for 4-6 hours.

4. Pluck one leaf from each plant. Test with iodine after boiling and alcohol steps.

Observation:

- Leaf from CO2-available plant turns blue-black.

- Leaf from CO2-absorbed plant does not change.

Conclusion:

Carbon dioxide is necessary for photosynthesis.

3. Experiment: To Show that Water is Necessary for Photosynthesis

Objective:

To prove that water is required for photosynthesis.

 Experiments to Show the Necessity of Factors for Photosynthesis

Materials Required:

- Two similar potted plants (one watered, one dry)

- Sunlight

- Alcohol, boiling water, iodine solution, burner

Procedure:

1. Water one plant, leave the other dry for 2-3 days.

2. Keep both in sunlight for 4-6 hours.

3. Pluck a leaf from each. Test with boiling, alcohol, and iodine.

Observation:

- Leaf from watered plant turns blue-black.

- Leaf from dry plant does not change.

Conclusion:

Water is essential for photosynthesis.

4. Experiment: To Show that Chlorophyll is Necessary for Photosynthesis

Objective:

To prove that chlorophyll is required for photosynthesis.

Materials Required:

- A variegated leaf plant (e.g., croton)

- Sunlight

- Alcohol, boiling water, iodine solution, burner

Procedure:

1. Destarch the plant by keeping it in darkness for 2-3 days.

2. Expose it to sunlight for 4-6 hours.

3. Pluck a variegated leaf. Mark green and white areas.

 Experiments to Show the Necessity of Factors for Photosynthesis

4. Boil the leaf in water, then in alcohol, and test with iodine.

Observation:

- Green parts turn blue-black.

- White parts do not change color.

Conclusion:

Chlorophyll is necessary for photosynthesis.

 How Plants Obtain Raw Materials for Photosynthesis

Carbon Dioxide:

Plants absorb carbon dioxide from the atmosphere through small openings on their leaves called

stomata. The stomata are typically found on the underside of leaves and are surrounded by guard

cells that regulate their opening and closing. Carbon dioxide enters the leaf cells through the

stomata and is then used in the Calvin cycle to produce glucose.

In aquatic plants, such as hydrophytes, carbon dioxide is absorbed from the water in the form of

bicarbonates. These bicarbonates are then converted into carbon dioxide, which is used for

photosynthesis.

Water:

Plants absorb water from the soil through their roots. The water is transported to the leaves through

the xylem, a type of vascular tissue. The water is then used in the photosynthetic process to

produce glucose and oxygen.

The importance of water in photosynthesis was demonstrated by experiments showing that oxygen

evolved during photosynthesis comes from water, not carbon dioxide.

Key Processes:

- Stomatal uptake: Carbon dioxide enters the leaf cells through stomata.

- Root absorption: Water is absorbed by the roots from the soil.

- Xylem transport: Water is transported to the leaves through the xylem.

- Calvin cycle: Carbon dioxide is fixed into glucose using energy from ATP and NADPH produced in

the light-dependent reactions.

 Site of Photosynthesis

- Chloroplasts are the organelles in green plant cells that contain chlorophyll and are the site of

photosynthesis.

- These chloroplasts are primarily located in the photosynthetic (mesophyll) cells of a green leaf,

especially in the palisade tissue just below the upper epidermis.

- In a cross-section of a leaf, chloroplasts appear as numerous disc-like structures inside the

mesophyll cells and can be observed using a light microscope.

- The mesophyll layer in a leaf contains cells that are rich in chlorophyll, making them highly

specialized for photosynthesis.

- A typical photosynthetic cell in green leaves may contain 100 or more chloroplasts, and an entire

leaf may have thousands of such cells.

- Carbon dioxide (CO2) required for photosynthesis enters the leaf through stomata and diffuses into

the mesophyll cells, eventually reaching the chloroplasts.

- Water (H2O) is transported to the leaf by xylem vessels and enters the mesophyll cells by

diffusion, reaching the chloroplasts.

- The leaf is covered by a waxy protective layer called the cuticle on both the upper and lower

surfaces, which helps to minimize water loss.