Photosynthesis

Nutrition in plants

Autotrophism

Autotrophism is the mode of nutrition used by green plants to manufacture their own food. The process by which green plants manufacture their food is known as photosynthesis.

Definition: Photosynthesis is the process in which green plants manufacture complex food substances from simple inorganic substances such as carbon dioxide and water in the presence of sunlight and chlorophyll.

This process primarily occurs in the green parts of the plant, most commonly in the leaves.

Structure of the leaf

External parts of the leaf

1. Petiole: The petiole is the stalk that attaches the leaf to the stem or branch of the plant.
2. Veins: Veins transport materials within the leaf, including water, nutrients, and food substances.
3. Midrib: The midrib is the central vein of the leaf that gives rise to smaller veins and transports materials to the leaf.
4. Lamina: The lamina, or leaf blade, is the flat, green part of the leaf where photosynthesis takes place.

External structure of a leaf

Internal parts of the leaf

The internal parts of the leaf consist of

1. Cuticle
2. Epidermis
3. Guard cell
4. Palisade layer
5. Spongy mesophyll layer
6. Leaf veins
7. Chloroplast

Internal structure of a leaf

Function of the internal parts of a leaf

1. Cuticle
   • Description: A thin, waxy, transparent, and waterproof layer found on both sides of the leaf.
   • Functions:
     • Reduces excessive water loss.
     • Protects inner tissues from damage.
     • Prevents the entry of pathogens.
2. Epidermis
   • Description: A thin protective layer found on both sides of the leaf.
   • Functions:
     • Secretes (produces) the cuticle.
     • Protects the inner tissue from damage.
3. Guard cells
   • Description: Bean-shaped epidermal cells found mostly on the lower surface of the leaf.
   • Functions:
     • Control the opening and closing of stomata.
     • Regulate water loss through transpiration.
     • Allow gaseous exchange (oxygen and carbon dioxide).
     • Contain chloroplasts with chlorophyll used to trap sunlight for photosynthesis.
4. Palisade layer
   • Description: Located between the upper epidermis and the spongy mesophyll layer, made up of palisade cells.
   • Functions:
     • Traps sunlight using chlorophyll for photosynthesis.
5. Spongy mesophyll layer
   • Description: Found between the palisade layer and the lower epidermis.
   • Functions:
     • Facilitates gaseous exchange.
     • Conducts photosynthesis.
6. Leaf veins
   • Description: Consist of both xylem and phloem.
   • Functions:
     • Xylem: Transports water and mineral salts from the roots to the rest of the plant.
     • Phloem: Transports manufactured food (sugars) from the leaves to the rest of the plant.
7. Chloroplasts
   • Description: Disc-shaped organelles containing chlorophyll.
   • Functions:
     • Absorb (trap) sunlight for photosynthesis.

Requirements and conditions necessary for photosynthesis

The raw materials necessary for photosynthesis are:

1. Carbon dioxide:
   • Obtained from the atmosphere.
2. Water:
   • Absorbed by the roots from the soil.

Conditions necessary for photosynthesis:

1. Chlorophyll:
   • Chlorophyll absorbs light energy needed for photosynthesis.
2. Sunlight:
   • Solar energy (sunlight) is converted into chemical energy (ATP) during photosynthesis, which is needed by all living organisms.

This summarizes the key functions of the leaf's internal parts and the necessary conditions for photosynthesis.

Process of photosynthesis

Photosynthesis occurs in two main stages:

1. The light stage
2. The dark stage

The light stage

1. Location: This stage occurs in the chloroplasts of plant cells.
2. Light absorption: Chlorophyll in the chloroplasts absorbs light energy from the sun.
3. Splitting of water molecules: The absorbed light energy is used to split water molecules (H₂O) into oxygen (O₂) and hydrogen (H₂).
4. Oxygen release: Oxygen is released into the atmosphere, and some is used in cellular respiration.
5. Hydrogen utilization: The hydrogen produced enters the dark stage of photosynthesis.
6. ATP formation: Some of the energy absorbed by chlorophyll is used to form an energy-rich compound called Adenosine Triphosphate (ATP), which is used in the dark stage of photosynthesis.

The dark stage

1. Location: This stage occurs in the stroma of the chloroplast, not requiring light.
2. Carbon dioxide fixation: In this stage, carbon dioxide (CO₂) combines with hydrogen to form sugars, such as glucose.
3. Energy requirement: This process requires the energy produced in the light stage (in the form of ATP) and specific enzymes to proceed.

End products of photosynthesis

The end products of photosynthesis are:

1. Glucose:
   • Used in respiration to release energy.
   • Some glucose is stored as starch or oils for later use.
2. Oxygen:
   • Some oxygen is used in respiration.
   • The rest is released into the atmosphere during gas exchange.
3. Water:
   • Some water is used in various chemical reactions within the cell.
   • Some water is released to the atmosphere during transpiration.

Overall chemical equation for photosynthesis

The general chemical equation representing photosynthesis is:

$6CO_2 + 6H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6O_2$

Where:

• $CO_2$ = Carbon dioxide
• $H_2O$ = Water
• $C_6H_{12}O_6$ = Glucose
• $O_2$ = Oxygen

Adaptation of the leaf in the process of photosynthesis

1. Cuticle and epidermis are transparent to allow easy penetration of light
2. Presence of stomata on the leaf allows easy diffusion of carbon dioxide
3. The network of veins allows movement of materials in and out of the leaf
4. The air spaces in the spongy mesophyll layer allow gases to circulate easily.
5. Presence of large numbers of chloroplasts in the palisade layer enables maximum sunlight absorption.
6. The leaves are well arranged to avoid overlapping and overshadowing
7. Broad and flat lamina allows maximum absorption of sunlight and carbon dioxide.

Factors affecting the rate of photosynthesis

1. Light intensity:
   • Light is essential for photosynthesis. A good quality of light enhances the rate of photosynthesis.
   • However, very bright light, such as intense sunlight, can damage the plant due to strong rays.
   • Plants that are under shade receive poor quality of light, reducing the rate of photosynthesis.
2. Carbon dioxide concentration:
   • An increase in carbon dioxide concentration boosts the rate of photosynthesis up to a certain level.
   • After reaching the maximum point, any further increase in carbon dioxide concentration induces the stomata to close, thereby cutting off the supply of carbon dioxide and reducing photosynthesis.
3. Temperature:
   • Photosynthesis is controlled by enzymes, which are affected by temperature.
   • High temperatures can destroy enzymes, while very low temperatures inactivate them.
   • The rate of photosynthesis increases as the temperature rises up to a maximum point, around 40°C.
4. Water:
   • Water is crucial for the various chemical reactions in plant cells and is also a raw material for photosynthesis.
5. Mineral salts:
   • Minerals like magnesium and iron are vital for the formation of chlorophyll, which is necessary for photosynthesis.
   • The availability of these minerals promotes the synthesis of chlorophyll, increasing the rate of photosynthesis.
6. Leaf age:
   • As a leaf ages, chlorophyll breaks down, which leads to a decrease in the rate of photosynthesis.
7. Presence of manufactured carbohydrates:
   • If carbohydrates (products of photosynthesis) are already present, they reduce the need for further photosynthesis, thereby lowering the rate of the process.

Importance of photosynthesis

1. Production of food substances:
   • Photosynthesis produces glucose, which serves as food for plants and animals.
2. Oxygen release:
   • Oxygen is released during photosynthesis, which is essential for the respiration of organisms.
3. Atmospheric purification:
   • Photosynthesis helps purify the atmosphere by removing carbon dioxide, a greenhouse gas.
4. Energy conversion:
   • Light energy is converted into a usable form, ATP (adenosine triphosphate), which can be used by all organisms for various energy-requiring processes.