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Photosynthesis

ICSE Class 7 Biology • Chapter 5 (Detailed Master Notes)

Chapter Overview

All living organisms require food for energy, growth, and repair. While animals must consume other organisms, green plants possess the unique ability to manufacture their own food. This process, powered by sunlight, is called Photosynthesis. In this chapter, we will explore how plants convert simple raw materials from the environment into complex organic food.

5.1 Nutrition in Organisms

Nutrition is the process by which living organisms obtain and utilize food. There are two primary modes of nutrition:

Autotrophic Nutrition (Auto = self, trophon = nourishment): Organisms synthesize their own food from simple inorganic substances like carbon dioxide and water. Green plants are autotrophs (producers).
Heterotrophic Nutrition (Hetero = others): Organisms cannot prepare their own food and depend directly or indirectly on plants for nutrition. Animals, fungi, and humans are heterotrophs (consumers).

5.2 What is Photosynthesis?

Photosynthesis (Photo = light, Synthesis = putting together) is the biological process by which green plants containing chlorophyll synthesize simple sugars (glucose) from carbon dioxide and water, using the energy of sunlight. Oxygen gas is released as a byproduct.

The General Chemical Equation

$6CO_2 + 6H_2O \xrightarrow{\text{Sunlight}}_{\text{Chlorophyll}} C_6H_{12}O_6 (\text{Glucose}) + 6O_2 \uparrow$

(Carbon Dioxide + Water $\rightarrow$ Glucose + Oxygen)

5.3 Essential Requirements for Photosynthesis

For photosynthesis to occur, four essential factors must be present simultaneously:

Sunlight (Solar Energy): The ultimate source of energy. It is captured by the plant leaves. The intensity of light directly affects the rate of photosynthesis.
Chlorophyll: The green pigment present in structures called chloroplasts (mainly located in leaves). It acts as a solar panel, trapping light energy and converting it into chemical energy. Leaves are often called the "food factories" of the plant.
Carbon Dioxide ($CO_2$): Obtained from the air. It enters the leaves through tiny microscopic pores present on their surface called stomata (singular: stoma). Each stoma is guarded by two kidney-shaped guard cells that open and close the pore.
Water ($H_2O$) and Minerals: Absorbed from the soil by the root hair system. The water is transported upwards through the stem to the leaves by specific conductive tissues called Xylem vessels.

AI Image Prompt: A vibrant, detailed educational diagram of a green plant showing the process of photosynthesis. Sun rays pointing to the green leaves, arrows showing Carbon Dioxide entering the leaf, Oxygen leaving the leaf, and roots drawing water from the soil. A magnifying glass callout showing a stoma on the leaf surface and chloroplasts inside a plant cell.

5.4 Site of Photosynthesis: The Leaf structure

Leaves are specially adapted to maximize photosynthesis:

Broad Flat Surface (Lamina): Provides a large surface area to absorb maximum sunlight.
Thinness: Allows sunlight to penetrate easily and reach the inner cells, and allows rapid diffusion of gases ($CO_2$ in, $O_2$ out).
Stomata: Present mainly on the lower surface (underside) of the leaf to minimize water loss by transpiration, while allowing efficient gas exchange.
Veins (Vascular Bundles): Extensive network containing Xylem (brings water) and Phloem (transports manufactured food away).

5.5 Fate of the Manufactured Food

The immediate product of photosynthesis is a simple carbohydrate called Glucose. What happens to it?

Immediate Use: Some glucose is utilized immediately by the plant cells during respiration to produce energy for daily life activities (growth, repair).
Storage (Starch): Plants do not store food as glucose. Excess glucose is rapidly converted into an insoluble complex carbohydrate called Starch. This starch is stored temporarily in leaves or permanently in roots, stems, fruits, or tubers (like potatoes).
Transport (Phloem): The food must be transported to non-green parts of the plant (roots, stem, flowers). This transport of soluble food is called translocation and occurs through a vascular tissue called Phloem.

5.6 Significance of Photosynthesis

Photosynthesis is arguably the most important biological process on Earth:

Primary Source of Food: It provides food directly or indirectly for all living organisms. Herbivores eat plants, and carnivores eat herbivores. Without plants, the food chain would collapse.
Release of Oxygen: It is the only natural process that constantly adds oxygen to the atmosphere, which is essential for the survival and respiration of all animals and humans.
Maintaining Carbon Dioxide Levels: By absorbing $CO_2$ during photosynthesis, plants help regulate the levels of this greenhouse gas in the atmosphere, thereby helping to control global warming.

Practice Zone

Q1. During photosynthesis, what energy conversion takes place?

Answer: Light energy (solar energy) is converted into chemical energy (stored in the bonds of glucose).

Q2. How would you test a leaf for the presence of starch?

Answer: The presence of starch in a leaf can be tested using the Iodine test. First, the leaf is boiled in water to kill cells and then in alcohol to remove the green chlorophyll pigment (decolorization). The pale leaf is washed in warm water to soften it. Finally, a few drops of Iodine solution are added. If starch is present, the leaf will turn a distinct blue-black color.