11.1. What do we Know?

All animals including human beings depend on plants for their food. Have you ever wondered from where plants get their food?

All living organisms, including humans, depend on plants as a source of food. While animals obtain their energy and nutrients from consuming plants or other animals, plants themselves generate their food through a unique process. This underscores the interdependence within ecosystems, where plants form the foundation of the food chain.

Think of plants as chefs in a restaurant. Just as chefs prepare meals for diners, plants produce food through photosynthesis. Without these 'chefs,' no one would have meals to enjoy, illustrating the vital role that plants play in supporting life.

Green plants have to make or rather synthesise the food they need and all other organisms depend on them for their needs. The green plants make or rather synthesise the food they need through photosynthesis and are therefore called autotrophs.

Plants, classified as autotrophs, have the ability to produce their own food through the process of photosynthesis. This process involves using sunlight, carbon dioxide, and water to create glucose, which serves as energy for the plant. In contrast, organisms that cannot make their own food and rely on others for nutrition are known as heterotrophs.

Imagine a factory producing goods. In this case, the factory represents a plant that creates its own products (food). Meanwhile, people who buy these goods from the factory represent heterotrophs, who depend on the factory for their supplies.

You have already learnt that autotrophic nutrition is found only in plants and all other organisms that depend on the green plants for food are heterotrophs. Green plants carry out ‘photosynthesis’, a physico-chemical process by which they use light energy to drive the synthesis of organic compounds.

Photosynthesis is a fundamental process performed by green plants that allows them to convert light energy into chemical energy stored in glucose. This process involves the absorption of light by chlorophyll, leading to a series of reactions that convert carbon dioxide and water into glucose and oxygen.

Consider a solar panel. Just as it converts sunlight into electricity that powers our homes, plants convert sunlight into chemical energy that fuels their growth and survival through photosynthesis.

Ultimately, all living forms on earth depend on sunlight for energy. The use of energy from sunlight by plants doing photosynthesis is the basis of life on earth. Photosynthesis is important due to two reasons: it is the primary source of all food on earth. It is also responsible for the release of oxygen into the atmosphere by green plants.

Photosynthesis is crucial for life on Earth as it provides the primary source of energy for nearly all organisms through food production. Moreover, it generates oxygen, which is essential for the survival of aerobic organisms, including humans. Without photosynthesis, the ecosystem would collapse, leading to a lack of food and oxygen.

Think of the sun as the ultimate power source for a city. Just as a city relies on electricity (from a power plant) for light, appliances, and productivity, all life on Earth relies on the sun as the primary energy source through the process of photosynthesis.

Some simple experiments you may have done in earlier classes have shown that chlorophyll (green pigment of the leaf), light and CO2 are required for photosynthesis to occur.

Experiments in classrooms have demonstrated the necessity of chlorophyll, light, and carbon dioxide for photosynthesis to happen. These experiments often show that green parts of plants (where chlorophyll is found) are essential for food production, indicating that without these elements, photosynthesis cannot occur.

Consider baking a cake. To make a cake, you need specific ingredients (like flour, sugar, and eggs). If any ingredient is missing, you simply can't bake. Similarly, for photosynthesis, the absence of chlorophyll, light, or carbon dioxide means plants can't produce their food.

You may have carried out the experiment to look for starch formation in two leaves – a variegated leaf or a leaf that was partially covered with black paper, and exposed to light. On testing these leaves for the presence of starch, it was clear that photosynthesis occurred only in the green parts of the leaves in the presence of light.

In this experiment, students typically observe that only the green parts of leaves produce starch when exposed to sunlight. This highlights that chlorophyll is essential for photosynthesis since starch is a product of this process. When parts of the leaf are covered and not exposed to light, starch formation does not occur in those areas.

This can be compared to a person trying to bake a cake by only mixing the ingredients but not turning on the oven. Without baking (or light), the cake (or starch) cannot form, illustrating the need for specific conditions for photosynthesis to occur.

Another experiment you may have carried out where a part of a leaf is enclosed in a test tube containing some KOH soaked cotton (which absorbs CO2), while the other half is exposed to air. The setup is then placed in light for some time. On testing for the presence of starch later in the two parts of the leaf, you must have found that the exposed part of the leaf tested positive for starch while the portion that was in the tube tested negative. This showed that CO2 was required for photosynthesis.

In this experiment, students set up a leaf to test for starch presence. The part of the leaf exposed to air (and thus CO2) produced starch, while the covered part did not. This experiment demonstrates the requirement of carbon dioxide for the photosynthesis process, reinforcing the earlier points regarding photosynthesis essentials.

Imagine a plant as a factory that needs all components to produce a product. If one essential component (like carbon dioxide in this case) is missing, the factory cannot produce goods, demonstrating how crucial each element of photosynthesis is for the plant's survival.

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