11.3. Where does Photosynthesis Take Place?

You would of course answer: in ‘the green leaf’ or ‘in the chloroplasts’, based on what you earlier read in Chapter 8. You are definitely right. Photosynthesis does take place in the green leaves of plants but it does so also in other green parts of the plants. Can you name some other parts where you think photosynthesis may occur?

Photosynthesis primarily occurs in the green parts of plants, most notably in the leaves. However, it is not limited to leaves alone. Other green parts of the plant, such as stems and unripe fruits, can also carry out photosynthesis. This is because these parts contain chlorophyll, the green pigment essential for the process, allowing them to capture light energy.

Think of a plant like a solar panel that harvests sunlight to produce energy. Just as a solar panel can work on rooftops as well as on the walls of buildings, a plant can use its chlorophyll-containing tissues not just in its leaves but through out its green surfaces.

You would recollect from previous unit that the mesophyll cells in the leaves, have a large number of chloroplasts. Usually the chloroplasts align themselves along the walls of the mesophyll cells, such that they get the optimum quantity of the incident light.

The mesophyll cells, found within the leaf, contain numerous chloroplasts, which are the sites of photosynthesis. These chloroplasts are strategically arranged along the walls of the mesophyll to capture the maximum amount of light that enters the leaf. This arrangement enhances their ability to perform photosynthesis efficiently.

Imagine a room filled with windows where people are sitting. If everyone positions themselves close to the windows, they can take in more sunlight, just like how chloroplasts gather light for the plant.

You have studied the structure of chloroplast in Chapter 8. Within the chloroplast there is membranous system consisting of grana, the stroma lamellae, and the matrix stroma. There is a clear division of labour within the chloroplast.

Chloroplasts have a complex structure that includes thylakoids, where the light reactions of photosynthesis occur, and the stroma, which is where the Calvin cycle takes place. The thylakoid membranes contain chlorophyll and other pigments that capture light energy, while the stroma is rich in enzymes that use this energy to convert carbon dioxide and water into glucose.

Think of the chloroplast like a factory. The thylakoids are the assembly lines where raw materials (light) are converted into products (energy). The stroma is the main office where decisions and calculations are made to ensure the factory runs smoothly, turning those products into usable energy for the plant.

The membrane system is responsible for trapping the light energy and also for the synthesis of ATP and NADPH. In stroma, enzymatic reactions synthesise sugar, which in turn forms starch. The former set of reactions, since they are directly light driven are called light reactions (photochemical reactions). The latter are not directly light driven but are dependent on the products of light reactions (ATP and NADPH). Hence, to distinguish the latter they are called, by convention, as dark reactions (carbon reactions). However, this should not be construed to mean that they occur in darkness or that they are not light-dependent.

Photosynthesis consists of two main phases: light reactions and dark reactions (Calvin cycle). Light reactions, which take place in the thylakoid membranes, require light to produce ATP and NADPH. The dark reactions, occurring in the stroma, use these products to convert carbon dioxide into sugar. It's important to note that while these reactions don't need light directly, they rely on the outputs from the light reactions.

Imagine a two-step process like making a smoothie. In the first step, you gather and blend all the fruits (light reactions). This energy-packed smoothie is then used later as a drink (dark reactions) to fuel your body. You can't make the smoothie without the fruit, but if it's already made, you can use it at any time, day or night.

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