The answer
Photosynthesis is the process by which green plants make their own food (glucose) using sunlight, water, and carbon dioxide. It happens mainly in the leaves inside tiny cell organelles called chloroplasts, which contain the green pigment chlorophyll. Chlorophyll captures sunlight energy, which is the driving force for the whole reaction.
The overall equation is:
Step 1: Light reaction (occurs in thylakoids) – Chlorophyll absorbs sunlight, splits water molecules into hydrogen and oxygen. Oxygen is released as a by-product. This step produces ATP (energy currency) and NADPH (reducing power).
Step 2: Dark reaction (Calvin cycle, occurs in stroma) – Carbon dioxide from air is fixed (combined) with hydrogen (from light reaction) using ATP and NADPH to form glucose. This step does not need light directly, but it uses the products of light reaction.
Micro-example: Imagine a factory: raw materials are CO₂ and H₂O, the machine is chlorophyll, and electricity is sunlight. Output is glucose (food) and oxygen (waste). Just as a factory needs power, plants need light to run the machine.
Why is it important? It provides food for the plant itself and for all other living beings (directly or indirectly). It also releases oxygen needed for respiration and balances atmospheric CO₂. Without photosynthesis, life on Earth would not exist.
Key points for exams: Remember the site (chloroplast), pigment (chlorophyll), raw materials (CO₂, H₂O), products (glucose, O₂), and the two main stages – light and dark. Also, remember that oxygen comes from water, not CO₂ (use isotope test).
