Photosynthesis
Easy Overview
Plants literally turn sunlight into food. Think about that — they take sunshine, water, and air, and make sugar. We can't do that. No machine can do it as well. This chapter is about how plants pull off this crazy magic trick.
What is Photosynthesis?
The big equation: 6CO₂ + 6H₂O + sunlight → C₆H₁₂O₆ + 6O₂. Carbon dioxide + water + light energy = glucose + oxygen. It happens in chloroplasts, specifically in the thylakoid membranes (where the green chlorophyll lives). The whole process is plants turning light energy into chemical energy.
Light Reaction — Catching the Sun
Light reaction happens when sunlight hits chlorophyll. The energy splits water molecules (photolysis), releasing oxygen. It also creates ATP and NADPH — energy carriers. This part needs light directly. It's like charging a battery.
Dark Reaction (Calvin Cycle) — Building Sugar
The Calvin Cycle doesn't need light directly — but it needs the ATP and NADPH from the light reaction. It takes CO₂ from the air and fixes it into glucose. It's like using that charged battery to actually do work. The first stable product is 3-PGA (a 3-carbon compound).
Factors Affecting Photosynthesis
Light intensity, CO₂ concentration, temperature, and water. Too little light? Slow photosynthesis. Too much light? The plant gets stressed. At a certain point, some other factor becomes the bottleneck — that's the law of limiting factors.
C3 vs. C4 Plants
Most plants are C3 — they make a 3-carbon compound first. C4 plants (like maize and sugarcane) have an extra step to concentrate CO₂. C4 plants are better adapted to hot, dry conditions. They waste less water. It's like having a turbocharger.
Importance of Photosynthesis
Without photosynthesis, there's no food. Period. Every bite of food you eat comes from plants either directly or indirectly. And that oxygen you're breathing right now? That came from photosynthesis too. Every second, plants produce thousands of tons of oxygen.
Key Points
- •6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂.
- •Light reaction: needs light, produces ATP, NADPH, and O₂.
- •Dark reaction (Calvin cycle): uses ATP and NADPH to fix CO₂ into glucose.
- •Chlorophyll absorbs light — mainly red and blue, reflects green.
- •C3 plants: 3-PGA is first product. C4 plants: oxaloacetate first.
- •Limiting factors: light, CO₂, temperature, water.
- •Photosynthesis provides food and oxygen for almost all life.
Practice Questions
- Write the balanced equation for photosynthesis.
- Explain the light reaction of photosynthesis.
- What is the Calvin cycle? Where does it occur?
- Differentiate between C3 and C4 plants.
- What are the factors affecting the rate of photosynthesis?