Photosynthesis: quick revision, 15 bites

The Core Reaction

Plants cook their own food.

**Photosynthesis** is the process by which green plants, algae, and some bacteria convert **light energy** into **chemical energy** stored in glucose. The overall equation is:

Takeaway. Photosynthesis turns light, water, and CO₂ into food and oxygen.

What Exactly Is Photosynthesis?

A precise definition.

**Photosynthesis** is the biochemical process in which **autotrophs** use sunlight to synthesise **glucose** from **carbon dioxide** and **water**, releasing **oxygen** as a by-product. It is the primary source of all organic matter on Earth.

Takeaway. It is the foundation of almost every food chain.

Where It Happens: The Chloroplast

Inside the green factory.

Photosynthesis occurs in **chloroplasts**, organelles found in plant cells. Chloroplasts contain **chlorophyll**, the green pigment that absorbs light energy, mainly in the blue and red parts of the spectrum.

Takeaway. Chloroplasts are the site; chlorophyll is the light catcher.

Two Stages: Light and Dark

First capture, then build.

Takeaway. Light reactions make energy; the Calvin cycle makes sugar.

The Balanced Equation

Six of each.

This equation summarises the overall process. Note that **oxygen** comes from water, not carbon dioxide.

Takeaway. Memorise the equation: 6 CO₂ + 6 H₂O → C₆H₁₂O₆ + 6 O₂.

Chlorophyll: The Green Pigment

Why leaves are green.

**Chlorophyll** absorbs red and blue light but reflects green light, which is why leaves appear green. It is embedded in the thylakoid membranes and is essential for capturing light energy.

Takeaway. Chlorophyll absorbs red/blue, reflects green.

Why Photosynthesis Matters

It fuels life on Earth.

Photosynthesis produces the **oxygen** we breathe and forms the base of nearly all **food chains**. It also removes **carbon dioxide** from the atmosphere, helping regulate Earth's climate.

Takeaway. Without photosynthesis, complex life would not exist.

Common Misconception: Plants Eat Soil

No, they make their own food.

Takeaway. Plants are producers, not consumers of soil food.

Exam Tip: Remember the Source of Oxygen

Oxygen comes from water.

In exams, a common question asks where the oxygen released during photosynthesis comes from. The answer is **water** (H₂O), not carbon dioxide. This is proven by experiments using heavy oxygen isotopes.

Takeaway. Oxygen is a by-product of water splitting.

Mnemonic for the Equation

Six CO₂, six water.

Remember: **6 CO₂ + 6 H₂O → C₆H₁₂O₆ + 6 O₂**. Think: 'Six carbon dioxides and six waters make one sugar and six oxygens.'

Takeaway. Use the 'six-six-one-six' pattern.

Did You Know? Oxygen Revolution

It changed the atmosphere.

About 2.4 billion years ago, photosynthetic cyanobacteria caused the **Great Oxidation Event**, transforming Earth's atmosphere and enabling aerobic life.

Takeaway. Photosynthesis reshaped the planet.

Factors Affecting Photosynthesis

Light, CO₂, temperature.

Takeaway. Rate depends on light, CO₂, and temperature.

Light vs Dark Reactions

Two stages, different roles.

Takeaway. Light reactions capture energy; Calvin cycle builds sugar.

Example: A Leaf in Sunlight

Follow the energy.

Sunlight hits a leaf. **Chlorophyll** absorbs it. Water from roots splits, releasing oxygen. ATP and NADPH form. In the stroma, CO₂ joins with these to make glucose. The plant uses glucose for energy or stores it as starch.

Takeaway. Light → chemical energy → food.

Study Strategy: Draw the Diagram

Visualise the chloroplast.

To master photosynthesis, draw a **chloroplast** and label **thylakoid**, **stroma**, and **grana**. Then trace the flow of electrons and protons. This active recall strengthens memory.

Takeaway. Diagram + labels = better recall.