Photosynthesis: quick revision, 12 bites

The Core Equation of Life

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. It is the foundation of almost all life on Earth.

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

What Exactly Is Photosynthesis?

Photo = light, synthesis = make.

**Photosynthesis** is the biochemical process where **autotrophs** use sunlight to synthesise **glucose** from **carbon dioxide** and **water**, releasing **oxygen** as a by-product.

Takeaway. It is the light-driven making of food from simple raw materials.

Where It Happens: The Chloroplast

Tiny green factories inside cells.

Photosynthesis occurs in **chloroplasts**, organelles found mainly in **mesophyll cells** of leaves. Chloroplasts contain the green pigment **chlorophyll**, which absorbs light energy.

Takeaway. Chloroplasts are the sites; chlorophyll is the light-catcher.

Light Reactions vs Calvin Cycle

Two stages, two jobs.

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

Step-by-Step: The Light Reactions

Water splits, energy is captured.

Takeaway. Light reactions convert light into ATP and NADPH, releasing O₂.

Step-by-Step: The Calvin Cycle

Sugar built without direct light.

Takeaway. The Calvin cycle uses ATP and NADPH to fix CO₂ into sugar.

Key Pigments and Their Roles

Chlorophyll a is the star.

Takeaway. Chlorophyll a is essential; others assist and protect.

Why Oxygen Matters

Every breath you take.

The **oxygen** released during photosynthesis is the source of the oxygen in Earth's atmosphere. It supports **aerobic respiration** in most living organisms, including humans.

Takeaway. Photosynthesis supplies the oxygen we breathe.

Factors Affecting Rate

Light, CO₂, temperature.

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

Plants Respire Too!

Not just oxygen producers.

Takeaway. Plants both photosynthesise and respire; net O₂ depends on light.

Remember the Locations

Thylakoid vs stroma.

Exams often ask where each stage occurs. **Light reactions** happen in the **thylakoid membrane**; the **Calvin cycle** happens in the **stroma**. Use the mnemonic: *'Light on the thylakoid, sugar in the stroma.'*

Takeaway. Thylakoid = light; stroma = sugar.

The Photosynthesis Equation

Six, six, one, six.

Memorise the equation as: **6 CO₂ + 6 H₂O → C₆H₁₂O₆ + 6 O₂**. Think: *six carbon dioxides, six waters, one glucose, six oxygens.*

Takeaway. Use the 6-6-1-6 pattern to recall the equation.