Topic 3.4 Notes – Photosynthesis

1. What Photosynthesis Is

At its core, photosynthesis uses light energy to convert carbon dioxide and water into carbohydrates and oxygen.

$$6\text{CO}_2 + 6\text{H}_2\text{O} + \text{light} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$$

What that actually means:

• Carbon dioxide (CO₂) → carbon source for building sugars
• Water (H₂O) → source of electrons and hydrogen
• Light energy → powers the reactions
• Glucose (C₆H₁₂O₆) → stored chemical energy
• Oxygen (O₂) → released as a byproduct

This happens in chloroplasts of plants and algae. It first evolved in prokaryotes, specifically cyanobacteria. Their oxygen-producing photosynthesis led to the oxygenation of Earth’s atmosphere, which allowed aerobic respiration and complex life to evolve.

Chloroplasts themselves came from cyanobacteria through endosymbiosis. That’s why they have their own DNA and ribosomes. When the AP gives you evidence about circular DNA or double membranes, they’re pointing at this evolutionary origin.

2. Chloroplast Structure and Where Each Stage Happens

Structure explains function here. If you can picture the chloroplast, you can predict where each reaction occurs.

Chloroplast structure and internal membranes

Stroma

• Fluid inside the inner membrane
• Surrounds the thylakoids
• Site of the Calvin cycle (carbon fixation)
• Contains enzymes that build carbohydrates

Thylakoids

• Flattened membrane sacs
• Contain chlorophyll pigments
• Contain photosystems I and II
• Contain electron transport chain (ETC) proteins
• Site of the light reactions

Grana

• Stacks of thylakoids
• Increase membrane surface area
• Major location of light reactions

Big spatial rule to memorize:

• Light reactions → thylakoid membrane
• Calvin cycle → stroma

If a question asks where a proton gradient forms, think thylakoid membrane. If it asks where CO₂ becomes organic, think stroma.

3. The Light Reactions How Light Energy Becomes ATP and NADPH

The goal is to convert light energy into ATP and NADPH, which are usable forms of energy for the cell.

As you read through the steps below, use the diagram to trace the path of electrons from water to NADPH and the movement of H⁺ across the thylakoid membrane.

Light-dependent reactions across the thylakoid membrane

Step 1: Light excites electrons

• Chlorophyll absorbs photons.
• Electrons jump to a higher energy level.
• Happens in Photosystem II (PSII) and Photosystem I (PSI).

Step 2: Water is split (photolysis)

$$2\text{H}_2\text{O} \rightarrow 4\text{H}^{+} + 4e^{-} + \text{O}_2$$

• Replaces electrons lost from PSII
• Releases oxygen
• Adds H⁺ to the thylakoid space

Students often forget this: the O₂ we breathe comes from water, not CO₂.

Step 3: Electron transport chain

• Electrons move PSII → plastoquinone (Pq) → cytochrome complex → plastocyanin (Pc) → PSI
• Each transfer is a redox reaction
• Energy released pumps H⁺ into the thylakoid space

This builds an electrochemical (proton) gradient:

• High H⁺ inside thylakoid
• Low H⁺ in stroma

Step 4: NADPH formation

• Electrons re-excited in PSI
• Passed to ferredoxin (Fd) and then to NADP⁺ reductase, forming NADPH

Step 5: ATP synthesis (photophosphorylation)

• H⁺ flows through ATP synthase
• Chemiosmosis drives ATP production

Outputs of light reactions:

• ATP
• NADPH
• O₂

The AP loves asking you to connect proton gradients across systems. The same logic appears in mitochondria and even prokaryotic membranes.

4. The Calvin Cycle How Carbon Is Fixed into Sugar

The Calvin cycle uses ATP and NADPH to build carbohydrates from CO₂.

• Occurs in the stroma
• Does not directly require light
• Requires ATP and NADPH from light reactions

Core idea:

• Carbon fixation converts inorganic CO₂ into organic molecules.
• Energy from ATP and high-energy electrons from NADPH become stored in sugar bonds.

You do not need to memorize enzyme names or every intermediate. Focus on the purpose:

Light reactions capture energy.
Calvin cycle stores that energy in stable carbohydrate molecules.

No ATP or NADPH means no carbon fixation.

5. How Photosynthesis Captures and Stores Energy

Energy flow looks like this:

1. Light excites electrons.
2. Electron energy drives proton pumping.
3. Proton gradient powers ATP synthesis.
4. Electrons reduce NADP⁺ → NADPH.
5. ATP and NADPH power carbon fixation.
6. Energy ends up stored in glucose bonds.

Photosynthesis is anabolic because it builds molecules.

Here’s how it complements respiration:

Energy entering ecosystems almost always starts with photosynthesis. When you analyze food webs or productivity data, you’re tracing stored solar energy.