AP Biology 3.4 Photosynthesis Study Notes - New Syllabus Effective 2025
AP Biology 3.4 Photosynthesis Study Notes- New syllabus
AP Biology 3.4 Photosynthesis Study Notes – AP Biology – per latest AP Biology Syllabus.
LEARNING OBJECTIVE
Describe the photosynthetic processes and structural features of the chloroplast that allow organisms to capture and store energy.
Key Concepts:
- Photosynthesis
3.4.A – Photosynthesis
☀️ What Is Photosynthesis? 
Photosynthesis is the process by which plants, algae, and some bacteria capture solar energy and use it to convert carbon dioxide (CO₂) and water (H₂O) into glucose (C₆H₁₂O₆). Oxygen (O₂) is released as a byproduct.
🧪 Basic Formula:
\[ 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{light energy} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \]
🌱 Where Does It Happen?
Photosynthesis takes place inside the chloroplasts, which have specialized structures that help in capturing light and converting it to chemical energy.
🧫 Key Structural Features of the Chloroplast
| Structure | Function |
|---|---|
| Thylakoid | Flat sacs where light-dependent reactions occur; contains chlorophyll |
| Grana | Stacks of thylakoids; increase surface area for light absorption |
| Stroma | Fluid surrounding thylakoids, where light-independent reactions (Calvin cycle) happen |
| Chlorophyll | Green pigment that absorbs light (mostly blue/red) to drive reactions |
| ATP Synthase | Enzyme embedded in thylakoid membrane; makes ATP during light reactions |
⚡ Two Main Stages of Photosynthesis:
1. Light-Dependent Reactions (in thylakoid membranes):
- Uses sunlight to split water (photolysis) → releases O₂
- Produces ATP and NADPH (energy carriers)
2. Light-Independent Reactions (Calvin Cycle) (in stroma):
- Uses ATP + NADPH + CO₂ → makes glucose
🔑 Key Takeaways:
- Photosynthesis captures solar energy and stores it in chemical bonds of glucose.
- The structure of the chloroplast is tightly linked to its function – like thylakoid membranes for light capture and ATP production.
- ATP synthase is the main enzyme involved in energy storage by forming ATP.
3.4.A.1 – Photosynthesis Basics
🌞 What Is Photosynthesis?
Photosynthesis is a biological process in which light energy is captured to convert carbon dioxide (CO₂) and water (H₂O) into carbohydrates (like glucose) and oxygen (O₂).
📌Key Concepts:
- Photosynthetic organisms (like plants, algae, and cyanobacteria):
- Use sunlight to make sugars (like glucose)
- Sugars can be used for cellular energy or stored for later use
🌍 Evolutionary Origin:
- Photosynthesis first evolved in prokaryotes, specifically cyanobacteria
- These were the first organisms to perform oxygenic photosynthesis
- Their activity released O₂ into Earth’s atmosphere — the start of the oxygen revolution
🔬 Evidence from Biology:
- Fossils and molecular biology suggest cyanobacteria were responsible for the first oxygenated atmosphere
- These early pathways laid the foundation for photosynthesis in eukaryotes (like plants and algae) via endosymbiosis
🧠 Big Idea:
Photosynthesis isn’t just about plants making food — it’s a major evolutionary milestone that made aerobic life possible on Earth 🌍 by producing oxygen and shaping the biosphere.
3.4.A.2 – Structure of a Chloroplast
🌿 What Are Chloroplasts?
Chloroplasts are the sites of photosynthesis in plants and algae. These double-membraned organelles convert light energy into chemical energy using specialized internal structures.
📍 Key Structures Inside a Chloroplast:
🧪 Stroma
- The fluid-filled space inside the inner membrane but outside the thylakoids.
- Calvin Cycle (Carbon Fixation) happens here → this is where glucose is built using CO₂.
- Also contains chloroplast DNA, enzymes, and starch granules.
🟩 Thylakoids
- Flattened membrane sacs containing chlorophyll pigments.
- These pigments are part of Photosystem I and II, which absorb sunlight.
- Light-dependent reactions (that make ATP & NADPH) occur in these membranes.
- Electron transport proteins are embedded in the thylakoid membrane.
📦 Grana
- Thylakoids are stacked like pancakes → each stack is a granum (plural: grana).
- Light reactions occur in the grana, using light energy to power the production of ATP & NADPH.
💡 Summary Table:
| Structure | Function |
|---|---|
| Stroma | Site of the Calvin Cycle (light-independent reactions) |
| Thylakoid | Contains chlorophyll and is where light-dependent reactions occur |
| Grana | Stacks of thylakoids that optimize light absorption |
3.4.A.3 – Light Reactions of Photosynthesis (Light-Dependent Reactions)
📌 Where It Happens
- Location: In the thylakoid membranes of chloroplasts.
- Purpose: To capture sunlight energy and convert it into ATP and NADPH, which are used later in the Calvin Cycle (light-independent reactions).
🔁 Step-by-Step Process
⚡ 1. Photon Hits Photosystem II (PSII)
- Sunlight excites electrons in chlorophyll of PSII.
- Excited electrons are passed to the primary electron acceptor.
💧 2. Photolysis of Water
- Water is split to replace lost electrons in PSII.
- Forms: 2e⁻, 2H⁺, and ½ O₂.
- O₂ is released as a by-product.
🚛 3. Electron Transport Chain (ETC)
- Electrons move down the ETC from PSII → PSI.
- H⁺ ions are pumped into the thylakoid lumen, creating a proton gradient.
⚙️ 4. ATP Production via ATP Synthase
- H⁺ diffuses back into the stroma through ATP synthase.
- This process makes ATP → called chemiosmosis.
💡 5. Photosystem I (PSI)
- Electrons are re-energized by sunlight at PSI.
- Electrons are passed to NADP⁺, forming NADPH (high-energy carrier).
📦 Outputs of Light Reactions
| Molecule | Purpose |
|---|---|
| ATP | Energy for Calvin Cycle |
| NADPH | Electron carrier for sugar production |
| O₂ | Byproduct released into atmosphere |
🔁 Cyclic vs Noncyclic Electron Flow
| Type | Involves | Product(s) | Notes |
|---|---|---|---|
| Noncyclic | PSII & PSI | ATP, NADPH, O₂ | Main pathway, most common |
| Cyclic | Only PSI | Only ATP | No NADPH or O₂ made; used when NADPH is not needed |
3.4.B – How Cells Capture Light Energy & Store It in Biological Molecules
🔆 Photosynthesis = Capturing & Storing Light Energy
Cells (specifically plant cells and some algae) use photosynthesis to:
- Capture light energy from the sun
- Convert it into chemical energy
- Store that energy in molecules like glucose (C₆H₁₂O₆)
🔬 Step-by-Step: How It Happens
☀️ 1. Light Reactions (Thylakoid Membrane)
- Sunlight excites electrons in chlorophyll (in Photosystem II & I).
- Electrons flow through the Electron Transport Chain (ETC).
- This powers two key processes:
- ATP synthesis (via ATP synthase)
- NADPH production (high-energy electron carrier)
- Water is split → releases O₂ (byproduct) and replaces lost electrons.
✅ ATP and NADPH = stored energy in chemical form (short-term)
🌱 2. Calvin Cycle (Stroma)
- Uses the ATP and NADPH from light reactions.
- CO₂ is fixed (captured from the atmosphere).
- CO₂ is used to build 3-carbon sugars like G3P, which are eventually assembled into glucose.
✅ Glucose = Long-term energy storage molecule
💡 Summary: Energy Capture & Transfer
| Step | Energy Form | Molecules Involved |
|---|---|---|
| Light Absorption | Light Energy | Chlorophyll (in PSII/PSI) |
| Conversion | → Chemical energy | ATP & NADPH |
| Storage | → Stable molecules | Glucose (C₆H₁₂O₆) |
🔁 Why It Matters
Glucose is later used in cellular respiration to make ATP, the usable energy for nearly all life processes.
3.4.B.1 – Electron Transport Chains in Energy Capture (Photosynthesis Focus)
🔁 What’s the Electron Transport Chain (ETC)?
The ETC is a series of proteins that pass electrons along a path to slowly release energy step by step.
📍 ETCs Are Found In:
- Chloroplasts (thylakoid membrane) – during photosynthesis
- Mitochondria – during cellular respiration
- Prokaryotic plasma membranes – since they don’t have organelles
🌞 In Photosynthesis: ETC in Thylakoid Membrane
- Light energy hits Photosystem II → energizes electrons in chlorophyll
- These excited electrons move through the ETC, embedded in the thylakoid membrane
- As electrons move:
- Protons (H⁺) are pumped into the thylakoid space → builds a proton gradient
- Protons flow back through ATP synthase → ATP is made
- Electrons reach Photosystem I, get re-energized by more light
- Electrons are transferred to NADP⁺ → forms NADPH (a high-energy electron carrier)
✅ ATP and NADPH = carry chemical energy to the Calvin Cycle
💡 Big Idea
- ETC Role in Photosynthesis:
- Transfers electrons step by step
- Releases energy gradually
- Powers ATP & NADPH formation
- Final electron acceptor = NADP⁺
3.4.B.2 – Light Absorption & Water Splitting in Photosynthesis
🌞 Step 1: Light Hits Chlorophyll
- Chlorophyll, the green pigment in Photosystems I and II, absorbs sunlight.
- The light excites electrons → they jump to a higher energy level.
- These high-energy electrons are passed down the Electron Transport Chain (ETC).
💧 Step 2: Water Splits (Photolysis)
- Photosystem II loses electrons (because they get excited and leave).
- To replace those electrons, water (H₂O) is split:
- Electrons go to Photosystem II
- H⁺ ions stay in thylakoid → used to help make ATP
- Oxygen (O₂) is released as a byproduct
🔁 Summary: Energy Capture Process
| Process | What Happens |
|---|---|
| Light Absorption | Chlorophyll absorbs sunlight → excites electrons |
| Water Splitting | Replaces lost electrons in PSII, releases O₂ |
| Energy Transfer | Excited electrons help make ATP & NADPH |
✅ This reaction is essential for life – it kickstarts photosynthesis and produces oxygen for the atmosphere.
3.4.B.3 – Photosystems & Electron Flow in Photosynthesis
🧬 Where Are Photosystems Found?
Photosystem I (PSI) and Photosystem II (PSII) are located in the thylakoid membranes of chloroplasts.
🔗 How Are They Connected?
- Electrons flow from PSII → through the Electron Transport Chain (ETC) → to PSI.
- This electron transfer chain is key to:
- Making ATP from ADP + Pi via ATP synthase
- Creating NADPH from NADP⁺ (energy-storing molecule)
🔋 Energy Flow Summary
| Step | What Happens |
|---|---|
| Light excites PSII | Electrons get energized and leave |
| ETC transfers electrons | Energy is used to pump H⁺ ions into the thylakoid lumen |
| ATP made | H⁺ flows back through ATP synthase |
| Electrons reach PSI | PSI re-energizes them with more light |
| NADPH formed | Final electron acceptor = NADP⁺ → NADPH |
✅ Both ATP and NADPH are then used in the Calvin Cycle to make glucose and other organic molecules.
3.4.B.4 – Proton Gradient in Photosynthesis
🔁 Electron Transfers in the ETC
- As electrons move through the Electron Transport Chain (ETC) in the thylakoid membrane, they go through oxidation-reduction (redox) reactions.
- Each step transfers electrons between molecules with increasing electronegativity.
🧪 What Happens During This Transfer?
- Energy released from redox reactions is used to pump protons (H⁺ ions) into the thylakoid lumen.
- This creates a proton gradient across the thylakoid membrane.
🌊 Electrochemical Gradient Breakdown
| Region | Proton (H⁺) Concentration |
|---|---|
| 🌊 Inside thylakoid (lumen) | High H⁺ concentration |
| 💧 Outside thylakoid (stroma) | Low H⁺ concentration |
⚡ Why This Matters
This electrochemical gradient powers ATP synthase, which allows protons to flow back into the stroma.
The flow of H⁺ ions spins ATP synthase → converts ADP + Pi → ATP
✅ This process is called chemiosmosis and is essential for energy storage during photosynthesis!
3.4.B.5 – ATP Formation via Chemiosmosis (Photophosphorylation)
🔁 Link Between Proton Gradient & ATP Production
- Proton gradient (high H⁺ inside thylakoid, low outside) is created by the ETC.
- This gradient powers the enzyme ATP synthase, embedded in the thylakoid membrane.
⚙️ How It Works (Chemiosmosis)
- Protons (H⁺) flow down their concentration gradient, from lumen → stroma, via ATP synthase.
- This flow spins the enzyme like a turbine.
- This mechanical energy is used to join:
- ADP + Pi → ATP
🌟 This process is called Photophosphorylation
- Photo = powered by light
- Phosphorylation = adding a phosphate to ADP → makes ATP
🎯 Why It’s Important
This light-dependent ATP synthesis provides the energy needed for the Calvin cycle (light-independent reactions).
3.4.B.6 – Light Reactions Power the Calvin Cycle
🔋 Where the Energy Goes
- ATP and NADPH made in the light reactions are energy carriers.
- These molecules store light energy in chemical form.
🌀 What They Power
They fuel the Calvin Cycle, which:
- Happens in the stroma (the fluid around the thylakoids).
- Uses CO₂ to build carbohydrates (like glucose).
- Does not need light directly (light-independent).
🧪 Calvin Cycle Needs:
- Carbon dioxide (CO₂) – from the atmosphere
- ATP – provides energy
- NADPH – provides electrons/hydrogen to build sugars
🧠 Summary:
The light reactions charge the battery (ATP/NADPH), and the Calvin cycle spends it to build sugar.