Skills you'll gain in this topic:
- Describe photosynthetic processes and chloroplast structures that capture and store energy
- Explain how cells capture light energy and transfer it to biological molecules
- Illustrate the light reactions and Calvin cycle in photosynthesis
- Relate chloroplast structure to its function in energy capture
- Connect photosynthesis to the evolution of life on Earth
The Energy of Life
All living systems require a constant input of energy to sustain life processes. This energy powers everything organisms do - from growing and moving to thinking and reproducing. Without energy, cells can't maintain their organized structure or carry out vital functions.
The primary source of energy for most living organisms is the food they eat, which gets converted into usable cellular energy through pathways like cellular respiration. Plants and other photosynthetic organisms can produce their own energy by converting sunlight into chemical energy through photosynthesis.
What is Photosynthesis?
Photosynthesis is the series of reactions that use carbon dioxide (CO₂), water (H₂O), and light energy to make carbohydrates and oxygen (O₂). This process is absolutely crucial because:
- Photosynthetic organisms capture energy from the sun and produce sugars that can be used in biological processes or stored
- It provides the oxygen we breathe
- It forms the base of most food chains on Earth
The overall equation for photosynthesis is: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂
The Chloroplast: Photosynthesis Factory
Photosynthesis occurs in specialized organelles called chloroplasts. Understanding chloroplast structure is key to understanding how photosynthesis works:
Key Structures Within Chloroplasts:
1. Stroma
- The fluid-filled space inside the inner chloroplast membrane
- Where the Calvin cycle (carbon fixation) reactions occur
- Contains enzymes necessary for making glucose from CO₂
2. Thylakoids
- Flattened membrane sacs inside the chloroplast
- Contain chlorophyll pigments organized into photosystems I and II
- Site of the light reactions of photosynthesis
- Contain electron transport proteins
3. Grana
- Stacks of thylakoids (think of them like stacks of pancakes!)
- Increase surface area for light absorption
- Where light energy is converted to chemical energy
The Two Stages of Photosynthesis
Photosynthesis happens in two main stages, each occurring in a different part of the chloroplast:
Stage 1: Light Reactions (in the Thylakoids)
The light reactions capture energy from sunlight and convert it into chemical energy in the form of ATP and NADPH. Here's how it works:
1. Light Absorption
- Chlorophyll molecules in photosystems I and II absorb light energy
- This boosts electrons to a higher energy level (excited state)
2. Water Splitting
- Water molecules are split (photolysis) to replace electrons lost from photosystem II
- This produces oxygen as a waste product (the O₂ we breathe!)
- Equation: 2H₂O → 4H⁺ + 4e⁻ + O₂
3. Electron Transport Chain (ETC)
- Excited electrons from photosystem II pass through an electron transport chain
- As electrons move through the chain, they release energy
- This energy is used to pump protons (H⁺) into the thylakoid space
4. NADPH Formation
- Electrons eventually reach photosystem I, get re-excited by light
- These electrons are then used to reduce NADP⁺ to NADPH
- NADPH carries high-energy electrons to the Calvin cycle
5. ATP Synthesis (Photophosphorylation)
- The pumping of protons creates a gradient across the thylakoid membrane
- High H⁺ concentration inside the thylakoid, low concentration in the stroma
- Protons flow back through ATP synthase (chemiosmosis)
- This flow drives the formation of ATP from ADP + Pi
Stage 2: Calvin Cycle (in the Stroma)
The Calvin cycle uses the ATP and NADPH from the light reactions to fix carbon dioxide into glucose:
Key Points:
- Occurs in the stroma of the chloroplast
- Uses CO₂ from the atmosphere
- Powered by ATP and NADPH from light reactions
- Produces glucose through a series of enzyme-catalyzed reactions
- Does NOT require light directly (but needs the products of light reactions)
Note: You don't need to memorize the specific steps of the Calvin cycle for the AP exam, just understand its overall function!
Evolutionary Significance of Photosynthesis
Photosynthesis has profoundly shaped life on Earth:
Ancient Origins
- Photosynthesis first evolved in prokaryotic organisms (bacteria)
- These early photosynthetic bacteria were much simpler than modern plants
The Great Oxygenation Event
- Cyanobacteria (blue-green bacteria) evolved oxygen-producing photosynthesis
- Their photosynthesis was responsible for producing Earth's oxygenated atmosphere
- This dramatically changed Earth's chemistry and allowed oxygen-breathing life to evolve
From Prokaryotes to Eukaryotes
- Prokaryotic photosynthetic pathways were the foundation of eukaryotic photosynthesis
- Chloroplasts actually evolved from ancient cyanobacteria through endosymbiosis
- This is why chloroplasts have their own DNA and ribosomes!
Order vs. Chaos: The Thermodynamics of Life
Living things are incredibly organized systems. This might seem to go against the second law of thermodynamics, which states that disorder (entropy) in closed systems always increases over time. So how do organisms stay organized?
The secret is that living organisms are open systems - they constantly exchange matter and energy with their environment. This allows them to maintain order by taking in energy from their surroundings.
Maintaining Order Requires Energy
To stay alive and organized, organisms must follow a simple rule: energy input must exceed energy loss. When an organism can no longer take in enough energy to offset what it's using, it can't maintain its organization - resulting in death. 💀
| Energy Balance | Result |
|---|---|
| Energy input > Energy loss | Life continues, organism maintains order |
| Energy input = Energy loss | Organism struggles to maintain functions |
| Energy input < Energy loss | Death, breakdown of organization |
Connecting Photosynthesis to Energy Flow
Photosynthesis is the crucial first step in energy flow through ecosystems:
- Plants capture solar energy through photosynthesis
- Energy is stored in glucose and other organic molecules
- Herbivores eat plants, obtaining this stored energy
- Carnivores eat herbivores, passing energy up the food chain
- Decomposers break down dead organisms, recycling nutrients
This means the energy in your burger actually came from the sun via photosynthesis in the plants the cow ate!
Photosynthesis vs. Cellular Respiration
These two processes are complementary:
| Photosynthesis | Cellular Respiration |
|---|---|
| Occurs in chloroplasts | Occurs in mitochondria |
| Uses CO₂ and H₂O | Produces CO₂ and H₂O |
| Produces glucose and O₂ | Uses glucose and O₂ |
| Stores energy | Releases energy |
| Anabolic (builds molecules) | Catabolic (breaks down molecules) |
Key Takeaways
- Photosynthesis converts light energy into chemical energy (glucose)
- It occurs in two stages: light reactions (thylakoids) and Calvin cycle (stroma)
- Chloroplast structure is perfectly adapted for capturing light and producing glucose
- The process produces oxygen as a byproduct, which transformed Earth's atmosphere
- Photosynthesis evolved in prokaryotes and was later incorporated into eukaryotes
- It forms the foundation of most food chains and ecosystems on Earth
Photosynthesis is the process that captures the sun's energy and makes it available to living things. Through the coordinated action of light reactions in the thylakoids and the Calvin cycle in the stroma, chloroplasts convert light energy into the chemical bonds of glucose. This ancient process, which first evolved in prokaryotes billions of years ago, not only feeds most life on Earth but also created the oxygen-rich atmosphere we depend on. Understanding photosynthesis helps us appreciate how energy flows through ecosystems and how all life is ultimately connected to the sun.
Vocabulary
The following words are mentioned explicitly in the College Board Course and Exam Description for this topic.
| Term | Definition |
|---|---|
| adenosine triphosphate | The primary energy currency of cells that powers cellular functions. |
| ADP | Adenosine diphosphate; a molecule that is phosphorylated to form ATP during oxidative phosphorylation. |
| ATP synthase | A membrane-bound enzyme that uses the proton gradient to drive the synthesis of ATP from ADP and inorganic phosphate. |
| Calvin cycle | The light-independent reactions of photosynthesis that use ATP and NADPH to produce carbohydrates from carbon dioxide in the stroma. |
| carbohydrates | Biological molecules composed of carbon, hydrogen, and oxygen that serve as a primary source of energy and structural support in living organisms. |
| carbon fixation | The process in the Calvin cycle that incorporates carbon dioxide into organic molecules. |
| chemiosmosis | The process by which the flow of protons across a membrane through ATP synthase drives ATP synthesis. |
| chlorophyll | A pigment in chloroplasts that absorbs light energy and transfers electrons to higher energy levels in photosystems. |
| chloroplast | An organelle in plant cells where photosynthesis occurs, containing thylakoids and stroma. |
| cyanobacteria | Prokaryotic photosynthetic organisms responsible for producing an oxygenated atmosphere through photosynthesis. |
| electrochemical gradient | The combined effect of the concentration gradient and electrical potential difference across a membrane that influences ion movement. |
| electron transport | A series of protein complexes in thylakoid membranes that transfer electrons and help generate ATP and NADPH during the light reactions. |
| electron transport chain | A series of protein complexes in membranes that transfer electrons and establish an electrochemical gradient to generate ATP during photosynthesis and cellular respiration. |
| grana | Stacks of thylakoid membranes organized within the chloroplast where light reactions of photosynthesis occur. |
| inorganic phosphate | A free phosphate group (Pi) that is added to ADP to form ATP during ATP synthesis. |
| light reactions | The light-dependent stage of photosynthesis that occurs in the thylakoid membrane and produces ATP and NADPH. |
| NADP⁺ | An electron carrier molecule that accepts electrons during photosynthesis and is reduced to NADPH to carry energy for the Calvin cycle. |
| NADPH | The reduced form of NADP⁺ that carries electrons and energy from the light reactions to power the Calvin cycle. |
| oxidation/reduction reactions | Chemical reactions in which electrons are transferred between molecules, occurring in the electron transport chain during photosynthesis. |
| photophosphorylation | The synthesis of ATP from ADP and inorganic phosphate using energy from the proton gradient established during the light reactions of photosynthesis. |
| photosynthesis | The series of reactions that use carbon dioxide, water, and light energy to produce carbohydrates and oxygen, allowing organisms to capture and store energy from the sun. |
| photosystem | Organized complexes of chlorophyll pigments and proteins in thylakoid membranes that capture light energy during the light reactions. |
| photosystem I | A light-harvesting complex embedded in the thylakoid membrane that uses light energy to boost electrons to a higher energy level and reduce NADP⁺ to NADPH. |
| photosystem II | A light-harvesting complex embedded in the thylakoid membrane that uses light energy to boost electrons and splits water to replace lost electrons. |
| prokaryotic photosynthesis | Photosynthetic processes in prokaryotic organisms, particularly cyanobacteria, that were the evolutionary foundation for eukaryotic photosynthesis. |
| proton gradient | A difference in proton concentration across a membrane, with higher concentration on one side than the other. |
| stroma | The fluid-filled space inside the chloroplast where the Calvin cycle occurs. |
| thylakoid | Membrane structures within the chloroplast that contain chlorophyll pigments and electron transport proteins, where light reactions occur. |
| thylakoid membrane | The membrane system within chloroplasts where light-dependent reactions of photosynthesis occur, containing photosystems and electron transport chains. |
| water splitting | The photolysis of water molecules during photosystem II that releases electrons, protons, and oxygen. |
Frequently Asked Questions
What is photosynthesis and how does it actually work?
Photosynthesis is the set of reactions that use light, CO2, and H2O to make sugars and O2. It happens in chloroplasts: light reactions in thylakoid membranes (grana) capture light with chlorophyll in photosystems II and I, boost electrons, and split water (photolysis) to replace PSII electrons—releasing O2. Electrons travel through an ETC, driving H+ into the thylakoid lumen and creating a proton gradient. H+ flow back through ATP synthase (chemiosmosis) to make ATP (photophosphorylation) and electrons reduce NADP+ to NADPH. ATP and NADPH power the Calvin cycle in the stroma to fix CO2 into carbohydrates. Prokaryotic (cyanobacterial) photosynthesis evolved first and oxygenated Earth. For AP exam focus on the big ideas/structures and processes (chloroplast, thylakoid, stroma, photosystems, ATP/NADPH, chemiosmosis); you don’t need to memorize every Calvin step. Review this CED-aligned study guide (https://library.fiveable.me/ap-biology/unit-3/cellular-energy/study-guide/pOhMYoE7Yc4VJi0Rk41H) and use the 1000+ practice questions (https://library.fiveable.me/practice/ap-biology) to prep.
Why do plants need both light reactions and the Calvin cycle - can't they just do one?
Short answer: they do different but linked jobs—you need both. Light reactions (in thylakoid membranes/grana) capture sunlight with photosystems I and II, split H2O, move electrons through an ETC, build a proton gradient, and make ATP + NADPH (photophosphorylation). The Calvin cycle (in the stroma) uses CO2 plus the chemical energy in ATP and the reducing power of NADPH to build sugars (carbon fixation). So light reactions convert light to chemical energy; the Calvin cycle uses that energy to store carbon as organic molecules. Without light reactions you’d have no ATP/NADPH; without the Calvin cycle the energy couldn’t be stored as sugar—one captures energy, the other uses it to make biomass (EK 3.4.A.1, EK 3.4.A.2, EK 3.4.A.3, EK 3.4.B.5–6). For a quick topic review, check the AP Bio Topic 3.4 study guide (https://library.fiveable.me/ap-biology/unit-3/cellular-energy/study-guide/pOhMYoE7Yc4VJi0Rk41H) and more practice questions at (https://library.fiveable.me/practice/ap-biology).
I'm confused about chloroplasts - what's the difference between stroma and thylakoids?
Short answer: stroma is the chloroplast’s watery interior outside the thylakoids, and thylakoids are the membrane sacs (often stacked as grana) where the light reactions happen. Why that matters for AP Bio (CED terms): the Calvin cycle (carbon fixation) happens in the stroma—it uses ATP and NADPH made in the light reactions to build sugars. The thylakoid membranes contain chlorophyll, photosystems I and II, and the electron transport chain; as electrons move through the ETC protons are pumped into the thylakoid lumen, creating a high H+ concentration inside the thylakoid compared to the stroma. Protons flow back through ATP synthase (photophosphorylation) to make ATP. Don’t worry about memorizing each Calvin step for the exam, but know where each set of reactions occurs (CED EK 3.4.A.2–3.4.B.6). For a quick review, check the Topic 3 study guide (https://library.fiveable.me/ap-biology/unit-3/cellular-energy/study-guide/pOhMYoE7Yc4VJi0Rk41H) and practice problems (https://library.fiveable.me/practice/ap-biology).
How do photosystems I and II work together in the light reactions?
Photosystems II and I work as a linked two-step light engine in the thylakoid membrane. PSII absorbs light first, exciting chlorophyll electrons and using photolysis (splitting H2O) to replace those lost—that releases O2. Electrons from PSII travel through an electron transport chain between the photosystems; as they move, proton pumps build a high [H+] inside the thylakoid (chemiosmosis). That proton gradient drives ATP synthase to make ATP (photophosphorylation). When electrons reach PSI, PSI absorbs light again to re-energize them so they can reduce NADP+ to NADPH. ATP and NADPH produced in the grana/stroma power the Calvin cycle in the stroma to fix CO2 into sugars. This is exactly what the CED describes: coordinated light reactions in thylakoids make ATP and NADPH for carbon fixation. For a concise AP review, see the Topic 3.4 study guide (https://library.fiveable.me/ap-biology/unit-3/cellular-energy/study-guide/pOhMYoE7Yc4VJi0Rk41H) and practice questions (https://library.fiveable.me/practice/ap-biology).
What happens when chlorophyll absorbs light energy step by step?
When chlorophyll absorbs a photon, here’s what happens step-by-step: 1. A chlorophyll molecule in a photosystem (PS II or PS I) in the thylakoid membrane absorbs light and one of its electrons is boosted to a higher energy level (excited state). 2. The excited electron is transferred to a nearby primary electron acceptor, starting electron flow through the thylakoid ETC (EK 3.4.B.2–3). 3. In PS II, the lost electron is replaced by electrons from water splitting (photolysis), producing O2 and H+ inside the thylakoid. 4. Electrons moving through the ETC release energy that pumps protons into the thylakoid lumen, creating a proton gradient (EK 3.4.B.4). 5. Protons flow back through ATP synthase to make ATP (photophosphorylation) by chemiosmosis (EK 3.4.B.5). 6. Electrons reach PS I, get re-excited by light, and are ultimately used to reduce NADP+ to NADPH. 7. ATP and NADPH produced in the light reactions power the Calvin cycle in the stroma to fix CO2 into sugars (EK 3.4.A.2–3). For a concise AP-aligned review, check the Topic 3 study guide (https://library.fiveable.me/ap-biology/unit-3/cellular-energy/study-guide/pOhMYoE7Yc4VJi0Rk41H). Practice problems are at (https://library.fiveable.me/practice/ap-biology).
Why does water split during photosynthesis and where do those electrons go?
Photosystem II loses high-energy electrons when chlorophyll absorbs light, so something has to replace them—that’s why water is split (photolysis). In the thylakoid lumen 2 H2O → 4 H+ + 4 e− + O2. The electrons replace those lost from PSII and then travel through the thylakoid electron transport chain to photosystem I and ultimately help reduce NADP+ to NADPH. Meanwhile the H+ released into the thylakoid builds the proton gradient used by ATP synthase to make ATP (photophosphorylation). ATP and NADPH produced in the light reactions are then used in the Calvin cycle in the stroma to fix CO2 into sugars (EK 3.4.A.1, EK 3.4.B.2–5). For a quick AP-aligned review of this and other Topic 3.4 concepts check the Unit 3 study guide (https://library.fiveable.me/ap-biology/unit-3/cellular-energy/study-guide/pOhMYoE7Yc4VJi0Rk41H).
Can someone explain the electron transport chain in photosynthesis in simple terms?
Think of the photosynthetic electron transport chain (ETC) as a solar-powered conveyor belt in the thylakoid membrane that turns light into chemical energy. Light excites chlorophyll in photosystem II (PSII), boosting electrons that move down the ETC to photosystem I (PSI). PSII replaces lost electrons by splitting water (photolysis), releasing O2 and H+. As electrons pass between proteins, energy pumps protons into the thylakoid lumen, creating a high H+ concentration inside and low H+ in the stroma (an electrochemical gradient). In PSI light re-excites electrons, which are ultimately used to reduce NADP+ to NADPH. The proton gradient then drives ATP synthase as H+ flows back to the stroma (chemiosmosis), making ATP (photophosphorylation). ATP and NADPH produced in the light reactions power the Calvin cycle in the stroma to make sugars. (You don’t need to memorize specific carrier names for the AP exam.) For a quick Topic 3.4 review see the study guide (https://library.fiveable.me/ap-biology/unit-3/cellular-energy/study-guide/pOhMYoE7Yc4VJi0Rk41H); broader Unit 3 resources are here (https://library.fiveable.me/ap-biology/unit-3). For extra practice, try the practice questions (https://library.fiveable.me/practice/ap-biology).
How does ATP synthase make ATP during photosynthesis - is it the same as in cellular respiration?
Yes—ATP synthase uses the same basic mechanism in photosynthesis and cellular respiration: chemiosmosis. In the light reactions (photophosphorylation) electrons energized by photosystems I and II move through an ETC in the thylakoid membrane. As they pass, protons are pumped (and water is split) to create a high H+ concentration inside the thylakoid lumen and low H+ in the stroma (EK 3.4.B.4–5). Protons flow back down that gradient through membrane-bound ATP synthase; that flow drives the enzyme to join ADP + Pi → ATP (EK 3.4.B.5). What’s different is where the gradient is made and the proton direction: chloroplasts build the gradient across the thylakoid membrane (lumen → stroma), while mitochondria build it across the inner mitochondrial membrane (intermembrane space → matrix). Functionally though, both use the same chemiosmotic coupling to make ATP—a concept AP asks you to know (see the Topic 3.4 study guide (https://library.fiveable.me/ap-biology/unit-3/cellular-energy/study-guide/pOhMYoE7Yc4VJi0Rk41H)). For more review and practice, check Unit 3 (https://library.fiveable.me/ap-biology/unit-3) and practice problems (https://library.fiveable.me/practice/ap-biology).
What's the difference between photophosphorylation and the phosphorylation that happens in cellular respiration?
Photophosphorylation and the phosphorylation in cellular respiration both make ATP by chemiosmosis (protons flowing back through ATP synthase), but they differ in energy source, location, and electron flow. Photophosphorylation occurs in chloroplast thylakoid membranes: light excites electrons in photosystems, electron transport pumps H+ into the thylakoid lumen, and ATP + NADPH are produced for the Calvin cycle (EK 3.4.A.3, EK 3.4.B.4–5). Respiration’s oxidative phosphorylation happens in the inner mitochondrial membrane: electrons from food oxidation move through an ETC, pumping H+ into the intermembrane space; O2 is the final electron acceptor and ATP is made for cellular work. Key differences: light vs chemical energy, NADP+ reduced to NADPH in photosynthesis (not NAD+), and different membrane compartments (thylakoid vs mitochondrial inner membrane). For review and practice questions on Topic 3.4, see the AP Bio Unit 3 study guide (https://library.fiveable.me/ap-biology/unit-3/cellular-energy/study-guide/pOhMYoE7Yc4VJi0Rk41H) or lots of practice problems (https://library.fiveable.me/practice/ap-biology).
I don't understand how the proton gradient forms across the thylakoid membrane - can someone break this down?
Think of the thylakoid like a battery being charged during the light reactions. Light excites electrons in chlorophyll (PSII and PSI). PSII loses electrons that are replaced when water is split (photolysis), releasing O2 and H+ into the thylakoid lumen (EK 3.4.B.2). Excited electrons flow through the electron transport chain in the thylakoid membrane (EK 3.4.B.1, 3), and as they move, the ETC proteins use that released energy to pump additional protons from the stroma into the thylakoid lumen. The result: high [H+] inside the thylakoid (lumen) and low [H+] in the stroma—an electrochemical (proton) gradient (EK 3.4.B.4). Protons flow back out through ATP synthase; that chemiosmotic flow drives ATP production (photophosphorylation) (EK 3.4.B.5). NADP+ is reduced to NADPH at PSI, so the light reactions make both ATP and NADPH used in the Calvin cycle in the stroma (EK 3.4.A.3, 3.4.B.6). For a focused review, see the Topic 3.4 study guide (https://library.fiveable.me/ap-biology/unit-3/cellular-energy/study-guide/pOhMYoE7Yc4VJi0Rk41H) and more unit practice (https://library.fiveable.me/ap-biology/unit-3).
Why did photosynthesis evolving in cyanobacteria change Earth's atmosphere so much?
When cyanobacteria evolved oxygenic photosynthesis (~2.7 billion years ago), they started splitting H2O and releasing O2 as a waste product (EK 3.4.A.1–3). Over millions of years that O2 built up in the atmosphere—the Great Oxygenation Event—and that changed Earth in several big ways: - Oxygen allowed much more efficient aerobic respiration, letting life harvest far more ATP and enabling the evolution of complex, energy-hungry cells. - Free O2 reacted with dissolved iron, creating banded iron formations and clearing oceans of dissolved iron. - O2 in the upper atmosphere formed an ozone layer, which reduced UV radiation and made colonization of land safer. - Many anaerobic microbes couldn’t tolerate rising O2 and declined (a major ecological turnover). This is exactly why cyanobacterial photosynthesis is credited with producing an oxygenated atmosphere in the CED. If you want a quick Topic 3.4 review, check the AP Bio Topic 3 study guide (https://library.fiveable.me/ap-biology/unit-3/cellular-energy/study-guide/pOhMYoE7Yc4VJi0Rk41H)—and practice questions are at https://library.fiveable.me/practice/ap-biology.
How do NADPH and ATP from the light reactions power the Calvin cycle?
The light reactions make ATP (by photophosphorylation) and NADPH (by reducing NADP+) in the thylakoid membranes. Those two molecules move into the stroma, where the Calvin cycle runs. ATP supplies the energy (phosphate transfers) needed to drive the endergonic steps of carbon fixation and regeneration of the CO₂ acceptor; NADPH supplies the high-energy electrons (reducing power) that convert the 3-carbon intermediates (3-PGA) into triose phosphates (G3P), which are then used to make sugars or to regenerate the CO₂ acceptor. Without enough ATP or NADPH the cycle stalls. This matches the CED: light reactions capture light to make ATP and NADPH, and the Calvin cycle uses them in the stroma to produce carbohydrates (EK 3.4.A.2–3 and EK 3.4.B.5–6). For a quick review, check the Topic 3.4 study guide (https://library.fiveable.me/ap-biology/unit-3/cellular-energy/study-guide/pOhMYoE7Yc4VJi0Rk41H) and more practice questions (https://library.fiveable.me/practice/ap-biology).
What's the overall equation for photosynthesis and how do all the parts connect?
Overall equation: 6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2. How the parts connect: light is captured by chlorophyll in photosystem II and I on thylakoid membranes (grana). Light excites electrons; PSII replaces them by splitting water (photolysis), releasing O2 and H+. Electrons move through the thylakoid ETC, pumping H+ into the thylakoid lumen and creating a proton gradient. Protons flow back through ATP synthase (photophosphorylation) to make ATP; electrons reduce NADP+ to NADPH at PSI. ATP and NADPH (products of the light reactions in the grana) diffuse into the stroma, where the Calvin cycle (carbon fixation) uses CO2, ATP, and NADPH to make carbohydrate (G3P → sugars). Remember: AP expects you to know locations (thylakoid vs stroma), roles of ATP/NADPH, photolysis, chemiosmosis and that you shouldn’t memorize every Calvin step (CED exclusion). For a focused review, check the Topic 3.4 study guide (https://library.fiveable.me/ap-biology/unit-3/cellular-energy/study-guide/pOhMYoE7Yc4VJi0Rk41H) and extra practice (https://library.fiveable.me/practice/ap-biology).
I missed the lab on chloroplast structure - where exactly do the light reactions and Calvin cycle happen?
Light reactions happen in the thylakoid membranes—mostly in the stacked thylakoids (grana). That’s where photosystems I and II, the ETC, proton pumping into the thylakoid lumen, ATP synthase, and photophosphorylation produce ATP and NADPH. The Calvin cycle (carbon fixation) takes place in the stroma—the fluid inside the inner chloroplast membrane but outside the thylakoids—where ATP and NADPH are used to build carbohydrates from CO2. So: light reactions = thylakoid membrane/grana (make ATP & NADPH, split H2O, make O2); Calvin cycle = stroma (uses ATP/NADPH to fix carbon). This matches the AP CED (EK 3.4.A.2, EK 3.4.B.5–6). Don’t worry about memorizing all enzyme steps for the exam—focus on locations and overall inputs/outputs. For a quick review, check the Topic 3.4 study guide (https://library.fiveable.me/ap-biology/unit-3/cellular-energy/study-guide/pOhMYoE7Yc4VJi0Rk41H) and more unit resources (https://library.fiveable.me/ap-biology/unit-3) or practice questions (https://library.fiveable.me/practice/ap-biology).
How is the electron transport chain in chloroplasts different from the one in mitochondria?
Short version: both use an electron transport chain and chemiosmosis to make ATP, but they differ in electron source, final acceptor, where the chain sits, and where protons build up. Details you should know for AP: in chloroplasts ETC is in the thylakoid membranes (photosystems I & II). Light excites electrons (from chlorophyll); water is split (photolysis) to replace electrons, releasing O2. Electrons end up reducing NADP+ → NADPH (used in the Calvin cycle). Proton pumping builds a high [H+] inside the thylakoid lumen; protons flow back into the stroma through ATP synthase (photophosphorylation) to make ATP (EK 3.4.B.1–5). In mitochondria ETC is in the inner mitochondrial membrane. Electrons come from NADH/FADH2 (from food) and ultimately reduce O2 → H2O. Protons are pumped into the intermembrane space, then flow back into the matrix through ATP synthase to make ATP (oxidative phosphorylation). Both use chemiosmosis and ATP synthase, but chloroplasts produce ATP + NADPH using light energy; mitochondria convert stored chemical energy to ATP using cellular respiration. For a quick Topic 3.4 review, check the Fiveable study guide (https://library.fiveable.me/ap-biology/unit-3/cellular-energy/study-guide/pOhMYoE7Yc4VJi0Rk41H) and more practice questions at (https://library.fiveable.me/practice/ap-biology).