AP Biology Photosynthesis — AP Biology
1. What Is Photosynthesis? ★★☆☆☆ ⏱ 3 min
Photosynthesis is the anabolic process carried out by photoautotrophs (plants, algae, cyanobacteria) that converts light energy from the sun into chemical energy stored in glucose and other organic carbohydrates.
6\text{CO}_2 + 12\text{H}_2\text{O} + \text{light energy} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 + 6\text{H}_2\text{O}
This is a redox reaction: carbon dioxide is reduced to form glucose, while water is oxidized to release oxygen gas as a byproduct. Photosynthesis contributes ~4-6% of your total AP Biology exam score, appearing regularly in both multiple-choice and free-response questions.
2. Chloroplast Structure and Light-Dependent Reactions ★★★☆☆ ⏱ 4 min
In eukaryotic photoautotrophs, photosynthesis occurs entirely within chloroplasts, double-membraned organelles. The gel-like stroma (site of the Calvin cycle) contains flattened, membrane-bound thylakoids stacked into grana, with a hollow thylakoid lumen. The thylakoid membrane hosts light-dependent reactions, with pigment molecules and electron transport protein complexes.
Chlorophyll a is the core reaction center pigment that absorbs light to excite electrons. Accessory pigments (chlorophyll b, carotenoids) absorb extra wavelengths, transfer energy to chlorophyll a, and provide photoprotection. Light excites electrons in Photosystem II (PSII) first; lost electrons are replaced by photolysis (splitting) of water, which releases oxygen. Excited electrons move down an electron transport chain (ETC) to Photosystem I (PSI), with energy used to pump H+ into the thylakoid lumen, generating a proton gradient. ATP synthase uses this gradient to make ATP via chemiosmosis. Electrons are re-excited in PSI and used to reduce NADP+ to NADPH. Final products: ATP, NADPH (sent to the Calvin cycle), and oxygen (waste).
Exam tip: On FRQs about chemiosmosis, always explicitly name ATP synthase and connect the proton gradient to ATP production to earn full points.
3. The Calvin Cycle and Photorespiration ★★★☆☆ ⏱ 3 min
The Calvin cycle occurs in the stroma, and uses ATP and NADPH from light-dependent reactions to fix inorganic CO2 into organic glucose. It has three core stages:
- **Carbon fixation**: RuBisCo attaches CO2 to the 5-carbon starting molecule RuBP, producing two 3-carbon 3-PGA molecules.
- **Reduction**: ATP phosphorylates 3-PGA, and NADPH reduces the product to G3P (glyceraldehyde 3-phosphate), a 3-carbon sugar.
- **Regeneration**: For every 6 G3P produced from 3 fixed CO2, only 1 G3P exits the cycle to make glucose. The remaining 5 G3P are rearranged with ATP to regenerate RuBP.
RuBisCo can bind either CO2 or O2 to RuBP. When O2 binds instead of CO2, photorespiration occurs: it consumes ATP, releases fixed CO2, and produces no net glucose, making it wasteful. Photorespiration increases when stomata close on hot, dry days to conserve water, leading to high O2 and low CO2 inside leaves.
Exam tip: Always remember only 1 out of 6 G3P exits the Calvin cycle for glucose production — forgetting the regeneration step is the most common calculation mistake on AP Bio exams.
4. C3, C4, and CAM Photosynthesis Adaptations ★★★★☆ ⏱ 3 min
To avoid wasteful photorespiration in hot, dry environments, many plants evolved modified pathways that concentrate CO2 around RuBisCo, reducing the chance RuBisCo binds O2:
- **C3 plants**: Most plants (wheat, rice) have no special adaptation. They fix CO2 directly into 3-PGA in mesophyll cells, with high photorespiration in hot, dry conditions.
- **C4 plants**: (corn, sugarcane) separate carbon fixation and the Calvin cycle *spatially*. CO2 is first fixed into a 4-carbon molecule in mesophyll cells, then transported to bundle sheath cells, where CO2 is released for the Calvin cycle. This keeps CO2 high around RuBisCo, eliminating most photorespiration.
- **CAM plants**: (cacti, pineapples) separate carbon fixation and the Calvin cycle *temporally*. They open stomata at night to take in CO2, fix it into 4-carbon molecules stored in vacuoles. During the day, they close stomata to conserve water, release stored CO2 for the Calvin cycle.
All three plant types use the same Calvin cycle to produce glucose; only the initial carbon fixation step differs.
Exam tip: When comparing C4 and CAM, explicitly name the separation type: spatial (location) for C4, temporal (time) for CAM — mixing these up causes automatic point loss.
5. Concept Check ★★★☆☆ ⏱ 3 min
Common Pitfalls
Why: Students misinterpret "light-independent" to mean "functions in the dark" instead of "does not directly use light"
Why: Students memorize the simplified reaction and assume O2 is a byproduct of CO2 reduction, since glucose is made from CO2
Why: The regeneration step is overlooked, leading to incorrect calculations of ATP/NADPH requirements
Why: Students see the different initial carbon fixation step and assume the entire pathway differs
Why: Both use chemiosmosis, but the compartment for high H+ concentration differs
Why: Students reverse absorption logic because leaves look green