AP® Biology review sheet from Aim for Five (aimforfive.com/bio/units/3/3-4)
Unit 3 · Topic 3.4
3.4 Photosynthesis
Photosynthesis captures light energy and stores it in sugar. In the thylakoid membranes, the light reactions use light to split water, release O₂, and make ATP and NADPH; in the stroma, the Calvin cycle uses that ATP and NADPH to build sugar from CO₂. Photosynthesis evolved first in prokaryotes, and cyanobacteria filled Earth's atmosphere with oxygen.
Key terms
- thylakoid
- stroma
- photosystems I and II
- electron transport chain
- ATP synthase
- Calvin cycle
The big picture
Overall: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂. Photosynthetic organisms, including plants, algae and some bacteria, capture energy from sunlight and store it in the chemical bonds of sugars. They use those sugars for their own energy and building materials, or store them, for example as starch (1.4). Almost every food chain starts here.
Photosynthesis evolved first in prokaryotes. Evidence from rocks and fossils supports the idea that photosynthesis by cyanobacteria released the oxygen that built up in Earth's atmosphere, starting roughly 2.4 billion years ago. Eukaryotes later gained photosynthesis through endosymbiosis (2.10), so plant and algae photosynthesis is built on these prokaryotic pathways.
Inside the chloroplast
- Thylakoids: flattened membrane sacs, stacked into piles called grana. The thylakoid membranes hold chlorophyll and other pigments, grouped into two light-capturing units called photosystems (photosystem II and photosystem I), plus electron transport proteins and ATP synthase. The light reactions happen here.
- Thylakoid space (lumen): the fluid-filled space inside each thylakoid, where protons build up.
- Stroma: the fluid inside the chloroplast's inner membrane but outside the thylakoids. The Calvin cycle happens here.
The light reactions, step by step
- Light hits photosystem II. Chlorophyll absorbs the energy, which boosts electrons to a higher energy level. The excited electrons leave the photosystem.
- Water is split to replace those lost electrons. Splitting water releases O₂ (as a by-product) and H⁺ ions into the thylakoid space.
- The excited electrons pass along an electron transport chain (ETC) in the thylakoid membrane through a series of oxidation-reduction (redox) reactions, where each molecule gains electrons (is reduced) and passes them on (is oxidized). Energy released along the way is used to pump H⁺ from the stroma into the thylakoid space.
- The electrons reach photosystem I, where light boosts them to a high energy level again. They're then passed to NADP⁺, reducing it to NADPH.
- Protons are now much more concentrated inside the thylakoid space than in the stroma. This proton gradient is stored energy.
- H⁺ flows back out to the stroma through ATP synthase, a membrane protein, and the flow drives ATP synthase to make ATP from ADP and inorganic phosphate (Pi). Using a proton gradient this way is called chemiosmosis. In photosynthesis, making ATP with light energy is called photophosphorylation.
The Calvin cycle
In the stroma, the Calvin cycle uses the ATP and NADPH from the light reactions to build sugar from CO₂. First, CO₂ from the air is attached to an organic molecule already in the cycle (carbon fixation). Then energy from ATP and electrons from NADPH convert the products into a three-carbon sugar. Some of that sugar leaves the cycle to make glucose and other organic molecules, and the rest is used to rebuild the starting molecule so the cycle can keep going.
The used ADP and NADP⁺ go back to the thylakoids to be recharged. So the two stages depend on each other: no light means no new ATP and NADPH, and the Calvin cycle stops. No CO₂ means the Calvin cycle stops, NADP⁺ runs out, and the light reactions slow down too.
You don't need to memorize the steps, intermediates or enzyme names of the Calvin cycle (ATP synthase is the one enzyme name you should know), or the full names of the individual electron carriers in the ETC.
Worked examples
Try each one yourself first, then open the solution.
- Example 1Calculator allowed
Net vs. gross photosynthesis
An aquatic plant in a sealed chamber is kept in bright light for 15 minutes, and the O₂ in the water increases by 1.8 mL. The same plant is then kept in the dark for 15 minutes, and O₂ decreases by 0.3 mL. Calculate the net rate of photosynthesis, the rate of respiration and the gross (total) rate of photosynthesis in mL O₂/min.
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- Step 1: In the light, the plant both photosynthesizes (makes O₂) and respires (uses O₂). The measured increase is the net rate: 1.8 mL ÷ 15 min = 0.12 mL O₂/min.
- Step 2: In the dark, there is no photosynthesis, so the O₂ decrease shows respiration alone: 0.3 mL ÷ 15 min = 0.02 mL O₂/min used.
- Step 3: Assume respiration runs at the same rate in the light.
- Step 4: Gross photosynthesis = net + respiration = 0.12 + 0.02 = 0.14 mL O₂/min.
Answer: Net photosynthesis = 0.12 mL O₂/min; respiration = 0.02 mL O₂/min; gross photosynthesis = 0.14 mL O₂/min.
- Example 2
A leaky thylakoid membrane
A chemical makes thylakoid membranes leaky to H⁺, so protons can cross freely without going through ATP synthase. Light is still on. Predict the effects on the proton gradient, ATP production and sugar production.
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- Step 1: Light still drives the ETC, so H⁺ is still pumped into the thylakoid space.
- Step 2: But H⁺ leaks right back out, so the proton gradient collapses.
- Step 3: ATP synthase needs H⁺ flowing through it to make ATP, so ATP production drops sharply.
- Step 4: The Calvin cycle needs ATP to build sugar, so sugar production drops, even though light, CO₂ and NADPH may still be available.
Answer: The proton gradient collapses, ATP synthesis falls, and the Calvin cycle slows, so less sugar is made.
- Example 3
Where does the oxygen come from? (classic trap)
In an experiment, a plant is given water containing a heavy oxygen isotope (¹⁸O), with normal CO₂. A second plant gets normal water and CO₂ containing ¹⁸O. In which plant does the O₂ released contain ¹⁸O?
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- Step 1: The trap: many people assume the released O₂ comes from CO₂, since CO₂ 'has the oxygen.'
- Step 2: Recall the light reactions: water is split at photosystem II to replace lost electrons, and that is what releases O₂.
- Step 3: So the oxygen atoms in released O₂ come from water.
- Step 4: The oxygen atoms from CO₂ end up in the sugar and in some of the water the overall reaction produces, not in the released O₂.
Answer: Only the first plant (given ¹⁸O-labeled water) releases ¹⁸O-labeled O₂, because O₂ comes from splitting water.
Common mistakes
- Saying the O₂ released by photosynthesis comes from CO₂. It comes from splitting water.
- Putting the Calvin cycle in the thylakoids or the light reactions in the stroma. Light reactions: thylakoid membranes. Calvin cycle: stroma.
- Saying the proton gradient is higher in the stroma. Protons are concentrated inside the thylakoid space and flow out to the stroma through ATP synthase.
- Saying plants photosynthesize instead of doing cellular respiration. Plants do both; they respire all the time, day and night.
On the exam
- Expect 'predict the effect' questions: a blocked ETC, a leaky membrane, no light or no CO₂. Trace the effect through the gradient, ATP/NADPH and the Calvin cycle.
- Lab-based questions (for example, floating leaf disks or O₂ sensors) often ask you to calculate a rate, identify variables, or separate net from gross photosynthesis.
Connected topics
Videos
Check yourself
4 questions on 3.4 Photosynthesis. Pick an answer to see if you got it, and why.
Plants are given water labeled with the heavy oxygen isotope ¹⁸O, along with normal CO₂. Where would the ¹⁸O first appear?
In a chloroplast in the light, where is the H⁺ concentration highest?
A herbicide blocks the transfer of electrons from photosystem II to photosystem I. Which of the following would most likely happen in treated plants in the light?
Rocks older than about 2.4 billion years contain minerals that break down quickly when oxygen is present, while younger rocks contain iron oxides that form only when free oxygen is present. Which of the following best explains this change?
0 of 4 answered