In the late 1940s and 1950s, Melvin Calvin and his colleagues at the University of California, Berkeley, including Andrew Benson and James Bassham, fed single-celled green algae carbon dioxide made with radioactive carbon-14. After a short time in the light, they dropped the algae into hot alcohol to stop every reaction, then looked for the molecules that had picked up the radioactive carbon. Run the experiment for longer and longer times, and the order of those molecules shows the path carbon takes on its way to sugar. Calvin received the 1961 Nobel Prize in Chemistry, and the pathway is often called the Calvin cycle (or the Calvin-Benson-Bassham cycle, recognizing his colleagues). Let's look closer at how a leaf turns light into that sugar. This is AP Biology Unit 3.
The big picture
Photosynthesis uses carbon dioxide, water and light energy to make carbohydrates and oxygen. It happens in two linked stages:
- The light reactions capture light energy and store it in ATP and NADPH.
- The Calvin cycle spends that ATP and NADPH to build sugar from CO₂.
Photosynthesis first evolved in prokaryotes. Evidence supports the claim that photosynthesis by cyanobacteria produced Earth's oxygenated atmosphere, and chloroplasts descend from photosynthetic prokaryotes, so their pathways are the foundation of photosynthesis in plants and algae.
The chloroplast
- Thylakoids are flattened membrane sacs, stacked into grana. The thylakoid membrane holds the pigments, organized into two photosystems, along with an electron transport chain and ATP synthase. The light reactions happen here.
- The stroma is the fluid around the thylakoids. The Calvin cycle happens here.
The light reactions
Chlorophyll absorbs red and blue light well and reflects much of the green. Accessory pigments such as carotenoids absorb other wavelengths and pass the energy along.
- In photosystem II, light energy boosts electrons to a higher energy level. They leave and enter the electron transport chain.
- Water is split to replace them. This releases electrons, H⁺ ions and O₂. The oxygen a plant gives off comes from water.
- As electrons pass along the chain in a series of oxidation-reduction reactions, H⁺ ions are moved into the thylakoid space. Together with the H⁺ from splitting water, this builds a gradient: high H⁺ inside the thylakoid, low in the stroma.
- In photosystem I, light re-energizes the electrons, which are used to reduce NADP⁺ to NADPH.
- H⁺ flows back into the stroma through ATP synthase, which makes ATP from ADP and phosphate. This is chemiosmosis, and using light energy to make ATP this way is called photophosphorylation.
Notice that photosystem II acts before photosystem I. They're numbered in the order they were discovered, not the order they work.
The Calvin cycle
In the stroma, CO₂ is attached to an organic molecule: carbon fixation. ATP and NADPH then provide the energy and electrons to turn the fixed carbon into a three-carbon sugar. Some of that sugar leaves the cycle to build glucose, sucrose, starch and other organic molecules; most is recycled to regenerate the molecule that accepts CO₂. ADP, phosphate and NADP⁺ go back to the thylakoids to be recharged.
This is the simple version, and for the AP Exam it's the right one: memorizing the steps of the Calvin cycle, the structures of its molecules and the names of its enzymes is beyond the scope of the exam. Know what goes in, what comes out and why.
Common mistakes students make
- "The Calvin cycle is the dark reaction, so it runs at night." It needs ATP and NADPH from the light reactions, so it slows soon after the light goes off.
- "The O₂ comes from CO₂." It comes from splitting water.
- "Photosynthesis creates oxygen." Atoms aren't created. Oxygen atoms already in water are released as O₂ gas.
- "Only plants photosynthesize." Algae and cyanobacteria do too.
- "Plants photosynthesize instead of respiring." Plant cells respire all the time.
Frequently asked questions
What are the inputs and outputs of the Calvin cycle?
In: CO₂, ATP and NADPH. Out: a three-carbon sugar used to build glucose and other molecules, plus ADP, phosphate and NADP⁺ that return to the light reactions.
Where do the light reactions and the Calvin cycle take place?
The light reactions take place in the thylakoid membranes, stacked in grana. The Calvin cycle takes place in the stroma.
Why is photosystem II before photosystem I?
The photosystems were numbered in the order they were discovered. In the light reactions, electrons pass from photosystem II to photosystem I.
Do I need to memorize the steps of the Calvin cycle for AP Biology?
No. The course excludes the individual steps, molecule structures and enzyme names, except ATP synthase. Focus on the inputs, outputs and how the two stages depend on each other.
Try it
Say each answer before you flip the card. Then picture a sprig of pondweed in a glass of water on a sunny windowsill, giving off small bubbles. What would you expect to see if you moved the glass farther from the window?
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