Photosynthesis: flashcards, quiz and summary
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The short version
Photosynthesis lets plants, algae and some bacteria capture light energy and store it as sugar, and it releases the oxygen we breathe. It has two linked stages: the light-dependent reactions in the thylakoid membranes make ATP, NADPH and O2 from water and light, and the Calvin cycle in the stroma uses that ATP and NADPH to fix CO2 into sugar.
Flashcards
1Where does photosynthesis take place in a plant cell?
Photosynthesis takes place in chloroplasts. The light-dependent reactions happen in the thylakoid membranes, and the Calvin cycle happens in the stroma.
2What is the overall chemical equation for photosynthesis?
6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2. Carbon dioxide and water are turned into glucose and oxygen using light energy.
3Where does the oxygen released in photosynthesis come from?
The oxygen comes from water. Light splits water in photosystem II, which releases O2 as a waste product, not from the carbon dioxide.
4What are the two products of the light-dependent reactions that move on to the Calvin cycle?
ATP and NADPH. The Calvin cycle uses them to power the building of G3P from CO2.
5How does chemiosmosis produce ATP during the light-dependent reactions?
Electron transport pumps H+ ions into the thylakoid space, creating a gradient. The H+ ions flow back out through ATP synthase, and that flow drives the making of ATP.
6Why do leaves look green?
Chlorophyll absorbs mostly red and blue light and reflects green light, so the reflected green light is what reaches our eyes.
7What does rubisco do in the Calvin cycle?
Rubisco is the enzyme that attaches CO2 to the 5-carbon sugar RuBP. This is the carbon fixation step that starts the cycle.
8Why does the Calvin cycle need ATP and NADPH if it is called light-independent?
The Calvin cycle does not use light directly, but it needs ATP and NADPH from the light-dependent reactions. Because those depend on light, the cycle runs in daylight in practice.
9How does a plant's response to water shortage affect photosynthesis?
A plant short on water closes its stomata to save water. This also cuts off CO2 entry, so the rate of carbon fixation falls.
Practice quiz
Pick an answer to check it.
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1Where does the oxygen released in photosynthesis come from?
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D. Water Light splits water during photolysis in photosystem II, and this releases O2. The carbon dioxide is used in the Calvin cycle and does not supply the released oxygen.
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2Which stage of photosynthesis produces ATP and NADPH?
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B. The light-dependent reactions in the thylakoid membranes The light-dependent reactions in the thylakoid membranes make ATP and NADPH. The Calvin cycle uses them, but does not make them.
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3Which enzyme attaches CO2 to RuBP in the Calvin cycle?
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A. Rubisco Rubisco is the enzyme for carbon fixation. ATP synthase makes ATP, photosystem I passes electrons to NADP+, and chlorophyll is a pigment that absorbs light.
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4Why do leaves usually look green?
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B. Chlorophyll absorbs mostly red and blue light and reflects green Chlorophyll absorbs mostly red and blue light and reflects green, and the reflected green is what we see. Reversing which colors are absorbed and reflected is a common mistake.
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5A plant is short of water. What is the most direct effect on photosynthesis?
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C. Stomata close, so less CO2 enters and the rate of carbon fixation falls The plant closes its stomata to save water, which also cuts off CO2 entry. Less CO2 slows the carbon fixation step.
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6In what way does the Calvin cycle depend on light?
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A. It does not use light directly, but it needs ATP and NADPH from the light-dependent reactions The Calvin cycle is called light-independent because it does not use light directly. It still depends on ATP and NADPH, which come from light-driven reactions, so it runs in daylight in practice.
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7Which pair of products from the light-dependent reactions goes into the Calvin cycle?
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C. ATP and NADPH ATP and NADPH from the light-dependent reactions power the Calvin cycle. Glucose is made later in the cycle, while CO2 and water are inputs to the whole process.
Test traps
What teachers like to test on this, and the mistakes students make.
- Saying the oxygen released comes from carbon dioxide. It comes from water, which is split during photolysis.
- Thinking the Calvin cycle runs at night because it is called light-independent. It does not need light directly, but it relies on ATP and NADPH from the light stage, so it runs in daylight.
- Mixing up the colors chlorophyll absorbs and reflects. It absorbs mostly red and blue and reflects green.
- Forgetting that 6 turns of the Calvin cycle are needed to make one glucose, while only 3 turns give one G3P that leaves.
- Confusing the location of the two stages: light-dependent reactions happen in the thylakoid membranes and the Calvin cycle in the stroma.
- Saying plants only respire at night. Plants do cellular respiration day and night, and photosynthesis in the light is simply the process that outweighs it.
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Summary
- Photosynthesis captures light energy and stores it as chemical energy in sugar, with the overall equation 6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2.
- In leaves, CO2 enters through stomata, water arrives from the roots through the xylem, and the reactions happen inside chloroplasts.
- Chlorophyll in the thylakoid membranes absorbs mostly red and blue light and reflects green, which is why leaves look green.
- The light-dependent reactions split water to replace electrons lost by photosystem II, releasing O2 as a waste product.
- Electron transport and chemiosmosis produce ATP, and electrons are loaded onto NADP+ to make NADPH.
- The Calvin cycle uses rubisco to fix CO2 onto RuBP, then uses ATP and NADPH to build G3P, which becomes glucose and rebuilds RuBP.
- Light, CO2, temperature and water availability all change the rate of photosynthesis, and the rate is limited by whichever factor is in shortest supply.
Key terms
- Chloroplast
- The organelle inside leaf cells where photosynthesis takes place, with an outer and inner membrane, thylakoids and stroma.
- Thylakoid
- A flat disc inside a chloroplast where the light-dependent reactions happen. Stacks of thylakoids are called grana.
- Chlorophyll
- The pigment in thylakoid membranes that absorbs mostly red and blue light and reflects green light.
- Stomata
- Small pores, usually on the underside of a leaf, that let CO2 in. Guard cells open and close them.
- Photolysis
- The splitting of water using light energy in photosystem II, which releases O2, H+ ions and electrons.
- Chemiosmosis
- The flow of H+ ions back through ATP synthase, driven by the H+ gradient, which makes ATP.
- Rubisco
- The enzyme that attaches CO2 to the 5-carbon sugar RuBP during carbon fixation in the Calvin cycle.
- G3P
- A 3-carbon sugar made in the Calvin cycle. Some leaves the cycle to build glucose and other organic molecules.
How to study it
Stage 1: light-dependent reactions
- Know the inputs and outputs: light and water go in, and ATP, NADPH and O2 come out.
- Trace the path of electrons from photosystem II through the electron transport chain to photosystem I.
- Explain how the H+ gradient across the thylakoid membrane drives ATP synthase.
- Remember that O2 leaves the leaf as a waste product.
Stage 2: Calvin cycle
- Learn the order: CO2 is fixed onto RuBP by rubisco, ATP and NADPH are used to make G3P, and RuBP is regenerated.
- Explain why the cycle is called light-independent but still runs only in daylight.
- Know that 3 CO2 make 1 G3P that leaves, and 6 turns make one glucose.
Factors and the big picture
- For any limiting-factor question, identify the factor in shortest supply and explain why raising it speeds the rate.
- Connect photosynthesis to cellular respiration as near-opposite processes, and note that plants do both.
The notes this pack came from
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Big idea
Plants, algae and some bacteria capture light energy and store it as chemical energy in sugar. Almost every food chain on Earth starts here, and it is where the oxygen we breathe comes from.
Overall equation: 6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2
Where it happens
- Leaves are the main site. CO2 enters through small pores called stomata, usually on the underside of the leaf. Guard cells open and close the stomata.
- Water comes up from the roots through the xylem.
- Inside leaf cells, photosynthesis happens in chloroplasts.
- Chloroplast parts: an outer and inner membrane, stacks of flat discs called thylakoids (a stack is a granum), and the fluid around them, the stroma.
- Chlorophyll sits in the thylakoid membranes. It absorbs mostly red and blue light and reflects green, which is why leaves look green.
Stage 1: the light-dependent reactions (thylakoid membranes)
- Light excites electrons in chlorophyll in photosystem II.
- To replace those electrons, water is split (photolysis). This releases O2 as a waste product, plus H+ ions.
- Electrons pass down an electron transport chain to photosystem I, pumping H+ into the thylakoid space.
- H+ flows back out through ATP synthase, making ATP (chemiosmosis).
- At the end, electrons and H+ are loaded onto NADP+ to make NADPH.
- Products that move on to stage 2: ATP and NADPH. Oxygen leaves the leaf.
Stage 2: the Calvin cycle (stroma)
- Also called the light-independent reactions. It does not need light directly, but it needs the ATP and NADPH from stage 1, so in practice it runs in daylight.
- Carbon fixation: the enzyme rubisco attaches CO2 to a 5-carbon sugar called RuBP.
- ATP and NADPH are used to turn the products into G3P, a 3-carbon sugar.
- Some G3P leaves the cycle to build glucose and other organic molecules. The rest is recycled to rebuild RuBP.
- It takes 3 turns of the cycle (3 CO2) to make 1 G3P that leaves, and 6 turns to make one glucose.
Factors that change the rate
- Light intensity: more light, faster rate, until another factor limits it.
- CO2 concentration: same pattern.
- Temperature: enzymes like rubisco work best in a range; too hot and the rate drops.
- Water: if a plant is short on water, it closes its stomata, which also cuts off CO2.
Connection to cellular respiration
Photosynthesis makes glucose and O2. Cellular respiration uses glucose and O2 and releases CO2 and H2O. The two processes are close to opposite, and plants do both: photosynthesis in chloroplasts, respiration in mitochondria.
Common mix-ups
- The oxygen released comes from water, not from CO2.
- Plants do cellular respiration too, day and night.
- "Light-independent" does not mean "happens at night".
Questions
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