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Grade 11-12 (age 16-18)

high school Photosynthesis

Photosynthesis is the process in which a chloroplast uses light energy to build glucose from carbon dioxide and water, releasing oxygen along the way. The light reactions on the thylakoid membranes split water, release O₂, and make ATP and NADPH; the Calvin cycle in the stroma then uses that ATP and NADPH to fix CO₂ into the raw material for glucose. Plants respire all the time, so in dim light they use more O₂ than they make; as the light gets brighter, photosynthesis pulls ahead until, past the light saturation point, it stops speeding up. In the diagram, raising the light intensity thickens the ATP and NADPH arrow leaving the light reactions, and the dot on the graph below moves past the compensation point and the saturation point.

Turning light energy into chemical energy

🌿 What photosynthesis turns into what
①Producers such as plants and algae capture light energy and store it as chemical energy in glucose
②Photosynthesis takes place in the chloroplast, where pigments such as chlorophyll in the thylakoid membranes absorb light
③The first stage, the light reactions, needs light and makes ATP, NADPH, and O₂
④The second stage, carbon fixation (the Calvin cycle), uses that ATP and NADPH to turn CO₂ into organic molecules
⑤The glucose made this way fuels the plant's own respiration and passes along the food chain to other organisms

Light intensity and the rate of photosynthesis

5
💡 Reading the graph
①With no light, photosynthesis stops and only respiration goes on, so the net rate is negative: the plant takes in O₂ and gives off CO₂
②As the light gets brighter, the light reactions speed up and make more ATP and NADPH, and the Calvin cycle keeps pace
③The light intensity at which the gross photosynthesis rate equals the respiration rate is the compensation point, where net gas exchange is zero
④The light intensity beyond which brighter light no longer raises the rate is the saturation point
⑤Above the saturation point, CO₂ concentration or temperature, not light, becomes the limiting factor

The equation and the two stages

Overall equation for photosynthesis
6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O
Writing water on both sides shows that all 12 oxygen atoms in the released O₂ come from the H₂O taken in.

Two stages of photosynthesis

Light reactions vs. the Calvin cycle
FeatureLight reactionsCalvin cycle
Locationthylakoid membranestroma
Lightrequirednot used directly, but runs only with products of the light reactions
InH₂O, light energy, ADP, NADP⁺CO₂, ATP, NADPH
OutO₂, ATP, NADPHG3P (raw material for glucose), ADP, NADP⁺
Main stepssplitting of water by light, photophosphorylationCO₂ fixation, reduction of 3-PGA, regeneration of RuBP

Photosynthesis and cellular respiration

Photosynthesis vs. respiration

Comparing photosynthesis and cellular respiration
FeaturePhotosynthesisCellular respiration
Locationchloroplastcytoplasm and mitochondria
ReactionCO₂ + H₂O → glucose + O₂glucose + O₂ → CO₂ + H₂O
Energystores light energy as chemical energyreleases glucose's chemical energy as ATP
Who does itphotosynthetic organisms such as plants and algaenearly all living things, plants included

Worked Examples and Exam Practice

Example 1
Where in the chloroplast do the light reactions and the Calvin cycle take place, and what do the two stages pass to each other?
1
The light reactions occur on the thylakoid membranes, which hold the light-absorbing pigments and the electron transport chain; the Calvin cycle occurs in the stroma, the fluid around the thylakoids.
2
The ATP and NADPH made by the light reactions move to the Calvin cycle to power CO₂ fixation, and the spent ADP and NADP⁺ return to the light reactions.
▸ Light reactions: thylakoid membranes. Calvin cycle: stroma. The light reactions send ATP and NADPH, and the Calvin cycle returns ADP and NADP⁺.
The two arrows in the diagram are this exchange. With the light intensity at 0, the ATP and NADPH arrow disappears and the Calvin cycle fades.
Example 2
When a plant is lit, its net gas exchange drops to zero. How do its gross photosynthesis rate and respiration rate compare at that moment?
1
Net photosynthesis rate = gross photosynthesis rate − respiration rate.
2
If the net rate is zero, gross photosynthesis equals respiration, and the light intensity at that point is called the compensation point.
▸ gross photosynthesis rate = respiration rate (compensation point)
At the compensation point, respiration uses up all the O₂ that photosynthesis releases, so no net change shows. With the light intensity at 1 in the diagram, the dot sits right on the horizontal axis.
CSAT-style problem
Which statement about photosynthesis in plants is correct?
①The light reactions take place in the stroma
②In the light reactions, water is split and O₂ is released
③The Calvin cycle keeps running even without products of the light reactions
④Glucose is a direct product of the light reactions
⑤Past the saturation point, brighter light keeps raising the rate of photosynthesis
▸ ② In the light reactions, water is split and O₂ is released
1
In the light reactions, light energy splits water and releases O₂, and the electrons freed in the process are used to make ATP and NADPH.
2
The light reactions occur on the thylakoid membranes (①), the Calvin cycle needs ATP and NADPH from the light reactions (③), glucose comes from the Calvin cycle (④), and above the saturation point brighter light does not raise the rate (⑤).

Summary

How photosynthesis flows
light → light reactions (make ATP and NADPH) → Calvin cycle (fix CO₂) → glucose
The light reactions store light energy in ATP and NADPH, and the Calvin cycle spends that energy turning CO₂ into glucose.
🎯 Exam Points
①The light reactions take place on the thylakoid membranes, where water is split to release O₂ and ATP and NADPH are made
②The Calvin cycle takes place in the stroma, where ATP and NADPH are used to fix CO₂ into G3P
③The O₂ released in photosynthesis comes from H₂O, not from CO₂
④At the compensation point, the gross photosynthesis rate equals the respiration rate, so the net rate is zero
⑤Above the saturation point, brighter light no longer raises the rate, and CO₂ concentration and temperature become the limiting factors
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