Plants use light energy to turn carbon dioxide and water into glucose and oxygen, and that process is photosynthesis.
It happens in the chloroplasts and speeds up as the light gets brighter, but only up to a point.
Cellular respiration runs the other way: it uses glucose and oxygen to release the energy cells need to live.
Raise the light and the leaf sends out more oxygen and glucose until 70%, where the rate levels off; further down, the two processes sit side by side.
Plants make their own food
🌿 What goes in and what comes out
①Ingredients: carbon dioxide (CO₂) and water (H₂O)
②Energy source: sunlight
③Where it happens: the chloroplasts in leaf cells
④Products: glucose (food) and oxygen (O₂)
Photosynthesis inside a leaf
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A leaf photosynthesizing at different light levels
60%
💡 Turning up the light
①CO₂ and H₂O enter on the left, and O₂ and glucose leave on the right
②The brighter the light, the more the chloroplasts glow and the stronger the outgoing arrows look
③In this model the rate reaches 100% at 70% light and climbs no further. That light level is the light saturation point
Photosynthesis stores light energy as chemical energy in glucose.
Example 2
In what way are photosynthesis and cellular respiration opposites, and how does the gas a plant gives off change between day and night?
1
Photosynthesis runs CO₂+H₂O→glucose+O₂ in the chloroplasts, and respiration runs glucose+O₂→CO₂+H₂O in the mitochondria, so they are reverse reactions.
2
By day photosynthesis outpaces respiration, so the plant releases O₂. At night photosynthesis stops and only respiration continues, so the plant releases CO₂.
▸ The equations run in opposite directions / day: O₂ released (photosynthesis > respiration); night: CO₂ released (respiration only)
Plants respire 24 hours a day, but they photosynthesize only while there is light.
Unit test
Which statement about photosynthesis and cellular respiration is correct?
①Photosynthesis takes place in the mitochondria
②The reactants of photosynthesis are glucose and oxygen
③Cellular respiration breaks down glucose to release usable energy
④Plants do not respire at night
⑤Photosynthesis and respiration run in the same direction
▸ ③ Cellular respiration breaks down glucose to release usable energy
1
Photosynthesis happens in chloroplasts (① is false), and its reactants are CO₂ and water, with glucose and oxygen as products (② is false).
2
Plants respire day and night (④ is false), and the two equations run in opposite directions (⑤ is false). Respiration breaks down glucose to release energy (③ is true).