Photosynthesis in higher plants: NEET short notes
Photosynthesis questions in NEET come from a few places again and again: where each reaction happens, what PS I and PS II absorb, the Calvin cycle's numbers, and how C4 plants differ from C3. Here they are in short notes, with a table and practice MCQs.
Short notes
- Photosynthesis happens in chloroplasts: the light reaction on the thylakoid membranes (grana), the dark (biosynthetic) reaction in the stroma.
- Photosystem II has its reaction centre at P680; Photosystem I at P700.
- Photolysis (splitting) of water happens on the PS II side; it releases O2, protons and electrons.
- The light reaction makes ATP and NADPH; the Calvin cycle uses them to fix CO2.
- In the Calvin cycle, RuBisCO adds CO2 to RuBP; the first stable product in C3 plants is 3-PGA, a 3-carbon compound.
- Fixing one CO2 needs 3 ATP and 2 NADPH; one glucose needs 6 turns of the cycle (18 ATP, 12 NADPH).
- C4 plants (maize, sugarcane, sorghum) have Kranz anatomy: bundle sheath cells around the veins.
- In C4 plants the first CO2 acceptor is PEP (enzyme PEP carboxylase, in mesophyll cells); the first product is OAA, a 4-carbon acid.
- Photorespiration: RuBisCO binds O2 instead of CO2; no ATP or sugar is made. It's significant in C3 plants and minimal in C4 plants.
- Factors that limit photosynthesis: light, CO2 concentration, temperature and water (Blackman's law of limiting factors).
At a glance
| C3 plants | C4 plants | |
|---|---|---|
| First CO2 acceptor | RuBP | PEP |
| First stable product | 3-PGA (3 carbon) | OAA (4 carbon) |
| Enzyme that fixes CO2 first | RuBisCO | PEP carboxylase |
| Kranz anatomy | No | Yes |
| Photorespiration | Present | Negligible |
| Examples | Wheat, rice | Maize, sugarcane, sorghum |
Memory tricks
Which photosystem is which
"One is seven": PS I = P700. So PS II = P680, the one that splits water.
Calvin cycle cost
"3-2-1": 3 ATP and 2 NADPH for 1 CO2.
C4 examples
C4 plants are "MaSS" crops: Maize, Sugarcane, Sorghum.
Practice questions
The first stable product of CO2 fixation in C4 plants is:
Show answer
OAA. PEP carboxylase fixes CO2 into oxaloacetic acid (OAA), a 4-carbon compound.
The reaction centre of photosystem I is:
Show answer
P700. PS I has P700; PS II has P680.
How many ATP and NADPH are needed to fix one molecule of CO2 in the Calvin cycle?
Show answer
3 ATP, 2 NADPH. 3 ATP and 2 NADPH per CO2.
Kranz anatomy is found in:
Show answer
Maize. Maize is a C4 plant with bundle sheath (Kranz) anatomy.
Questions
Where does the light reaction take place?
On the thylakoid membranes of the chloroplast (the grana). The Calvin cycle takes place in the stroma.
Why is photorespiration wasteful?
RuBisCO fixes O2 instead of CO2, and the process uses energy without making ATP or sugar. C4 plants avoid most of it by concentrating CO2 around RuBisCO.
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