Photosynthesis in Higher Plants — Practice

Question 1

What is the Z-scheme of electron transport in photosynthesis?

Accepted Answer

It is the path of electrons during the light reaction: light excites P680 in PS II, which after splitting water passes electrons through plastoquinone, the cytochrome b6f complex, and plastocyanin to PS I. Light then excites P700 in PS I, which passes electrons via ferredoxin to NADP+ to form NADPH. ATP is made along the way (photophosphorylation). Plotted on an energy scale the path looks like the letter Z.

Question 2

Differentiate between C3 and C4 plants.

Accepted Answer

C3 plants: first stable product is 3-PGA (3C), CO2 fixed by RuBisCO in mesophyll, normal leaf anatomy, high photorespiration, optimal at 15-25 C (e.g. wheat, rice). C4 plants: first product is OAA (4C), CO2 fixed first by PEP carboxylase in mesophyll then by RuBisCO in bundle sheath, show Kranz anatomy, very low photorespiration, optimal at 30-45 C (e.g. maize, sugarcane).

Question 3

Why is photorespiration considered wasteful?

Accepted Answer

In photorespiration RuBisCO fixes O2 instead of CO2, producing a 2-carbon compound that is processed to release already-fixed CO2. It consumes ATP and reducing power but produces no sugar and no net ATP, so it reduces the net efficiency of photosynthesis.

Question 4

Name the enzyme that fixes CO2 in the Calvin cycle and state what is special about it.

Accepted Answer

RuBisCO (ribulose bisphosphate carboxylase-oxygenase). It is the most abundant enzyme on Earth and can act on both CO2 (carboxylation) and O2 (oxygenation, leading to photorespiration).

Question 5

What is the significance of Kranz anatomy?

Accepted Answer

The wreath-like arrangement of mesophyll and bundle sheath cells in C4 plants spatially separates initial CO2 fixation (in mesophyll) from the Calvin cycle (in bundle sheath), concentrating CO2 around RuBisCO and minimising photorespiration.

Practice — Photosynthesis in Higher Plants

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