What is the C4 pathway? Briefly explain how this adaptation increases the effectiveness of ...

Answer to q. 1

In the C4 pathway, CO2 reacts with PEP in a reaction catalyzed by PEP carboxylase, and oxaloacetate
is formed as the product. This reaction occurs in mesophyll cells. In turn, oxaloacetate is converted to
malate, the malate diffuses into bundle sheath cells, and is decarboxylated to pyruvate. The CO2 thus
formed enters the Calvin cycle. The C4 pathway enables photosynthesis to occur more efficiently in
hot, dry environments when plants normally close their stomata to prevent water loss. In such cases,
the C3 pathway is inhibited because the O2 that accumulates in photorespiration outcompletes low
levels of CO2 for the active site of RuBP carboxylase/oxygenase. However, because PEP carboxylase
binds only CO2 and not O2, plants having the C4 pathway can thrive when the CO2 levels are low

Answer to q. 2

The proton gradient is established in the thylakoid lumen as electrons flow down the photosynthetic
electron transport chain. As the electrons lose energy some of that energy is used to pump protons
across the thylakoid membrane into the lumen. As the protons accumulate, they form a gradient, with
the proton concentration in the lumen higher than in the stroma. Eventually the protons flow
downhill through the ATP synthase complex in the thylakoid membrane. The exergonic process of
diffusion down this gradient is used to power the endergonic process of ATP synthesis

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