Photosynthesis: 17 MCQs with answers

Chlorophyll a

Chlorophyll a is the main pigment that directly participates in the light reactions, converting light energy into chemical energy.

Thylakoid membrane

The thylakoid membrane contains the photosystems and electron transport chain, where light energy is converted into ATP and NADPH.

Stroma

The Calvin cycle (light-independent reactions) occurs in the stroma, the fluid-filled space surrounding the thylakoids.

Glucose

Glucose is produced in the Calvin cycle (light-independent reactions), not directly in the light-dependent reactions.

False

Photosynthesis occurs in chloroplasts, not mitochondria. Mitochondria are involved in cellular respiration.

False

The Calvin cycle does not directly require light; it uses ATP and NADPH produced by the light-dependent reactions. It is often called the light-independent reactions.

ATP and NADPH

The light reactions produce ATP and NADPH, which provide energy and reducing power for the Calvin cycle to fix CO2 into sugars.

Water

Oxygen is released from the splitting of water molecules (photolysis) during the light-dependent reactions, not from CO2.

Photosystem II

Photosystem II (PSII) is the first in the electron transport chain and uses light energy to split water, releasing oxygen and electrons.

To fix carbon dioxide into glucose

The Calvin cycle uses ATP and NADPH to convert CO2 into G3P, which is then used to form glucose and other carbohydrates.

RuBisCO

RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) catalyzes the reaction of CO2 with RuBP to form two molecules of 3-PGA.

RuBP

CO2 is fixed by combining with ribulose-1,5-bisphosphate (RuBP), a five-carbon molecule, in a reaction catalyzed by RuBisCO.

It provides reducing power (electrons) for the Calvin cycle

NADPH carries electrons and hydrogen ions to the Calvin cycle, where they are used to reduce 3-PGA to G3P.

Photophosphorylation

ATP is produced via photophosphorylation, which uses light energy to create a proton gradient across the thylakoid membrane, driving ATP synthase.

All of the above

Photosynthesis is affected by light intensity, CO2 concentration, and temperature, as they influence the light reactions and the Calvin cycle.

NADP+

Electrons from the electron transport chain are ultimately transferred to NADP+, reducing it to NADPH.

To produce additional ATP

Cyclic electron flow involves only photosystem I and produces ATP without generating NADPH, helping balance the ATP/NADPH ratio needed for the Calvin cycle.