Biochemistry : Light Reactions

Study concepts, example questions & explanations for Biochemistry

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Example Questions

Example Question #1 : Light Reaction Energetics

In photosynthesis, what is the purpose of absorbing light into chloroplasts?

Possible Answers:

The energy from the absorbed light generates NADPH from the electrons of water molecules, ultimately leading to the creation of carbon dioxide

The energy from the absorbed light generates NADPH from the electrons of water molecules, ultimately setting up a proton gradient

The energy from the absorbed light causes all of the electrons in the chloroplast to jump to higher energy levels

The energy from the absorbed light moves the electrons from NADPH to water molecules, ultimately setting up a proton gradient

The energy from the absorbed light causes the generation of NADH from the electron of water molecules, ultimately setting up a proton gradient

Correct answer:

The energy from the absorbed light generates NADPH from the electrons of water molecules, ultimately setting up a proton gradient

Explanation:

In photosynthesis, light is absorbed in order to move electrons from water molecules to NADPH.  The reduction of  to NADPH is accompanied by the movement of protons across a membrane which sets up a gradient similar to that of oxidative phosphorylation.  The protons eventually run through ATP synthase and ATP is formed.

Example Question #1 : Light Reactions

Photosynthesis consists of a light phase and a dark phase. The light phase precedes the dark phase and supplies it with __________.

Possible Answers:

Correct answer:

Explanation:

During photosynthesis, the light phase is responsible for creating  which the dark phase then consumes as a part of its process. In addition to  is used in the dark phase as a high energy substrate to work properly.

Example Question #1 : Other Light Reaction Concepts

In photosynthesis, if photosystem II absorbs 12 photons, how many molecules of  would be produced?

Possible Answers:

Correct answer:

Explanation:

Absorbing four photons by photosystem II creates one oxygen molecule, so absorbing 12 would produce 3 molecules of .

Example Question #2 : Light Reactions

During photosynthesis there is both a light phase and a dark phase. If the light phase were to continue unabated, but the dark phase came to a halt, which of the following is most likely to occur?

Possible Answers:

A proportional decrease in  

Decrease in the level of 

An increase in the production of carbohydrates 

Decreased sensitivity of the chloroplast to incoming light

Increased sensitivity of the chloroplast to incoming light

Correct answer:

Decrease in the level of 

Explanation:

During photosynthesis, the dark phase follows the light phase. The light phase produces  which, along with , is fed into the dark phase where it is consumed (becomes  and ). If the light phase continues working, but the dark phase does not, the  and  created during the light phase will not be consumed. Thus, there will be a decrease in the level of  and  (the correct answer). The relative proportion of  will actually increase.

Carbohydrate production is a result of the dark phase working properly, so their levels would decrease in this instance. The sensitivity of the chloroplast to light would not change.

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