AP Biology : Understand light dependent reactions

Study concepts, example questions & explanations for AP Biology

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Example Questions

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Example Question #1 : Understand Light Dependent Reactions

What is the first step in photosynthesis?

Possible Answers:

Photons present in light strike and excite electrons in chlorophyll

Nutrients from the soil are turned into carbohydrates when combined with water

Water is split by chlorophyll

Light causes oxygen to pair with hydrogen, forming water for the plant

Light reduces sugars present in the plant cells to energized derivatives

Correct answer:

Photons present in light strike and excite electrons in chlorophyll

Explanation:

Photosynthesis begins with energy from light being converted to chemical energy. The only answer explanation accurately explaining this process describes the conversion of light energy (photons) to chemical energy (excited electrons).

Photons present in light strike and excite electrons in chlorophyll.

The photons are highly energized light particles, which strike chlorophyll and excite and free electrons in the chlorophyll molecule. These excited electrons pass through the electron transport chain and their energy is collected to make ATP.

Example Question #1 : Understand Light Dependent Reactions

Which of the following is a by-product of the light reactions in photosynthesis?

Possible Answers:

Carbon dioxide

Oxygen

Sugar

Water

Correct answer:

Oxygen

Explanation:

During the light reactions water is split and the by-product is oxygen. Oxygen is then released back into the atmosphere. Sugars are produced during the Calvin cycle and are not a direct product of the light reactions.

Example Question #3 : Understand Light Dependent Reactions

The light reactions of photosynthesis are responsible for the production of which of the following molecules?

Possible Answers:

Producing sugars

Converting solar energy to chemical energy

Producing carbon dioxide

Producing water

Correct answer:

Converting solar energy to chemical energy

Explanation:

The light reactions are the steps of the photosynthesis that function to convert solar energy into chemical energy that can be used during the Calvin cycle. The light reactions use solar energy to reduce   to  and to convert  to .  

Example Question #1 : Understand Light Dependent Reactions

Where do the light reactions of photosynthesis occur?

Possible Answers:

In the thylakoid membranes

In the phospholipid bilayer

In the stroma

In the cell wall

In the mitochondrial membrane

Correct answer:

In the thylakoid membranes

Explanation:

The light reactions of photosynthesis occur in the thylakoid membranes inside of chloroplasts in plant cells. Photosynthesis depends on the buildup of a proton gradient across a membrane to generate ATP. The thylakoid is an organelle present only in plants. The phospholipid bilayer and cell wall surround plant cells, and are not involved in photosynthesis.

Example Question #2 : Understand Light Dependent Reactions

When a molecule of NAD⁺ gains a hydrogen atom, the molecule becomes?

Possible Answers:

hydrolyzed

oxidized

plasmolyzed

redoxed

reduced

Correct answer:

reduced

Explanation:

The molecule is reduced because when a molecule gains an electron, it is said to be reduced. The answer oxidized would have been chosen if the molecule lost an electron. Plasmolysis has nothing to do with the gain or loss of an electron, and the term redoxed does not fit into this question. Hydrolysis is when a bond is broken using water, so that is incorrect as well.

Example Question #2225 : Ap Biology

What is the purpose of the light dependent reaction?

Possible Answers:

To convert solar energy into chemical energy

To convert solar energy into electrical energy

To produce CO2 necessary for light independent reaction

To convert chemical energy to solar energy

Correct answer:

To convert solar energy into chemical energy

Explanation:

The light dependent reaction converts solar energy to chemical energy, using the photosystem to capture light energy, and ultimately produce ATP and NADPH. Pigment molecules in the photosystem absorb photons of light, which then reach the chlorophyll, exciting electrons in the chlorophyll. The excited electron then is transferred to a primary electron acceptor of the electron transport chain (and is replaced by the splitting of water (to produce O2, H+ and e-)).

Example Question #2226 : Ap Biology

During the light dependent reaction, what is the purpose of creating a proton gradient in the thylakoid lumen?

Possible Answers:

To power ATP hydrolysis

To create 

To generate 

To power ATP synthase

Correct answer:

To power ATP synthase

Explanation:

The ATP synthase protein is embedded in the thylakoid membrane. When sunlight is captured by photosystem II in the light dependent reaction, water is split, producing O2 and 2 H+ ions and 2 electrons. As the electrons flow down the electron transport chain, this energy is used to pump hydrogen ions from the stroma into the thylakoid space, producing a high concentration of hydrogen ions inside the thylakoid lumen. Eventually, the hydrogen ions then flow from the area of high concentration (in the thylakoid lumen) to an area of low concentration (in the stroma ), through the ATP synthase protein. The movement of the hydrogen ions through this structure combines ADP with phosphate, producing ATP.

Example Question #2227 : Ap Biology

Which does not occur during the light dependent reaction?

Possible Answers:

NADPH is produced

ATP is produced

 is split, producing H+ ions, oxygen, and electrons

carbon fixation is aided by the enzyme Rubisco

Correct answer:

carbon fixation is aided by the enzyme Rubisco

Explanation:

Carbon fixation, catalyzed by Rubisco, occurs during the first step of the light independent reaction.

Example Question #2231 : Ap Biology

What products of the light dependent reaction are necessary for the light independent reaction to occur?

Possible Answers:

ATP and NADP+

ATP and NAPH

ADP and NADP+

ADP and NADP+

Correct answer:

ATP and NAPH

Explanation:

 ATP and NADPH are produced during the light dependent reaction, and are reactants necessary for the light independent reaction (Calvin Cycle) to occur. During the light independent reaction, ATP and NADPH are used to convert phosphoglycerate molecules (PGA) to glyceraldehyde-3-phosphate molecules (G3P). G3P is then used to produce glucose.

Example Question #2232 : Ap Biology

Which is true of light dependent reactions?

Possible Answers:

GTP is a reactant

 is a reactant

NADP+ is a product

ATP is a product

Correct answer:

ATP is a product

Explanation:

Light dependent reactions convert solar energy to chemical energy (in the form of ATP and NADPH). Sunlight and H2O are the reactant of the light dependent reaction. The sunlight is captured by photosystem II, and is used to excite electrons. Splitting of H2O produces hydrogen ions used to establish the proton gradient, and electrons produced replace the electrons transferred to the primary electron acceptor.

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