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What is the primary purpose of the light-dependent reactions in photosynthesis?

• Production of glucose

• Conversion of light energy to chemical energy

• Formation of chlorophyll

• Reduction of carbon dioxide

Answer explanation

The primary purpose of the light-dependent reactions in photosynthesis, as stated in the question, is the conversion of light energy to chemical energy. In this step, sunlight is used to produce ATP and NADPH which are energy-rich compounds. These are crucial for the next stage of photosynthesis. Hence, the correct option is 'Conversion of light energy to chemical energy'.

In which membrane discs within the chloroplast do the light-dependent reactions occur?

• Stroma

• Thylakoids

• Lamellae

• Inner membrane

Answer explanation

The light-dependent reactions of photosynthesis occur in the thylakoids. These are disc-shaped structures found within the chloroplast. They contain the photosynthetic pigments that capture light energy and convert it to chemical energy. The other options, stroma, lamellae, and inner membrane, are parts of the chloroplast but do not directly participate in the light-dependent reactions.

How are the two photosystems involved in the light-dependent reactions classified?

• By their location in the thylakoid membrane

• By their maximal absorption wavelengths

• By the number of electrons they contain

• By their role in ATP production

Answer explanation

The two photosystems involved in the light-dependent reactions are classified based on their maximal absorption wavelengths. Each photosystem has a specific range of wavelengths it can absorb due to the types and arrangements of pigments in its antenna system. This distinct characteristic allows them to play unique roles in capturing solar energy during photosynthesis.

What happens to the delocalized electrons within photosynthetic pigments when they absorb light energy?

• They become excited

• They form chlorophyll

• They combine with H+ ions

• They are used to reduce NADP+

Answer explanation

In photosynthesis, pigments such as chlorophyll absorb light energy. This absorbed energy is used to excite delocalized electrons within the photosynthetic pigments. When these electrons become excited, they are able to participate in the photosynthetic reactions. Therefore, the answer to the question is 'They become excited'.

How are the excited electrons from Photosystem II used to generate a proton gradient?

• They are used to synthesize ATP.

• They are transferred to carrier molecules in the ETC.

• They combine with oxygen.

• They contribute to the formation of glucose.

Answer explanation

In photosynthesis, the excited electrons from Photosystem II are not used to synthesize ATP or combine with oxygen or contribute to the formation of glucose. Instead, they are transferred to carrier molecules in the Electron Transport Chain (ETC). This process initiates a series of redox reactions which eventually leads to the generation of a proton gradient.

What is the electrochemical gradient or proton motive force used for in light-dependent reactions?

• To synthesize ATP

• To release oxygen

• To transport electrons

• To produce glucose

Answer explanation

The electrochemical gradient or proton motive force in the light-dependent reactions is primarily used to synthesize ATP. This process is a crucial part of photosynthesis where the energy from light is used to create a high-energy electron donor and a lower-energy electron acceptor. Energy from the subsequent electron transfer is utilized to synthesize ATP, not for oxygen release, electron transport, or glucose production.

Which enzyme is responsible for catalyzing the synthesis of ATP using the proton gradient in the thylakoid membrane?

• ATPase

• ATP synthase

• ATP translocase

• ATPase synthetase

Answer explanation

The correct answer is ATP synthase. This enzyme is found in the thylakoid membrane of chloroplasts, where it plays a key role in photosynthesis. It uses the proton gradient, also known as chemiosmotic potential, to catalyze the synthesis of ATP. The other options, such as ATPase, ATP translocase, and ATPase synthetase, are not responsible for this process.

What process is responsible for ATP production in the light-dependent reactions?

• Chemiosmosis

• Photolysis

• Glycolysis

• Fermentation

Answer explanation

The process responsible for ATP production in the light-dependent reactions is chemiosmosis. In this process, the energy derived from the light-dependent reactions is used to pump hydrogen ions across the thylakoid membrane, creating a gradient. This gradient is then used to drive ATP synthesis.

How are the de-energized electrons from Photosystem II replenished?

• By de-energized electrons from Photosystem I

• By electrons from ATP

• By electrons from glucose

• By electrons released from water via photolysis

Answer explanation

In the process of photosynthesis, the de-energized electrons in Photosystem II are replenished by the electrons released from water via photolysis. This is a critical step called 'oxidative photophosphorylation', where water molecules are split to release oxygen and hydrogen ions, producing electrons to replace those lost in Photosystem II.

What is the role of NADPH in photosynthesis?

• To carry oxygen in the blood

• To reduce carbon dioxide

• To transport electrons

• To form glucose

Answer explanation

In photosynthesis, NADPH is pivotal for the reduction of carbon dioxide. It plays a role in the Calvin cycle, where it provides the necessary electrons and protons to help transform carbon dioxide into glucose. Hence, the role of NADPH in photosynthesis is to reduce carbon dioxide, not to carry oxygen, transport electrons, or directly form glucose.