Question

When a photosystem absorbs light, _____ . a. water forms and exits the cell b. electrons are transferred to ATP c. its special pair releases electrons d. rubisco fixes carbon

Short answer

The photogenerated excitation releases electrons from the special pair. Option c is correct.

Step-by-step solution

Locate the Role of Photosystems

Photosystems are protein complexes involved in the light-dependent reactions of photosynthesis. Their primary role is to absorb light energy and convert it into chemical energy. Focus on what happens to electrons when these complexes are activated by light.

Determine the Process Activated by Absorbing Light

Photosystems contain chlorophyll molecules that absorb photons, which excite electrons to a higher energy level. This change initiates a series of events related to electron movement.

Identify the Function of the Special Pair

Within the photosystem, there exists a chlorophyll molecule known as the 'special pair'. When it absorbs light energy, it becomes excited and is responsible for releasing high-energy electrons.

Select the Correct Multiple Choice Answer

Given that the special pair of chlorophyll molecules releases high-energy electrons when excited by light, the correct answer from the choices is 'its special pair releases electrons', which corresponds to option c.

Key concepts

Light-Dependent Reactions

In photosynthesis, the light-dependent reactions are a crucial first step. Unlike other stages, these reactions harness light energy directly and convert it into chemical energy.

This process takes place in the chloroplasts of plant cells, specifically within structures known as photosystems.

The light-dependent reactions are called so because they require sunlight to proceed. They generate two main forms of energy carrier molecules: ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate). These molecules are essential for driving the subsequent processes in photosynthesis known as the Calvin cycle, or light-independent reactions.

• ATP serves as an energy currency, providing power for other cellular activities.
• NADPH acts as a reducing agent, donating electrons for biosynthetic reactions.

Understanding light-dependent reactions provides insight into the energy conversion processes that are vital for life on Earth.

Chlorophyll

Chlorophyll is a pigment found in plants, algae, and cyanobacteria. It plays a central role in photosynthesis by capturing light energy from the sun. This pigment is primarily responsible for the green color of plants, as it reflects and transmits green wavelengths of light.

Chlorophyll molecules are embedded in the thylakoid membranes of chloroplasts and are a part of the core structure of photosystems. There are several types of chlorophyll, with chlorophyll a being the most common and crucial for the photosystems’ activity.

When chlorophyll absorbs light, it transfers energy via electrons to initiate the electron transport chain. This is a pivotal moment that leads to the creation of energy carriers necessary for plant growth and reproduction.

Electron Movement

When a photosystem absorbs light, it triggers a fascinating series of events involving electron movement. Energized by photons, chlorophyll releases electrons that enter the electron transport chain.

This electron movement is essential because it drives the synthesis of ATP and NADPH, both high-energy molecules. The process starts when electrons are excited to a higher energy level, allowing them to move from molecule to molecule within the photosystem.

• As electrons are passed along, they lose energy.
• The lost energy is used to pump hydrogen ions across the thylakoid membrane.
• This creates a gradient used to produce ATP through a process known as chemiosmosis.

Understanding how electrons move within photosystems is key to grasifying how plants convert solar energy into a usable form.

Special Pair

Within every photosystem, there is a unique component called the "special pair". These are two chlorophyll a molecules that are crucial for the release of high-energy electrons.

The special pair is strategically situated in the reaction center of the photosystem. When it absorbs light, it becomes excited and releases electrons to the electron transport chain, which is key in the conversion of light energy into chemical energy.

• The special pair is crucial in initiating the electron transport process.
• It ensures that electrons are at a high enough energy level for them to be useful in creating ATP and NADPH.

This special component highlights the incredible efficiency and specialization found in photosynthetic processes.