Question

Which of the following processes requires sunlight, ADP, and water? (A) Photosystem I (B) Photosystem II (C) Glycolysis (D) Calvin cycle (E) Electron transport system

Short answer

The process that requires sunlight, ADP and water is Photosystem II.

Step-by-step solution

Understanding the Biological Processes

Photosynthesis is the process by which green plants and some other organisms convert light energy, usually from the sun, into chemical energy in the form of glucose or other sugars. This process involves several different stages, including light-dependent reactions (which occurs in Photosystem I and II), and the light-independent reactions also known as the Calvin cycle or dark reactions.

Identifying the requirements of each process

Photosystem I is a type of photosynthetic system that requires light and is capable of performing photosynthesis, converting light energy into chemical energy. On the other hand, Photosystem II is the only known biological system that can take electrons from water, in the presence of sunlight and ADP, to produce ATP (Adenosine Triphosphate) and NADPH (Nicotinamide Adenine Dinucleotide Phosphate), which are then used in the Calvin cycle. Both Glycolysis and the Electron Transport System are steps in the process of cellular respiration, not photosynthesis, and thus do not need sunlight.

Choosing the correct process

Given the requirements of sunlight, ADP, and water, the only system among the choices that fits the criteria is Photosystem II, which is part of the light-dependent reactions in photosynthesis.

Key concepts

Photosystem I

Photosystem I is an essential part of photosynthesis, the process by which plants, algae, and some bacteria harness energy from sunlight to create chemical energy stored in glucose.

This system is specifically designed to absorb light and use it to energize electrons. These excited electrons are ultimately transferred to the molecule NADP+, forming NADPH.

• Photosystem I predominantly absorbs light at around 700 nm wavelength, commonly referred to as P700.
• It plays a critical role in the light-dependent reactions occurring in the thylakoid membranes of the chloroplasts.

Photosystem I works in tandem with Photosystem II, yet it occurs later in the process, ensuring that enough ATP and NADPH are available to support the energy-intensive reactions of the Calvin Cycle. Despite its later role, it is essential to completing the light reactions of photosynthesis.

Photosystem II

Photosystem II initiates the process of photosynthesis and is crucial because it utilizes sunlight, water, and ADP to produce ATP and NADPH, which are then used in further photosynthetic processes.

One of its unique features is its ability to split water molecules, releasing oxygen as a byproduct.

• This system is called Photosystem II because it was discovered after Photosystem I, despite occurring first in the photosynthesis sequence.
• The core pigment of Photosystem II absorbs light at a wavelength of about 680nm, often termed P680.

When sunlight hits Photosystem II, it excites electrons that are then passed along through various carrier proteins in the thylakoid membrane. These electrons eventually reach Photosystem I after passing through a chain which assists in the production of ATP, a key energy carrier within cells.

Calvin Cycle

The Calvin Cycle, also known as the light-independent reactions or dark reactions, takes place after the light-dependent reactions and utilizes the ATP and NADPH generated by Photosystem I and II.

It occurs in the chloroplasts' stroma, turning carbon dioxide into glucose, which serves as an energy source for the plant.

• The Calvin Cycle does not directly require light; however, it relies heavily on the energy carriers produced by the previous steps in photosynthesis.
• Often called "carbon fixation," the cycle converts atmospheric CO2 into an organic molecule that the plant can use, like glucose.

The Calvin Cycle cycles through three main phases: carbon fixation, reduction phase, and regeneration of RuBP (ribulose bisphosphate). This cycle is vital for the synthesis of high-energy sugars, essential to the survival of plants and fuel sources in ecosystems.

Light-dependent reactions

Light-dependent reactions are the first phase of photosynthesis where sunlight is absorbed and converted into chemical energy in the form of ATP and NADPH.

These reactions occur in the thylakoid membranes of chloroplasts and involve both Photosystem I and II.

• The absorption of sunlight energizes electrons in chlorophyll, and these electrons move through a sequence of proteins known as the electron transport chain.
• The process produced ATP through chemiosmosis and forms NADPH, both of which are utilized in the Calvin Cycle.

In these reactions, water is split, providing electrons and releasing oxygen as a byproduct. This phase is essential to capturing and storing energy from sunlight, setting the stage for the Calvin Cycle to synthesize glucose from carbon dioxide.