Question

The water necessary for photosynthesis a. is split into \(\mathrm{H}_{2}\) and \(\mathrm{O}_{2}\) b. is directly involved in the synthesis of carbohydrates. c. provides the electrons to replace those lost in photosystem II. d. provides the \(\mathrm{H}^{+}\) needed to synthesize G3P. e. does none of the above.

Short answer

The correct answer is option c. Water splitting provides the electrons needed to replace those lost in photosystem II during photosynthesis.

Step-by-step solution

Analyse each option

Let's analyse each option. a. Water does split into \(\mathrm{H}_{2}\) and \(\mathrm{O}_{2}\). However, it's not the water that is split but the water molecule. So this option is incorrect. b. Water is not directly involved in synthesising carbohydrates, so this option is also incorrect. c. The splitting of water does provide the electrons that replenish those lost in photosystem II. d. While it's true that water splitting provides \(\mathrm{H}^{+}\) ions, these ions are not directly used to synthesize G3P. G3P synthesization primarily involves carbon dioxide. e. This statement is not true because water does play a role in photosynthesis.

Identify the correct answer

Having analysed each option, the correct answer is the one that provides a true statement regarding the role of water in photosynthesis. Based on this, the correct answer is option c. Water splitting provides the electrons that are used to replace those lost in photosystem II during photosynthesis.

Key concepts

Photosystem II

Photosystem II (PSII) is of paramount significance in the photosynthetic process, acting as the initial step in the light-dependent reactions of photosynthesis. It is here where the energy from sunlight is first harnessed by chlorophyll and other pigments. When light is absorbed by PSII, it propels electrons to a higher energy state.

These high-energy electrons are critical because they are passed down through a series of molecules, known as the electron transport chain, to ultimately produce energy-rich compounds that plants require for growth and maintenance. However, the elevation of electrons leaves PSII electron-deficient, which must be promptly rectified to sustain the photosynthetic process.

The deficit is filled by splitting water molecules, a process known as photolysis. When water (H_2O) molecules are split, they release electrons, hydrogen ions (H^+), and oxygen gas (O_2). The electrons are then supplied back to PSII, permitting the continuation of photosynthesis. This elucidates why water is not only vital for the hydration of the plant but also for its biochemical energy processing.

Water Splitting in Photosynthesis

During the light-dependent reactions of photosynthesis, specifically within photosystem II, water splitting plays a critical role. The splitting of water molecules, or photolysis, can be represented by the chemical equation:

2H_2O -> 4H^+ + 4e^- + O_2.

This reaction illustrates how water is enzymatically divided in the presence of light to yield protons (H^+), free electrons (e^-), and dioxygen (O_2). The electrons freshly liberated from water are essential because they replenish the electron supply in photosystem II, which loses electrons as it absorbs light. Additionally, the protons contribute to the formation of a proton gradient across the thylakoid membrane. Oxygen, as a byproduct, is expelled from the leaf stomata into the atmosphere, an action essential for aerobic life on Earth.

To easily remember, consider that the 'splitting' of water not only provides chemical ingredients that drive the photosynthetic machinery but also sustains the Earth's oxygen levels.

Photosynthesis Electron Transport

The electron transport chain in photosynthesis is a sequence of protein complexes and other molecules embedded in the thylakoid membrane of chloroplasts. Upon the activation of photosystem II by light, the chain is instrumental in shuttling electrons from the water split by PSII all the way to a molecule called NADP+ to form NADPH, an important carrier molecule for energy and reducing power.

This flow of electrons is fundamental to creating an electrochemical gradient across the thylakoid membrane, thanks to the translocation of protons into the lumen. The gradient powers an enzyme called ATP synthase to synthesize ATP from ADP and inorganic phosphate. ATP and NADPH then partake in the Calvin cycle, which occurs in the stroma of chloroplasts, to fabricate glucose from carbon dioxide, closing the loop on the conversion of solar energy into usable chemical energy.

In essence, the photosynthesis electron transport comprises the primary steps where light energy initializes a cascade of events converting it into chemical energy, forming the basis for virtually all life forms on our planet.