Question

If green plant cells are incubated with \(0^{18}\) - labelled water, which of the following molecules will become radioactive when the cells are exposed to light? (a) \(\mathrm{O}_{2}\) (b) \(\mathrm{CO}_{2}\) (c) \(\mathrm{H}_{2} \mathrm{O}\) (d) Sugar

Short answer

The molecule that will become radioactive when green plant cells incubated with 18-labelled water are exposed to light is (a) 2.

Step-by-step solution

Understanding the Process of Photosynthesis

Recognize that photosynthesis in green plants involves the conversion of carbon dioxide and water into sugars using sunlight as energy. Oxygen is released as a byproduct. This process takes place in chloroplasts where the light-dependent and light-independent (Calvin cycle) reactions occur.

Identifying the Role of Water in Photosynthesis

Acknowledge that during the light-dependent reactions, water molecules are split into oxygen, protons, and electrons in a process known as photolysis. If the water molecules are labeled with an isotope, such as oxygen-18 (18), the byproducts of the water molecule's photolysis will carry this radioactive label.

Determining the Radioactive Molecule

Since oxygen is released during the photolysis of water in the light-dependent reactions of photosynthesis, this oxygen will carry the 18 label if the water used is labeled with 18. Therefore, 18-labelled oxygen will be the molecule that becomes radioactive when exposed to light.

Key concepts

Light-Dependent Reactions

The light-dependent reactions are the initial phase of photosynthesis, where light energy harnessed by chlorophyll and other pigments is used to generate ATP and NADPH. These reactions take place within the thylakoid membranes of chloroplasts. During these reactions, light photons excite electrons to a higher energy state, initiating a flow of electrons through the photosystems and electron transport chain.

The energy derived from this electron flow is used to pump protons across the thylakoid membrane, creating a proton gradient. Subsequently, ATP synthase utilizes this proton gradient to produce ATP, which stores this energy in a usable form. Concurrently, the electrons reduce NADP+ to NADPH, another molecule that carries and transfers energy. Both ATP and NADPH are then used in the subsequent stage of photosynthesis, the Calvin cycle, to produce glucose.

Photolysis of Water

Photolysis of water is an integral part of the light-dependent reactions in photosynthesis. During photolysis, water molecules (H2O) are split by specific enzymes within the thylakoid lumen. This process produces oxygen, protons (H+), and electrons. The equation for photolysis can be represented as: 2H2O → 4H+ + 4e- + O2.

The protons contribute to the proton gradient used for ATP synthesis, while the electrons are used to replenish the electrons lost by chlorophyll in photosystem II. Photolysis is crucial because it not only supplies the electrons needed for the light reactions but also contributes to the formation of oxygen, the vital by-product of photosynthesis that sustains life on Earth.

Oxygen-18 Tracking in Photosynthesis

Oxygen-18 (18O) is a stable isotope of oxygen used to trace the path of oxygen atoms during photosynthesis. When water molecules labeled with 18O are utilized in the light-dependent reactions, any oxygen produced through photolysis will include the heavier isotope. In research, this allows scientists to track and confirm the source of oxygen released by plants.

During experiments, when plants are exposed to light in the presence of 18O-labeled water, the resulting oxygen gas released contains the distinct isotope, indicating that the oxygen atoms in 18O water were incorporated into molecular oxygen (O2). This process aids in illustrating the overall flow of atoms in photosynthesis and underscores that the oxygen produced during photosynthesis comes solely from water molecules, not carbon dioxide (CO2).

Calvin Cycle

The Calvin cycle, also known as the light-independent reactions, occurs in the stroma of chloroplasts and does not directly require light to proceed. Instead, it utilizes the ATP and NADPH produced during the light-dependent reactions to catalyze the conversion of carbon dioxide (CO2) into organic molecules like glucose. The cycle consists of three main stages: carbon fixation, reduction phase, and regeneration of the starting molecule ribulose bisphosphate (RuBP).

During the carbon fixation stage, CO2 is attached to RuBP by the enzyme Rubisco, resulting in a six-carbon compound that quickly splits into two three-carbon compounds. In the reduction phase, these compounds are converted into glyceraldehyde-3-phosphate (G3P) using ATP and NADPH. Finally, the majority of G3P molecules are utilized to regenerate RuBP, enabling the cycle to continue. The net result of multiple turns of the Calvin cycle is the production of a sugar molecule, such as glucose, which can be used by the plant for energy or building materials.