In both lecture and lab, the significance of water (H2O), molecular oxygen (O2), and carbon dioxide (CO2) has been emphasized with regard to their energy content or participation in metabolic oxidation and reduction reactions. This lab has demonstrated that green leaf tissue shows both (A) photosynthetic light reactions and carbon fixation and (B) aerobic respiratory activities.

For each of the three molecules (water, oxygen, and carbon dioxide) describe the context of its production or consumption in photosynthesis or respiration. Be specific with regard to cellular locations and processes where production or consumption occurs.

In photosynthesis, there are two main stages: the light reactions and the carbon fixation reactions. Let's go through each of the three molecules (water, oxygen, and carbon dioxide) and explain their role in both processes:

1. Water (H2O):
Water plays a crucial role in both photosynthesis and respiration. In the light reactions of photosynthesis, water molecules are split via a process called photolysis. This occurs in the thylakoid membranes of the chloroplasts, which are specialized structures found in plant cells. Photolysis is driven by light energy and involves the splitting of water molecules into oxygen (O2), protons (H+), and electrons (e-). The oxygen produced is released into the atmosphere as a byproduct of photosynthesis, while the protons and electrons are used in the subsequent steps of the light reactions to generate energy-rich molecules.

In respiration, water is produced as a byproduct of the electron transport chain (located in the inner mitochondrial membrane in plant and animal cells) during the process of oxidative phosphorylation. Oxygen serves as the final electron acceptor in this chain, joining with electrons and protons to form water (H2O). The water produced in respiration is essential for maintaining the balance of water within the cell.

2. Molecular Oxygen (O2):
In photosynthesis, molecular oxygen is a byproduct of the light reactions. As discussed earlier, it is produced through the process of photolysis, where water molecules are split. Oxygen gas (O2) is released into the atmosphere as a waste product, while the protons and electrons generated from the photolysis process are utilized in further steps of the light reactions to produce energy carriers like ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate).

In respiration, molecular oxygen is consumed in the final step known as the electron transport chain. This process takes place in the mitochondria of plant and animal cells. Oxygen acts as the final electron acceptor, combining with electrons and protons to form water as a byproduct. This allows the cells to generate ATP, the primary energy currency of cells.

3. Carbon Dioxide (CO2):
In photosynthesis, carbon dioxide is incorporated into organic molecules through a process called carbon fixation. This occurs during the second stage of photosynthesis, known as the Calvin cycle or the dark reactions. In the stroma of chloroplasts, carbon dioxide molecules are combined with the energy-rich molecules generated in the light reactions (ATP and NADPH). This process is catalyzed by an enzyme called Rubisco. The incorporation of carbon dioxide enables the synthesis of glucose and other organic compounds, which are vital for the growth and maintenance of plants.

During respiration, carbon dioxide is produced as a waste product when organic molecules are broken down to release energy. The breakdown of glucose and other organic compounds takes place in the cytoplasm and mitochondria of animal and plant cells. Carbon dioxide is then released into the surrounding environment, eventually diffusing out of the organism.

To summarize, water is involved in both the light reactions and respiration, with water splitting occurring in the thylakoid membranes during photosynthesis and its formation as a byproduct of the electron transport chain in respiration. Oxygen is produced during photosynthesis by the photolysis of water and is consumed during respiration as the final electron acceptor in the electron transport chain to form water. Carbon dioxide is assimilated during photosynthesis in the Calvin cycle and released as a waste product during respiration when organic molecules are broken down for energy.