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. (3 points; one each for a thorough treatment of each molecule)
1. Water (H2O) in Photosynthesis and Respiration:
Photosynthesis:
In photosynthesis, water molecules play a crucial role in the light-dependent reactions that occur in the chloroplasts of plant cells. During this process, water molecules are consumed as reactants and are split into oxygen (O2), protons (H+), and electrons (e^-) through a process called photolysis. Photolysis takes place in the thylakoid membrane of the chloroplasts, specifically in the photosystem II (PSII) complex. The electrons released during photolysis replace the ones lost by chlorophyll, ensuring the continuation of the light reactions.
Respiration:
In cellular respiration, water molecules are produced as a byproduct of the electron transport chain (ETC) during oxidative phosphorylation. The ETC occurs within the inner mitochondrial membrane. During respiration, glucose and other organic molecules are broken down in the presence of oxygen through a series of chemical reactions. The electrons derived from these reactions are passed along the ETC, and oxygen acts as the final electron acceptor. The combination of electrons, protons, and oxygen produces water.
2. Oxygen (O2) in Photosynthesis and Respiration:
Photosynthesis:
In photosynthesis, oxygen gas is released as a byproduct of the light-dependent reactions. As mentioned earlier, water molecules are split during photolysis in the thylakoid membrane of chloroplasts. The released oxygen atoms combine to form molecular oxygen (O2). This process is often referred to as oxygenic photosynthesis, as it is responsible for oxygen generation in the atmosphere.
Respiration:
In cellular respiration, oxygen serves as the final electron acceptor in the ETC. Within the mitochondria, oxygen molecules accept electrons at the end of the electron transport chain, combining with protons to form water. This process, known as oxidative phosphorylation, produces energy in the form of ATP through a series of redox reactions. The oxygen consumed during respiration is vital for efficiently extracting energy from glucose and other organic molecules.
3. Carbon Dioxide (CO2) in Photosynthesis and Respiration:
Photosynthesis:
During photosynthesis, carbon dioxide is consumed during the process of carbon fixation. This occurs in the stroma of the chloroplasts, specifically in the enzyme-catalyzed reaction known as the Calvin cycle or the dark reactions. Here, carbon dioxide molecules are converted into organic compounds, primarily glucose, by incorporating carbon atoms into sugar molecules. This process is essential for the synthesis of complex carbohydrates and the subsequent storage of energy in plants.
Respiration:
In cellular respiration, carbon dioxide is produced as a byproduct of the breakdown of organic molecules, particularly glucose, during glycolysis, the citric acid cycle, and oxidative phosphorylation. These processes take place in the cytoplasm and mitochondria of cells. Carbon dioxide is released into the surrounding environment, where it can be removed from the body through exhalation. The production of carbon dioxide during respiration is indicative of the complete oxidation of glucose and other organic molecules in the presence of oxygen.
Water (H2O):
Water plays a crucial role in both photosynthesis and respiration. In photosynthesis, water acts as a source of electrons for the light-dependent reactions. During the light reactions, water molecules are split through a process called photolysis, which occurs in the thylakoid membrane of the chloroplast. The splitting of water molecules releases oxygen as a byproduct and provides electrons for the photosynthetic electron transport chain.
In respiration, water is produced as a byproduct of the electron transport chain during oxidative phosphorylation. This occurs in the inner mitochondrial membrane of eukaryotic cells or the plasma membrane of prokaryotic cells. As electrons flow through the electron transport chain, they combine with molecular oxygen and protons (H+) to form water (H2O). This final step in respiration helps to generate ATP, the energy currency of the cell.
Oxygen (O2):
Oxygen is essential for aerobic respiration, which is the most efficient way to produce ATP in cells. During respiration, molecular oxygen acts as the final electron acceptor in the electron transport chain. As electrons pass through the various protein complexes in the inner mitochondrial membrane, they combine with oxygen and protons to form water. This process, known as oxidative phosphorylation, takes place in the mitochondria of eukaryotic cells or the plasma membrane of prokaryotic cells.
In photosynthesis, oxygen is released as a byproduct of the light-dependent reactions. These reactions occur in the thylakoid membrane of the chloroplast. During photosynthesis, water molecules are split, and oxygen is released into the surrounding environment. This oxygen originates from the photolysis of water and is essential for supporting aerobic life on Earth.
Carbon dioxide (CO2):
Carbon dioxide is a key molecule in photosynthesis as it serves as the carbon source for carbon fixation. In photosynthesis, carbon dioxide is taken up by plants or other photosynthetic organisms and enters the chloroplast. Within the chloroplast, the enzyme RuBisCO catalyzes the fixation of carbon dioxide into organic molecules during the process known as the Calvin cycle. This carbon fixation occurs in the stroma of the chloroplast.
In respiration, carbon dioxide is generated as a byproduct of the breakdown of organic molecules in a process called oxidative metabolism. During the citric acid cycle (also known as the Krebs cycle or TCA cycle), carbon dioxide is released as a waste product when acetyl-CoA is fully oxidized. This process takes place in the mitochondrial matrix of eukaryotic cells or the cytoplasm of prokaryotic cells. The generated carbon dioxide is then expelled from the cell into the surrounding environment.