The Calvin Cycle A light-dependent cycle occurs in the chloroplast's thylakoid membrane. Using water, chlorophyll absorbs solar energy and converts it to chemical energy in this process. Water (H2O) is divided, and as a byproduct, oxygen (O2) is produced. Carbon dioxide (CO2) combines with nitrogen (N2) from the atmosphere to form ammonia (NH3), which is then converted into amino acids by bacteria in soil.
Carbon dioxide is also fixed by green plants through the process of photosynthesis. The majority of carbon dioxide is taken up by the leaf through the stomata for use in making carbohydrates. However, some carbon dioxide is lost through transpiration. That is, water is released into the atmosphere from the leaves during the process of photosynthesis.
Thus, light is required for photosynthesis to take place, since it is only under light conditions that electrons are transferred through the electron transport chain. This causes formation of NADPH and reduction of ferricyanide, which is used as a marker in enzyme assays.
Photosynthesis can occur only if there is light with wavelengths that can stimulate pigment molecules. Pigments such as chlorophylls and carotenoids absorb light at specific wavelengths and thus help plants capture light energy at different wavelengths.
Chlorophyll collects energy from sunshine and turns it into chemical energy via the light-dependent processes that occur at the thylakoid membrane. As water is broken down, the light-dependent processes produce oxygen as a byproduct. The reverse process occurs in darkness when carbon dioxide is fixed back into carbohydrate.
Oxygen is thus an essential part of plant metabolism. The amount of oxygen in the atmosphere is constant but because plants use oxygen to break down organic matter, remove pollutants from the soil and resist pathogens, their activity can have a large impact on the level it in the environment. Plants take up oxygen from the atmosphere and store it as glucose during photosynthesis. They release this stored oxygen when they need it for growth or during periods of stress such as when their roots are submerged in water. This allows them to avoid using energy storing molecules such as starch which would be needed if oxygen were being used up constantly converting carbon dioxide into sugar.
The light-dependent process that produces oxygen takes place inside chloroplasts which are organelles within cells that perform different functions depending on the type of cell. Under normal conditions these membranes self-repair themselves but under certain conditions such as excessive light exposure or lack of nutrients, they may become damaged causing the light-dependent process to start in the mitochondria instead.
The energy from sunshine is collected by chlorophyll and transformed into chemical energy in the form of electron carrier molecules such as ATP and NADPH in light-dependent processes. Photosystems I and II, both of which are found in chloroplast thylakoid membranes, capture light energy. The reaction centers of photosystem II transfer electrons to oxygen, forming water while protons are moved across the membrane, creating a transmembrane voltage difference. This voltage difference is used by complexes on the other side of the membrane to create an electrochemical gradient that can be used for synthesis of organic compounds via mitochondria or secretion of chemicals through vacuoles.
Photosystem I uses sunlight to oxidize reduced ferredoxin, reducing molecular oxygen while at the same time pumping hydrogen ions across the membrane, generating a potential difference that can be used by power-consuming reactions inside the cell. Reduced ferredoxin is also needed to reduce acetaldehyde to ethanol during fermentation. Fermentation produces carbon dioxide, which is then released when ethanol is oxidized in the respiration process described below.
Ethanol is further oxidized to acetic acid by alcohol dehydrogenase. Acetic acid is removed by esterases, which convert it to acetic acid. Finally, acetic acid is converted back to ethanol by acetate kinase using adenosine triphosphate (ATP) as a substrate. ATP is generated by mitochondrial oxidation of food nutrients.
As a consequence of the light-dependent processes, oxygen is released from the hydrolysis of water. The chemical energy obtained from light-dependent processes drives both the uptake of carbon in carbon dioxide molecules and the subsequent construction of sugar molecules in the Calvin cycle, which occurs in the stroma. Carbon dioxide leaves the reaction center in the form of CO2, and the energy stored in the sugar molecules fuels other parts of the plant or animal body.
In photosynthesis, sunlight is used to split water into hydrogen protons and oxygen atoms. The oxygen is then transported through the leaf to the atmosphere where it helps fuel plants and animals. The hydrogen protons are retained by the leaf's stomata to help make organic compounds such as glucose for storage in the stem or root tissues. Animals that use this method of oxygen transport to live mostly include fish, reptiles, and birds. These organisms can close their stomata during times of darkness when there is no risk of damage from oxygen, thus preventing more oxygen from entering their bodies than what they need at any given moment.
In some species of bacteria, the role of hydrolysis in photoreactions has been replaced by enzymes called "photolyases". Photolyases are only found in certain types of bacteria including Rhodobacter sp. And Heliobacterium sp. They repair ultraviolet radiation (UV) induced DNA damage without the need for water.
The Calvin cycle in the chloroplast stroma uses ATP and NADPH generated by photosystems' light-dependent processes. In a process mediated by the enzyme Rubisco, molecules of CO2 gas are fixed into molecules of 3-phosphoglycerate. This reaction is the first step in the synthesis of carbohydrates. As such, it is also called carbon fixation.
Enzymes that participate in light-dependent reactions include: ribulose bisphosphate carboxylase/oxygenase (Rubisco); phosphoenolpyruvate carboxykinase (PEPCK); pyruvate kinase (PK); glucose 6-phosphate dehydrogenase (G6PD). Other enzymes involved in carbon fixation include: sedoheptulose 1,7-bisphosphatase; fructose-1,6-bisphosphatase; glyceraldehyde-3-phosphate dehydrogenase.
In addition to the enzymes mentioned above, other proteins require light for their activity.
Light-dependent processes, as the name indicates, require sunshine. Light-dependent processes in the thylakoid membrane utilise light energy to produce ATP and NADPH. The Calvin cycle, which occurs in the stroma, draws energy from these molecules to produce GA3P from CO2. Carbon dioxide is reduced by photosynthesis, while water is oxidised to produce oxygen.
The term "light-dependent" means that these processes cannot happen without light. Photosynthesis uses light to break down water into hydrogen protons and oxygen atoms. These particles are then used by other enzymes in the cell to create more organic compounds. Organic chemicals are the building blocks of life. Without photosynthesis, none of us would be here today.
Some organisms can use other sources of energy instead. For example, some cyanobacteria can use heat for carbon fixation. However, they need sunlight for this process to work. Heat will not drive the reduction of carbon dioxide through the Calvin cycle.
Organisms can also use chemical compounds as sources of energy. For example, some chemoautotrophs (organisms that can make their own nutrients) use the energy from hydrogen ions to fix carbon dioxide into glucose. But they need sunlight to do this process efficiently. Chemically driven processes can replace some parts of photosynthesis, but not all of it.