Photosynthesis converts these energy- depleted compounds (ADPand NADP+) back to the high energy forms (ATP and NADPH) and theenergy thus produced in this chemical form is utilized to drivethe chemical reactions necessary for synthesis of sugars andother carbon containing compounds (e.g., proteins, fats). Theproduction of high energy ATP and NADPH in plants occurs in whatis known as Light Phase Reactions (Z Scheme) (requiressunlight). The energy releasing reactions which converts themback to energy-depleted ADP and NADP is known as Dark PhaseReactions (Calvin Cycle) (does not require light) in whichthe synthesis of glucose and other carbohydrates occurs.
So we can summarize by saying that the photosynthetic plantstrap solar energy to form ATP and NADPH (Light Phase) and thenuse these as the energy source to make carbohydrates and otherbiomolecules from carbon dioxide and water (Dark Phase),simultaneously releasing oxygen in to the atmosphere. Thechemoheterotrophic animals reverse this process by using theoxygen to degrade the energy-rich organic products ofphotosynthesis to CO2 and water in order to generate ATP fortheir own synthesis of biomolecules.
Chloroplasts have many shapes in different species but aregenerally fusiform shaped (and much larger than mitochondria) andhave many flattened membrane-surrounded vesicles called thylakoidswhich are arranged in stacks called grana. Thesethylakoid membranes contain all of the photosynthetic pigments ofthe chloroplast and all of the enzymes required for Light Phasereactions. The fluid in the stroma surrounding the thylakoidvesicles contains most of the enzymes for Dark phase reactions.
The plants and cyanobacteria (which use water as a hydrogendonor and produce oxygen) have Photosystems I and II, whereas theless highly evolved other photosynthetic bacteria(which do notuse water as their hydgrogen donor and do not produce oxygen)have only Photosystem I.
Plant photosynthesis, both the Light Phase and Dark phasereactions, takes place in chloroplasts, which may be regarded asthe "power plants" of the green leaf cells. At night,when there is no sunlight energy, ATP continues to be generatedfor the plant's needs by respiration, i.e., oxidation of(photosynthetically produced) carbohydrate in mitochondria(similar to animals).
Plants absorb water through their roots, and carbon dioxide through their leaves. Some glucose is used for respiration, while some is converted into insoluble for storage. The stored starch can later be turned back into glucose and used in respiration. Oxygen is released as a by-product of photosynthesis.
While oxygen is necessary for the process of respiration, glucose plays a crucial role in the diet; and that explains why the photosynthesis is important for all the lifeforms on the planet - including humans.
In PS I, the electrons are again excited by harnessing the energy from photons, and the reduction of NADP to NADPH2 is achieved by utilizing electrons and protons.
Stroma is the site for the dark or light-independent reactions of photosynthesis.
In a broad chemical sense, the opposite of photosynthesis isrespiration. Most of life on this planet (all except in the deepsea vents) depends on the reciprocal photosynthesis-drivenproduction of carbon containing compounds by a series of reducing(adding electrons) chemical reactions carried out by plants andthen the opposite process of oxidative (removing electrons)chemical reactions by animals (and plants, which are capable ofboth photosynthesis and respiration) in which these carboncompounds are broken down to carbon dioxide and water.
When the chlorophyll molecule is excited by light, the energylevel of an electron in its structure is "boosted to ahigher energy level and this "excited" chlorophyll (nowis called an ) moves rapidly the the reactioncenter of the Photosystem I where it transfers its extra energyto an electron which is then expelled from the reaction centerand is accepted by the first member of a chain of electroncarriers and ultimately reaches NADP+, reducing it to NADPH. Thereaction center has lost an electron and this "electronhole" is filled by by stripping electrons from water whichleaves hydrogen ion (H+) and molecular oxygen (O2). The pathwayof electrons from water to NADP+ has "Z" shape whendiagramed and is refered to as the Z Scheme.
During this reaction, and water are converted into glucose and . The reaction requires , which is absorbed by a green substance called chlorophyll.
Photosynthetic organisms can be divided into two classes:those which produce oxygen and those which do not. Photosyntheticbacteria do not produce oxygen (in fact some of them calledanaerobes cannot tolerate oxygen) and this is considered a moreprimitive type of photosynthesis (in which the hydrogen donor ishydrogen sulfide, lactate or other compounds, but not water).Plants and one type of bacteria (cyanobacteria) do produceoxygen, an evolutionarily more advanced type of photosynthesis(in which the hydrogen donor is water).
They are also the sites for nitrogen fixation, the reactions of which are interconnected with the processes of photosynthesis and photorespiration, and play a protective role in plant cells.