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**Photosynthesis Overview and Biological Processes:**
Photosynthesis converts sunlight into chemical energy and fixes carbon dioxide into sugar.
– Water is split to liberate oxygen, and most photosynthetic organisms are photoautotrophs.
– Oxygenic photosynthesis releases oxygen and is essential for fueling activities of photosynthetic organisms.
– Light-dependent reactions involve absorption of light energy, while light-independent reactions like the Calvin cycle synthesize sugars.
– Bacteria perform anoxygenic photosynthesis using different mechanisms.

**Impact of Photosynthesis on Climate and Chemical Reactions:**
Photosynthesis captures carbon dioxide from the air and binds carbon in plants, soils, and harvested products.
– Approximately 130 terawatts of energy is captured globally through photosynthesis.
– Photosynthetic organisms convert around 100-115 billion tons of carbon into biomass yearly.
Photosynthesis is vital for climate processes and maintaining oxygen levels.
– The general equation for photosynthesis was proposed by Cornelis van Niel, and water is used as the electron donor in oxygenic photosynthesis.

**Light-dependent Reactions, Z Scheme, and Water Photolysis:**
Chlorophyll absorbs photons, leading to electron transfer and ATP synthesis.
– The Z scheme involves cyclic and non-cyclic light-dependent reactions.
– Water photolysis in photosystem II releases oxygen and hydrogen ions.
– Electrons from water oxidation in photosystem II are crucial for the electron flow.

**Photosynthetic Bacteria Structures and Plant Adaptations:**
– Photosynthetic bacteria have light-gathering proteins in cell membranes.
– Chloroplasts in plants and algae contain chlorophyll for light absorption.
– Leaves are the main photosynthetic organs in plants, containing chloroplasts in mesophyll cells.
– Waxy cuticle on leaf surfaces reduces water loss, and palisade mesophyll cells are primary sites of photosynthesis.
– Antenna proteins in chloroplasts contain various pigments for light absorption.

**Carbon Fixation Mechanisms and Efficiency Comparison:**
– C4 plants and CAM plants have carbon concentrating mechanisms for efficient carbon fixation.
– Plants convert light into chemical energy with 3-6% efficiency.
– Solar panels have higher efficiency in converting light into electric energy.
Photosynthesis efficiency varies with light frequency, intensity, and CO2 levels.
– Alarm photosynthesis in calcium-oxalate-accumulating plants operates as a biochemical pump for carbon collection.

Photosynthesis (Wikipedia)

Photosynthesis (/ˌftəˈsɪnθəsɪs/ FOH-tə-SINTH-ə-sis) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their activities. Photosynthetic organisms use intracellular organic compounds to store the chemical energy they produce in photosynthesis. Photosynthesis is usually used to refer to oxygenic photosynthesis, a form of photosynthesis where the photosynthetic processes produce oxygen as a byproduct and synthesize carbohydrate molecules like sugars, starches, glycogen, and cellulose to store the chemical energy. To use the chemical energy stored in these organic compounds, the organisms' cells metabolize the organic compounds through another process called cellular respiration. Photosynthesis is largely responsible for producing and maintaining the oxygen content of the Earth's atmosphere, and it supplies most of the biological energy necessary for complex life on Earth.

Schematic of photosynthesis in plants. The carbohydrates produced are stored in or used by the plant.
Composite image showing the global distribution of photosynthesis, including both oceanic phytoplankton and terrestrial vegetation. Dark red and blue-green indicate regions of high photosynthetic activity in the ocean and on land, respectively.

Some bacteria also perform anoxygenic photosynthesis, which use bacteriochlorophyll to split hydrogen sulfide as a reductant instead of water, and sulfur is produced as a byproduct instead of oxygen. Archaea such as Halobacterium also perform a type of non-carbon-fixing anoxygenic photosynthesis, where the simpler photopigment retinal and its microbial rhodopsin derivatives are used to absorb green light and power proton pumps to directly synthesize adenosine triphosphate (ATP). Such archaeal photosynthesis might have been the earliest form of photosynthesis evolved on Earth, going back as far as the Paleoarchean, preceding that of cyanobacteria (see Purple Earth hypothesis).

Although photosynthesis is performed differently by different species, the process always begins when energy from light is absorbed by proteins called reaction centers that contain photosynthetic pigments or chromophores. In plants, these proteins are chlorophyll (a porphyrin derivative that absorbs the red and blue spectrums of light, thus reflecting a green color) held inside organelles called chloroplasts, which are most abundant in leaf cells, while in bacteria they are embedded in the plasma membrane. In these light-dependent reactions, some energy is used to strip electrons from suitable substances, such as water, producing oxygen gas. The hydrogen freed by the splitting of water is used in the creation of two further compounds that serve as short-term stores of energy to later drive other reactions: reduced nicotinamide adenine dinucleotide phosphate (NADPH) and adenosine triphosphate (ATP), the "energy currency" of cells.

In plants, algae, and cyanobacteria, sugars are synthesized by a subsequent sequence of light-independent reactions called the Calvin cycle. In the Calvin cycle, atmospheric carbon dioxide is incorporated into already existing organic carbon compounds, such as ribulose bisphosphate (RuBP). Using the ATP and NADPH produced by the light-dependent reactions, the resulting compounds are then reduced and removed to form further carbohydrates, such as glucose. In other bacteria, different mechanisms such as the reverse Krebs cycle are used to achieve the same end.

The first photosynthetic organisms probably evolved early in the evolutionary history of life and most likely used reducing agents such as hydrogen or hydrogen sulfide, rather than water, as sources of electrons. Cyanobacteria appeared later; the excess oxygen they produced contributed directly to the oxygenation of the Earth, which rendered the evolution of complex life possible. Today, the average rate of energy capture by photosynthesis globally is approximately 130 terawatts, which is about eight times the current power consumption of human civilization. Photosynthetic organisms also convert around 100–115 billion tons (91–104 Pg petagrams, or a billion metric tons), of carbon into biomass per year. That plants receive some energy from light — in addition to air, soil, and water — was first discovered in 1779 by Jan Ingenhousz.

Photosynthesis is vital for climate processes, as it captures carbon dioxide from the air and then binds carbon in plants and further in soils and harvested products. Cereals alone are estimated to bind 3,825 Tg (teragrams) or 3.825 Pg (petagrams) of carbon dioxide every year, i.e. 3.825 billion metric tons.

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