Introduction:
Photosynthesis occurs in a wide variety of organisms and it comes in different forms. These include a form of photosynthesis that does not produce oxygen (anoxygenic) and a form that does (oxygenic). Anoxygenic photosyntehsis is found in four different bacterial groups: purple bacteria, green sulfur bacteria, green nonsulfur bacteria and heliobacteria. oxygenic phtosyntehsis is found in cyanobacteria, seven groups of algae, and essentially all land plants. These two types of phtosynthesis share similariteis in the types of pigments they use to trap light energy, but they differ in the arrnagement and action of these pigments.
The overall equation for photsynthesis is the following: 6CO2 (carbon dioxide) + 12 H2O (water) + light —-C6H12O6 (glucose) + 6H2O (water) + 6O2 (oxygen).
This is the reverse of the recaction for respiration where glucose is oxidized to CO2 using O2 as the electron acceptor. In photosynthesis, CO2 is reduced to glucose using electrons gained from the oxdiation of water. The oxdiation of H2O and the reduction of CO2 requires energy that is provided by light.
Photosynthesis requires light dependent reactions which use the energy in sunlight to make ATP and reduce the electron carreir NADP+ to NADPH. This also oxidzes water to provide electrons and produces O2. Photosynthesis also requires light independent reactions that use this ATP and NADPH to power the synthesis of organic molecules from CO2 in the air.
In plants, photosynthesis takes place in chloroplasts. The internal membrane of chloroplasts, called the thylakoid membrane, is a continous phospholipid bilayer organized into flattened sacs called thylakoid disks. These are stacked in columns called grana. This form three compartments: the thylakoid membrane itself, the spaces inside and ouside this membrane. The thylakoid membrane contains the enzmatic machinery to make ATP, and chlorophll and other photosynthetic pigments that capture lgiht energy. The compartment outside the thylakoid membrane system is called the stroma.
The thylakoid membrane contains the enzymatic machinery to make ATP, and chlorophyll and other photosynthetic pigments that capture light energy. In the thyladoid membrane, photosyntetic pigments, are orgnaized into photosystems that absorb light, which excites an electron that can be passed to an electorn carrier.
Carbohydrates contain many C-H bonds and are highly reduced compared with CO2. To build carbohydrates, cells use energy and a source of electrons produced by the light dependent reactions of the thylakoids.
The cycle of reactions that allow carbon fixation is called the Calvin cycle. The key step that makes the reduction of CO2 possible is the attachment of CO2 to a highly specilized organic molecule, ribulose 1,5-bisphosphate (RuBP). CO2 reacts with RuBP to form a transient 6 carbon intermediate that splits into two molecules. The overall reaction is called the carbon fixation reaction because inorgnaic carbon (CO2) is incorporated into an organic form. The enzyme that carries out this reaction, ribulose biphosphate carboxylase/oxygenase (rubisco), is a large, 16 subunt enzyme found int he chloroplast stroma. The Calvin cycle can be thought of as divided into 3 phases: (1) carbon fixation, (2) reduction and (3) regeneration of RuBP.
The carboxylation and oxidation of RuBP are catalyzed at the same active site on rubisco, and CO2 and O2 compete with each other at this site. Under normal conditions at 25C, the rate of the carboxylation reaction is fourt times that of the oxidation reaction, meaning that 20% of photosythetically fixed carbon is lost to photorespiration. This loss reises usbstantially as temperature increases, becasue under hot, arid conditions, specilized openings in the leaft called stomata close to conserve water. This cuts off the supply of CO2 entering the elaf and does not allow O2 to exit. As a result, the low CO2 and high O2 conditions within the leaf favor photorespiration which incorproates O2 into RuBP which undergoes additional reactions that actually release CO2.
The reduction in the yield of carbohydrate as a result of photorespiration is not trivial. C3 plants losoe between 25-50% of their photosynthetically fixed carbon in this way. The rate depends largely on temeprature. However, C4 plants include corn, sugarcane and other grasses can fix carbon using PEP carboxylase in mesophyll cells. This reaction produces the orgnic acid oxalacetate, which is coverted to malate and transported to bundle sheath cells that surround the leaft veins. Wihting the bundle sheath cells, malate is decarboxyalted to produce pyruvate and CO2. Becasue the bundle sheath cells are impemeable to CO2, the local level of CO2 is high and carbon fixation by rubisco and the Calvin cycle is efficient.
Chlorophylls absorb photons within narrow energy ranges. Two kinds of chlorophyll in plants are chlorophill a and chlorophll b preferentially absorb violet blue and red light. Chlrophyll a is the main photosynthetic pigment in plans and cyanobacterai and is the only pigment that can act direclty to convert light energy to chemical energy. Chlorophyll b, acting as an accessory pigment, complements and adds to the lgiht absorption of chlorophyll a. Chlrophylls absorb photons by means of an excitation process analogous to the photoelectric effect. These pigments contain a complex ring structure, called a porphyrin ring, with alterating single and double bonds. At the center of the ring is a magnesium atom. Photons excite electrons in the porphyrin ring, which are then channeled away through the latenrating carbon single and double bond systems.
Types of Organisms that use Photosynthesis:
Phytoplankton, (microalgae): are similar to terrestrial plants in that they contain chlorophyll and require sunlight in order to live and grow. Most phytoplankton are buoyant and float in the upper part of the ocean, where sunlight penetrates the water. Phytoplankton also require inorganic nutrients such as nitrates, phosphates, and sulfur which they convert into proteins, fats, and carbohydrates.
The two main classes of phytoplankton are dinoflagellates and diatoms.
–Dinoflagellates use a whip-like tail, or flagella, to move through the water and their bodies are covered with complex shells.
–Diatoms also have shells, but they are made of a different substance and their structure is rigid and made of interlocking parts. Diatoms do not rely on flagella to move through the water and instead rely on ocean currents to travel through the water.
In a balanced ecosystem, phytoplankton provide food for a wide range of sea creatures including shrimp, snails, and jellyfish. When too many nutrients are available, phytoplankton may grow out of control and form harmful algal blooms (HABs). These blooms can produce extremely toxic compounds that have harmful effects on fish, shellfish, mammals, birds, and even people.
Marine alae are responsible for half of Earth’s primary production and form the basis of the oceanic food chain.
Algal Blooms: are ephermeral events of phytoplankton proliferation that occur annually across the glove covering thousands of square kilometers. Upon bloom demise, most of teh fixed carbon is trnsferred to higher torpic levels either via herbivorous predtion or through heteterotrophic bacteria and their predators (a process called the microbial loop), being largely recycled and respired along teh way. Only a minor fraction of teh algal biomass is sequestered into teh deep sea. (Vardi, “Viral infection switches teh balance between bacterail and eukaryotic recyclers of organic matter during coccolithophore blooms” Nature communications, 2023).
Viral infection enhances lysis of host cells and release of dissolved organic matter, leading to bacterail growth and respiration at the expense of organic carbon sinking, in a process coined the “viral shunt”. Thus, infection could accelerate teh biologcially driven sequenstration of carbon into the deep sea. One group used enclosures immersed with water containing natural planktonic communiteis and nutrients. Each enclosure showed viral infection of E. hyxleyi by its large double stranded DNA virus EhV.