Or, What I’ve Learned in 12 Years Writing about Energy ( words, about 25 minutes reading time) Folks who pay attention to energy and climate issues are regularly treated to two competing depictions of society’s energy options. Nikola Tesla's article The Problem of Increasing Human Energy which first appeared in the June Century Magazine. Written shortly after his return from Colorado, this piece contains a comprehensive description of Tesla's vision regarding man's technological future. Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that can later be released to fuel the organisms' activities. This chemical energy is stored in carbohydrate molecules, such as sugars, which are synthesized from carbon dioxide and water – hence the name photosynthesis, from the Greek φῶς, phōs, "light", and σύνθεσις. The Ultimate Burrito has all your nutrients from 9 whole ingredients in the most cost effective, time efficient, and environmentally friendly form. THE TRAGEDY OF THE COMMON REVISITED by Beryl Crowe () reprinted in MANAGING THE COMMONS by Garrett Hardin and John Baden W.H. Freeman, ; ISBN
Chloroplast and Thylakoid In photosynthetic bacteria, the proteins that gather light for photosynthesis are embedded in cell membranes. In its simplest form, this involves the membrane surrounding the cell itself.
A typical plant cell contains about 10 to chloroplasts. The chloroplast is enclosed by a membrane. This membrane is composed of a phospholipid inner membrane, a phospholipid outer membrane, and an intermembrane space.
Enclosed by the membrane is an aqueous fluid called the stroma. Embedded within the stroma are stacks of thylakoids grana , which are the site of photosynthesis. The thylakoids appear as flattened disks. The thylakoid itself is enclosed by the thylakoid membrane, and within the enclosed volume is a lumen or thylakoid space.
Embedded in the thylakoid membrane are integral and peripheral membrane protein complexes of the photosynthetic system. Plants absorb light primarily using the pigment chlorophyll. The green part of the light spectrum is not absorbed but is reflected which is the reason that most plants have a green color. Besides chlorophyll, plants also use pigments such as carotenes and xanthophylls.
These pigments are embedded in plants and algae in complexes called antenna proteins. In such proteins, the pigments are arranged to work together.
Such a combination of proteins is also called a light-harvesting complex. Although all cells in the green parts of a plant have chloroplasts, the majority of those are found in specially adapted structures called leaves.
Certain species adapted to conditions of strong sunlight and aridity , such as many Euphorbia and cactus species, have their main photosynthetic organs in their stems. The cells in the interior tissues of a leaf, called the mesophyll , can contain between , and , chloroplasts for every square millimeter of leaf. The surface of the leaf is coated with a water-resistant waxy cuticle that protects the leaf from excessive evaporation of water and decreases the absorption of ultraviolet or blue light to reduce heating.
The transparent epidermis layer allows light to pass through to the palisade mesophyll cells where most of the photosynthesis takes place. Light-dependent reactions Main article: Light-dependent reactions In the light-dependent reactions , one molecule of the pigment chlorophyll absorbs one photon and loses one electron.
This electron is passed to a modified form of chlorophyll called pheophytin , which passes the electron to a quinone molecule, starting the flow of electrons down an electron transport chain that leads to the ultimate reduction of NADP to NADPH. In addition, this creates a proton gradient energy gradient across the chloroplast membrane , which is used by ATP synthase in the synthesis of ATP.
The chlorophyll molecule ultimately regains the electron it lost when a water molecule is split in a process called photolysis , which releases a dioxygen O2 molecule as a waste product. The overall equation for the light-dependent reactions under the conditions of non-cyclic electron flow in green plants is: The photosynthetic action spectrum depends on the type of accessory pigments present. For example, in green plants, the action spectrum resembles the absorption spectrum for chlorophylls and carotenoids with absorption peaks in violet-blue and red light.
In red algae, the action spectrum is blue-green light, which allows these algae to use the blue end of the spectrum to grow in the deeper waters that filter out the longer wavelengths red light used by above ground green plants.
The non-absorbed part of the light spectrum is what gives photosynthetic organisms their color e. The light-dependent reactions are of two forms: In the non-cyclic reaction, the photons are captured in the light-harvesting antenna complexes of photosystem II by chlorophyll and other accessory pigments see diagram at right. The absorption of a photon by the antenna complex frees an electron by a process called photoinduced charge separation.
The antenna system is at the core of the chlorophyll molecule of the photosystem II reaction center. That freed electron is transferred to the primary electron-acceptor molecule, pheophytin. The electron enters a chlorophyll molecule in Photosystem I. There it is further excited by the light absorbed by that photosystem. The electron is then passed along a chain of electron acceptors to which it transfers some of its energy. The energy delivered to the electron acceptors is used to move hydrogen ions across the thylakoid membrane into the lumen.
The cyclic reaction takes place only at photosystem I. Once the electron is displaced from the photosystem, the electron is passed down the electron acceptor molecules and returns to photosystem I, from where it was emitted, hence the name cyclic reaction. Water photolysis Main articles: Photodissociation and Oxygen evolution Linear electron transport through a photosystem will leave the reaction center of that photosystem oxidized. Elevating another electron will first require re-reduction of the reaction center.
The excited electrons lost from the reaction center P of photosystem I are replaced by transfer from plastocyanin , whose electrons come from electron transport through photosystem II. Photosystem II, as the first step of the Z-scheme, requires an external source of electrons to reduce its oxidized chlorophyll a reaction center, called P The source of electrons for photosynthesis in green plants and cyanobacteria is water.
Two water molecules are oxidized by four successive charge-separation reactions by photosystem II to yield a molecule of diatomic oxygen and four hydrogen ions. The electrons yielded are transferred to a redox-active tyrosine residue that then reduces the oxidized P This resets the ability of P to absorb another photon and release another photo-dissociated electron.
Food web and food chain
The oxidation of water is catalyzed in photosystem II by a redox-active structure that contains four manganese ions and a calcium ion; this oxygen-evolving complex binds two water molecules and contains the four oxidizing equivalents that are used to drive the water-oxidizing reaction Dolai's S-state diagrams.
The hydrogen ions are released in the thylakoid lumen andd therefore contribute to the transmembrane chemiosmotic potential that leads to ATP synthesis. Oxygen is a waste product of light-dependent reactions, but the majority of organisms on Earth use oxygen for cellular respiration , including photosynthetic organisms.
Light-independent reactions and Carbon fixation In the light-independent or "dark" reactions, the enzyme RuBisCO captures CO2 from the atmosphere and, in a process called the Calvin cycle , it uses the newly formed NADPH and releases three-carbon sugars, which are later combined to form sucrose and starch. The overall equation for the light-independent reactions in green plants is : The simple carbon sugars produced by photosynthesis are then used in the forming of other organic compounds, such as the building material cellulose , the precursors for lipid and amino acid biosynthesis, or as a fuel in cellular respiration.
The latter occurs not only in plants but also in animals when the energy from plants is passed through a food chain. The fixation or reduction of carbon dioxide is a process in which carbon dioxide combines with a five-carbon sugar, ribulose 1,5-bisphosphate , to yield two molecules of a three-carbon compound, glycerate 3-phosphate , also known as 3-phosphoglycerate. This product is also referred to as 3-phosphoglyceraldehyde PGAL or, more generically, as triose phosphate.
Most 5 out of 6 molecules of the glyceraldehyde 3-phosphate produced is used to regenerate ribulose 1,5-bisphosphate so the process can continue.
The triose phosphates not thus "recycled" often condense to form hexose phosphates, which ultimately yield sucrose , starch and cellulose. The sugars produced during carbon metabolism yield carbon skeletons that can be used for other metabolic reactions like the production of amino acids and lipids. Carbon concentrating mechanisms Overview of C4 carbon fixation In hot and dry conditions, plants close their stomata to prevent water loss.
Some plants have evolved mechanisms to increase the CO2 concentration in the leaves under these conditions. C4 carbon fixation Plants that use the C4 carbon fixation process chemically fix carbon dioxide in the cells of the mesophyll by adding it to the three-carbon molecule phosphoenolpyruvate PEP , a reaction catalyzed by an enzyme called PEP carboxylase , creating the four-carbon organic acid oxaloacetic acid.
Oxaloacetic acid or malate synthesized by this process is then translocated to specialized bundle sheath cells where the enzyme RuBisCO and other Calvin cycle enzymes are located, and where CO2 released by decarboxylation of the four-carbon acids is then fixed by RuBisCO activity to the three-carbon 3-phosphoglyceric acids.
The physical separation of RuBisCO from the oxygen-generating light reactions reduces photorespiration and increases CO2 fixation and, thus, the photosynthetic capacity of the leaf.
Many important crop plants are C4 plants, including maize, sorghum, sugarcane, and millet. Plants that do not use PEP-carboxylase in carbon fixation are called C3 plants because the primary carboxylation reaction, catalyzed by RuBisCO, produces the three-carbon 3-phosphoglyceric acids directly in the Calvin-Benson cycle. CAM plants have a different leaf anatomy from C3 plants, and fix the CO2 at night, when their stomata are open.
CAM plants store the CO2 mostly in the form of malic acid via carboxylation of phosphoenolpyruvate to oxaloacetate, which is then reduced to malate. Decarboxylation of malate during the day releases CO2 inside the leaves, thus allowing carbon fixation to 3-phosphoglycerate by RuBisCO.
Sixteen thousand species of plants use CAM. They cannot cross the membrane as they are charged, and within the cytosol they turn back into CO2 very slowly without the help of carbonic anhydrase.