Photosynthesis

Photosynthesis Photosynthesis is a process in which green plants use energy from the sun to transform water, carbon dioxide, and minerals into oxygen and organic compounds. It is one example of how people and plants are dependent on each other in sustaining life. Photosynthesis happens when water is absorbed by the roots of green plants and is carried to the leaves by the xylem, and carbon dioxide is obtained from air that enters the leaves through the stomata and diffuses to the cells containing chlorophyll. The green pigment chlorophyll is uniquely capable of converting the active energy of light into a latent form that can be stored (in food) and used when needed. Photosynthesis provides us with most of the oxygen we need in order to breathe. We, in turn, exhale the carbon dioxide needed by plants. Plants are also crucial to human life because we rely on them as a source of food for ourselves and for the animals that we eat.

Light Reaction Plants have to break the bonds of two stable compounds, CO2 and H2O, rearrange electrons, and produce two compounds which are less stable relative to the first two, ATP Adenosine Triphosphate (ATP) : A common form in which energy produced in the light reaction of photosynthesis stored in living systems; consists of a nucleotide (with ribose sugar) with three phosphate groups. and NADPH Nicotine adenine dinucleotide phosphate (NADP+): A substance to which electrons are transferred from photosystem I during photosynthesis; the addition of the electrons reduces NADP, which acquires a hydrogen ion to form NADPH, which is a storage form of energy that can be transferred to the Calvin Cycle for the production of carbohydrate. . It would not be profitable for the plants to do this using their own energy. Instead plants use an energy source that is readily available to them- light. In the light dependent processes (light reactions Light reaction : The photosynthetic process in which solar energy is harvested and transferred into the chemical bonds of ATP; can occur only in light. ) light strikes chlorophyll in such a way as to excite electrons to a higher energy state. In a series of reactions, called a redox reaction, the energy is converted (by an electron transport process) into ATP and NADPH, or the energy components of plants. Water is split in Electron transport : C oupled series of oxidation/reduction reactions during which ATP is generated by energy transfer as electrons move from high reducing state to lower reducing state. the process, releasing oxygen as a by-product of the reaction. The ATP and NADPH are Dark reaction : The photosynthetic process in which food (sugar/carbohydrate) molecules are formed from carbon dioxide from the atmosphere with the use of ATP and NADPH; can occur in the

dark as long as energy source is present. then used to make C-C bonds in the Light Independent Process (Dark Reactions). The total process of the "light reactions" are the net result of two net reactions and result in the formation of ATP and NADPH, or plant energy components. One reaction involves the splitting of water. This process is an oxidative reaction that requires light, and may be written as: 12 H2O -----------------------> 6 O2 + 24 H+ + 24elight or radiant energy

The oxidation Oxidation reaction : The combination of a substance with oxygen; a reaction in which the atoms in an element lose electrons and the valence of the element is correspondingly increased. In photosynthesis this means that electrons are removed from oxygen by light energy in order to reconstitute them by way of a reduction reaction to produce ATP and NADPH. of water is accompanied by a reduction reaction Reduction reaction : A decrease in positive valence or an increase in negative valence by the gaining of electrons. resulting in the formation of a compound, called nicotinamide adenine dinucleotide phosphate (NADPH). The total reaction is written here: NADP+ + (oxidized form)

H20 ------------> NADPH + (reduced form)

H+

+

O (oxygen)

The second reaction involved in the light reactions is yet another reaction resulting in the formation of a highly energetic compound, called adenosine triphosphate, (ATP). As this reaction involves the addition of a phosphate group (labeled, as Pi) to a compound called, adenosine diphosphate (ADP) during the light reaction, it is called photophosphorylation: ADP + Pi ------------> ATP Think of the light reaction, as a process by which organisms "capture and store" radiant energy as they produce oxygen gas. This energy is stored in the form of chemical bonds of the compounds NADPH and ATP.

Dark Reaction In the light independent process (dark reaction), carbon dioxide from the atmosphere is captured and modified by the addition of hydrogen to form carbohydrates. Why is supplemental Carbohydrate : Any of a group of organic compounds that includes sugars, starches, celluloses, and gums and serves as a major energy source in the diet of plants and animals. These compounds are produced by photosynthetic plants and contain only carbon, hydrogen, and oxygen, usually in the ratio 1:2:1. Glucose is the major plant carbohydrate and the end product of photosyntheis. CO2 not used at night when it is needed in the dark reactions? The answer is that the dark reaction takes place in the presence of usable (or already created by light reaction) energy, or available ATP and NADPH. It happens that the peak in available energy is during photosythetically active periods, i.e. when the lights are on. So the term "dark reactions" can be a little misleading. The incorporation of carbon dioxide into organic compounds is known as the Calvin Cycle (after Melvin Calvin for which he won a 1961 Nobel Prize in chemistry), or carbon fixation and is the major process involved in the dark reaction. The energy for this comes from the first phase of the photosynthetic process with the production of ATP and NADPH and takes place in the stroma Stroma : The connective tissue framework of an organ, gland, or other structure, as distinguished from the tissues performing the special function of the organ or part. Site of the dark reaction of photosynthesis. of plant leaves. Living systems cannot directly utilize light energy, but can, through a complicated series of reactions, convert it into C-C bond energy that can be released by glycolysis Glycolysis : An ATP-generating metabolic process that occurs in nearly all living cells in which glucose is converted in a series of steps to pyruvic acid. The metabolic breakdown of glucose and other sugars that releases energy in the form of ATP. and other metabolic processes. The energy contained in both NADPH and ATP is used to reduce carbon dioxide to glucose, a type of sugar (C6H12O6). This reaction, shown below, does not require light, and it is often referred to as the "dark reaction". The 24 hydrogen ions and 24 electrons represent the energy obtained from ATP and NADPH of which the specifics will be skipped here for simplicity. A simple web search can garner this information if need be. The total dark reaction is as follows: 6 CO2 + 24 H+ + 24 e- ------> C6H12O6 (glucose) + 6 H2O The chemical bonds present in glucose Glucose : A product of photosynthesis and an

important source of physiological energy for plants and animals. Glucose is a sugar, or carbohydrate. contain a considerable amount of potential energy. This stored energy is released whenever glucose is catabolized Catabolism : The metabolic breakdown of complex molecules into simpler ones, often resulting in a release of energy. to drive cellular processes. The carbon skeleton in glucose also serves as a source of carbon for the synthesis of other important biochemical compounds such as lipids, amino acids, and nucleic acids. A lot of glucose is transformed into cellulose Cellulose : A polysaccharide made up of many glucose molecules chemically bonded together. The most abundant compound on earth. It comprises the bulk of cell walls of plants where it occurs as microfibrils. , which comprises the bulk of Lipids : Diverse class compounds, including fats, oils, fatty acids, triglycerides and steroids essential for membrane formation, energy stores, and fuel molecules. cell Amino acid : A ny one of a class of simple organic compounds containing carbon, hydrogen, oxygen, nitrogen, and in certain cases sulfur. These compounds are the building blocks of proteins and enzymes. They are characterized by the presence of a carboxyl group (COOH) and an amino group (NH2) attached to the same carbon at the end of the compound. walls vital to plant structure. In simplest terms, the process of photosynthesis can be viewed as one-half of the carbon cycle in plants. In this half, energy from the sun is captured and transformed into plant usable energy, which can be utilized by higher organisms in the food chain through ingestion or for plant energy. The release of energy during the metabolic reconversion of glucose to water and carbon dioxide represents the second half of the carbon cycle and is termed cellular respiration.