• Autotrophs are organisms capable of sustaining themselves without ingesting organic molecules.  Photoautotrophs are the producers in ecological systems, using the energy of sunlight to synthesize organic molecules from CO₂ and H₂O. Some bacteria are  chemoautotrophs, using inorganic substances rather than sunlight as the source of energy for the formation of their organic molecules.
• Heterotrophs must ingest other organisms or their byproducts to obtain energy and carbon skeletons. They are completely dependent on photosynthesizers to produce food and oxygen to drive aerobic respiration in their mitochondria.

• In autotrophic eukaryotes, photosynthesis occurs inside chloroplast, organelles enclosing an elaborate system of thylakoid membranes that are layered in places in stack like grana and separate and outer stroma from an inner thylakoid space.
• All chloroplasts contain the green pigment chlorophyll, which resides in the thylakoid membranes and absorbs the light energy that initiates photosynthesis.
• The chloroplast in plants are especially dense in the cells of the mesophyll, a green tissue in the interior of the leaf, which obtains its CO₂ from and exports its O₂ to the environment through stomata.
• The conversion of CO₂  to sugar occurs in the stroma. Vascular bundles transport the sugar to other parts of the plant and provide chloroplasts in the leaves with water from the roots.
• Photosynthetic prokaryotes lack chloroplasts, but their chlorophyll is built into the plasma membrane or vesicle membranes inside the cell.

How Plants Make Food: An Overview

• The complex process of photosynthesis can be summarized by the following equation:

• The chloroplast splits water into hydrogen and oxygen, incorporating the electrons of hydrogen into the energy rich bonds of sugar molecules.  Photosynthesis is thus an endergonic redox process in which water is oxidized and carbon dioxide is reduced.
• There are two, linked stages of photosynthesis, the light reactions and the Calvin cycle.  The light reactions in grana produce ATP by photophosphorylation and split water, evolving oxygen and forming NADPH by transferring electrons from water to NADP⁺.
• The Calvin cycle occurs in the stroma and uses ATP for energy and NADPH for reducing power to form sugar from CO₂. Although the Calvin cycle, sometimes called the dark reactions, does not require light directly, it usually occurs during the day, when the light reactions are providing ATP and NADPH.

How the Light Reactions Capture Solar Energy

• Sunlight is a form of electromagnetic energy that travels in waves.  The range of wavelengths of this radiation constitutes the electromagnetic spectrum, part of which is detected by us as the colors of visible light.
• The wavelengths of light are emitted in discrete energy packets called photons, each with a fixed quantity of energy inversely proportional to its wavelength.  The two wavelengths most effective in driving photosynthesis are those perceived by the human eye as red and blue.
• A pigment is a substance that absorbs specific wavelengths of light, determined by using a spectrophotometer.  The action spectrum of photosynthesis and the absorption spectrum of chlorophyll a, however, do not directly correspond.  Accessory pigments, chlorophyll b and various carotenoids, have molecular structures that enable them to absorb different wavelengths of light and pass their energy on to chlorophyll a.
• A pigment goes from a ground state to an excited state when a photon boosts one of its electrons to higher energy orbital.   In isolated pigments, the electron immediately returns to the ground state, releasing the energy as light (fluorescence) and or heat.
• The pigments of chloroplasts are built into the thylakoid membrane near molecules known as primary electron acceptors, which trap the high energy electrons before they return to the ground state in a light driven redox reaction.  Energy stored in these electrons powers the synthesis of ATP and NADPH.
• Accessory pigments in chloroplasts are clustered in an antenna complex of a few hundred molecules surrounding a molecule of chlorophyll a at the reaction center.  Photons absorbed anywhere in the antenna can pass this energy along to energized this chlorophyll a, which then passes its electrons to a nearby primary electron acceptor.  The antenna complex, the reaction center chlorophyll, and the primary electron acceptor make up one of many photosystems, light harvesting units built into the thylakoid membrane.
• There are two kinds of photosystems .  Photosystem I contains P700, and photosystem II P680, chlorophyll a molecules at the reaction center that have different absorption characteristics due to specific properties of associated proteins.
• The flow of energized electrons in the light reactions can be cyclic or noncyclic.  Cyclic electron flow starts when photosystem I absorbs two photons of light, causing P700 to donate two electrons to an electron transport chain located on the thylakoid membrane.   A series of redox reactions returns two electrons to the ground state in P700, generating a proton motive force across the thylakoid membrane.  The passage of hydrogen ions through an ATP synthase enzyme drives the chemiosmotic formation of ATP from ADP.
• Noncyclic electron flow, the more prevalent pathway in nature, involves both photosystems and produces NADPH and oxygen in addition to ATP.  Photons of light boost two electrons from each photosystem to higher energy levels.  Electrons from P700 in photosystem I are trapped by NADP⁺, which stores them in the form of cyclic photophosphorylation when they pass from the primary electron acceptor in photosystem II to the same transport chain of photosystem I that functions in cyclic electron flow..  Electrons at the end of the chain are accepted by P700, and the electron "holes" in P680 are filled by electrons from water, which is split into hydrogen ions and oxygen.  The overall process powers the endergonic transfer of electrons from water to NADP⁺ and the endergonic phosphorylation of ADP by chemiosmosis.
• Both chloroplasts and mitochondria use similar ATP synthases to generate ATP by chemiosmotic proton gradients.  These gradients are driven by a redox transfer of electrons down a sequence of increasingly electronegative components in a transport chain of a specialized membrane.  In the chloroplasts, the thylakoid membrane pumps protons from the stroma into the thylakoid compartment.  In the mitochondrion, the cristae pump protons from the matrix to the intermembrane space.  The major  difference is that light, rather than food energy, powers the process in the chloroplast.

How the Calvin Cycle Makes Sugar

• The Calvin cycle is organized in a cyclic metabolic pathway in the stroma that combines carbon dioxide with ribulose bisphosphate a five carbon sugar.  Then, using electrons from NADPH and energy from the hydrolysis of ATP, the cycle synthesizes the three carbon sugar glyceraldehyde phosphate in a series of reactions.  Most of the glyceraldehyde phosphate is reused in the cycle as an intermediate for reconversion to RuBP, by some can exit the cycle and be converted to glucose or other essential organic molecules.

Photorespiration

• On dry, hot days, plants close their stomata to conserve water, and oxygen from the light reactions builds up.  Oxygen is a competitive inhibitor of rubisco, the enzyme that incorporates CO₂ into RUBP.  When O₂ substitutes for CO₂  in the active site of rubisco, an intermediate is formed that leaves the cycle to be oxidized to CO₂ and H₂O in the mitochondria.  This process, called photorespiration, consumes oxygen, evolves carbon dioxide, produces no ATP, and decreases photosynthetic output.

C₄ Plants

• Most plants are C₃ plants, named after the number of carbons in the first stable intermediate of the Calvin cycle.
• C₄ plants are adapted to dry conditions and avert photorespiration by prefacing the Calvin cycle with a series of reactions that incorporate carbon dioxide into four carbon compounds in specialized mesophyll cells.  The final compound, malic acid, is then exported to photosynthetic bundle sheath cells, where carbon dioxide is released for use in the Calvin cycle.

CAM Plants

• Some plants use CAM metabolism as an adaptation for carbon fixation in a hot and dry environment.  These plants open stomata during the night and incorporate the carbon dioxide that enters into a variety of organic acids that are store in the vacuoles of mesophyll cells.  During the day, the stomata close completely which conserves water, and the carbon dioxide is released from the organic acids for use in the Calvin cycles while light reactions are supplying ATP and NADPH.

The Fate of Photosynthetic Products

• Vascular bundles export carbohydrates made in green cells to nonphotosynthetic parts of the plant.  Mitochondria degrade about one half of the carbohydrate made by photosynthesis to form ATP.  Much of the remaining carbohydrate is converted to a variety of molecules, including cellulose.
• Excess organic material is stockpiled as starch and stored in the leaves, roots, tubers, and fruit.  Heterotrophs consume much of this organic material.