Biology  5/e   Raven/Johnson
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Glucose: Glucose is a six-carbon sugar – a monosaccharide. Its molecular formula is C₆H₁₂0₆. It is the primary energy-storing molecule of living organisms. Each of its seven C-H bonds are energy-storing bonds. When those bonds are broken during the oxidation of glucose, energy is released. Many different food molecules can be converted to glucose, and all living organisms contain large amounts of glucose. Cellular respiration essentially begins with glucose.

Chemiosmosis: The vast majority of ATP that is produced by aerobic organisms comes from chemiosmosis. In this process, a proton pump forces protons across a cell membrane through special channels, making the proton concentration higher outside the membrane than inside. Following the concentration gradient, the protons diffuse back in across the membrane through a second type of special channel. Passage through this second channel causes ATP to be synthesized. It takes energy to run the proton pump, and therefore it takes energy to synthesize ATP by chemiosmosis.

Oxidation-reduction: Oxidation is the loss of an electron by an atom or molecule, reduction is the addition of an electron to an atom or molecule. The electrons are frequently accompanied by a hydrogen proton. Since electrons represent energy, oxidation involves the release of energy from the atom or molecule, and reduction stores energy in the atom or molecule. Whenever one substance is oxidized, another substance is reduced since the electrons taken from the oxidized substance must go somewhere (i.e., to the reduced substance). Oxidation-reduction reactions transfer energy within living organisms.

ATP: ATP is the universal energy currency of all cells. Almost every type of work that cells and organisms do is powered by ATP. High-energy covalent bonds link the three phosphate groups. When one of those bonds is broken, ADP and an inorganic phosphate are left, and 7.3 kcal/mole of energy are released. This is enough energy to drive endergonic reactions in the cell or perform other work. ATP is constantly being synthesized and broken down in living cells as it cycles and recycles between ADP + P_i and ATP. The energy required to synthesize ATP from ADP + P_i comes from photosynthesis or cellular respiration.

Exergonic and endergonic reactions: Free energy is the energy in a system that is available to do work. If the products of a reaction contain more free energy than the reactants, the reaction is endergonic. If the reactants contain more free energy than the products, the reaction is exergonic. Spontaneous reactions are exergonic. Endergonic and exergonic reactions are often coupled in living systems: the energy released by exergonic reactions is used to drive endergonic reactions.

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