1Life requires energy. A cell uses energy to builc and maintain is structure, transport materials, manufacture products, move, grow,and reproduce. This energy ultimately materials, manufacture products, move, grow,and reproduce. This energy ultimately comes from the sun. The figure shows that in these energy conversions some energy is lost as heat. In photosynthesis the energy of sunlight is used to rearrange the atoms of CO2 and H2O to produce glucose and O2. In cellular respiration, O2 consumed as glucose is broken down to CO2 and H2O: the cell captures the energy released in ATP.2Respiration refers to an exchange of gases: An organism obtains O2 from its environment and releases CO2 as a waste product. Cellular respiration is the aerobic harvesting of and releases CO2 as a waste product. Cellular respiration is the aerobic harvesting of energy from food molecules by cells. As the runner in picture breathes in air, her lungs take up O2 and pass it to her bloodstream. The bloodstream carries the O2 to her muscle. Mitochondria in the muscle cells use the O2 in cellular respiration, harvesting energy from glucose and other organic molecules to generate ATP, which the cells then use to contract.3Generating ATP for cellular work is the fundamental function of cellular respiration. The balanced chemical equation summarizes cellular respiration as carried out by cells that use balanced chemical equation summarizes cellular respiration as carried out by cells that use O2 in harvesting energy from glucose. The equation tells us that atoms of the starting (reactants) molecules glucose and O2 regroup to form the products CO2 and H2O. Cellular respiration can produce up to 38 ATP molecules for each glucose molecule. 45The energy available to a cell is contained in the arrangement of electrons in the chemical bonds that hold an organic molecule like glucose. During cellular respiration, electrons are bonds that hold an organic molecule like glucose. During cellular respiration, electrons are transferred to oxygen as the carbon-hydrogen bonds of glucose are broken and the hydrogen-oxygen bonds of water form. The movement of electrons from one molecule to another is an oxidation-reduction reaction, or REDOX REACTION. IN a redox reaction, the loss of electrons from one substance is called OXIDATION, and the addition of electrons to another substance is called REDUCTION. A molecule is said to become oxidized when it looses one or more electrons and reduced when it gains one or more electrons. 6Two key players in the process of oxidizing glucose are an enzyme, called DEHYDROGENASE and a coenzyme called NAD+. NAD+ (nicotinamideadenine dinucleotide) is an organic and a coenzyme called NAD+. NAD+ (nicotinamideadenine dinucleotide) is an organic molecule that cells make from the vitamin niacin and use to shuttle electrons in redoxreactions. 7Cellular respiration consists of a sequence of steps that can be divided into three main stages. stages. STAGE 1: GLYCOLYSIS- occurs in the cytoplasmic fluid of the cell, that is, outside the organelles. Glycolysis begins respiration by breaking glucose into two molecules of a three-carbon compound called Pyruvate. STAGE 2: CITRIC ACID CYCLE- takes place within the mitochondria. It completes the breakdown of glucose by decomposing a derivative of pyruvate to carbon dioxide.-The main function of glycolysis and the citric acid cycle is to supply the third stage of respiration with electrons.STAGE 3: OXIDATIVE PHOSPHORYLATION involves the electron transport chain and a process known as chemiosmosis. NADH and related electron carrier,FADH2, shuttle electrons to the electron transport chain embedded in the inner mitochondrion memberane. Most the ATP produced by cellular respiration is generated by oxidative phosphorylation, which uses the energy released by the downhill fall of electrons from NADH and FADH2 to O2 to phosphorylate ADP. AS the electron transport chain passes electrons down the energy staircase, it also pumps hydrogen ions (H+) across the inner mitochondrial membrane into the narrow intermembrane space. The result is a concentration gradient of H+ across the membrane. In CHEMIOSMOSIS, the potential energy of this concentration gradient is used to make ATP. The concentration gradient drives the diffusion of H+ through ATP SYNTHASES, protein complexes built into the inner membrane that synthesize ATP. 8GLYCOLYSIS means the splitting of sugar. GLYCOLYYSIS begins with a single molecule of glucose and concludes with two molecules of pyruvate. In SUBSTRATE-LEVEL glucose and concludes with two molecules of pyruvate. In SUBSTRATE-LEVEL PHOSPHORYLATION an enzyme transfers a phosphate group from a substrate molecule directly to ADP, forming ATP. This process produces a small amount of ATP in both glycolysis and the citric acid cycle. 9The compounds that form between the initial reactant, glucose, and the final product, pyruvate, are known as INTERMEDIATES. The energy investment phase, actually consume pyruvate, are known as INTERMEDIATES. The energy investment phase, actually consume energy. IN this phase, ATP is used to energize a glucose molecule, which is then split into two small sugars that are now primed to release energy. 10The energy payoff phase, yield energy for the cell. In this phase, two NADH molecules are produced for each initial glucose molecule, and four ATP molecules are generated. Since produced for each initial glucose molecule, and four ATP molecules are generated. Since the first phase uses two molecules of ATP, the net gain to the cell is two ATP molecules for each glucose molecules that enters glycolysis. These two ATP molecules from glycolysis account for only 5% of the energy that a cell can harvest from a glucose molecule. 11As pyruvate forms at the end of glycolysis, it is transported from the cytoplasm into a mitochondrion, the site of the citric acid cycle. Pyruvate itself does not enter the citric acid mitochondrion, the site of the citric acid cycle. Pyruvate itself does not enter the citric acid cycle. A large, multi enzyme catalyzes three reactions.1.) A carboxyl group (COO) is removed from pyruvate and given off as a molecule of CO2 (this is the first step in which CO2 is produced)2.) The two-carbon compound remaining is oxidized while a molecule of NAD is reduced to NADH3.) A compound called COENZYME A, derived from B vitamin, joins with the two-carbon group to form a molecule called acetyl coenzyme A. Acetyl coenzyme A,