Biology 107 Cellular Respiration October 2, 2002

Cellular Respiration I Student Objectives:As a result of this lecture and the assigned reading, you should understand the following: 1.Cell release chemical energy by means of an exergonic process called cellular respiration, the aerobic harvesting of energy from food molecules by cells. Cellular respiration is the energy-releasing chemical breakdown of molecules and the storage of energy from that breakdown in a form (e.g., ATP) the cell can use to perform work. 2.Cellular respiration is the energy-releasing chemical breakdown of molecules and the storage of energy from that breakdown in a form (e.g., ATP) the cell can use to perform work.

Cellular Respiration II 3.Normally there is an oxidation of the organic molecule causing the hydrogen atoms (electrons and their accompanying protons) to be removed from the carbon atoms and eventually combined with oxygen (which is thereby reduced). 4.In cellular respiration, the electrons go from higher energy levels to lower energy levels, and energy is released. This energy is released over many steps as electrons move to successively lower energy levels. Some of that energy is lost as heat; a portion of that energy (40%) is captured in the terminal phosphate bonds of ATP. 5.The efficiency in living systems is due to the fact that energy release occurs over the course of a series of controlled reaction steps.

Cellular Respiration I 6.The harvesting of energy involves the rearrangement of electrons in chemical bonds. The common theme is that a cell transfers energy from one molecule to another by coupling an exergonic reaction (energy-releasing) to an endergonic reaction (energy- storing). The energy released was stored in the specific arrangement of a molecule's covalent bonds, and the energy stored is in the new covalent bonds formed. 7.In summary, cellular respiration rearranges electrons in chemical bonds. These are redox reactions. Because an electron transfer requires both a donor and an acceptor, an electron leaves one molecule only when it contacts another molecule that attracts it more strongly. 8. In respiration, there are two main coenzymes derived from B complex vitamins. First is NAD +,(nicotinamide adenine dinucleotide) which in part is derived from B 3, niacin. The second coenzyme is FAD (flavin adenine dinucleotide), which in part is derived from B 2, riboflavin.

Cellular Respiration I 9.Glucose supplies energy to form ATP by two related processes: 1) glycolysis and 2) cellular respiration. The products of glycolysis are reactants used in respiration. 10.In glycolysis ("splitting of sugar"), the 6-carbon glucose molecule is split into two 3-carbon molecules, pyruvate. The net energy harvested from the glycolysis reactions is in the form of ATP and NADH. a.This production of ATP in glycolysis is by the direct, enzyme- mediated transfer of a phosphate group from a substrate to ADP by the mechanism called substrate phosphorylation. This is different from electron transport (oxidative) phosphorylation, which requires oxygen and a transport system. b.Glycolysis occurs in the cytosol and does not require oxygen (i.e., it is an anaerobic process).

Cellular Respiration I 11.In the presence of oxygen, the pyruvates are fed into the second stage of energy capturing, cellular respiration. 12.In the absence of oxygen, the pyruvate is converted to either lactic acid or ethanol. This conversion process is known as fermentation, and it produces no ATP. Fermentation is a mechanism for cells to replenish the supply of NAD + that the cell is using in glycolysis. 13.The reactions of glycolysis follow essentially the same routes in prokaryotes and eukaryotes, except the products of fermentation are more varied under anaerobic conditions.

Energy Cycle in Ecosystems

ATP Supplies Energy for Cellular Work

Exergonic Reactions – Advantage of Multistep Process in Transfer of Energy to ATP

Summary of Multistep Reactions Used to Generate ATP in Eukaryotic Cells

Structure of NAD + /NADH

Early Steps in Glycolysis

Later Steps in Glycolysis

Substrate Phosphorylation

Summary of Net Products of Gycolysis Two ATPs Two water molecules Two NADHs (+2H + ) Two pyruvates The electrons in the NADHs can yield ATPs through the electron transport system, and the pyruvate can be metabolized in the Krebs cycle.

In the Presence of Oxygen, Pyruvates Enter the Mitochondrion and are Oxidized

In the Absence of Oxygen, Pyruvates are Fermented to Liberate NAD +

Lactic Acid Fermentation Compared to Alcohol Fermentation