Biology 107Biology 107

Cellular RespirationCellular Respiration

October 2, 2002October 2, 2002

Cellular Respiration ICellular Respiration I

Student Objectives:Student Objectives: As a result of this lecture and the assigned As a result of this lecture and the assigned reading, you should understand the following:reading, you should understand the following:

1.1. Cell release chemical energy by means of an exergonic process Cell release chemical energy by means of an exergonic process called called cellular respirationcellular respiration, the aerobic harvesting of energy from , the aerobic harvesting of energy from food molecules by cells.food molecules by cells.

2. Cellular respiration is the energy-releasing chemical breakdown Cellular respiration is the energy-releasing chemical breakdown of molecules and the storage of energy from that breakdown in a of molecules and the storage of energy from that breakdown in a form (e.g., ATP) the cell can use to perform work.form (e.g., ATP) the cell can use to perform work.

Cellular Respiration IICellular Respiration II3.3. Normally there is an oxidation of the organic molecule causing the Normally there is an oxidation of the organic molecule causing the

hydrogen atoms (electrons and their accompanying protons) to be hydrogen atoms (electrons and their accompanying protons) to be removed from the carbon atoms and eventually combined with removed from the carbon atoms and eventually combined with oxygen (which is thereby reduced). oxygen (which is thereby reduced).

4.4. In cellular respiration, the electrons go from higher energy levels In cellular respiration, the electrons go from higher energy levels to lower energy levels, and energy is released. This energy is to lower energy levels, and energy is released. This energy is released over many steps as electrons move to successively released over many steps as electrons move to successively lower energy levels. Some of that energy is lost as heat; a portion 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 that energy (40%) is captured in the terminal phosphate bonds of ATP.of ATP.

5.5. The efficiency in living systems is due to the fact that energy The efficiency in living systems is due to the fact that energy release occurs over the course of a series of controlled reaction release occurs over the course of a series of controlled reaction steps.steps.

Cellular Respiration ICellular Respiration I6.6. The harvesting of energy involves the rearrangement of electrons The harvesting of energy involves the rearrangement of electrons

in chemical bonds. The common theme is that a cell transfers in chemical bonds. The common theme is that a cell transfers energy from one molecule to another by coupling an exergonic energy from one molecule to another by coupling an exergonic reaction (energy-releasing) to an endergonic reaction (energy-reaction (energy-releasing) to an endergonic reaction (energy-storing). The energy released was stored in the specific storing). The energy released was stored in the specific arrangement of a molecule's covalent bonds, and the energy arrangement of a molecule's covalent bonds, and the energy stored is in the new covalent bonds formed. stored is in the new covalent bonds formed.

7.7. In summary, cellular respiration rearranges electrons in chemical In summary, cellular respiration rearranges electrons in chemical bonds. These are redox reactions. Because an electron transfer bonds. These are redox reactions. Because an electron transfer requires both a donor and an acceptor, an electron leaves one requires both a donor and an acceptor, an electron leaves one molecule only when it contacts another molecule that attracts it molecule only when it contacts another molecule that attracts it more strongly.more strongly.

8.8. In respiration, there are two main coenzymes derived from B In respiration, there are two main coenzymes derived from B complex vitamins. First is complex vitamins. First is NADNAD+ + ,(nicotinamide adenine ,(nicotinamide adenine dinucleotide) which in part is derived from Bdinucleotide) which in part is derived from B33, niacin. The second , niacin. The second coenzyme is coenzyme is FADFAD (flavin adenine dinucleotide), which in part is (flavin adenine dinucleotide), which in part is derived from Bderived from B22, riboflavin., riboflavin.

Cellular Respiration ICellular Respiration I9.9. Glucose supplies energy to form ATP by two related processes: Glucose supplies energy to form ATP by two related processes:

1) 1) glycolysisglycolysis and 2) and 2) cellular respirationcellular respiration. The products of . The products of glycolysis are reactants used in respiration.glycolysis are reactants used in respiration.

10.10. In glycolysis ("splitting of sugar"), the 6-carbon glucose molecule In glycolysis ("splitting of sugar"), the 6-carbon glucose molecule is split into two 3-carbon molecules, is split into two 3-carbon molecules, pyruvatepyruvate. The net energy . The net energy harvested from the glycolysis reactions is in the form of ATP and harvested from the glycolysis reactions is in the form of ATP and NADH.NADH.

a.a. This production of ATP in glycolysis is by the direct, This production of ATP in glycolysis is by the direct, enzyme-enzyme- mediated transfer of a phosphate group from a mediated transfer of a phosphate group from a substrate to substrate to ADP by the mechanism called ADP by the mechanism called substrate substrate phosphorylationphosphorylation. This . This is different from is different from electron transport electron transport (oxidative) phosphorylation(oxidative) phosphorylation, , which requires oxygen and a which requires oxygen and a transport system.transport system.

b.b. Glycolysis occurs in the cytosol and does not require Glycolysis occurs in the cytosol and does not require oxygen oxygen (i.e., it is an anaerobic process).(i.e., it is an anaerobic process).

Cellular Respiration ICellular Respiration I

11.11. In the presence of oxygen, the pyruvates are fed into the second In the presence of oxygen, the pyruvates are fed into the second stage of energy capturing, cellular respiration. stage of energy capturing, cellular respiration.

12.12. In the absence of oxygen, the pyruvate is converted to either In the absence of oxygen, the pyruvate is converted to either lactic acid or ethanol. This conversion process is known as lactic acid or ethanol. This conversion process is known as fermentationfermentation, and it produces no ATP. Fermentation is a , and it produces no ATP. Fermentation is a mechanism for cells to replenish the supply of NADmechanism for cells to replenish the supply of NAD++ that the cell that the cell is using in glycolysis.is using in glycolysis.

13.13. The reactions of glycolysis follow essentially the same routes in The reactions of glycolysis follow essentially the same routes in prokaryotes and eukaryotes, except the products of fermentation prokaryotes and eukaryotes, except the products of fermentation are more varied under anaerobic conditions.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 ATPsTwo ATPs

Two water moleculesTwo water molecules

Two NADHs (+2HTwo NADHs (+2H++))

Two pyruvatesTwo pyruvates

The electrons in the The electrons in the NADHs can yield ATPs NADHs can yield ATPs through the electron through the electron transport system, and transport system, and the pyruvate can be the pyruvate can be metabolized in the metabolized in the Krebs cycle.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