Cellular Respiration

1. Cellular Respiration Cellular Respiration: Harvesting Chemical Energy

2. Life Requires Energy • Living cells require energy from outside sources • Some animals, such as the giant panda, obtain energy by eating plants; others feed on organisms that eat plants

4. Energy flows into an ecosystem as sunlight and leaves as heat • Photosynthesis uses sunlight to generate oxygen and glucose sugar. • Cell respiration uses chemical energy in the form of carbohydrates, lipids, or proteins, to produce ATP.

5. ATP • ATP stands for Adenosine Tri-Phosphate • ATP is a molecule that serves as the most basic unit of energy • ATP is used by cells to perform their daily tasks

6. ATP • ATP can be broken down into a molecule of ADP by removing one of the phosphate groups. • This releases energy. • ADP can be remade into ATP later when the cell has food that can be broken down (i.e. glucose)

7. NADH • NADH is a molecule that can “carry” H+ ions and electrons from one part of the cell to another. • NADH is the “energized” version of this molecule that is carrying the H+ ion and two high-energy electrons. • NAD+ is the “non-energized” version of this molecule that does not have the ion or the extra two electrons.

8. LE 8-9 P P P Adenosine triphosphate (ATP) H2O + P P P + Energy i Adenosine diphosphate (ADP) Inorganic phosphate

9. Light energy LE 9-2 ECOSYSTEM Photosynthesis in chloroplasts Simple sugars (Glucose) CO2 + H2O + O2 Cellular respiration in mitochondria ATP powers most cellular work Heat energy

10. Cell Respiration and Production of ATP • The breakdown of organic molecules (carbohydrates, lipids, proteins) releases energy. • Cellular respiration consumes oxygen and organic molecules and yields ATP • Although carbohydrates, fats, and proteins are all consumed as fuel, it is helpful to trace cellular respiration with the sugar glucose: C6H12O6 + 6O2 6CO2 + 6H2O + Energy

11. Glycolysis • Glycolysis is the first stage of cellular respiration. • Occurs in cytoplasm. • During glycolysis, glucose is broken down into 2 molecules of the 3-carbon molecule pyruvic acid. • ATP and NADH are produced as part of the process.

12. ATP Production • 2 ATP molecules are needed to get glycolysis started.

13. ATP Production • Glycolysis then produces 4 ATP molecules, giving the cell a net gain of +2 ATP molecules for each molecule of glucose that enters glycolysis.

14. NADH Production • During glycolysis, the electron carrier 2 NAD+become 2 NADH. • 2 NADH molecules are produced for every molecule of glucose that enters glycolysis.

15. Glycolysis • Glycolysis uses up: • 1 molecule of glucose (6-carbon sugar) • 2 molecules of ATP • 2 molecules of NAD+ • Glycolysis produces • 2 molecules of pyruvic acid (3-carbon acids) • 4 molecules of ATP • 2 molecules of NADH

16. Advantages of Glycolysis • Glycolysis produces ATP very fast, which is an advantage when the energy demands of the cell suddenly increase. • Glycolysis does not require oxygen, so it can quickly supply energy to cells when oxygen is unavailable.

17. Movement to the Citric Acid Cycle • Before the next stage can begin, pyruvic acid must first be transported inside the mitochondria. • Pyruvic acid is combined with an enzyme called Coenzyme A. This enzyme helps with the transportation. • Pyruvic acid + Coenzyme A make Acetyl CoA • One more molecule of NADH is produced. • This also releases one molecule of CO2 as a waste product.

18. LE 9-10 MITOCHONDRION CYTOSOL NAD+ NADH + H+ Acetyl Co A CO2 Coenzyme A Pyruvate Transport protein

19. Krebs Cycle • During the citric acid cycle, pyruvic acid produced in glycolysis is broken down into carbon dioxide and more energy is extracted.

20. Citric Acid Cycle • Acetyl-CoA from glycolysis enters the matrix, the innermost compartment of the mitochondrion. • Once inside, the Coenzyme A is released.

21. Citric Acid Cycle • The molecule of acetate that entered from glycolysis joins up with another 4-carbon molecule already present. • This forms citric acid.

22. Citric Acid Cycle • Citric acid (6-carbon molecule) is broken down one step at a time until it is a 4-carbon molecule. • The two extra carbons are released as carbon dioxide.

23. Citric Acid Cycle • Energy released by the breaking and rearranging of carbon bonds is captured in the forms of ATP, NADH, and FADH2. • FADH2has the same purpose as NADH – to transport high-energy electrons and H+ ions.

24. Citric Acid Cycle • For each turn of the cycle, the following are generated: • 1 ATP molecule • 3 NADH molecules • 1 FADH2 molecule

25. Citric Acid Cycle • Remember! Each molecule of glucose results in 2 molecules of pyruvic acid, which enter the Krebs cycle. • So each molecule of glucose results in two complete “turns” of the Krebs cycle. • Therefore, for each glucose molecule: • 6 CO2 molecules, • 2 ATP molecules, • 8 NADH molecules, • 2 FADH2 molecules are produced.

26. Pyruvic acid (from glycolysis, 2 molecules per glucose) LE 9-11 Citric acid cycle Glycolysis Oxidation phosphorylation CO2 NAD+ CoA NADH ATP ATP ATP + H+ Acetyl CoA CoA CoA Citric acid cycle 2 CO2 FADH2 3 NAD+ 3 NADH FAD + 3 H+ ADP + P i ATP

27. Electron Transport Chain • The electron transport chain occurs in the inner membrane of the mitochondria. • Electrons are passed along the chain, from one protein to another. • Each time the electron is passed, a little bit of energy is extracted from it. • Electrons drop in energy as they go down the chain and until they end with O2, forming water

28. Electron Transport Chain • NADH and FADH2 pass their high-energy electrons to electron carrier proteins in the electron transport chain.

29. Electron Transport Chain • At the end of the electron transport chain, the electrons combine with H+ ions and oxygen to form water.

30. Electron Transport Chain • Energy generated by the electron transport chain is used to move H+ ions (from NADH and FADH2) against a concentration gradient. • This creates a “dam” of H+ ions in the outer fluid of the mitochondria.

32. The electron transport chain generates no ATP • The chain’s function is to break the large free-energy drop from food to O2 into smaller steps that release energy in manageable amounts. • The end result is a “reservoir” of H+ ions that can be tapped for energy, much like a reservoir in a hydroelectric dam.

33. Chemiosmosis • The electron transport chain has created a high concentration of H+ ions in the outer fluid of the mitochondria. • H+ then moves back across the membrane, into the inner fluid. • H+ ions pass through a channel protein called ATP Synthase • ATP synthase uses this flow of H+ to convert ADP molecules (low energy) into ATP (high energy)

34. INTERMEMBRANE SPACE A rotor within the membrane spins as shown when H+ flows past it down the H+ gradient. H+ LE 9-14 H+ H+ H+ H+ H+ H+ A stator anchored in the membrane holds the knob stationary. A rod (or “stalk”) extending into the knob also spins, activating catalytic sites in the knob. H+ Three catalytic sites in the stationary knob join inorganic phosphate to ADP to make ATP. ADP + ATP P i MITOCHONDRAL MATRIX

35. Total ATP Production • During cellular respiration, most energy flows in this sequence: glucose  NADH   electron transport chain chemiosmosis ATP • About 40% of the energy in a glucose molecule is transferred to ATP during cellular respiration, making about 38 total ATP • Remainder is lost as waste heat

36. Fermentation • Cellular respiration requires O2 to produce ATP • Glycolysis can produce ATP with or without O2 (in aerobic or anaerobic conditions) • In the absence of O2, glycolysis can couples with a process called fermentation to produce ATP.

37. Types of Fermentation • Fermentation consists of glycolysis + reactions that regenerate NAD+, which can be reused by glycolysis • Two common types are alcohol fermentation and lactic acid fermentation

38. Alcohol Fermentation • Yeast and a few other microorganisms use alcoholic fermentation that produces ethyl alcohol and carbon dioxide. • This process is used to produce alcoholic beverages and causes bread dough to rise. Pyruvic acid + NADH → Alcohol + CO2 + NAD+

39. Lactic Acid Fermentation • Most organisms, including humans, carry out fermentation using a chemical reaction that converts pyruvic acid to lactic acid. • Pyruvic acid + NADH  Lactic acid + NAD+

40. In lactic acid fermentation, pyruvate is reduced to NADH, the only end product is lactic acid. No carbon dioxide is released. • Lactic acid fermentation by some fungi and bacteria is used to make cheese and yogurt • Human muscle cells use lactic acid fermentation to generate ATP when O2 is scarce (out of breath) • Result: Soreness!

41. Yeast and many bacteria are facultative anaerobes, meaning that they can survive using either fermentation or cellular respiration • Most other organisms cannot survive in the long-run using glycolysis and fermentation, they require oxygen. • These are obligate aerobic organisms.

42. Glucose LE 9-18 CYTOSOL Pyruvate O2 present Cellular respiration No O2 present Fermentation MITOCHONDRION Acetyl CoA Ethanol or lactate Citric acid cycle

43. The Evolutionary Significance of Glycolysis • Glycolysis occurs in nearly all organisms • Glycolysis probably evolved in ancient prokaryotes before there was oxygen in the atmosphere

44. Other Energy Sources • Catabolic pathways funnel electrons from many kinds of organic molecules into cellular respiration • Glycolysis accepts a wide range of carbohydrates • Proteins must be digested to amino acids; amino groups can feed glycolysis or the citric acid cycle • Fats are digested to glycerol (used in glycolysis) and fatty acids (used in generating acetyl CoA) • An oxidized gram of fat produces more than twice as much ATP as an oxidized gram of carbohydrate

45. Proteins Carbohydrates Fats LE 9-19 Amino acids Sugars Glycerol Fatty acids Glycolysis Glucose Glyceraldehyde-3- P NH3 Pyruvate Acetyl CoA Citric acid cycle Oxidative phosphorylation