CH7: Cellular Respiration

1. CH7: Cellular Respiration

2. Key Ideas • How does glycolysis produce ATP? • How is ATP produced in aerobic respiration? • Why is fermentation important?

3. Glycolysis • Before you can use energy from food, it must be released and transferred to ATP. • The primary fuel for cellular respiration is glucose. • Glycolysis occurs in the cytoplasm.

4. Glycolysis, continued • In glycolysis, enzymes break down one six-carbon molecule of glucose into two three-carbon pyruvate molecules. • The breaking of a sugar molecule by glycolysis results in a net gain of two ATP molecules. • This process of glycolysis is anaerobic, or takes place without oxygen.

5. Glycolysis, continued • Glycolysis is the only source of energy for some prokaryotes. • Other organisms use oxygen to release even more energy from a glucose molecule. Metabolic processes that require oxygen are aerobic. • In aerobic respiration, the pyruvate product of glycolysis undergoes another series of reactions to produce more ATP molecules.

6. Glycolysis, continued • Step 1, Breaking Down Glucose • Two ATP molecules are used to break glucose into two smaller units. • Phosphate is attached to these 2 – three carbon sugars

7. Glycolysis, continued • Step 2, NADH Production • Each of the three carbon sugars react with another phosphate (not ATP) • Hydrogen atoms from these compounds transfer to two molecules of NAD+ resulting in 2 NADH molecules

8. Glycolysis, continued • Step 3, Pyruvate Production • In a series of 4 reactions, each of the 3 – carbon sugars are converted into a 3 – carbon molecule of pyruvate, resulting in 4 ATP molecules. • 2 ATP molecules were used in step one, 4 ATP molecules were created so net total for glycolysis is 2 ATP molecules

9. Glycolysis

10. Aerobic Respiration • The first stage of aerobic respiration is the Krebs cycle, a series of reactions that produce electron carriers. • The electron carriers enter an electron transport chain, which powers ATP synthase. • Up to 34 ATP molecules can be produced from one glucose molecule in aerobic respiration.

11. Aerobic Respiration, continued Krebs Cycle • Pyruvate (from glycolysis) is broken down and combined with other carbon compounds. • Each time the carbon-carbon bonds are rearranged during the Krebs cycle, energy is released. • The total yield of energy-storing products from one time through the Krebs cycle is one ATP, three NADH, and one FADH2.

12. Aerobic Respiration, continued Electron Transport Chain • The second stage of aerobic respiration takes place in the inner membranes of mitochondria, where ATP synthase enzymes are located. • Electron carriers (NADH and FADH2), produced during the Krebs cycle, transfer energy through the electron transport chain.

13. Aerobic Respiration, continued ATP Production • Hydrogen ions diffuse through ATP synthase, providing energy to produce several ATP molecules from ADP. • ATP synthase is present on the inner membrane of the mitochondria. • H+ ions diffuse through a channel in this enzyme which provides energy to produce ATP from ADP.

14. Aerobic Respiration, continued The Role of Oxygen • At the end of the electron transport chain, the electrons combine with an oxygen atom and two hydrogen ions to form two water molecules. • If oxygen is not present, the electron transport chain stops. The electron carriers are not recycled, so the Krebs cycle also stops.

15. Fermentation • To make ATP during glycolysis, NAD+ is converted to NADH. Organisms must recycle NAD+ to continue making ATP through glycolysis. • The process in which carbohydrates are broken down in the absence of oxygen is called fermentation. • Fermentation enables glycolysis to continue supplying a cell with ATP in anaerobic conditions.

16. Fermentation, continued • In lactic acid fermentation, pyruvate is converted to lactic acid. • During alcoholic fermentation, one enzyme removes carbon dioxide from pyruvate. A second enzyme converts the remaining compound to ethanol, recycling NAD+ in the process.

17. Two Types of Fermentation

18. Fermentation, continued Efficiency of Cellular Respiration • In the first stage of cellular respiration, glucose is broken down to pyruvate during glycolysis, an anaerobic process. • Glycolysis results in a net gain of two ATP molecules for each glucose molecule that is broken down. • In the second stage, pyruvate either passes through the Krebs cycle or undergoes fermentation. Fermentation recycles NAD+ but does not produce ATP.

19. Fermentation, continued Efficiency of Cellular Respiration • Cells release energy most efficiently when oxygen is present because they make most of their ATP during aerobic respiration. • For each glucose molecule that is broken down, as many as two ATP molecules are made during the Krebs cycle. • The Krebs cycle feeds NADH and FADH2 to the electron transport chain, which can produce up to 34 ATP molecules.

20. Visual Concept: Comparing Aerobic and Anaerobic Respiration

21. Summary • The breaking of a sugar molecule by glycolysis results in a net gain of two ATP molecules. • The total yield of energy-storing products from one time through the Krebs cycle is one ATP, three NADH, and one FADH2.

22. Summary, continued • Electron carriers transfer energy through the electron transport chain, which ultimately powers ATP synthase. • Fermentation enables glycolysis to continue supplying a cell with ATP in anaerobic conditions.