Understanding Cellular Respiration: Photosynthesis, Redox Reactions, and Biochemistry Basics

1. Cellular Respiration LA Charter School Science Partnership 28 Apr 2012 Nick Klein

2. Today’s Talk • Part 1: Big picture: review of photosynthesis, redox • Part 2: Macromolecules, enzymes, and catalysis • Part 3: Respiration & Fermentation

3. Part 1: The big picture • Let’s think back to photosynthesis. • Photosynthesis is the process by which organisms use the energy in sunlight to chemically transform carbon dioxide (CO2) into organic carbon compounds such as sugars 12H2O + 6CO2 C6H12O6 + 6O2 + 6H2O

4. Part 1: The big picture • Photosynthesis and respiration both involve reduction/oxidation (redox) reactions— chemical reactions that involve the movement of electrons from one molecule to another • In photosynthesis, when carbon dioxide is fixed, it is reduced (electrons are added to it) which produces organic carbon compounds

5. Part 1: The big picture Loss ofElectrons isOxidation goesGain ofElectrons isReduction

6. Part 1: The big picture • Respiration is in many ways photosynthesis BACKWARDS. Photosynthesis uses sun energy to turn CO2 into glucose. Respiration releases that stored energy from glucose. C6H12O6+ 6O26CO2+ 6H2O

7. Part 1: The big picture • So if photosynthesis involves the chemical reduction of CO2 into glucose, and respiration is very similar to photosynthesis backwards… • Respiration is the oxidation of glucose back into CO2, which releases the stored chemical energy!

8. Part 1: The big picture • Organisms that make their own food are called autotrophs. Organisms that make food using photosynthesis are photoautotrophs • All animals, including humans, are heterotrophs—we have to consume other organisms as food

9. Part 1: The big picture • Photosynthesis respiration work together in what is called the carbon cycle

10. Part 1: The big picture Image courtesy NASA Earth Observatory

11. Break!

12. Part 2: Macromolecules & Catalysis • Before we get to the details of cellular respiration, let’s cover a few more basics of biochemistry that will help us understand both photosynthesis and respiration better! • Specifically, we’re going to briefly discuss the basic building blocks and machinery of biology

13. Part 2: Macromolecules & Catalysis • What are the basic building blocks of life? • Amino acids (proteins) • Sugars (carbohydrates) • Lipids (fats) • Nucleic acids (DNA & RNA) • All of these “building blocks” string together to form chains called macromolecules or biopolymers

14. Part 2: Macromolecules & Catalysis • Remember glucose? Glucose is the basic unit of a large number of different sugars (carbohydrates)

15. Part 2: Macromolecules & Catalysis • Glucose can bond with other glucose molecules in several different ways Sucrose (table sugar)

16. Part 2: Macromolecules & Catalysis • Glucose can bond with other glucose molecules in several different ways Lactose (milk sugar)

17. Part 2: Macromolecules & Catalysis • Glucose can also form long chains Cellulose (woody part of plants)

18. Part 2: Macromolecules & Catalysis • Glucose can also form long chains Starch

19. Part 2: Macromolecules & Catalysis • Amino acids chain together to form proteins Catalase

20. Part 2: Macromolecules & Catalysis • Nucleic acids chain together to form DNA & RNA DNA

21. Part 2: Macromolecules & Catalysis • Our body has to break down sugar polymers into the individual sugar monomers (glucose) before we can use it in cellular respiration • Can our bodies use cellulose? Why or why not? • We don’t have the right biochemical machinery to digest cellulose! We would need a cellulase enzyme…

22. Part 2: Macromolecules & Catalysis • Enzymes are proteins (chains of amino acids) that act as biological catalysts: they speed the rate of a chemical reaction, but are left unchanged by the reaction • Example demo: catalase

23. Part 2: Macromolecules & Catalysis • Catalase speeds the reaction:2H2O2 2H2O + O2 • What do you think will happen when I pour H2O2 on the potato? Catalase

24. Part 2: Macromolecules & Catalysis • Enzymes work by lowering the activation energy. The activation energy is a measure of how much chemical energy a molecule must have before it will undergo a reaction. • Enzymes lower this “hill” and cause reactions to happen that would otherwise only go very slowly

25. Part 2: Macromolecules & Catalysis

26. Part 2: Macromolecules & Catalysis • Other examples of enzymes: lactase, cellulase, amylase • If you’re lactose intolerant, your body does not produce enough lactase to digest lactose sugar very well • Similarly, we cannot digest the woody part of plants since our bodies don’t produce cellulase—cows and other herbivores have bacteria in their guts that make cellulase

27. Part 2: Macromolecules & Catalysis • We explored the action of amylase in one of our morning activities

28. Break!

29. Part 3: Respiration & Fermentation 12H2O + 6CO2 C6H12O6 + 6O2 + 6H2O C6H12O6 + 6O2 6H2O + 6CO2 Respiration is photosynthesis backwards!

30. Part 3: Respiration & Fermentation Pigments Photosystem 2H2O 4e- + 4H+ + O2

31. Part 3: Respiration & Fermentation Electron TransportChain Pigments Photosystem 4e- NADPH

32. Part 3: Respiration & Fermentation ATP + NADPH CO2 C6H12O6 The Calvin Cycle (light independent reactions)

33. Part 3: Respiration & Fermentation • In photosynthesis, we used light energy to split electrons out of a water molecule, then used the electron transport chain to take energy from those electrons and convert it into ATP • Then we used ATP and the leftover electrons (in the form of NADPH) to fix (reduce) CO2 into glucose using the Calvin Cycle

34. Part 3: Respiration & Fermentation • In respiration, we oxidize glucose (add oxygen to transform it into 6CO2) to “pull” electrons out of it • These electrons are then put through an electron transport chain to generate ATP • What do we need for respiration? • Glucose • Oxygen

35. Part 3: Respiration & Fermentation • First step in respiration is glycolysis • In glycolysis, glucose (6 carbons) is split into two molecules of pyruvate (3 carbons each) • This yields 2 ATP and 2 NADH (electron carriers) • If no O2 is available, glycolysis is the only way to get energy from glucose and fermentation occurs

36. Part 3: Respiration & Fermentation • In fermentation, we get 2 ATP from glycolysis but can’t continue to the Krebs cycle, which requires O2 to function • Have to recycle the NADH, so the electrons the NADH carries are transferred to pyruvate and glycolysis can continue • Different organisms transform pyruvate to different waste molecules in fermentation—in humans, lactic acid.

37. Part 3: Respiration & Fermentation • But, if we have O2 we can put the pyruvate into the Krebs cycle and yield 38 ATP total instead of 2 for each glucose! • Krebs cycle is complex, but in basic terms pyruvate is added to a 4-carbon molecule to make citrate, which is then oxidized one CO2 at a time • Each time a carbon is removed from citrate, CO2 is produced and we pull electrons out and transfer them to NADH or FADH2

39. Part 3: Respiration & Fermentation • In the Krebs Cycle, we’ve oxidized pyruvate into CO2 and produced NADH and FADH2 (electron carriers) • These electron carriers now move the electrons to the electron transport chain (remember from photosynthesis?) • As the electrons flow through the transport chain, their energy is used to create a proton gradient • Then, when the protons flow back in, they drive ATP synthase (an enzyme!) which makes ATP

40. Part 3: Respiration & Fermentation

41. Part 3: Respiration & Fermentation • Recap: in glycolysis, we split 6-carbon glucose into two 3-carbon pyruvate and yield 2 ATP • Stop at glycolysis if no oxygen available, then fermentation • If oxygen is available, Krebs Cycle oxidizes pyruvate and strips the electrons from it • NADH and FADH2 carry electrons stripped from glucose to electron transport chain where they are used to make ATP (energy)

42. Part 3: Broader context