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

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

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 (CO 2 ) into organic carbon compounds such as sugars 12H 2 O + 6CO 2  C 6 H 12 O 6 + 6O 2 + 6H 2 O

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

Part 1: The big picture L oss of E lectrons is O xidation goes G ain of E lectrons is R eduction

Part 1: The big picture Respiration is in many ways photosynthesis BACKWARDS. Photosynthesis uses sun energy to turn CO 2 into glucose. Respiration releases that stored energy from glucose. C 6 H 12 O 6 + 6O 2  6CO 2 + 6H 2 O

Part 1: The big picture So if photosynthesis involves the chemical reduction of CO 2 into glucose, and respiration is very similar to photosynthesis backwards… Respiration is the oxidation of glucose back into CO 2, which releases the stored chemical energy!

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

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

Part 1: The big picture Image courtesy NASA Earth Observatory

Break!

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 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 Part 2: Macromolecules & Catalysis

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

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

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

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

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

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

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

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… 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 Part 2: Macromolecules & Catalysis

Catalase speeds the reaction: 2H 2 O 2  2H 2 O + O 2 What do you think will happen when I pour H 2 O 2 on the potato? Part 2: Macromolecules & Catalysis Catalase

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 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 Part 2: Macromolecules & Catalysis

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

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Part 3: Respiration & Fermentation 12H 2 O + 6CO 2  C 6 H 12 O 6 + 6O 2 + 6H 2 O C 6 H 12 O 6 + 6O 2  6H 2 O + 6CO 2 Respiration is photosynthesis backwards!

2H 2 O 4e - + 4H + + O 2 Pigments Photosystem Part 3: Respiration & Fermentation

Pigments Photosystem 4e - Electron Transport Chain NADPH Part 3: Respiration & Fermentation

ATP + NADPH CO 2 C 6 H 12 O 6 The Calvin Cycle (light independent reactions) 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) CO 2 into glucose using the Calvin Cycle Part 3: Respiration & Fermentation

In respiration, we oxidize glucose (add oxygen to transform it into 6CO 2 ) 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 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 O 2 is available, glycolysis is the only way to get energy from glucose and fermentation occurs Part 3: Respiration & Fermentation

In fermentation, we get 2 ATP from glycolysis but can’t continue to the Krebs cycle, which requires O 2 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. Part 3: Respiration & Fermentation

But, if we have O 2 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 CO 2 at a time Each time a carbon is removed from citrate, CO 2 is produced and we pull electrons out and transfer them to NADH or FADH 2 Part 3: Respiration & Fermentation

In the Krebs Cycle, we’ve oxidized pyruvate into CO 2 and produced NADH and FADH 2 (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 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 FADH 2 carry electrons stripped from glucose to electron transport chain where they are used to make ATP (energy) Part 3: Respiration & Fermentation

Part 3: Broader context