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 (CO2) into organic carbon compounds such as sugars

12H2O + 6CO2 C6H12O6 + 6O2 + 6H2O

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 pictureLoss of

Electrons is

Oxidation

goes

Gain of

Electrons is

Reduction

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

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!

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:2H2O2 2H2O + O2

• What do you think will happen when I pour H2O2 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

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

Break!

Part 3: Respiration & Fermentation

12H2O + 6CO2 C6H12O6 + 6O2 + 6H2O

C6H12O6 + 6O2 6H2O + 6CO2

Respiration is photosynthesis backwards!

2H2O

4e- + 4H+ + O2

Pigments

Photosystem

Part 3: Respiration & Fermentation

Pigments

Photosystem

4e-

Electron TransportChain

NADPH

Part 3: Respiration & Fermentation

ATP + NADPH

CO2 C6H12O6

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) CO2 into glucose using the Calvin Cycle

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

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

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.

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

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

Part 3: Respiration & Fermentation

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)

Part 3: Respiration & Fermentation

Part 3: Broader context