NOTES: Ch 9 Cellular Respiration: Harvesting Chemical Energy Part 1: The Overview

Overview: Life Is Work ● 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

● Energy flows into an ecosystem as sunlight and leaves as heat ● Photosynthesis generates oxygen and organic molecules, which are used in cellular respiration ● Cells use chemical energy stored in organic molecules to regenerate ATP, which powers work

ECOSYSTEM Light energy Photosynthesis in chloroplasts Cellular respiration in mitochondria Organic molecules + O 2 CO 2 + H 2 O ATP powers most cellular work Heat energy

9.1 - Catabolic Pathways & Production of ATP: ● The breakdown of organic molecules is EXERGONIC ● Fermentation is a partial degradation of sugars that occurs without oxygen ● Cellular respiration consumes oxygen and organic molecules and yields ATP

Principles of Energy Harvest ● Although carbohydrates, fats, and proteins are all consumed as fuel, it is helpful to trace cellular respiration with the sugar glucose: Chemical Equation: C 6 H 12 O 6 + 6O 2  6CO 2 + 6H 2 O + Energy (ATP + heat)

Recall: the ATP cycle

Redox Reactions: Oxidation and Reduction ● The transfer of electrons during chemical reactions releases energy stored in organic molecules ● This released energy is ultimately used to synthesize ATP

● REDOX REACTIONS: involve electron transfers from 1 substance to another

● OXIDATION: loss of electrons (“energy releasing”) ● REDUCTION: gain of electrons (“energy storing”) The Principle of Redox

Example of Redox: Na + Cl  Na + + Cl - Oxidation (loss of electrons) Reduction (gain of electrons)

● The electron donor is called the reducing agent ● The electron receptor is called the oxidizing agent

Example of Redox: Na + Cl  Na + + Cl - Na is oxidized; it is the reducing agent Cl is reduced; it is the oxidizing agent

Oxidation of Organic Fuel Molecules During Cellular Respiration: ● During cellular respiration, the fuel (such as glucose) is oxidized and oxygen is reduced:

Respiration: C 6 H 12 O 6 + 6O 2  6CO 2 + 6H 2 O *the transfer of electrons from 1 element to another, more electronegative element (e.g. from H to O) releases stored potential energy - - this chemical energy can be put to work! oxidized reduced

● Glucose & other fuels are broken down gradually, in a series of steps ● at key steps, H atoms are stripped from glucose and passed to a coenzyme: NAD + –(this is performed by enzymes: dehydrogenase) NAD + + 2H  NADH + H +

Stepwise Energy Harvest via NAD + and the Electron Transport Chain ● As an electron acceptor, NAD + functions as an oxidizing agent during cellular respiration ● Each NADH (the reduced form of NAD + ) represents stored energy that is tapped to synthesize ATP

NAD + Nicotinamide (oxidized form) Dehydrogenase 2 e – + 2 H + 2 e – + H + NADH H+H+ H+H+ Nicotinamide (reduced form) + 2[H] (from food) +

● Respiration uses an ELECTRON TRANSPORT CHAIN to break the fall of electrons into several energy- releasing steps (instead of one explosive reaction) e-

● NADH passes the electrons to the electron transport chain ● Oxygen pulls electrons down the chain in an energy-yielding tumble ● The energy yielded is used to regenerate ATP

2 H e – 2 H (from food via NADH) Controlled release of energy for synthesis of ATP 2 H + 2 e – H2OH2O + 1 / 2 O 2 H2H2 + H2OH2O Explosive release of heat and light energy Cellular respiration Uncontrolled reaction Free energy, G Electron transport chain

ELECTRON TRANSPORT CHAIN: ● consists of molecules, mostly proteins, built into the inner membrane of mitochondria ● NADH carries the electrons to the “top” of the chain

● oxygen captures electrons at the “bottom” of the chain and combines with H +, forming water. Reduction of oxygen!

SUMMARY OF ENERGY FLOW IN THE CELL: Food  NADH  E.T.C.  oxygen (glucose) **oxygen is the final electron acceptor!

The Stages of Cellular Respiration: A Preview ● Cellular respiration has three stages: 1) Glycolysis (breaks down glucose into two molecules of pyruvate) 2) The Citric Acid Cycle, a.k.a. Krebs Cycle (completes the breakdown of glucose) 3) E.T.C. & Oxidative phosphorylation (accounts for most of the ATP synthesis)

Mitochondrion Glycolysis Pyruvate Glucose Cytosol ATP Substrate-level phosphorylation

Mitochondrion Glycolysis Pyruvate Glucose Cytosol ATP Substrate-level phosphorylation ATP Substrate-level phosphorylation Citric acid cycle

Mitochondrion Glycolysis Pyruvate Glucose Cytosol ATP Substrate-level phosphorylation ATP Substrate-level phosphorylation Citric acid cycle ATP Oxidative phosphorylation Oxidative phosphorylation: electron transport and chemiosmosis Electrons carried via NADH Electrons carried via NADH and FADH 2

● The process that generates most of the ATP is called oxidative phosphorylation because it is powered by redox reactions ● Oxidative phosphorylation accounts for almost 90% of the ATP generated by cellular respiration ATP Synthesis:

● A small amount of ATP is formed in glycolysis and the citric acid cycle by substrate-level phosphorylation ATP Synthesis:

Enzyme ADP P Substrate Product Enzyme ATP +