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

Light energy ECOSYSTEM Photosynthesis in chloroplasts CO2 + H2O Organic molecules + O2 Cellular respiration in mitochondria 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: C6H12O6 + 6O2  6CO2 + 6H2O + 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

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

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 Na + Cl  Na+ + Cl- 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: C6H12O6 + 6O2  6CO2 + 6H2O oxidized C6H12O6 + 6O2  6CO2 + 6H2O *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! 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+ 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

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

● 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

Electron transport chain + 1/2 O2 2 H + 1/2 O2 (from food via NADH) Controlled release of energy for synthesis of ATP 2 H+ + 2 e– ATP Explosive release of heat and light energy ATP Free energy, G Free energy, G Electron transport chain ATP 2 e– 1/2 O2 2 H+ H2O H2O Uncontrolled reaction Cellular respiration

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)

Glycolysis Glucose Pyruvate Cytosol Mitochondrion ATP Substrate-level phosphorylation

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

Electrons carried via NADH Electrons carried via NADH and FADH2 Oxidative phosphorylation: electron transport and chemiosmosis Glycolysis Citric acid cycle Glucose Pyruvate Cytosol Mitochondrion ATP ATP ATP Substrate-level phosphorylation Substrate-level phosphorylation Oxidative phosphorylation

ATP Synthesis: ● 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

Enzyme Enzyme ADP P Substrate + ATP Product