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Chapter 9 - Cellular Respiration and Fermentation (very IMPORTANT for MCAT)

9.1 Catabolic pathways yield energy by oxidizing organic fuels

energy flows into an ecosystem as sunlight and leaves as heat

Catabolic Pathways and Production of ATP

fermentation — a partial degradation of sugars or other organic fuel that occurs without the use of oxygen (catabolic process)

aerobic respiration — oxygen is consumed as a reactant along with the organic fuel

the cell of most eukaryotic and many prokaryotic organisms can carry out aerobic respiration

anaerobic respiration — some prokaryotes use substances other than oxygen as reactants in a similar process that harvests chemical energy without oxygen

cellular respiration includes both aerobic and anaerobic processes

→ cellular respiration is often more refer to aerobic process

→ overall process can be summarized as:

organic compounds + oxygen → carbon dioxide + water + energy

cellular respiration by the following degradation of sugar glucose:

C6H12O6 + 6O2 → <inside of cytoplasm & mitochondria>6CO2 + 6H2O + energy (36/38 ATP + heat)

→ this breakdown of glucose is exergonic

Redox Reactions: Oxidation and Reduction

The Principle of Redox

redox reaction — a transfer of one or more electrons from one reactant to another

oxidation — the loss of electrons from one substance

reduction — the addition of electrons to another substance

reducing agent — the electron donor

oxidizing agent — the electron acceptor

→ not all redox reactions involve the complete transfer of electrons from one substance to another; some change the degree of electron sharing in covalent bonds

→ energy must be added to pull an electron away from an atom

→ the more electronegative the atom, the more energy is required to take an electron away from it

Oxidation of Organic Fuel Molecules During Cellular Respiration

→ in respiration, the oxidation of glucose transfers electrons to a lower energy state, liberating energy that becomes available for ATP synthesis

Stepwise Energy Harvest via NAD+ and the Electron Transport Chain

NAD+ — the coenzyme that is well suited as an electron carrier because it can cycle easily between its oxidized form

coenzyme - an organic cofactor and helps with the enzyme function

NADH — reduced form of NAD+

→ Nicotine Amide dinucleotide

each NADH represents stored energy that is tapped to synthesize ATP

NADH passes the electrons to the electron transport chain

electron transport chain — consists of a number of molecules, mostly proteins, built into the inner membrane of the mitochondria of eukaryotic cells (and the plasma membrane of respiring prokaryotes)

Electrons removed from glucose are shuttled by NADH to the “top,” higher-energy end of the chain. At the “bottom,” lower-energy end, O2 captures these electrons along with hydrogen nuclei (H+), forming water

Summary of cellular respiration, most electrons travel this “downhill” route:

glucose → NADH → electron transport chain → oxygen

The Stages of Cellular Respiration: A Preview

Glycolysis

Pyruvate Oxidation

Citric Acid Cycle

Oxidative Phosphorylation: Electron transport and chemiosmosis

Glycolysis — occurs in the cytosol, begins the degradation process by breaking glucose into two molecules of a compound called pyruvate

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→ in eukaryotes, pyruvate enters the mitochondrion and is oxidized to a compound called acetyl CoA

Citric acid cycle — acetyl CoA enters, the breakdown of glucose to carbon dioxide is completed

→ the carbon dioxide produced by respiration represents fragments of oxidized organic molecules

some of the steps of glycolysis and the citric acid cycle are redox reactions in which dehydrogenases transfer electrons from substances to NAD+ or the related electron carrier FAD, forming NADH or FADH2

oxidative phosphorylation — One mode of ATP synthesis; the energy released at each step of the chain is stored in a form the mitochondrion (or prokaryotic cell) can use to make ATP from ADP

substrate-level phosphorylation (a mechanism) — a smaller amount of ATP is formed directly in a few reactions of glycolysis and the citric acid cycle

Three stages of respiration: glycolysis, citric acid cycle, oxidative phosphorylation

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9.2 Glycolysis harvests chemical energy by oxidizing glucose to pyruvate

glycolysis — sugar splitting

a six-carbon sugar, is split into two three-carbon sugars (pyruvate)

c-c-c-c-c-c → c-c-c | c-c-c

two phases of glycolysis:

the energy investment phase — cell spends ATP

the energy payoff phase

→ glycolysis occurs whether or not O2 is present

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9.3 After pyruvate is oxidized, the citric acid cycle completes the energy-yielding oxidation of organic molecules

Oxidation of Pyruvate to Acetyl CoA

acetyl CoA — pyruvate is first converted to acetyl coenzyme A

linking glycolysis complex that catalyzes three reactions:

pyruvate’s carboxyl group,already somewhat oxidized and thus carrying little chemical energy, is now fully oxidized and given off as a molecule of CO2. This is the first step in which CO2 is released during respiration

the remaining two-carbon fragment is oxidized and the electrons transferred to NAD+, storing energy in the form of NADH

coenzyme A is attached via its sulfur atom to the two-carbon intermediate, forming acetyl CoA.

The Citric Acid Cycle

functions as a metabolic furnace that further oxidizes organic fuel derived from pyruvate

→ the cycle has 8 steps, each catalyzed by a specific enzyme

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9.4 During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis

The Pathway of Electron Transport

the electron transport chain is a collection of molecules embedded in the inner membrane of the mitochondrion in eukaryotic cells

cytochromes — proteins that remaining electron carriers between ubiquinone and oxygen

→ their prophetic group, called a heme group, has an iron atom that accepts and donates electrons

NADH & FADH2 — sources of electrons for the ETC

Chemiosmosis: The Energy-Coupling Mechanism

ATP synthase — the enzyme that makes ATP from ADP and inorganic phosphate; populating the inner membrane of the mitochondrion or the prokaryotic plasma membrane are many copies of a protein complex

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feedback inhibition is the most common mechaism for controlling producing ATP

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