chapter 7: cellular respiration and fermentation

fermentation

A partial degradation of sugars or other organic fuel that occurs without the use of oxygen 

• Fermentation is a less efficient catabolic process

aerobic respiration

the most efficient catabolic pathway in which oxygen is consumed as a reactant along with organic fuel

• Cells of most eukaryotic and prokaryotic organisms carry out aerobic respiration

anaerobic respiration

Prokaryotes can perform anaerobic respiration to harvest chemical energy without oxygen

cellular respiration

Both aerobic and anaerobic processes 

• All organisms do respiration

• Mostly occurs in mitochondria

cellular respiration in redox process

CH4 becomes oxidized

O2 becomes reduced

NAD+

 Oxidized form (less electrons, less charged)

• NAD+ functions as an oxidizing agent during respiration

• Functions in several of the redox steps during the breakdown of glucose 

• Electrons lose very little of their potential energy when they are transferred from glucose to NAD+

NADH

Reduced form (more electrons, more charged) 

• H in NADH shows hydrogen has been received, reducing the coenzyme 

• Each NADH molecule formed during respiration represents stored energy that can be tapped to make ATP when the electrons complete their fall down energy gradient from NADH to oxygen

ETC

Consists of a number of molecules, mostly proteins, built into the inner membrane of the mitochondria of eukaryotic cells and the plasma membrane of aerobically respiring prokaryotes 

• Instead of occurring in one explosive reaction, respiration uses an ETC to break the fall of electrons to oxygen into several energy-releasing steps

• Electrons removed from glucose are shuttled by NADH to the “top,” higher-energy end of the chain

• At the “bottom,” lower-energy end of an ETC, O2 captures these electrons along with hydrogen nuclei (H+), forming water

• Electrons cascade down the chain from one carrier molecule to the next in a series of redox reactions, losing a small amount if energy with each step until they reach oxygen

Downhill route of ETC: Glucose → NADH → ETC → Oxygen

substrate level phosphorylation

The mechanism that produces a small amount of ATP in a few reactions of glycolysis and krebs cycle 

• Occurs when an enzyme transfers a phosphate group from a substrate molecule to ADP, rather than adding an inorganic phosphate to ADP as in oxidative phosphorylation

3 stages of celular respiration

1. glycolysis

2. pyruvate oxidation and TCA

3. oxidative phosphorylation

glycolysis

Glucose, a six-carbon sugar, is split into two three-carbon sugars

• These smaller sugars are oxidized and their remaining atoms rearranged to form two molecules of pyruvate

Glycolysis is divided into two phases:

1. Energy investment

• Cell spends 2 ATP

2. Energy payoff

• ATP spent during energy investment is regained as ATP is produced by substrate-level phosphorylation

• NAD+ is reduced to NADH by electrons released from oxidation of glucose 

2 ATP and 2 NADH

pyruvate oxidation

• When O2 is present, the pyruvate enters mitochondria, where the oxidation of glucose is complete

• Pyruvate undergoes a series of reactions: CO2 is removed and the two-carbon fragment is oxidized, forming NADH

• Product is called Acetyl CoA, which is used in TCA

TCA

• Pyruvate is broken down to three CO2 molecules, including the CO2 released during the conversion of pyruvate to Acetyl CoA

• Generates 1 ATP per turn by substrate-level phosphorylation

• Most chemical energy is transferred to NAD+ and FAD during the redox reactions

• Reduced coenzymes, NADH and FADH2, shuttle high-energy electrons into ETC

• For each turn of citric acid cycle, two carbons enter in the relatively reduced form of an acetyl group

• Two different carbons also leave in the completely oxidized form of CO2 molecules

• Next seven steps decompose the citrate back to oxaloacetate; the regeneration of oxaloacetate makes the process a cycle

• 6 NADH, 2 FADH2, and 2 ATP

oxidative phosohporylation

ETC accepts electrons from NADH or FADH2 generated during first two stages at complex 1

Electrons at end combine with oxygen and hydrogen forming water

The energy released at each step of the chain is stored in a form that can make ATP from ADP

Free-energy change down ETC during stage 3:

ETC accepts electrons from NADH and FADH2 at complex I

In next redox reaction, the flavoprotein returns to its oxidized form as it passes electrons to an iron-sulfur protein

The iron-sulfur protein then passes the electrons to the only non-protein compound in the ETC called ubiquinone

cytochromes

Proteins that make up most of the remaining electron carriers between ubiquinone and oxygen

FADH2 adds its electrons to the ETC within the complex II, at a lower energy level than NADH does

Although NADH and FADH2 each donate 2 electrons, the ETC provides about ⅓ less energy for ATP synthesis when the electron donor is FADH2 rather than NADH

ATP synthase

The enzyme that actually makes ATP from ADP and inorganic phosphate

• Works like an ion pump in reverse

• ATP synthase uses energy of an existing ion gradient to power ATP synthesis

chemiosmosis

The electron transport chain uses energy from electrons carried by NADH and FADH₂ to pump H⁺ ions from the mitochondrial matrix to the intermembrane space, creating a hydrogen ion gradient.

This gradient stores potential energy, which drives ATP synthesis through chemiosmosis. As electrons are transferred, H⁺ ions are also released into the surrounding solution.

proton-motive force

Describes the resulting H+ gradient, emphasizing the capacity of the gradient to perform work 

• The force drives H+ back across the membrane through the H+ channels provided by ATP synthases

types of fermentattion

alochol and lactic acid

alcohol fermentation

Pyruvate is converted to ethanol in two steps:

1. First step releases carbon dioxide from pyruvate

2. In second step, acetaldehyde is reduced by NADH to ethanol

- This regenerates the supply of NAD+ needed for continuation of glycolysis

- Yeast is an example of an organism which carries out this fermentation

lactic acid fermentation

Pyruvate is reduced directly by NADH to form lactate as an end product, with no release of CO2

• Bacteria/fungi which do this are used by dairy industry to make cheese

• Human muscle cells make ATP by lactic acid fermentation when oxygen is scarce

• This occurs during strenuous exercise

obligate anerobes

Organisms that carry out only fermentation or anaerobic respiration

facultative anaerobes

Organisms, such as yeast and many bacteria, that can make enough ATP to survive using either fermentation or respiration

• Muscle cells behave as facultative anaerobes