Chapter 9: Cellular Respiration and Fermentation
How is the chemical energy stored in food used to generate ATP; the molecule that drives most
cellular work?
- Plant and animal cells break down organic molecules by cellular respiration in the
mitochondria
- The chemical energy in food is transformed into chemical energy in ATP
- Some energy is released to the environment as heat
9.1: Catabolic pathways yield energy by oxidizing organic fuels
Energy enters ecosystems as light and exits as heat
- The chemical elements essential to life are recycled
o Photosynthesis uses CO2 and H2O to make organic molecules and O2
o Cellular respiration uses O2 and organic molecules to make ATP; CO2 and H2O
are produced as waste
- Catabolic pathways release stored energy by breaking down complex molecules
- Electron transfer from food molecules to other molecules plays a major role in these
pathways
- These processes are central to cellular respiration.
Catabolic pathways and production of ATP
The breakdown of organic molecules is exergonic
- Fermentation: partial degradation of sugars that occur without oxygen
- Aerobic respiration: consumes organic molecules and oxygen and yields ATP
- Anaerobic respiration is similar but consumes compounds other than oxygen.
Cellular respiration includes both aerobic and anaerobic respiration but is often used to refer to
aerobic respiration
- Helpful to trace cellular respiration with glucose
- C6H12O6 + 6 O2 →6 CO2 + 6 H2O + Energy (ATP + heat)
Catabolic pathways DO NOT directly power work in the cell; they are linked to work by
ATP
- Cells must constantly regenerate their supply of ATP from ADP and phosphate.
Redox reactions: Oxidation and Reduction
The transfer of electrons during chemical reactions releases energy stored in organic molecules
- This energy is ultimately used to synthesize ATP
THE PRINCIPLE OF REDOX-
, - Chemical reactions that transfer electrons between reactants are called oxidation-
reduction reactions, or redox reactions
- Oxidation: loss of electrons from a substance
- Reduction: addition of electrons to a substance (the amount of positive charge is
reduced)
Na + Cl ----> Na+ + Cl-
- Na becomes oxidized (loses electron) and Cl becomes reduced (gains electron)
Oxidation and reduction go hand in hand
- Reducing agent: electron donor, reduces the electron acceptor
- Oxidizing agent: electron acceptor, oxidizes the electron donor
Some redox reactions change electron sharing in covalent bonds
- Oxygen atoms are very electronegative; they attract electrons and don’t share them
equally
- The partial “gain” of electrons by O atoms and the partial “loss” of electrons by
their bonding partners constitutes a redox reaction
o Electrons are not completely transferred in the redox reaction between methane
and oxygen
An electron loses potential energy when it shifts from a less electronegative atom toward a
more electronegative one
- Redox reactions that move electrons closer to electronegative O atoms release energy
OXIDATION OF ORGANIC FUEL MOLECULES DURING CELLULAR RESPIRATION
- Fuel molecules (glucose) are oxidized and O2 is released
- Organic molecules with an abundance of hydrogen are excellent sources of high
energy electrons
- Cellular respiration is a redox process; energy is released as hydrogen and electrons
are transferred to O atoms
The oxidation of glucose transfers electrons from a higher state (in glucose) to a lower state w
O atoms
- Releases energy that is used to synthesize ATP
STEPWISE ENERGY HARVEST VIA NAD+AND THE ELECTRON TRANSPORT CHAIN
- Each electron travels with a proton, as a hydrogen atom
- Hydrogen atoms are usually first passed to electron carriers rather than directly to O2
Nicotinamide adenine dinucleotide, NAD+, is a coenzyme that functions as an electron carrier
, - As an electron acceptor, it functions as an oxidizing agent in cellular respiration
- Each NADH (reduced form of NAD+) represents stored energy that is tapped to
synthesize ATP
- Enzymes called dehydrogenases remove a pair of hydrogen atoms (2 electrons and 2
protons) from the substrate
o 2 electrons and 1 proton is transferred to NAD+ forming NADH
o The other proton is released as a hydrogen ion (H+) into the surrounding solution
- If NADH transferred electrons directly to oxygen, energy would be released in one
explosive reaction.
o Instead, cellular respiration uses an electron transport chain to break the fall of
electrons to O2 into several energy-releasing steps
Electron transport chain: a series of molecules built into the inner membrane of the
mitochondria (or plasma membrane of prokaryotes)
- NADH passes electrons to the electron transport chain where they are transferred in a
series of redox reactions, each releasing a small amount of energy
- O2, the final electron acceptor, captures the electrons and the hydrogen nuclei (H+),
forming H2O
- The energy yielded is used to regenerate ATP
Stages of cellular respiration
1. Glycolysis breaks down glucose into two molecules of pyruvate
2. Pyruvate oxidation and the citric acid cycle complete the breakdown of glucose to CO2
3. During Oxidative phosphorylation, the electron transfer chain and chemiosmosis
facilitate the synthesis of most of the cell’s ATP
a. Generates almost 90% of ATP bc its powered by redox reactions
- Some ATP is also formed in glycolysis and the citric acid cycle by substrate-level
phosphorylation
o Occurs when an enzyme transfers a phosphate group directly from a substrate to
ADP
9.2: Glycolysis harvests chemical energy by oxidizing glucose to pyruvate
Glycolysis occurs in the cytoplasm and has two major phases
- Energy investment phase: 2 ATP are used to split glucose into 2 three-carbon sugar
molecules.
o 2 ATP --> 2 ADP + 2P
- Energy payoff phase: 4 ATP are synthesized, 2 NAD+ are reduced to NADH, the small
sugars are oxidized to form 2 pyruvate and 2 H2O
- Net of 2 ATP is produced by substrate-level phosphorylation
Glycolysis doesn’t release any CO2 and occurs whether or not O2 is present
, 9.3: After pyruvate is oxidized, the citric acid cycle completes the energy-yielding oxidation
of organic molecules
- Most energy in glucose remains stored in the pyruvate molecules produced by
glycolysis
- Eukaryotic cells, if O2 is present, pyruvate enters a mitochondrion to complete
glucose oxidation
o Occurs in the cytosol for aerobic prokaryotes
Oxidation of pyruvate to acetyl CoA-
- Pyruvate is converted to acetyl coenzyme (acetyl CoA) before entering the citric acid
cycle
- Pyruvate dehydrogenase catalyzes three reactions
o Oxidation of pyruvate’s carboxyl group
o Reduction of NAD+ to NADH
o Combination of the remaining two-carbon fragment with coenzyme A to
form acetyl CoA
The citric acid cycle-
- Krebs cycle oxidizes fuel derived from pyruvate generating 1 ATP, 3 NADH, and 1
FADH2 per turn
- Another 2 CO2 are produced as a waste product
- Bc 2 pyruvate are produced per glucose, the cycle runs twice per glucose molecule
consumed
Eight steps of the citric acid cycle-
1. Acetyl group of acetyl CoA joins the cycle by combining with oxaloacetate, forming
citrate
- Next seven steps decompose the citrate back to oxaloacetate, making the process a
cycle
- The NADH and FADH2 produced by the cycle carry electrons to the electron
transport chain
9.4: During oxidative phosphorylation, chemiosmosis couples electron transport to ATP
synthesis
- Molecules of NADH and FADH2 produced during glycolysis and the citric acid cycle
account for most of the energy extracted from glucose
- NADH and FADH2 donate electrons to the electron transport chain, which powers
ATP synthesis via oxidative phosphorylation
The pathway of electron transport-
How is the chemical energy stored in food used to generate ATP; the molecule that drives most
cellular work?
- Plant and animal cells break down organic molecules by cellular respiration in the
mitochondria
- The chemical energy in food is transformed into chemical energy in ATP
- Some energy is released to the environment as heat
9.1: Catabolic pathways yield energy by oxidizing organic fuels
Energy enters ecosystems as light and exits as heat
- The chemical elements essential to life are recycled
o Photosynthesis uses CO2 and H2O to make organic molecules and O2
o Cellular respiration uses O2 and organic molecules to make ATP; CO2 and H2O
are produced as waste
- Catabolic pathways release stored energy by breaking down complex molecules
- Electron transfer from food molecules to other molecules plays a major role in these
pathways
- These processes are central to cellular respiration.
Catabolic pathways and production of ATP
The breakdown of organic molecules is exergonic
- Fermentation: partial degradation of sugars that occur without oxygen
- Aerobic respiration: consumes organic molecules and oxygen and yields ATP
- Anaerobic respiration is similar but consumes compounds other than oxygen.
Cellular respiration includes both aerobic and anaerobic respiration but is often used to refer to
aerobic respiration
- Helpful to trace cellular respiration with glucose
- C6H12O6 + 6 O2 →6 CO2 + 6 H2O + Energy (ATP + heat)
Catabolic pathways DO NOT directly power work in the cell; they are linked to work by
ATP
- Cells must constantly regenerate their supply of ATP from ADP and phosphate.
Redox reactions: Oxidation and Reduction
The transfer of electrons during chemical reactions releases energy stored in organic molecules
- This energy is ultimately used to synthesize ATP
THE PRINCIPLE OF REDOX-
, - Chemical reactions that transfer electrons between reactants are called oxidation-
reduction reactions, or redox reactions
- Oxidation: loss of electrons from a substance
- Reduction: addition of electrons to a substance (the amount of positive charge is
reduced)
Na + Cl ----> Na+ + Cl-
- Na becomes oxidized (loses electron) and Cl becomes reduced (gains electron)
Oxidation and reduction go hand in hand
- Reducing agent: electron donor, reduces the electron acceptor
- Oxidizing agent: electron acceptor, oxidizes the electron donor
Some redox reactions change electron sharing in covalent bonds
- Oxygen atoms are very electronegative; they attract electrons and don’t share them
equally
- The partial “gain” of electrons by O atoms and the partial “loss” of electrons by
their bonding partners constitutes a redox reaction
o Electrons are not completely transferred in the redox reaction between methane
and oxygen
An electron loses potential energy when it shifts from a less electronegative atom toward a
more electronegative one
- Redox reactions that move electrons closer to electronegative O atoms release energy
OXIDATION OF ORGANIC FUEL MOLECULES DURING CELLULAR RESPIRATION
- Fuel molecules (glucose) are oxidized and O2 is released
- Organic molecules with an abundance of hydrogen are excellent sources of high
energy electrons
- Cellular respiration is a redox process; energy is released as hydrogen and electrons
are transferred to O atoms
The oxidation of glucose transfers electrons from a higher state (in glucose) to a lower state w
O atoms
- Releases energy that is used to synthesize ATP
STEPWISE ENERGY HARVEST VIA NAD+AND THE ELECTRON TRANSPORT CHAIN
- Each electron travels with a proton, as a hydrogen atom
- Hydrogen atoms are usually first passed to electron carriers rather than directly to O2
Nicotinamide adenine dinucleotide, NAD+, is a coenzyme that functions as an electron carrier
, - As an electron acceptor, it functions as an oxidizing agent in cellular respiration
- Each NADH (reduced form of NAD+) represents stored energy that is tapped to
synthesize ATP
- Enzymes called dehydrogenases remove a pair of hydrogen atoms (2 electrons and 2
protons) from the substrate
o 2 electrons and 1 proton is transferred to NAD+ forming NADH
o The other proton is released as a hydrogen ion (H+) into the surrounding solution
- If NADH transferred electrons directly to oxygen, energy would be released in one
explosive reaction.
o Instead, cellular respiration uses an electron transport chain to break the fall of
electrons to O2 into several energy-releasing steps
Electron transport chain: a series of molecules built into the inner membrane of the
mitochondria (or plasma membrane of prokaryotes)
- NADH passes electrons to the electron transport chain where they are transferred in a
series of redox reactions, each releasing a small amount of energy
- O2, the final electron acceptor, captures the electrons and the hydrogen nuclei (H+),
forming H2O
- The energy yielded is used to regenerate ATP
Stages of cellular respiration
1. Glycolysis breaks down glucose into two molecules of pyruvate
2. Pyruvate oxidation and the citric acid cycle complete the breakdown of glucose to CO2
3. During Oxidative phosphorylation, the electron transfer chain and chemiosmosis
facilitate the synthesis of most of the cell’s ATP
a. Generates almost 90% of ATP bc its powered by redox reactions
- Some ATP is also formed in glycolysis and the citric acid cycle by substrate-level
phosphorylation
o Occurs when an enzyme transfers a phosphate group directly from a substrate to
ADP
9.2: Glycolysis harvests chemical energy by oxidizing glucose to pyruvate
Glycolysis occurs in the cytoplasm and has two major phases
- Energy investment phase: 2 ATP are used to split glucose into 2 three-carbon sugar
molecules.
o 2 ATP --> 2 ADP + 2P
- Energy payoff phase: 4 ATP are synthesized, 2 NAD+ are reduced to NADH, the small
sugars are oxidized to form 2 pyruvate and 2 H2O
- Net of 2 ATP is produced by substrate-level phosphorylation
Glycolysis doesn’t release any CO2 and occurs whether or not O2 is present
, 9.3: After pyruvate is oxidized, the citric acid cycle completes the energy-yielding oxidation
of organic molecules
- Most energy in glucose remains stored in the pyruvate molecules produced by
glycolysis
- Eukaryotic cells, if O2 is present, pyruvate enters a mitochondrion to complete
glucose oxidation
o Occurs in the cytosol for aerobic prokaryotes
Oxidation of pyruvate to acetyl CoA-
- Pyruvate is converted to acetyl coenzyme (acetyl CoA) before entering the citric acid
cycle
- Pyruvate dehydrogenase catalyzes three reactions
o Oxidation of pyruvate’s carboxyl group
o Reduction of NAD+ to NADH
o Combination of the remaining two-carbon fragment with coenzyme A to
form acetyl CoA
The citric acid cycle-
- Krebs cycle oxidizes fuel derived from pyruvate generating 1 ATP, 3 NADH, and 1
FADH2 per turn
- Another 2 CO2 are produced as a waste product
- Bc 2 pyruvate are produced per glucose, the cycle runs twice per glucose molecule
consumed
Eight steps of the citric acid cycle-
1. Acetyl group of acetyl CoA joins the cycle by combining with oxaloacetate, forming
citrate
- Next seven steps decompose the citrate back to oxaloacetate, making the process a
cycle
- The NADH and FADH2 produced by the cycle carry electrons to the electron
transport chain
9.4: During oxidative phosphorylation, chemiosmosis couples electron transport to ATP
synthesis
- Molecules of NADH and FADH2 produced during glycolysis and the citric acid cycle
account for most of the energy extracted from glucose
- NADH and FADH2 donate electrons to the electron transport chain, which powers
ATP synthesis via oxidative phosphorylation
The pathway of electron transport-
