CHAPTER 9CELLULAR RESPIRATION:

HARVESTING CHEMICAL ENERGY

Cells require energy

• Catabolic pathways• Fermentation- partial degradation of sugars without

oxygen.

• Cellular respiration- uses oxygen to complete the breakdown of organic molecules, more efficient

Cellular respiration and fermentation are catabolic, energy-yielding pathways

Organic compounds + O2 → CO2 + H2O + Energy

Glucose

C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy (ATP + heat)

ΔG of - 686 kcal per mole of glucose

Cellular respiration

• Phosphorylation- Used for Transport, Mechanical, Chemical Work

• Redox reactions- release energy when electrons move closer to electronegative atoms• Oxidation- loss of electrons

• Reduction- gain of electrons

Na + Cl → Na+ + Cl-

• Na is oxidized

• Cl is reduced (its charge drops from 0 to -1).

Redox reactions

• Xe- + Y → X + Ye-

• X, the electron donor, is the reducing agent and reduces Y.

• Y, the electron recipient, is the oxidizing agent and oxidizes X.

• Redox reactions require both a donor and acceptor.

Redox reactions

• Change in the degree of electron sharing in covalent bonds.

Redox reactions

C6H12O6 + 6O2 → 6CO2 + 6H2O

• Glucose is oxidized• Oxygen is reduced

Electrons “fall” from organic molecules to oxygen during cellular respiration

• Molecules with lots of hydrogen are excellent fuels• Ex: Carbohydrates and Lipids

Fuel for the Cell

• Hydrogen atoms are stripped from glucose and passed first to a coenzyme, NAD+ (nicotinamide adenine dinucleotide).

NAD+ and an electron transport chain

Dehydrogenase enzymes strip two hydrogen atoms from the fuel (glucose), pass two electrons and one proton to NAD+ and release H+.• H-C-OH + NAD+ → C=O + NADH + H+

Dehydrogenase enzymes

• Cellular respiration uses an electron transport chain to break the fall of electrons to O2 into several steps

Respiration involves glycolysis, the Krebs cycle, and electron transport

• Glycolysis (cytoplasm)• Glucose broken down into two pyruvate.

• The Krebs cycle (mitochondrial matrix)• Pyruvate degrades to carbon dioxide.

• Transfer electrons from substrates to NAD+, forming NADH for the Electron transport chain

Respiration involves Glycolysis, the Krebs cycle, and Electron transport

• Electron transport chain, the electrons move from molecule to molecule until they combine with oxygen and hydrogen ions to form water.

• ATP made via oxidative phosphorylation.

ATP also generated in glycolysis and the Krebs cycle by substrate-level phosphorylation.

38 ATP per mole of glucose

• Glucose (six carbon-sugar) splits into two, three-carbon sugars.

• Three-carbon sugars are oxidized and rearranged to form two molecules of pyruvate.

• Ten steps, each catalyzed by a specific enzyme.

Glycolysis harvests chemical energy by oxidizing glucose to pyruvate:

Energy investment phase2 ATP provide activation energy by phosphorylating

glucose.

• Energy payoff phase• ATP is produced by substrate-level

phosphorylation and NAD+ is reduced to NADH.

4 ATP (net) and 2 NADH are produced per glucose.

• Net yield from glycolysis is 2 ATP and 2 NADH per glucose.• No CO2 is produced

• Glycolysis occurs whether O2 is present or not.

• Hans Krebs

• More than 75% of the original energy in glucose is still present in two molecules of pyruvate.

• Requires oxygen

The Krebs cycle

Two pyruvates enter the mitochondrion.

1. A carboxyl group is removed as CO2.

2. A pair of electrons is transferred from the remaining two-carbon fragment to NAD+ to form NADH.

3. The oxidized fragment, acetate, combines with coenzyme A to form acetyl CoA.

The Krebs cycle

This cycle begins when acetate from acetyl CoA combines with oxaloacetate to form citrate.

The oxaloacetate is recycled and the acetate is broken down to CO2.

Each cycle produces one ATP by substrate-level phosphorylation, three NADH, and one FADH2 (another electron carrier) per acetyl CoA.

Large quantities of electron carriers produced

• Most ATP comes from the energy in the electrons carried by NADH (and FADH2) to power ATP synthesis

Electron transport chain

• Occurs in the matrix. • Electrons drop in free energy as they pass down

the electron transport chain.

Electron transport chain

• Electrons carried by NADH and FADH2 are transferred to molecules in the electron transport chain.

• Electrons ultimately pass to oxygen.

• For every two electron carriers (four electrons), one O2 molecule is reduced to two molecules of water.

• No ATP generated directly.

Electron transport chain

• A protein complex, ATP synthase, in the cristae actually makes ATP from ADP and Pi.

• A proton gradient is used to power ATP synthesis.

• Several chain molecules pump H+ from the matrix to the intermembrane space.• This concentration of H+ is

the proton-motive force.

• Chemiosmosis is an energy-coupling mechanism that uses energy stored in the form of an H+ gradient across a membrane to drive cellular work.• Mitochondrion

• Chloroplasts

• Prokaryotes

• During respiration, most energy flows from glucose → NADH → electron transport chain → proton-motive force → ATP.

• Carbon: one six-carbon glucose molecule is oxidized to six CO2 molecules.

• ATP: substrate-level phosphorylation during glycolysis and the Krebs cycle and oxidative phosphorylation in the ETC

Cellular respiration generates many ATP molecules for each sugar molecule it oxidizes- REVIEW

• Each NADH from the Krebs cycle and the conversion of pyruvate contributes enough energy to generate a maximum of 3 ATP.• The NADH from glycolysis may also yield 3 ATP.

• Each FADH2 from the Krebs cycle can be used to generate about 2ATP.

• In some eukaryotic cells, NADH produced in the cytosol by glycolysis may be worth only 2 ATP.• The electrons must be shuttled to the mitochondrion.

• In some shuttle systems, the electrons are passed to NAD+, in others the electrons are passed to FAD.

Counting Energy

• Assuming the most energy-efficient shuttle of NADH from glycolysis, a maximum yield of 34 ATP is produced by oxidative phosphorylation.

• This plus the 4 ATP from substrate-level phosphorylation gives a bottom line of 38 ATP.

Counting Energy

• How efficient is respiration in generating ATP?• Complete oxidation of glucose releases 686 kcal per

mole.

• Formation of each ATP requires at least 7.3 kcal/mole.

• Efficiency of respiration is 7.3 kcal/mole x 38 ATP/glucose/686 kcal/mole glucose = 40%.

• The other approximately 60% is lost as heat.

• Cellular respiration is remarkably efficient in energy conversion.

• Glycolysis generates 2 ATP whether oxygen is present (aerobic) or not (anaerobic).

• Fermentation generates ATP from glucose by substrate-level phosphorylation with NAD+ to accept electrons.

• NAD+ is recycled by transferring electrons from NADH to pyruvate or derivatives of pyruvate.

Fermentation enables some cells to produce ATP without the help of oxygen

• Alcohol fermentation, pyruvate is converted to ethanol in two steps.• 1. Pyruvate is converted to a two-carbon compound,

acetaldehyde by the removal of CO2.

• 2. Acetaldehyde is reduced by NADH to ethanol.

• Yeast used for brewing and winemaking.

Lactic acid fermentation, pyruvate is reduced directly by NADH to form lactate (ionized form of lactic acid). • Some fungi and bacteria make cheese and yogurt.

• Muscle cells switch from aerobic respiration to lactic acid fermentation to generate ATP when O2 is scarce.

• The waste product, lactate, may cause muscle fatigue, but ultimately it is converted back to pyruvate in the liver.

Fermentation and cellular respiration• Both use glycolysis to oxidize sugars to pyruvate with a

net production of 2 ATP by substrate-level phosphorylation.

• Both use NAD+ as an electron acceptor.

• Fermentation, the electrons of NADH are passed to an organic molecule, regenerating NAD+.

• Respiration, the electrons of NADH are ultimately passed to O2, generating ATP by oxidative phosphorylation.

Fermentation and cellular respiration• Krebs cycle oxidizes pyruvate to make ATP• Without oxygen, the energy still stored in pyruvate

is unavailable to the cell.• Aerobic respiration- glucose yields 38 ATP• Anaerobic respiration- glucose yields only 2 ATP

• Facultative anaerobes, (yeast and many bacteria), can use either fermentation or respiration.

• Human muscle cells can behave as facultative anaerobes, but nerve cells cannot.

• From pyruvate there are two metabolic routes.

History• The oldest bacterial fossils are over 3.5 billion

years old, before O2 accumulated in the atmosphere.

• Therefore, the first prokaryotes may have generated ATP exclusively from glycolysis.

• The fact that glycolysis is also the most widespread metabolic pathway and occurs in the cytosol without membrane-enclosed organelles, suggests that glycolysis evolved early in the history of life.

• Glycolysis can accept a wide range of carbohydrates.• Polysaccharides, starch or glycogen, can be hydrolyzed to

glucose monomers

• Other hexose sugars, galactose and fructose, also modified

• Proteins and fats, can Glycolysis and the Krebs cycle

Glycolysis and the Krebs cycle connect to many other metabolic pathways

Proteins for energy• Digested to individual amino acids.• Amino groups removed via deamination.

• The nitrogenous waste is excreted as ammonia, urea, or another waste product.

• Carbon skeletons are modified by enzymes and enter as intermediaries into glycolysis or the Krebs cycle depending on their structure.

Fats for energy• Fats digested to glycerol and fatty acids.

• Glycerol converted to glyceraldehyde phosphate, an intermediate of glycolysis.

• Fatty acids are split into two-carbon fragments via beta oxidation.

• These molecules enter the Krebs cycle as acetyl CoA.

• Per gram, fat will generate twice as much ATP as a carbohydrate via aerobic respiration.

• Carbohydrates, fats, and proteins all use the same pathways

Anabolic pathways• Intermediaries in glycolysis and the Krebs cycle

diverted to anabolic pathways.• 10 amino acids by modifying compounds from the

Krebs cycle.

• Glucose can be synthesized from pyruvate

• Fatty acids can be synthesized from acetyl CoA.

• Glycolysis and the Krebs cycle function as metabolic interchanges• Excess carbohydrates and proteins can be converted to

fats through intermediaries of glycolysis and the Krebs cycle.

• Supply and demand • Excess of a certain amino acid, feedback inhibition

prevents more synthesis

• Catabolic rate is regulated by ATP level

Feedback mechanisms control cellular respiration

Enzyme regulation

Phosphofructokinase- strategic point of regulation

Allosteric regulation

Sets the pace of respiration.

Inhibited by ATP and stimulated by AMP

Citrate is also an inhibitor

Synchronizes the rate of glycolysis and the Krebs cycle.