Chapter 9: Cellular Respiration

Energy Transfer In Life

Reaction Coupling•Catabolic reactions: reactants act as “fuels,” broken down with the help of enzymes

•Fermentation: sugar degradation without oxygen (anaerobic)

•Cellular Respiration: most efficient and prevalent means of respiration

•Energy released from respiration can be used for cellular work (endergonic reactions) or as heat

•Ex: ATP regeneration from ADP + Pi

•Glucose (C6H12O6) – ΔG = - 686 kcal/mol

•spontaneous

•ADP + Pi + 7.3 kcal/mol ATP

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

Oxidation/Reduction Reactions (Redox)•The relocation of e- releases E from organic molecules.

•Loss of is e- oxidation.

•Gain of is e- reduction.

LEO the lion goes GER!!!

• e- donor – reducing agent (is oxidized)

• e- acceptor – oxidizing agent (is reduced)

•Note: Redox can happen without a complete transfer of electrons – can change e-

•Highly electronegative atoms are strong oxidizers (ex. Oxygen)

Redox Reactions

• The substance being reduced actually gets “bigger” because the increased number of electrons allows for more bonds

• Glucose oxidation transfers electrons (of hydrogen) to a lower energy state as it bonds with oxygen– Energy released is used in ATP regeneration

Pulling e- away from an atom – requires energy

•This is the activation energy of cell respiration

e- lose energy when then move from a less electronegative atom to a more electronegative atom (ex. Oxygen)

•This powers ATP regeneration, and creates water

•Hydrogen – low electronegativity

•Oxygen – high electronegativity

•Hydrocarbons – many uphill e- (high ∆G), such as found in our foods, including lipids and carbohydrates

Hydrocarbons – many uphill e-

•Excellent fuel source – lots of e- to travel downhill – energy released.

•Therefore, the oxidation of glucose is an exergonic reaction

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•The energy of glucose’s electrons is harvested in a series of stepwise reactions relying on NAD+ and the electron tranport chain

•Glucose is broken down in steps.

•Electrons are removed and transported with protons

•Both are carried by NAD+ - nicotinamide adenine dinucleotide

•NAD+ is an e- acceptor and a proton carrier

•Dehydrogenase: removes 2 hydrogen atoms from a substrate, thereby oxidizing it

•e- transfer to O2 from NADH – ΔG = - 53 kcal/mol

•NADH holds stored energy that can be used in the future to power the creation of ATP

Glucose

OXYGEN

ETC

NADH

H+ and e-

Relies on proteins in inner membrane of mitochondria of eukaryotes

Final e- acceptor (because oxygen is highly electronegative)

The Stages of Cellular Respiration: A Preview•Glycolysis

•Turns glucose into 2 pyruvate molecules

•No O2

•Occurs in the cytoplasm

•Relies on Substrate Level Phosphorylation

•Substrate level phosphorylation uses an enzyme to add a substrate’s phosphate group to ADP

•Catabolic: -∆G

•Dehydrogenases and NAD+

•Citric Acid Cycle

•Uses oxygen

•Occurs in Mitochondrial Matrix

•Substrate Level Phosphorylation

•Catabolic

•Dehydrogenases and NAD+ used, transfer of e- to NAD+ making NADH

•Oxidative Phosphorylation: accepts e- from NADH and passes them through a chain of proteins

•Uses oxygen

•Mitochondrial Inner Membrane

•Includes the Electron Transport Chain and Chemiosmosis

•Anabolic – requires an energy input

•Proton Pump and ATP synthase

Glycolysis•Hexose to Triose

•6C to 3C

•Oxidized to Pyruvate

•Energy Investment Phase

•Glucose becomes G3P, requiring an input of 2 ATP

•Energy Payoff Phase

•G3P becomes pyruvate, substrate level phosphorylation occurs twice, 2 ATP created

•2 net ATP

Energy Investment Phase

Step 1:

•Hexokinase

•Phosphate ‘traps’ glucose

•Increases reactivity

Step 2:

•Isomerases

Step 3:

•Activated for cleavage

•Phosphofructokinase (PFK) phosphorylates glucose

•Allosterically regulated: PFK is inhibited by ATP (ATP is an allosteric inhibitor)

Step 4:

•Cleavage

•Creation of Structural Isomers

Step 5:

•Isomerase

•Active molecule G-3-P

2 ATP have been used

Energy Payoff Phase

Step 6:

•G3P is oxidized

•Very exergonic

•Phosphorylation of oxidized sugar

Step 7:

•Substrate Level phosphorylation

•Sugar oxidized to an organic acid

Step 8:

•Phosphate relocated

Step 9:

•Dehydration reaction

•Creation of double bond

•Phosphate bond unstable

Step 10:

•Substrate level phosphorylation

•Net 2 ATP produced.

Glycolysis – a review•ATP used: 2

•ATP produced: 4

•2 per G3P

•All via substrate level phosphorylation

•NADH produced: 2

•1 per G3P

The Glycolysis/Citric Acid Intermediate•Oxygen Required

•Occurs in the Mitochondrial Matrix

•Active transport of pyruvate into matrix, Pyruvate is converted to Acetyl Coenzyme A

Fully oxidized – very little E 2-C molecule

Sulfur-containing

Very Reactive

The Citric Acid Cycle•Tricarboxylic Acid Cycle

•Krebs Cycle – Hans Krebs – 1930s

•8 Steps

•Specific enzymes

•Cycle – 2 time per glucose (1 time per pyruvate)

•FAD – flavin adenine dinucleotide

•Electron carrier similar to NAD+

Step 1:

•2-C + 4-C = 6-C

•Acetyl CoA + Oxaloacetate

•Coenzyme A recycled

Step 2:

•Isomerase

Step 3:

•CO2 released

•NAD+ NADH

Step 4:

•CO2 released

•NAD+ NADH

•Coenzyme A added

Step 5:

•Coenzyme A removed

•GDP GTP

•Substrate-level phosphorylation – ATP

Step 6:

•FAD FADH2

Step 7:

•Hydration reaction

•Bond rearrangement

Step 8:

•OAA regenerated

•NAD+ NADH

The Citric Acid Cycle – A Review•CO2

•Per glucose: 6

•Per pyruvate: 3

•NADH

•Per glucose: 6

•Per pyruvate: 3

•FADH2

•Per glucose: 2

•Per pyruvate: 1

KREBS CYCLE ANIMATION

•ATP

•Per glucose: 2

•Per pyruvate: 1

Pathway of the Electron Transport Chain•Inner membrane of the mitochondria: contains a chain of several complexes (some are protein, others are non-protein)

•Cristae

•4 protein components I- IV

•Prosthetic groups: non-protein components that help transport e-

•e- carriers arranged in a ‘downhill’ formation via e- carriers such as NADH and FADH2

•E- carriers alternate between reduced and oxidized forms

•NADH begins at Protein Complex I

•FADH2 begins at Protein Complex II

The Path of Electrons:

Protein Complex I

•Flavoprotein

•Flavin mononucleotide

•Iron-sulfide

Ubiquinone (Coenzyme Q)

•Non-protein prosthetic group

•Hydrophobic

•Mobile

Protein Complex II

•FAD

•Iron-sulfide

Protein Complex III

•Cytochrome b (heme)

•Iron-sulfide

•Cytochrome c1 (heme)

Cytochrome c

•Not in a protein – prosthetic group

Protein Complex IV

•Cytochrome a (heme)

•Cytochrome a3 (heme)

Oxygen

•Final electron acceptor

The Electron Transport Chain•Makes no ATP

•ΔG = -53 kcal/mol (exergonic – power the endergonic creation of ATP)

•Proton gradient created

Chemiosmosis – Energy coupling

•Inner mitochondrial memebrane

•ATP synthase

•Reverse ion pump

•Endergonic reaction powered by ETC

•Relies on proton-motive force

•Bacteria: use gradient across cell membrane

•Cells use chemiosmosis to generate ATP, do active transport and rotate flagella

•32-34 ATP produced

•Glucose → NADH → ETC → Proton Motive Force → ATP

ETC animation

Accounting 101

•1 NADH = generates ~3 ATP

•10 H+ across membrane

•3-4 H+ = 1 ATP

•1 FADH2 = generates ~ 2 ATP

Electron shuttlesspan membrane

CYTOSOL 2 NADH

2 FADH2

2 NADH 6 NADH 2 FADH22 NADH

Glycolysis

Glucose2

Pyruvate

2AcetylCoA

Citricacidcycle

Oxidativephosphorylation:electron transport

andchemiosmosis

MITOCHONDRION

by substrate-levelphosphorylation

by substrate-levelphosphorylation

by oxidative phosphorylation, dependingon which shuttle transports electronsfrom NADH in cytosol

Maximum per glucose:About

36 or 38 ATP

+ 2 ATP + 2 ATP + about 32 or 34 ATP

or

•Electron Shuttle (via active transport) into Mitochondrion from Cytoplasm varies with different cells

•NAD+: liver cells

•FAD: brain cells

•Total 36-38 ATP produced

•40% efficient (rest is lost as heat)

Fermentation

•No O2 – anaerobic

•Substrate-level phosphorylation

•As in glycolysis

•NAD+ is needed at start and therefore must be regenerated

Alcoholic Fermentation

•Yeast – for use in brewing and baking

•Bacteria

Lactic Acid Fermentation

•Bacteria – used in making yogurt

•Fungi

•Muscle cells – use LAF when oxygen is scarce

•Liver recycles lactic acid back to lactate

•Former thinking: Lactic Acid build-up in muscle was the cause of muscle cramping

•Facultative Anaerobes: aerobic or anerobic

•Obligate Anaerobes: cannot live in the presence of oxygen

Fermentation v. Aerobic Respiration• Both use glycolysis to oxidize glucose

and other organic fuels to pyruvate• Fermentation yields 2 ATP via

substrate level phosphorylation– Aerobic respiration yields as much as

38 ATP via oxidative phosphorylation• NAD+ is the oxidizing agent in

fermentation so oxygen is not involved. In fermentation, the final e- acceptor is pyruvate

– Aerobic respiration’s final e- acceptor is oxygen

• In fermentation, the energy of pyruvate is still unavailable to the cell

Glucose

CYTOSOLPyruvate

No O2 presentFermentation

O2 present Cellular respiration

Ethanolor

lactate

Acetyl CoAMITOCHONDRION

Citricacidcycle

Evolutionary Significance•Glycolysis is performed by almost all living things

•Glycolysis does not require organelles

•Probably evolved in ancient prokaryotes before there was oxygen in the atmosphere

•Oldest bacterial fossils date to 3.5 bya, while scientists believe oxygen was not present until 2.7 bya

•Heterotroph Hypothesis

•Anaerobic Heterotrophs → Anaerobic Autotrophs (cyanobacteria) → Aerobic Heterotrophs → Aerobic Autotrophs

Metabolic Pathways - Catabolism

•Glycolysis derives sugar from many sources

•Carbohydrates are digested into simple sugars

•Proteins: are digested into amino acids

•Deamination: removal of the amino group from amino acids

•Removed amino acids eventually become ammonia (then uric acid or urea)

•Fats

•Glycerol is converted into G-3-P

•Beta oxidation: changes fatty acids to 2-C fragments which are then converted into acetyl-CoA

•Hydrocarbons of fats are an excellent source of fuel

•1 g of fat oxidized yields twice the ATP of a carbohydrate

Metabolic Pathways – Anabolism •Biosynthesis: food molecules are reused to make needed molecules other than ATP

•Create ½ amino acids

•Nonessential amino acids are made in cells. Essential amino acids must be obtained in the diet.

•Acetyl CoA is created from fatty acids

•Dihydroxacetone Phosphate – fat precursor for glycolysis

Regulation•Feedback Inhibition

•Enzyme regulation

•Phosphofructokinase

•Allosteric: contains sites for inhibitors and activators

•Inhibitors

•ATP

•Citrate: synchronized the rate of the CAC and glycolysis

•Activators

•AMP