How Cells Harvest Energy

How Cells Harvest Energy

Chapter 7

2

Respiration

• Organisms can be classified based on how they obtain energy:

• autotrophs: are able to produce their own organic molecules through photosynthesis

• heterotrophs: live on organic compounds produced by other organisms

• All organisms use cellular respiration to extract energy from organic molecules.

3

Respiration

• Respiration is a metabolic pathway of Redox Reactions

• Respiration typically oxidizes carbohydrates

• The type of molecule that is reduced determines the type of respiration

• The energy produced is in the form of ATP 3

4

Respiration• Cellular respiration a metabolic pathway of redox reactions:

-oxidation – loss of electrons– dehydrogenations: loss of hydrogen e-’s

-reduction - gain of electrons– gain on hydrogen e-’s

• Oxidized molecules actually loose a hydrogen atom (1 electron, 1 proton)

• Both the protons and electrons are used by cellular respiration to produce ATP

5

Respiration

• During respiration, high energy electrons are passed along chains of molecules - Electron Transport Chains

• Energy is released as molecules in the electron transport chains are oxidized

• The energy released is used to power the production of ATP

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Three Types of Respiration

• Respiration type is determined by the final electron acceptors:

1. Aerobic Respiration: final electron receptor is oxygen (O2)• Anaerobic Respiration: final electron acceptor is an inorganic molecule other than O2

• Fermentation: final electron acceptor is an organic molecule

Aerobic Respiration• Glucose contains chemical energy that

can be transferred and stored as ATP• Aerobic Respiration is a metabolic

pathway that oxidizes glucose and transfers the energy to produce ATP

• Oxygen is the final electron acceptor:• Recall:

C6H12O6 + 6 O2 6 H2O + 6 CO2 + EnergyGlucose Oxygen Water Carbon Dioxide

• The Energy is in the form of ATP

Aerobic Respiration

C6H12O6 + 6 O2 6 H2O + 6 CO2 + Energy

-Now-

C6H12O6 + 6O2 + 38 ADP + 38 P 6 H2O + 6CO2 + 38 ATP

Aerobic Respiration• Aerobic Respiration is a three stage

process:Stage 1: Glycolysis

Stage 2: The Krebs Cycle

Stage 3: Oxidative Phosphorylation

• Each of these stages produce ATP• At the end of all three stages, there is a

net gain of 38 ATP molecules (profit)– recall: cells are very efficient because of

enzymes

Stage 1: Glycolysis

• Glycolysis is a 10 step metabolic pathway that cleaves glucose

• Glyo-lysis = “splitting glucose”

• Glycolysis occurs in the cell’s cytoplasm– that’s where the enzymes for glycolysis are

located

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Stage 1: GlycolysisGlycolysis converts glucose to pyruvate

(pyruvic acid).

- a 10-step biochemical pathway

- occurs in the cytoplasm- 2 molecules of pyruvate are formed from

each glucose

- net production of 2 ATP molecules

-2 NADH produced by reduction of 2 NAD+

–recall NADH just is an electron carrier

–(see ch. 6)

Stage 1: Glycolysis• During Glycolysis, glucose (a 6 carbon molecule) is

chopped up into 2 Pyruvates (each pyruvate is a 3 carbon molecule)

• As glucose is cleaved, it is also being

oxidized - loosing electrons (hydrogens)

Figure 6_07

• Glucose is cut up into 2 Pyruvates in 10 steps

Figure 6_07

• 2 ATP must be invested during first two steps of glycolysis

• In step 1 ATP is used to phosphorylate glucose to make G-6-P• Phosphorylation destabilizes the glucose molecule so it can be cleaved• Phosphorylation reactions are carried out by enzymes known as Kinases - see chapter 6 and slide 38

Figure 6_07


• In step 2, G-6-P is converted to F-6-P • This step is carried out by an isomerase enzyme• recall isomers from ch. 3 and slide 32

Figure 6_07


•In step 3, 1 ATP is used to phosphorylate F-6-P to become F-1,6-bP • this step is carried out by another kinase

Figure 6_07

• In steps 4 and 5, the six-carbon molecule, F-1,3-bP is cleaved into 2 three-carbon molecules• G-3-P and Dihydroxyacetone phosphate (DHAP)• Dihydroxyacetone phosphate is immediately converted into another G-3-P

Figure 6_07

Very Important!

• Steps 6-10 occur twice for every glucose that enters glycolysis• because there are now two G-3P’s

Figure 6_07

• In step 6, G-3-P’s are oxidized• one NAD+ is reduced to produce one NADH

• Also in step 6, G-3-P’s are phosphorylated to produce 1,3-BPG

Figure 6_07

• One ATP is produced in step 7• 1,3-BPG is dephosphorylated to become 3-BPG• ADP is phosphorylated to ATP

Figure 6_07

• Steps 8 and 9 involve structural changes of 3-BPG to become Phosphoenolpyruvate

Figure 6_07

• In step 10, one more ATP is produced as Phosphoenolpyruvate is dephosphorylated to become Pyruvic Acid• another kinase

Fig. 7.6-1

Fig. 7.6-2

Fig. 7.6-3

Fig. 7.7-1

Fig. 7.7-2

Fig. 7.7-3

GlycolysisTotals

Per GlucoseCosts

– 2 ATPYield

– 2 NADH– 4 ATP

Net Gain from Glycolysis– 2 ATP– 2 NADH

Glucose

Pyruvate1 Pyruvate 2

2 ATP

4 ATP, 2 NADH

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NAD+, NADH

• During redox reactions, electrons carry energy from one molecule to another

• NAD+ is an electron carrier• NAD+ functions to carry electrons by carrying Hydrogen atoms

- NAD+ accepts 2 electrons and 1 proton to become NADH- The reaction is reversible

- NAD+ + 2e-’s + 1p+ NADH

NAD+ Reduced to NADH

FAD• FAD is very similar to NAD+

• It has the same function of collecting and carrying Hydrogen atoms from one molecule to another

• FAD can carry 2 Hydrogen atoms

• FAD is Reduced to FADH2

Stage 2: The Krebs Cycle• Also known as The Citric Acid Cycle

– citrate is the first molecule produced in this cycle

• The Krebs cycle is a metabolic pathway that further cleaves and oxidizes pyruvate

• The Krebs Cycle occurs in the cell membrane of Prokaryotic Cells and in the mitochondria of Eukaryotic Cells

• In mitochondria, a multienzyme complex called pyruvate dehydrogenase catalyzes the reaction


• The Krebs cycle is fueled with pyruvates from glycolysis– recall, there are 2 Pyruvates made from each Glucose, so there

are 2 Krebs Cycles for every glucose molecule

• Prep Step - before pyruvate enters the mitochondria for the Krebs cycle it is cleaved, oxidized and converted to become Acetyl-Coenzyme A (Acetyl-CoA)

Pyruvate

Acetyl-CoACO2

Krebs Cycle

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Prep Step: Pyruvate Oxidation

• Pyruvates are oxidized to form Acetyl-CoA– a CO2 moiety of pyruvate is exchanged for a

Coenzyme A(CoA) moiety

• The products of pyruvate oxidation include:- 1 CO2 - 1 NADH- 1 acetyl-CoA which consists of 2 carbons

from pyruvate attached to coenzyme A

• Acetyl-CoA proceeds to the Krebs cycle

Prep Step: Pyruvate Oxidation

• Prep Step: Pyruvates are converted to Acetyl-CoA with the release of CO2 in a preparation step

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• The Krebs cycle further oxidizes the acetyl group from pyruvate.

- Occurs in the matrix of the mitochondria

- Biochemical pathway of 5 steps

- First Step: Each Acetyl-CoA (2 carbon per molecule) is bonded to an Oxaloacetate (a 4 carbon molecule) to produce Citrate

acetyl group + oxaloacetate citrate

(2 carbons) (4 carbons) (6 carbons)


First Step of Krebs Cycle:

• Each Acetyl CoA (2 carbon per molecule) is bonded to an Oxaloacetate (a 4 carbon molecule)

• The new molecule made is Citrate (a 6 carbon molecule)

acetyl-CoA + oxaloacetate citrate(2 carbons) (4 carbons) (6 carbons)


• Citrate undergoes a five step cycle that builds additional ATPS

• During the Krebs Cycle additional NADH’s and FADH2’s are produced

• Citrate is eventually converted back into oxaloacetate and the cycle continues

Fig. 7.12-2

In step 1, acetyl-CoA enters the mitochondria and combines with oxaloacetate to form citrate

Fig. 7.12-2

Citrate is further oxidized to produce 3 NADH and and FADH2

Fig. 7.12-2

2 CO2’s are

produced as Citrate is converted back into oxaloacetate

Fig. 7.12-2

The regenerated oxaloacetate is ready for another Acetyl-CoA and the Krebs cycle continues

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• The remaining steps of the Krebs cycle:

- release 2 molecules of CO2

- reduce 3 NAD+ to 3 NADH

- reduce 1 FAD (electron carrier) to FADH2

- produce 1 ATP

- regenerate oxaloacetate

The Krebs Cycle Totals(per pyruvate)

TotalsCosts• 2 ATP

Yields• 2 CO2

• 3 NADH• 1 FADH2

• 1 GTP (immediately converted to 1 ATP)

Glucose


2 ATP

4 ATP, 2 NADH

2 ATP

1 ATP, 2 CO2

3 NADH, 1 FADH2

Pyruate converted to Acetyl Co-A

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Aerobic Respiration Review

• After glycolysis, pyruvate oxidation, and the Krebs cycle, one glucose has been completely cleaved and oxidized to produce:

- 6 CO2

- 4 ATP

- 10 NADH

- 2 FADH2

These electron carriers proceedto the electron transport chain for stage 3 of aerobic respiration

Glucose


2 ATP

4 ATP, 2 NADH

2 ATP

2 ATP, CO2

8 NADH, 2 FADH2

Electron Transport Chain NADH, FADH234 ATP


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Stage 3: Electron Transport Chain

• The electron transport chain (ETC) is a series of membrane-bound electron carrier molecules called cytochromes

- embedded in the mitochondrial inner membrane

- electrons from NADH and FADH2 are transferred to cytochromes of the ETC

- each cytochrome transfers the electrons to the next cytochrome in the chain

Fig. 7.13a

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Energy from the Electrons

• As the electrons are transferred, some electron energy is released with each transfer

• This energy is used by the cytochromes to pump protons (H+) across the membrane from the matrix to the inner membrane space

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Energy from the Protons

• Electron energy is used by the cytochromes to pump protons (H+) across the membrane from the matrix to the inner membrane space

• A proton gradient is established– There are more protons on the

Stage 3: Electron Transport Chain• The cytochromes are channel proteins

and use the electron energy to pump protons (H+) across the inner mitochondrial membrane– Now there are more protons on the inside of

the membrane than the outside

• A proton gradient is established

• This proton gradient is potential energy that can be utilized to make more ATP’s– Recall diffusion: The protons want to equalize

their number on both sides of the membrane


• There are other channel proteins in the membrane known as ATP synthases

• ATP synthases provide a channel for the protons to diffuse through

• The rushing protons provides the energy for ATP synthase to phosphorylate ADP to ATP


• In Aerobic Respiration, oxygen is the final molecule to receive the hydrogens as they are passed down the Electron Transport Chain

• The result is water: O2 + 4e- + 4H+ 2H2O

• Oxygen is reduced to water

Oxygen is the Final Electron Acceptor in Aerobic Respiration

The Electron Transport Chain

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Energy Yield of Respiration

• The ETC is very efficient and produces most of the ATP for cellular respiration (34 of the 38)

• Theoretical energy yields– 38 ATP per glucose for bacteria– 36 ATP per glucose for eukaryotes

• Actual energy yield– 30 ATP per glucose for eukaryotes– reduced yield is due to “leaky” inner membrane – reduced yield also due to chemiosis - the use of the

proton gradient for purposes other than ATP synthesis

Glucose


2 ATP

4 ATP, 2 NADH

2 ATP

2 ATP, CO2

8 NADH, 2 FADH2

Electron Transport Chain NADH, FADH230+ ATPAnd H2O


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Respiration Without O2

• Respiration occurs without O2 via either:

1. Anaerobic Respiration• use of inorganic molecules (other than O2) as the final electron acceptor

• Fermentation1. use of organic molecules as the final electron acceptor

Oxidation Without O2

• Anaerobic respiration produces fewer ATPs per glucose molecule compared to Aerobic Respiration

• Anaerobic respiration is much less efficient than aerobic respiration

• The exact amount of ATP production depends on the organism and the final electron acceptors that are used


• Anaerobic respiration is much less efficient than aerobic respiration– the ETC is bypassed– not all molecules are as readily reduced as O2 – other final electron acceptors may be reduced

to produce harmful products • fermentation of organic molecules produces acids

and alcohols

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• Anaerobic respiration by methanogens– methanogens reduce CO2 to regenerate NAD+

– CO2 is reduced to CH4 (methane)

• Anaerobic respiration by sulfur bacteria– inorganic sulphate (SO4) is reduced to

hydrogen sulfide (H2S)

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Fermentation reduces organic molecules in order to regenerate NAD+

1. Ethanol fermentation occurs in yeast

2. Lactic acid fermentation occurs in animal cells (especially muscles)

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Fermentation reduces organic molecules in order to regenerate NAD+

1. Ethanol fermentation• NADH reduces acetaldehydes to produce ethanol (an alcohol)

• CO2, ethanol, and NAD+ are produced

• Lactic acid fermentation1. NADH reduces pyruvate to produce lactic acid

Fig. 7.19-1

Fig. 7.19-2

Other Nutrients Serve as Energy Sources

• In addition to Glucose, many other molecules can be used by cells to produce energy through cellular respiration

• A variety of Carbohydrates, Lipids and Proteins can be catabolized for energy

• All of these must go through preparatory steps before they can enter into glycolysis

• For example: – Amino acids must go through a deamination process– Fatty Acids must go through beta oxidation

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Catabolism of Protein & FatCatabolism of proteins:

• Amino acids undergo deamination to remove the amino group (H2N-)- amino group removed as urea in the urine

• Remainder of the amino acid is converted to a molecule that enters glycolysis or the Krebs cycle- for example:

• amino acid alanine is converted to pyruvate to enter Krebs cycle

• animo acid aspartate is converted to oxaloacetate to enter Krebs cycle

Fig. 7.21

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Catabolism of Protein & Fat

• Catabolism of fats:– triglycerides are broken down to fatty acids

and glycerol– fatty acids are converted to acetyl groups by

β-oxidation – acetyl groups can enter the Krebs cycle

• The respiration of a 6-carbon fatty acid yields 20% more energy than glucose

Preparatory Steps

Amino Acids

Lipids

How Cells Harvest Energy

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