Metabolic pathways

• A membrane proton gradient supplies proton motive force to drive chemiosmotic ATP synthesis by ATP synthase

• Glycolysis, pyruvate oxidation, and the citric acid cycle extract electrons from food, to generate NADH, which delivers electrons to the electron transport chain, to replenish the membrane proton gradient

• In the absence of respiration, cells can still make ATP through fermentation and substrate-level phosphorylation

Chemiosmotic synthesis of ATP

Free energy changes in the electron transport chain

Q: In prokaryotes, where is the electron transport chain located?

A. In the mitochondrial outer membraneB. In the mitochondrial inner membrane C. In a vacuolar membraneD. In the cytoplasmE. In the plasma membrane

Glucose metabolism

• C6H12O6 + 6 O2 + 6 H2O → 6CO2 + 12 H2O

Overview of eukaryotic energy pathways

Cellular locations of energy pathwaysEukaryotes Prokaryotes Cytoplasm

Glycolysis Fermentation

Cytoplasm Glycolysis Fermentation Pyruvate oxidation Citric acid cycle

Mitochondria Inner membrane

Electron transport chain

Matrix Pyruvate oxidation Citric acid cycle

Plasma membrane Electron transport chain

Initial steps of glycolysis use 2 ATPs, split glucose to 2 G3PGlucose

ATP

ADP

Hexokinase

Glucose-6-phosphate

Phosphoglucoisomerase

Fructose-6-phosphate

ATP

ADP

Phosphofructokinase

Fructose-1, 6-bisphosphate

Aldolase

Isomerase

Dihydroxyacetonephosphate

Glyceraldehyde-3-phosphate

1

2

3

4

5

Aldolase

Isomerase

Fructose-1, 6-bisphosphate

Dihydroxyacetonephosphate

Glyceraldehyde-3-phosphate

4

5

Q: In the first reaction of glycolysis, hexokinase couples phosphorylation of glucose to hydrolysis of ATP. The phosphorylation of glucose has a free energy change of +3.3 kcal/mol, whereas the hydrolysis of ATP has a free energy change of -7.3 kcal/mol. This coupled reaction is:

a) respiration.b) a redox reaction.c) overall exergonic.d) overall endergonic.e) fermentation.

As 2 G3Ps are oxidized to 2 pyruvates, 2 NADH are produced...

and 4 ATPs via substrate-level phosphorylation

Q: Which of the following is NOT an input to glycolysis?

A) GlucoseB) ATPC) ADPD) NAD+E) All of the above are inputs to glycolysis

Q: During glycolysis, for each mole of glucose oxidized to pyruvate

a) 6 moles of ATP are produced.b) 4 moles of ATP are used, and 2 moles of

ATP are produced.c) 2 moles of ATP are used, and 4 moles of

ATP are produced.d) 2 moles of ATP are used, and 2 moles of

ATP are produced.e) net 4 moles of ATP are produced.

Changes in free energy during glycolysis

Pyruvate pathways

• How is pyruvate metabolized?– Through respiration– Through fermentation

Q: Which photo shows plant mitochondria?

A

B

CD. None of the above – plants don’t have mitochondria

Cellular Energy Pathways from PyruvateGlucose (C6H12O6)2 NAD+ + 2 ADP

Respiration Fermentationglycolysis 2 ATP

2 Pyruvate 2 NADH

Q: Glycolysis is found in all domains of life and is therefore believed to be ancient in origin. What about the citric acid cycle, the electron transport chain, and the F1 ATP synthase?

a) They evolved after accumulation of atmospheric oxygen began

b) They evolved before accumulation of atmospheric oxygenc) Individual enzymes were present before accumulation of

atmospheric oxygen, but served other functions such as amino acid metabolism

d) They evolved only in eukaryotese) They evolved only in animals

In respiring cells, pyruvate is oxidized to CO2 + acetyl CoA

Two molecules of pyruvate are oxidized to 2 acetyl CoA + 2CO2, generating 2 NADH per molecule of glucose.

http://www.micro.siu.edu/micr201/chapter8N.html

Citric acid cycle generates ATP, NADH and FADH2 by oxidizing acetate to CO2

Citric acid cycle: hub of cellular metabolism

http://en.wikipedia.org/wiki/File:Citricacidcycle_ball2.png

Citric acid cycle releases (and captures) more free energy than does glycolysis

Q: Which of the following is produced during the citric acid cycle?

a) FADb) Pyruvatec) Reduced electron carriersd) Lactic acide) Water

Citric acid cycle inputs and products

Respiratory Electron Transport Chain & F1 ATPase

• Inputs

• Outputs

Q: In the absence of respiration (no oxidative phosphorylation), a cell can still generate ATP from a high rate of glycolysis if it:

a) Reduces NAD+ to NADH.b) Generates oxygen.c) Makes more ADP.d) Makes Acetyl-CoA from pyruvate.e) Oxidizes NADH to NAD+.

Fermentation reduces pyruvate to lactic acid or ethanol, to regenerate NAD+

2 ADP + 2 Pi 2 ATP

Glucose Glycolysis

2 NAD+ 2 NADH

2 Pyruvate

+ 2 H+

2 Acetaldehyde2 Ethanol

(a) Alcohol fermentation

2 ADP + 2 Pi 2 ATP

Glucose Glycolysis

2 NAD+ 2 NADH+ 2 H+

2 Pyruvate

2 Lactate

(b) Lactic acid fermentation

2 CO2

Q: Why are fermentation reactions not required in a cell undergoing oxidative phosphorylation?

A. Glycolysis is not needed because oxidative phosphorylation supplies enough ATP

B. NADH is oxidized to NAD+ by the electron transport chain

C. NADH is oxidized to NAD+ by oxygenD. NAD+ is not required for pyruvate oxidation and

the citric acid cycleE. Fermentation reactions are required to supply

NAD+ for glycolysis, pyruvate oxidation, and the citric acid cycle

Diagram cellular energy pathways

Carbohydrates (CH2O) NAD+ + ADP

Respiration Fermentationglycolysis

Total ATP production from glycolysis and aerobic respiration in eukaryotes

Metabolism of fats and proteins

• Fats (triglycerides) are broken down into glycerol and fatty acids. – Glycerol has to be phosphorylated (expends

ATP)– Fatty acids broken down and converted to acetyl-

coA in mitochondria (beta-oxidation)• Proteins hydrolyzed to amino acids

– Individual amino acids converted to citric acid cycle intermediates, pyruvate, or acetyl-coA

Q: During intense exercise, as muscles go into anaerobiosis, the body will increase its consumption of:a) fatsb) proteinsc) carbohydratesd) All of the above

Q: How will excess acetyl-CoA affect the rates of glycolysis and the citric acid cycle?a) Both will increaseb) Both will decreasec) Glycolysis will increase,

citric acid cycle will decrease

d) Glycolysis will decrease; the citric acid cycle will increase

e) Neither will change

What do the following have in common?

Thermogenesis in philodendrons

Brown fat in newborn mammals

Q: Some drugs known as uncouplers facilitate diffusion of protons across the mitochondrial inner membrane. When such a drug is added, what will happen to ATP synthesis and oxygen consumption?a) Both ATP synthesis and oxygen consumption will

decreaseb) ATP synthesis will increase, oxygen consumption

will decreasec) Both ATP synthesis and oxygen consumption will

increased) ATP synthesis will decrease, oxygen consumption

will stay the samee) There will be no effect on ATP synthesis or oxygen

consumption

Q: Petite mutants of yeast have defective mitochondria incapable of oxidative phosphorylation. What carbon sources can these mutants use to grow?a) Glucoseb) Fatty acidsc) Pyruvated) All of the abovee) None of the above