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Biology 1010

Fall 2009


RESPIRATION

Virtually all energy on earth comes from sunlight. Plants use energy from the sun to make the bonds which hold organic molecules together. When these bonds are broken during respiration the energy is ultimately transferred to ATP, which is then moved about cells and organisms to power their needs.

I. How cells acquire ATP 

A. Cellular Respiration

1. Cellular respiration includes the various metabolic pathways that break down carbohydrates and other metabolites to make ATP

2. Some processes of respiration take place under aerobic (with oxygen) and some take place under anaerobic (without oxygen) conditions

3. Overall equation for complete aerobic breakdown of glucose:

C6H12O6 + 6 O2 ----------> 6 CO2 + 6 H2O + energy

Note that respiration is the complement of photosynthesis

4. Pathways of aerobic respiration allow energy in glucose to be released slowly; ATP is produced gradually

5. Rapid breakdown of glucose would lose most energy as nonusable heat

6. Complete aerobic breakdown of glucose yields synthesis of 36 ATP; this preserves 38% of energy available in glucose

B. NADH and FADH2 are prime energy carrying molecules of respiration

C. Under anaerobic conditions only glycolysis takes place, which yields only a small amount of energy

D. Under aerobic conditions the products of glycolysis enter the krebs cycle and electron transport system which provide much greater energy yields

II. Glycolysis ("sugar splitting")

A. Occurs in the cytoplasm outside the mitochondria

B. It is an anaerobic process, universal in organisms; therefore, most likely evolved before krebs cycle and electron transport system

C. Requires an input of ATP to start

D. Glucose is broken down to two C3 molecules of pyruvate and there is a small yield of ATP (2) and NADH (2)

E. If oxygen is present the two pyruvates will move to the krebs cycle

III. Krebs cycle

A. Takes place within the interior (matrix) of mitochondria, which consists of a dense solution of enzymes, coenzymes, water, phosphates and other molecules involved in respiration

B. The two pyruvates (from glycolysis) enter the mitochondria and one carbon is stripped from each, resulting in the carbon dioxide you exhale

C. The remaining portion of the pyruvates are combined with an enzyme helper (Coenzyme A) to form acetyl-CoA.

D. Acetyl-CoA combines with oxaloacetate and this compound goes through a series of chemical reactions to form a small amount of ATP (2) plus several energy carrying molecules called NADH (6) and FADH2 (2).

E. The energy captured in the NADH and FADH2 must be converted to ATP via the electron transport system 

IV. Electron transfer phosphorylation 

A. Takes place on the cristae of mitochondria

B. Its purpose is to extract the energy from the NADH and FADH2 molecules created during the previous processes and transfer them to ATP

C. Complicated process driven by a proton gradient across the membrane

D. The net result is that each NADH from the Krebs cycle yields 3 ATP and each FADH2 yields 2 ATP

1 NADH = 3 ATP, 1 FADH2 = 2 ATP

V. Net yield from one glucose molecule via aerobic respiration 

A. Net yield of 36 ATP

B. Energy difference between total reactants (glucose and O2) and products (CO2 and H2O) is 686 kcal per mole

C. ATP phosphate bond has energy of 7.3 kcal; 36 x 7.3 = 263 kcal per mole

D. Efficiency is 263/686 or 38% of available energy in glucose is transferred to ATP

VI. Fermentation

A. Fermentation = anaerobic respiration

1. Fermentation consists of glycolysis plus reduction of pyruvate to either lactate or alcohol and CO2

2. NADH passes its electrons to pyruvate instead of to an electron transport system; NAD+ is then free to return and pick up more electrons during earlier reactions of glycolysis

3. Examples

a. Anaerobic bacteria produce lactic acid in the manufacture of some cheeses

b. Anaerobic bacteria produce industrial chemicals: isopropanol, butyric acid, propionic acid, and acetic acid

c. Yeasts use CO2 to make bread rise, produce alcohol in winemaking

d. Animals reduce pyruvate to lactate when it is produced faster than it can be oxidized by krebs cycle

B. Advantages and disadvantages of fermentation

1. Despite low yield of two ATP molecules, fermentation provides quick burst of ATP energy for muscular activity

2. Disadvantage is that lactate is toxic to cells

a. When blood cannot remove all lactate from muscles, lactate changes pH and causes muscles to fatigue

b. Individual is in oxygen debt because oxygen is still needed after exercising

c. Recovery occurs after lactate is sent to liver, converted into pyruvate; then respired or converted into glucose

C. How efficient is fermentation?

1. Two ATP produced per glucose molecule during fermentation is equivalent to 14.6 kcal

2. Complete glucose breakdown to CO2 and H2O during aerobic respiration yields 686 kcal.

3. Efficiency for fermentation is 14.6/686 or about 2%; much less than complete breakdown of glucose

4. Not surprising that most organisms rely on aerobic respiration 

 VII. Alternative Energy Sources in the Body

A. Glucose

1. After eating a meal, glucose is absorbed into the blood

a. Insulin levels rise, causing greater uptake of glucose by cells for entry into glycolysis

b. Excess glucose is converted into glycogen for storage in muscles and the liver

2. Between meals blood glucose levels fall. If this were not checked it would be bad

a. The hormone glucagon prompts liver cells to convert glycogen back to glucose

b. Glycogen levels are adequate but can be depleted in 12 hours

B. Fats

1. Excess fats are stored away in cells of adipose tissue, primarily as triglycerides. Stored fats insulate and pad parts of our bodies in addition to their role as stored energy

2. When blood glucose levels fall, fats are digested into glycerol, which enters glycolysis, and fatty acids, which enter the Krebs cycle

3. Because fatty acids have many more carbon and hydrogen atoms than carbohydrates, they are degraded more slowly and yield greater amounts of ATP

4. Excess glucose can be converted to fat by a diversion of acetyl-CoA into a pathway that synthesizes fatty acids

C. Proteins

1. During digestion, proteins are split into their amino acid subunits and absorbed into the bloodstream

2. In some cases, amino acids are broken down into their subunits and fed into the Krebs cycle

E. Just as glucose was broken down in cellular respiration, these other molecules undergo catabolism