ATP Production via Cellular Respiration: Glycolysis, ETS, and Oxidative Phosphorylation - , Study notes of Biology

Download ATP Production via Cellular Respiration: Glycolysis, ETS, and Oxidative Phosphorylation - and more Study notes Biology in PDF only on Docsity!

BIOL 1110 REEDER

SUMMARY AND REVIEW OF CELLULAR RESPIRATION

Cellular respiration is an oxidative process which occurs as a series of small step-by-step reactions, each catalyzed by an enzyme. The release of packets of energy is coupled with phosphorylation reactions that synthesize ATP and inorganic P.

GLYCOLYSIS

The oxidation of carbohydrate to pyruvic acid is called GLYCOLYSIS. The process can occur whether or not oxygen is present, because no molecular oxygen is used for this reason it is frequently termed "anaerobic respiration". We can summarize the results of glycolysis as follows:

1. The breakdown of a molecule of glucose (a 6-C compound) to two molecules of pyruvic acid (a 3-C compound).
2. The use of two molecules of ATP as "activation energy" and the synthesis of four new molecules of ATP, for a net gain of two molecules of ATP (substrate level phosphorylation)
3. The production of four molecules of H (goes to NAD and eventually through Hydrogen transfer system to produce 6 ATP's).

If no oxygen is present, i.e., if the conditions are anaerobic, pyruvic acid formed by glycolysis will accept the hydrogen from NADH 2 , freeing the NAD freeing the NAD for further use as a hydrogen acceptor and enabling glycolysis to continue. This addition of hydrogen to pyruvic acid results in the formation of LACTIC ACID (A 3-C compound) and CO 2 in plant cells and in many unicellular organisms.

The process whereby the glycolytic pathway leads to production of alcohol or lactic acid from pyruvic acid is called FERMENTATION. Lactic acid fermentation by certain fungi and bacteria is used in the dairy industry to make cheese and yogurt. Human muscle cells can make ATP by lactic acid fermentation when oxygen is scarce (anaerobic) as in the early stages of strenuous exercise (weightlifting), when sugar catabolism for ATP production outpaces the muscle's oxygen supply from the blood. Lactic acid accumulates as a waste leading to muscle fatigue. It is gradually carried away by the blood to the liver, where pyruvic acid is reformed from the lactate in a process requiring oxygen. This results in an "oxygen debt" that is paid back when you pant after you stop exercising.

ELECTRON TRANSPORT SYSTEM (hydrogen transfer system or the cytochrome carrier system): If oxygen is present (after glycolysis) the NADH 2 molecules are not forced to donate hydrogen to pyruvic acid; instead molecular oxygen can act as the ultimate hydrogen acceptor and water is formed. The NAD does not, however, pass its hydrogen directly to the oxygen. Rather, the H is passed down a "respiratory chain" of carrier compounds, most of which are cytochromes. As the H is lowered down this energy gradient, energy is released and used in synthesis of ATP from ADP and inorganic P. This process, often called OXIDATIVE PHOSPHORYLATION, is very similar to the electron transport and ATP synthesis of photophosphorylation of plants.

It can be shown that for every 2 Hydrogens (actually electrons) moved through the transfer system from NAD to H 2 O, three new ATP molecules are synthesized.

BIOL-1110-REVIEW-CELLULAR-RESPIRATION 12/20/

BIOL-1110-REVIEW-CELLULAR-RESPIRATION 12/20/

SUMMARY OF THE ATP YIELD OF THE RESPIRATION OF GLUCOSE:

We can summarize the yield of ATP molecules from the complete cellular respiration of one molecule of glucose to carbon dioxide and H 2 O as follows:

1. A grand total of 38 (net) ATP molecules are formed. Thus, almost 40% of the energy initially trapped in glucose is utilized, the rest lost as heat. This is remarkably good - most internal combustion engines fall far below this mark.
2. Only 2 of the 38 ATP molecules are synthesized anaerobically; the other 36 are the product of aerobic respiration. We now see why oxygen is essential to the life of human beings and most other organisms.
3. Of the 36 aerobically synthesized ATP molecules, 34 result from H transport via (or similar electron acceptors) and the cytochrome system.

FAT RESPIRATION: Glycerol (a 3-C compound) converted to PGAL and fed into glycolytic pathway. Fatty acids broken down into 2-C fragments and fed into Kreb's Cycle. Fats yield slightly more than twice as much energy as 1 gram of carbohydrate. Initial pathway in liver: Beta Oxidation

PROTEIN RESPIRATION: Some amino acids converted into pyruvic acid; some into acetic acid, and some into different compounds of Kreb's Cycle. One gram protein yields slightly less energy than 1 gram o f CH 2 O. Initial pathway in liver: deamination