|MadSci Network: Anatomy|
Thank you Frank for the follow-up question.
I reviewed the answer to which you were referring before proceeding with answering your questions.
For the question regarding walking and starving, as opposed to just starving in the original instance, I think the same factors apply. Only now, it becomes even more of a factor as to which person is more "muscled". According to the same breakdown discussed by Carol alongisde the fact that the more muscle you have, the more energy is consumed by metabolic processes. The muslce also becomes an energy store on which to draw. Thus, the answer is largely a "depends"...on muscle content and amount of fat stores.
In the second question and third question, the fat person may draw on more, if that was the only thing being considered. Imagine if you will a person with a large build, with a significant mass of muscle walking alongside our average person who is slightly obese. Muscle being heavier than fat and burning alot more stores through metabolism, I would wager to believe that the built individual will fall short before the other person. So, you see, it becomes complicated and we need to specify the others factors in the situation.
I also want to remind of the fact that the process of breaking down different stores does not follow a defined order, as Carol explained. You could begin consuming muscle tissue before all of the fat reserves are depleted, which makes sense, because the body can sometimes decide that the muscles are more of a draw on energy for metabolism than other processes...In effect your body concludes that you need to hold on to your fat, since fat is your most efficient emergency food store, but you can do without some of that muscle that you apparently aren't using. In terms of daily metabolism, muscle is ten times more costly to maintain than fat. If you were forced to live for 90 days without food, you might survive if your body consumed its muscle first and its fat last, rather than the other way round. But once it consumes its high-metabolism muscle, your body needs less energy to maintain itself.
In response to your final question, the liver creates glucose out of fatty acid through the process of glycogenolysis. In glycogenolysis, glycogen stored in the liver and muscles, is converted first to glucose-1- phosphate and then into glucose-6-phosphate. Two hormones that control glycogenolysis are a peptide, glucagon, from the pancreas and epinephrine from the adrenal glands.
Glucagon is released from the pancreas in response to low blood glucose and epinephrine is released in response to a threat or stress. Both hormones act upon enzymes to stimulate glycogen phosphorylase to begin glycogenolysis and inhibit glycogen synthetase (to stop glycogenesis, which is essentially the storage process...).
Glycogen is a highly branched polymeric structure containing glucose as the basic monomer. First individual glucose molecules are hydrolyzed from the chain, followed by the addition of a phosphate group. In the next step the phosphate is moved to create glucose-6-phosphate.
Glucose-6-phosphate is the first step of the glycolysis pathway if glycogen is the carbohydrate source and further energy is needed. If energy is not immediately needed, the glucose-6-phosphate is converted to glucose for distribution in the blood to various cells such as brain cells.
As for the formation of glucose from lactate by the hepatic tissues, that works similar (but not the exact reverse) to this discussion through the pathway of gluconeogenesis. The conversion of lactate to glucose by hepatic tissue is termed the Cori cycle.
On this last part I wanted to draw in a little more detail...bear with me.
During muscle contractions, ATP is constantly being used to supply energy and more ATP is produced to replenish supplies.
At first glycolysis produces pyruvic acid which is then converted into acetyl CoA and is metabolized in the citric acid cycle to make ATP using the electron transport chain.
If muscular activity continues, the availability of oxygen for use at the end of the electron transport chain becomes the limiting factor and the cells soon exhaust their supplies of oxygen. When this happens, the citric acid cycle is inhibited and causes pyruvic acid to accumulate.
However, glycolysis continues even under anaerobic conditions even though the citric acid cycle works only under aerobic conditions. When the cells become anaerobic, glycolysis continues if pyruvic acid is converted to lactic acid. The formation of lactic acid buys time and shifts part of the metabolic burden to the liver.
Even though not as much ATP can be furnished by glycolysis alone, it is a significant source of ATP when muscular activity continues for any length of time. The final limiting factor in continued muscular activity is the build up of lactic acid. The lactic acid eventually produces muscular pain and cramps which force discontinuation of activity. Usually before this happens and after activity has ceased, lactic acid diffuses out of the muscle cells and into the blood where it enters the liver.
The body is very efficient in that lactic acid is sent in the blood to the liver which can convert it back to pyruvic acid and then to glucose through gluconeogenesis. The starting point of gluconeogenesis is pyruvic acid, although oxaloacetic acid and dihydroxyacetone phosphate also provide entry points. Lactic acid, some amino acids from protein and glycerol from fat can be converted into glucose.
Oxaloacetic acid is synthesized from pyruvic acid in the first step. Oxaloacetic acid is also the first compound to react with acetyl CoA in the citric acid cycle. The concentration of acetyl CoA and ATP determines the fate of oxaloacetic acid. If the concentration of acetyl CoA is low and concentration of ATP is high then gluconeogenesis proceeds. I have included a link to a nice little presentation of the gluconeogenesis process: Gluconeogenesis Animation Link
Gluconeogenesis occurs mainly in the liver with a small amount also occurring in the cortex of the kidney. Very little gluconeogenesis occurs in the brain, skeletal muscles, heart muscles or other body tissue. In fact, these organs have a high demand for glucose. Therefore, gluconeogenesis is constantly occurring in the liver to maintain the glucose level in the blood to meet these demands.
The glucose can then enter the blood and be carried to muscles and immediately used. If by this time the muscles have ceased activity, the glucose can be used to rebuild supplies of glycogen through glycogenesis.
This recycling of lactic acid is referred to, as stated above, the Cori Cycle. The Cori cycle also operates more efficiently when the muscular activity has stopped. At this time the oxygen debt can be made up so that the citric cycle and electron transport chain also begin to function again. In order for most of the lactic acid to be converted to glucose, some must be converted to pyruvic acid and then to acetyl CoA. The citric acid cycle and electron transport chain must provide ATP to "fuel" the gluconeogenesis of the remainder of the lactic acid to glucose.
I hope this was not an overload of information, but I was attemtping to answer as much of your questions as possible. Hope it helped.
Tye "Mad Scientist"
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