Basic Human Metabolism

end result is the absorption of amino acids that get used, converted into fats, or turned into

acetyl CoA for the Krebs cycle. The process of breaking down proteins into amino acids is called

proteolysis.

The urea cycle takes the nitrogenous waste products that come from the breakdown of amino

acids and turns it into a molecule that is safer in the body than the ammonium ions made in

amino acid breakdown. This cycle occurs in the liver and the kidneys. Since the amino group in

the amino acid is not a part of any metabolic pathway, to become a metabolically-active

substance it must undergo transamination, which turns the amino group into a keto group.

This creates a molecule that can enter the Krebs cycle plus an ammonium ion that goes into the

urea cycle. While in the urea cycle, ammonium goes along with CO2 to make urea and water;

the urea is excreted by the kidneys.

Amino acids can be metabolized into several different molecules, including acetyl CoA,

pyruvate, oxaloacetate, acetoacyl CoA, and alpha-ketoglutarate. Each of these can participate

somewhere in the basic metabolic processes in the body that lead to the Krebs cycle and

aerobic metabolism.

BASIC HUMAN METABOLISM

Food gets absorbed during eating, used or stored as necessary, and then metabolized during

times when a person isn’t eating. In other words, you don’t have to eat continually in order to

have nutrients readily available.

Right after eating, the body is in an absorptive state. The food is digested and transported into

the body via the enterocytes. The sugars, amino acids, and lipids go to the liver, adipose tissue,

or muscle tissue in order to be processed and used for energy. This absorptive state can last for

up to four hours; insulin is released in order to put glucose into the muscle cells, fat cells, and

liver cells. Glucose gets immediately turned into glucose-6-phosphate, which means there is a

concentration gradient that pushes more glucose into the cell. Liver glucose goes to make

glycogen; the same is true of muscle glucose that isn’t directly needed.

The post-absorptive state is when the food has already been digested, absorbed, and stored.

The body can use glycogen that was previously made in order to maintain a normal glucose

level. There is less storage of fats and sugars (as insulin production is decreased) and more

production of glucose under the influence of glucagon. Gluconeogenesis also occurs in the liver

to make more glucose molecules to replace the depleted glycogen stores. Once this is done,

the liver will convert leftover glucose into fat.

During starvation or prolonged fasting, the immediate priority is to make glucose for the brain.

The second priority is to conserve amino acids to make proteins. The body will use ketones for

fuel and glycolysis is shut down in order for cells to use alternative fuels. Muscles will use fatty

acids rather than glucose for fuel by being converted into acetyl CoA (to go into the Krebs

cycle). The alanine, lactate, and pyruvate will not go into acetyl CoA in the muscle cells but will

go to make necessary glucose in the liver.

As starvation continues, ketone bodies get used for fuel in the heart and major organs. Fatty

acids and triglyceride stores will be used to make ketones. This helps spare proteins from being

used up to make glucose molecules. If this source of fuel is used up as well, the proteins from

muscles are broken down to make glucose. This means that survival is dependent upon the

amount of protein and fat stored in the body.

The process of thermoregulation is keeping the body temperature within certain boundaries.

The core body temperature is maintained within 97.7 and 99.5 degrees Fahrenheit. About 60

percent of the energy in the ATP production process is used to maintain the body temperature.

The hypothalamus in the brain is highly involved in the regulation of body temperature. If the

temperature is too high, there are mechanisms in place to lower the temperature. There is

more heat dissipated through the skin surface. On the other hand, if there is a sensation of low

body temperature, shivering will generate some heat. Thyroid hormone will also generate more

heat through metabolic processes in order to gain more body heat.

Heat can be lost from the body, flowing from a high heat area to a low heat area. This is done

via several mechanisms: 1) Conduction (the direct transfer of heat between two objects,

accounting for 3 percent of heat loss; 2) Convection (the transfer of heat through the skin,