BIOLOGY Volume 1 - A Guide to General Biology - 2004

8. HETEROTROPHIC NUTRITION

8.5. Fate of Absorbed Nutrients

Monosaccharides and Amino Acids are absorbed into the Blood Vessels of the villi and are then transported via the HEPATIC PORTAL VEIN to the Liver. The liver stores the greater part of the glucose, which is also deposited in Muscles as Glycogen or fat. Some glucose leaves the liver via the hepatic vein and is distributed throughout the body, where it is used for oxidation during Respiration or for other Functions. If the body requires additional energy between meals, the glycogen stored in the liver can be converted back into glucose, which is carried via the bloodstream to the Tissues that need it.

Amino acids are used for Protein Synthesis. The Functions of Proteins are listed in Table 3.9. Proteins play a particularly vital role in growth and repair processes, being among the primary Components of the Cytoplasm. Enzymes and certain Hormones are proteins. Excess amino acids cannot be stored and are therefore deaminated in the liver. During deamination, amino groups (-NH₂) are removed and converted into urea. Urea is carried by the blood to the Kidneys, from where it is excreted in the urine. The remaining parts of The amino acid molecules are converted into glycogen and stored.

Absorbed fats bypass the liver, entering The Lymphatic system and traveling through its vessels via the Thoracic duct into the Veins near The Heart. Fats serve as the body's primary energy reserve. Under normal conditions, the body receives an adequate supply of glucose, and fats are not required for energy production. In this situation, they are deposited in the subcutaneous adipose tissue, in the fat depots surrounding The Heart and kidneys, and in the mesentery. Certain Lipids (Phospholipids) are incorporated into Cell membranes.

Many of the processes described here will be examined in greater detail in other sections of the book.