Metabolic Reactions

Metabolic Reactions

Metabolism consists of the chemical changes thattake place inside living cells. As a result of metabolism, organisms grow, maintain body functions, release or store energy, produce and eliminate waste, digest nutrients, or destroy toxins. These reactions alter the chemical nature of a chemical substance, maintaining homeostasis.

Two major types of metabolic reactions control how cells use energy. The buildup of larger mole-cules from smaller molecules is called anabolism. An example of anabolism is when amino acids bond together and form proteins. The breakdown of larger molecules into smaller ones is called catabolism. An example of catabolism is when foods are hydrolyzed in the digestive tract. Both anabolism and catabolism require the use of energy.

Anabolism

Anabolism is the process of building complex molecules in the body from simpler materials. When a person is healthy and has adequate nutrition, simple nutrients (such as amino acids, fats, and glucose) are used by the body to build the basic chemicals that sup-port cellular functioning and sustain life.

Anabolism supplies biochemicals needed for cells to grow and repair themselves. An example of anab-olism is when simple sugar molecules called mono-saccharides are linked to form a chain, making upmolecules of glycogen (a carbohydrate). This anabolic process is called dehydration synthesis. As the links in this chain are formed, an OH (hydroxyl group) is removed from one molecule, whereas an H (hydrogen atom) from another is removed. Together, the OH and H produce a water molecule (H₂O). The monosaccha-rides are then joined by a shared oxygen atom, making the chain grow (FIGURE 4-1).

Dehydration synthesis, which links glycerol and fatty acid molecules in adipose (fat) cells, forms fat molecules (triglycerides). This occurs when three hydrogen atoms are removed from a glycerol molecule. An OH group is removed from each of three fatty acid molecules (FIGURE 4-2 ). This creates three water molecules and one fat molecule. Oxygen atoms are then shared between the glycerol and fatty acid portions.

Cells also use dehydration synthesis to join amino acid molecules, eventually forming protein molecules. As two amino acids unite, one OH mol-ecule is removed from one of them, whereas one H molecule is removed from the NH₂ group of another. This forms a water molecule. The amino acid mole-cules are then joined by a bond created between a nitrogen atom and a carbon atom, called a peptidebond (FIGURE 4-3).

A dipeptide is formed from two amino acids bound together and a polypeptide is formed from many amino acids bound into a chain. Polypeptides usu-ally have specialized functions. When a ­polypeptidehas more than 100 molecules, it is considered to be a protein­. Certain protein molecules have more than one polypeptide.

The three primary stages involved in processing nutrients for energy release are:

■■ Stage 1: Digestion in the gastrointestinal tract. Absorbed nutrients are transported to the tissue cells by the blood.

■■ Stage 2: In the tissue cells, nutrients are built into glycogen, lipids, and proteins or are broken down to pyruvic acid and acetyl coenzyme A (CoA) in the cell cytoplasm.

■■ Stage 3: In the mitochondria, there is much catabolic activity, requiring oxygen. This finalizes food breakdown, produces water and carbondioxide, and collects large amounts of adenosine triphosphate (ATP). Carbohydrates such as glucose combine with oxygen to produce large amounts of ATP.The glycolysis occurring in Stage 2 and all events in Stage 3 make up cellular respiration, which is ­discussed in detail later in this chapter.

Catabolism

Catabolism can be defined as the metabolic breakdown of stored carbohydrates, fats, or proteins to provide energy. It occurs continuously to differing degrees. Excessive catabolism leads to wasting of tissues. An example of catabolism is the process of hydrolysis, which is actually the opposite of dehydration synthe-sis. This involves the decomposition of carbohydrates, lipids, and proteins.

Hydrolysis splits a water molecule; for example, hydrolysis of sucrose (a disaccharide) gives off glucose and fructose (two monosaccharides) as the water mol-ecule splits. The equation is:

C₁₂H₂₂O₁₁+H₂O  →  C₆H₁₂O₄+C₆H₁₂O₆

 (Sucrose) (Water) (Glucose) (Fructose)

As shown in the equation, inside the sucrose mol-ecule the bond between the simple sugars breaks. The water molecule supplies a hydrogen atom to one of the sugar molecules while supplying a hydroxyl group to the other.

Both dehydration synthesis and hydrolysis are reversible and are summarized in the following equation:­

Hydrolysis → Disaccharide + Water ↔ Monosaccharide + Monosaccharide ← Dehydration synthesis

During digestion, hydrolysis breaks down carbo-hydrates into monosaccharides. It also breaks down fats into glycerol and fatty acids, nucleic acids into nucleotides, and proteins into amino acids.

1. What are the functions of catabolism and anabolism?

2. Explain the differences between dehydration synthesis and hydrolysis.

3. What are the three primary stages involved in processing nutrients for energy release?