Carbohydrates

amino acid to be hydrophilic (water-loving) or hydrophobic (lipid-loving)—having a major effect on the ability of the amino acid to interact with other amino acids. They can add to the charge on the amino acid and can help to create the three-dimensional shapes of proteins.

You need to know that, aside from the simple amino acid glycine, the molecule will be chiral. Chiral means that there can be enantiomers, which in biochemistry are referred to as L-enantiomers and D-enantiomers (for left-sided and dextro- or right-sided enantiomers). Chemists use the same structural characteristics as glyceraldehyde to identify D-amino acids and L-amino acids because it structurally resembles amino acids. Figure 77 shows the difference between D-amino acids and L-amino acids:

Figure 77.

Nearly all amino acids are found in L-form except for a few amino acids seen in bacteria and in some antibiotics.

CARBOHYDRATES

All carbohydrates have carbon, hydrogen, and oxygen atoms in them and nothing else. Examples include fiber, glycogen, and starch (which are large molecules) as well as sugars, such as glucose (a simple monosaccharide) and sucrose (a disaccharide). According to organic chemistry, carbohydrates can be described as polyhydroxy aldehydes, shown in figure 78:

Figure 78.

Green plants undergo photosynthesis to create glucose, known as C6H12O6 from CO2 (carbon dioxide) and water using solar energy. It can be determined that it takes 686 kilocalories of solar energy to make a mole of glucose in reactions. After glucose is made, it is stored as starch molecules or in cellulose, which makes up the carbohydrate cell wall of plants. Cellulose cannot be broken down by humans for energy but starch can be broken down to be used as food. In cellular respiration, oxygen is used to break the glucose back down to CO2 and water (plus energy that is used for cellular activities).

Cellular respiration is a series of reactions that start with glucose and end with CO2 and water. Humans can, of course, break down fats and amino acids, but they must be broken down into components that are fed into the cellular respiration process. Figures 79 and 80 show some of the biochemical reactions that take place in cellular respiration:

Figure 79.

Figure 80.

In the same way, carbohydrates can be used to make nucleic acids, lipids, and many proteins, being the base molecules that go into making these complex organic molecules.