Chemical Building Blocks of Life

Related to hydrogen bonding is what are called van der Waals forces but van der Waals forces are even weaker than hydrogen bonds. Van der Waals forces attract neutral molecules to one another in the same way as is seen in hydrogen bonding. Because of the polarity of these types of molecules, the atoms that are more electronegative will attract atoms that are more electropositive. You can determine the electronegativity of a substance by looking at the periodic table. Those atoms in the upper right-hand corner of the table are considered the most electronegative.

CHEMICAL BUILDING BLOCKS OF LIFE

There are four elements that make up the most common elements in living things. The largest contribution by weight is oxygen, followed by carbon, hydrogen, and nitrogen. Others that are less commonly seen include phosphorus, calcium, iron, and other metals or ions.

There are four classes of biomolecules seen in the biochemistry of living things. These include lipids, carbohydrates, protein, and nucleic acids. As you will see, these tend to be polymers made up of certain monomers linked together. As you will see, when monomers turn into polymers, they do so by participating in dehydration reactions.

Carbohydrates are molecules that are used for the storage and production of energy. Carbohydrates are made from sugars linked together. These molecules are the most abundant of the different biomolecules on earth. The simplest carbohydrate is called a monosaccharide. Two monosaccharides together will yield a disaccharide, while many together make a polysaccharide. They are made from carbon, hydrogen, and oxygen atoms only.

A monosaccharide involves just a few different sugars. There are the six-carbon sugars, including glucose, fructose, and galactose, and five-carbon sugars, including ribose and deoxyribose. The five-carbon sugars are used to make RNA and DNA, respectively. Monosaccharides can be in an open-chain form or a closed-chain or “cyclic” form. Figure 6 shows the structure of several monosaccharides and disaccharides:

Figure 6.

As mentioned, two monosaccharides undergo a dehydration reaction to make a disaccharide. Examples of disaccharides include lactose, sucrose, and maltose. Sucrose is ordinary table sugar, made from glucose and fructose together. It takes a hydrolysis reaction to separate a disaccharide into two monosaccharides.

A few sugars together make an oligosaccharide, which is used in chemical signaling. Many monosaccharides together make a polysaccharide, which can be linear or

branched. Cellulose, chitin, and glycogen are common polysaccharides. Of these, glycogen is used as an energy storage molecule in various animals.

Lipids include thing like fatty acids and sterols like cholesterol. Triglycerides are free fatty acid molecules attached to glycerol. These look different from one another but share the fact that they are water-insoluble or hydrophobic. Some lipids have ring structures, like cholesterol, while others are linear, like fatty acids. Figure 7 shows the structure of the cholesterol molecule:

Figure 7.

Saturated fatty acids have no double bonds in the molecule, while unsaturated fatty acids have at least one double bond. A polyunsaturated fatty acid has many double bonds. Figure 8 shows what some unsaturated fatty acids look like:

Figure 8.

In general, fatty acids are nonpolar. When they are attached to a phosphate molecule, the molecule, called a phospholipid, becomes amphipathic, which means that it is hydrophobic on one end and hydrophilic on the other end. In this case, the phosphate component of the phospholipid is hydrophilic and the long tail is hydrophobic.

Lipids are seen in the membranes of cells and are a common part of the human diet. Things like vegetable oils have polyunsaturated fatty acids, while butter contains saturated fatty acids. These are broken down in the diet and made into triglycerides in the body.

Proteins and amino acids are called nitrogenous compounds because they contain nitrogen. Amino acids are the building blocks of proteins. Amino acids have a carboxyl group or a -COOH group and an amino group or NH3. Peptides are short proteins,

while proteins can consist of hundreds of amino acids strung together. There are twenty standard amino acids used in living things. They have different degrees of lipophilicity and hydrophilicity, which contribute to their three-dimensional shape.

Amino acids can be modified or unmodified to make neurotransmitters in neurologic systems. Glutamate is an amino acid that also acts as a neurotransmitter. Figure 9 shows the structure of amino acids in neutral and zwitterion form:

Figure 9.

Proteins can be found in all biological systems. Things like actin and myosin, used to contract muscles, are made from proteins, albumin, and antibodies are all made from amino acids and are considered proteins. Glycoproteins are mixtures of proteins and sugars in one molecule. Enzymes are made from proteins. As you will see in a later chapter, there are different ways to describe the structure of proteins.

Proteins are important parts of the human diet. Proteins are consumed and are broken down into amino acids. They get absorbed by the GI tract and get reconfigured to make new proteins. In the cell, the pentose phosphate pathway and the citric acid cycle, which will be discussed later, are used to create new amino acids. There are nine amino acids that are considered essential to humans because they cannot be synthesized.

Nucleic acids are found in the nuclei of cells as well as in the mitochondria. DNA and RNA are the polymers that are made from monomers called nucleotides. The consist of purines and pyrimidines, which are the “nitrogenous bases”, a sugar group, and a phosphate group. Figure 10 shows what a nucleotide looks like chemically:

Figure 10.