Basics of Biological Macromolecules

negative charge will adhere to water molecules to a greater degree. Glass has a negative charge but things like wax do not have this negative charge so you will see less of this phenomenon.

Another interesting thing you will see related to cohesion is called capillary action. This is seen as the tendency of water to be more cohesive when in contact with another surface. In a capillary tube, you will see a rise in the water level at the edges and a sinking in the middle. If you have a thin enough tube, water will rise against gravity, which is what is known as capillary action. It works better with glass tubes because the glass is so negative in its own charge.

This is also seen in the meniscus of water. The meniscus is the upper layer of a solvent in a narrow tube. With nonpolar solvents, you will see a flat meniscus because there is no adhesion to the sides of the tube. With water, though, you will see a concave meniscus, which is what you’ll see in figure 3:

Figure 3.

As you will learn later, it’s this amazing property of cohesion and the phenomenon of capillary action that keeps plants watered internally. Water rises from the roots to the upper plant parts by capillary action of water inside the circulatory system of the plant.

HYDROGEN BONDING AND OTHER BOND TYPES

In case you need some brushing up on your chemistry skills, we will talk now about the different bonding types and how they apply to biological systems. The three main types of bonds in biology are covalent bonds, ionic bonds, and what you can call attractive forces, of which hydrogen bonding is just one of these.

Hydrogen bonding is what takes place between water molecules as you have just discovered. It requires that a hydrogen atom be at least one component of the attractive force or bond. In nearly all cases, the attraction involves a hydrogen atom being attracted to some other atom that is itself already attached to a larger molecule covalently.

Hydrogen best bonds to those atoms that are the most electronegative, such as chlorine, oxygen, or fluorine but it will bond with others as well. An example of hydrogen bonding within the same molecule is how it is used to zip together two DNA strands in the nucleus. Proteins also can retain their shape because of hydrogen bonding between atoms within the same molecule. Figure 4 shows the periodic table and the degree of electronegativity within the table itself:

Figure 4.

As you will see later, hydrogen bonding is sometimes included as a type of van der Waals force in a molecule. Van der Waals forces are attractive forces that don’t have to involve hydrogen and are weaker than hydrogen bonds.

As was just said earlier, hydrogen bonding depends entirely on the dipole to dipole interaction between some electronegative atom and any hydrogen atom that is itself bound covalently to some other molecule. Water is often that molecule. These are weak forces that are only about five percent of the strength of a typical covalent bond. It is the slight positive charge on the bonded hydrogen atom in a molecule that allows for this type of bonding to happen.

Hydrogen bonds are extremely important in all aspects of biology. They help the transcription factors bind to DNA during DNA transcription and help antigens and antibodies pair up together. Proteins will gain their shape with hydrogen bonding and DNA strands are held together using these bonds.

Figure 6.

Lipids are another form of major biomolecule or biological molecule. These are generally made from a combination of fatty acids attached to a glycerol molecule to make a triglyceride molecule of varying sizes. These molecules are the long-term storage form of energy in biological systems, often used when sugars are used up or are unavailable. There are several other forms of lipids besides triglycerides, such as steroids, phospholipids, waxes, grease, and oils. Figure 7 shows what a triglyceride looks like as well as what cholesterol, a steroid lipid, looks like: