We will build up to how gas exchange takes place, but we will do it in in steps. In this section, we will look at the chemical nature of membranes and solutions. The chemistry has to do with the different types of bonds formed by oil and water. Some chemical bonds are lopsided, while others are not. This lopsidedness explains why some substances (like salts and sugars) dissolve in water, while others (like oils) will not.

Why this is important…A basic understanding of how chemicals move around the body starts with the understanding of why oil and water don’t mix. Water-soluble and fat-soluble substances behave very differently in the body, whether they are vitamins, drugs, or hormones.

Foundations

Every cell in the body faces a similar challenge: how to allow some things into the cell but keep others out. The answer begins with the lipids that make up the cell membrane. These lipids are special types of lipids – phospholipids. The cell adds other molecules to customize how the cell membrane works, but phospholipids are the foundation.

Oil and water don’t mix. The chemical reason for this is simple:

Water has lopsided chemical bonds and is able to dissolve other “lopsided molecules,” like sugars, amino acids and DNA, as well as ions.
Oils do not have lopsided bonds, and therefore, do not mix well with water. They only mix well with molecules with similar chemical properties.

Key terms

Solution – mixture of solvent and solute that remains mixed without separating (EX: salt or sugar in water)

Solute – chemical substance being dissolved in a liquid solvent

Solvent – liquid portion of a solution (in living beings this is usually water)

Polar covalent bond – bond between atoms where electrons are shared unequally; this results in lopsided charges where some atoms carry partially positive or negative charges

Nonpolar covalent bond – bond between atoms where electrons are shared equally

Hydrophilic (adj.) – chemical property of substances that mix well with water. If a substance dissolves in water, we can also call it water-soluble. (Greek: hydro = water, philos = love, or “water-loving”)

Saturated – describes fatty acids that have only single covalent bonds between carbons

Unsaturated – describes fatty acids that have at least one double covalent bond between carbons

Details

The reason that lipids form a chemical barrier to water is the nature of their chemical bonds. Lipids are mostly long hydrocarbon chains (Figure 4-2). Carbon and hydrogen share their electrons fairly equally (they have nonpolar covalent bonds). Water is composed of hydrogen and oxygen, which share electrons unequally (they have polar covalent bonds with partial electrical charges). (Video 4a explains this further.)

Lipids don’t have monomers that link together in chains like other biomolecules. The fatty acid is the building-block molecule common to most lipids (Figure 4-2). On our food plate, most lipids are oils or fats. These are usually triglycerides – three fatty acids joined by a 3-carbon molecule (glycerol).

Video 4a: Lipids
After watching the video, you should be able to:

describe where you find lipids
recognize different lipids and describe their functions
differentiated saturated and unsaturated fatty acids

Structural diagrams of several lipids.

FIGURE 4-2. Lipids. Fatty acids, small triglyceride, phospholipid and steroid. (“Lipids” by OpenStax College is licensed under CC BY 3.0)

Variations in the structure of fatty acids change their physical properties. Longer fatty acids tend to form solids. Waxes, for example, are made of very long fatty acids. Another significant difference has to do with the bonds between carbons. On the hydrocarbon chains in Figure 4-2 you can see that most of the carbons are bound by single covalent bonds. However, some have a double bond (C=C) which creates a bend in the molecule. This bend makes those lipids spread out rather than pack together tightly. Lipids with bends are more fluid than those with only single bonds. Generally, unsaturated triglycerides are oils (liquid at room temperature), while saturated triglycerides are fats (solid at room temperature).

Skeletal structures are a shortcut way to draw large molecules. In Figure 4-2 you can see that large molecules are complicated to draw using all the chemical symbols, but it is simplified when using the skeletal structure. We assume all organic molecules have a core structure (or backbone) of carbon, so we assume carbon is present at every “bend or end” of a line. The shortcut is simply to not draw carbon or the hydrogens directly attached to them. Anything else is drawn using chemical symbols. Video 4b explains this further.

Stop & Think #1: Draw a skeletal formula for two or more of the lipids in Figure 4-2.

VIDEO 4b: Skeletal formulas

After watching this video, you should be able to:

draw a molecule using the skeletal formula
interpret molecules drawn with skeletal formulas to know what elements are present

Like dissolves like. This is the reverse of our rule about “opposites attract.” When it comes to solutions, chemicals with similar chemical properties tend to “get along.” Study Table 4-1 for a moment – do you notice how the examples in the polar group tend to “get along?” For example, sugar will dissolve in water. The same is true of the nonpolar group – if you have an oil stain, then a chemical solvent may be necessary to dissolve it. As a starting point, we can think of the chemical world of the body divided into substances that are water-soluble and fat-soluble (or polar and nonpolar).

VIDEO 4c: Covalent bonds – polar and nonpolar
After watching this video, you should be able to:

explain the difference between polar and nonpolar covalent bonds, as well as ionic bonds
identify molecules that are mostly polar or nonpolar
determine whether a molecule is likely to dissolve in water or oil

TABLE 4-1 Polar and nonpolar molecules. Polar molecules are those with many polar covalent bonds, while nonpolar molecules have few or none.

Polar

Nonpolar

Atom combinations

O-H
N-H
C-O

C-H

Same-element pairs, such as C-C or H-H

Examples of molecules

· Water

· Sugars like glucose or sucrose

· Most amino acids

· DNA

· Triglycerides (oils and fats)

· Sterols (cholesterol, estrogen, etc.)

· Oxygen (O₂)

· Carbon dioxide (CO₂)

· Vitamins A, D, E and K

· Also, chemical solvents like paint thinner or acetone

Substances with electrical charges will dissolve in polar solvents. A substance that dissolves in a solvent is called a solute. Since water is the only polar solvent we will discuss, we can also refer to solutes as water-soluble or hydrophilic. Compounds that dissolve completely in water without settling out are:

Ionic compounds. Most salts break into individual ions when added to water. The partial charges on water molecules tend to attract their opposite charges of ions and surround them, keeping them suspended in water.
Polar molecules, like those shown in Figure 4-3, have partial charges. These are attracted to the opposite partial charges of water molecules. This keeps them suspended in water.

Stop & Think #2: Around your kitchen, more substances dissolve in water than in oil. What kitchen products can you think of that dissolve better in oil than water?

Structural diagrams of 3 molecules showing partial charges on certain atoms - those bound to oxygen.

FIGURE 4-3. Polar molecules. Structural diagrams of several polar molecules – serine (an amino acid), glucose, and water. Partial charges are also shown – the delta (δ) symbol indicates “partial.” (Chemical structures are in the public domain)

Nonpolar molecules dissolve in oils. We can also refer to nonpolar molecules as fat-soluble or hydrophobic. Examples include:

Fatty acids – the cell can use these for energy or to make other lipids
Steroids – many of these are hormones (like testosterone or estrogen) that signal cells
Oxygen (O₂) & Carbon dioxide (CO₂) – gasses exchanged between air and blood in the lungs

Learning objectives

Explain the meaning of polar and nonpolar. Give examples of molecules in each category.
Explain the chemical basis for oil and water remaining separate.
Predict if a chemical will dissolve in water or oil.
Identify lipids from structural diagrams or common examples.

Stop & Think Questions Explained

Skeletal formulas for Figure 4-3.
The list is probably fairly limited: other fats or oils, alcohol, aromatic chemicals from many herbs, and some spices like turmeric