Atoms, molecules, and macromolecules of the cell

DNA is housed inside cells, but what are cells made of? How does DNA fit in the picture? Let’s start with atoms: Are you familiar with the periodic table of chemical elements? You can find one at the end of this book. Take a moment to explore it. The periodic table is color coded to tell you where each of the elements is used in cells. Atoms such as carbon (C), hydrogen (H), oxygen (O), phosphorous (P), nitrogen (N), and sulfur (S) are brought together by machinery in cells to form molecules.

A molecule is the combination of two or more atoms that are bound together. Carbon, hydrogen, oxygen, nitrogen, phosphorous and sulfur are generally considered organic elements because they are frequently used by organisms. As you completed the hands-on DNA extraction, keep in mind that most of the “cellular stuff” that you manipulated was made of these elements, and in the case of extracting DNA from cells, DNA is made of CHOPN.

A cell has the necessary machinery to combine atoms into molecules, and it also has the machinery inside to combine molecules together to form macromolecules. The term macromolecule is used to describe the four basic components of cells: proteins, lipids, carbohydrates, and nucleic acids. In Chapter 1, the primary focus is on nucleic acids, specifically, deoxyribonucleic acid (DNA) in cells.

What would it look like if you were to peek into a cell to look at the atoms, molecules, and macromolecules? The inside might look like a ball pit, but rather than colorful plastic spheres, there is a sea of different molecules packed together and bumping into one another (Figure 1-14). Imagine being immersed in the depths of a ball pit - completely surrounded. If you were shrunk down to the size of atoms and molecules - the size of DNA, the colored balls packed against you would be comparable to other molecules of the cell.

Figure 1-14. A ball pit is a good analogy for thinking about how the inside of cells are packed with molecules

The molecules of living cells range in size from very small, such as oxygen gas (0₂) with only two atoms, to water (H₂0) with three atoms, to DNA which is by far the most enormous at millions or billions of atoms. In the DNA extraction exercise, you saw with your own eyes how much DNA was in the strawberry cells - that glob of DNA was made of trillions of atoms. And the other macromolecules, such as the proteins, lipids, and carbohydrates bound up by the surfactants into micelles, were made up of trillions more.

Keep in Mind! Sometimes memory tricks help to remember specific facts. You can try to think of CHOPNS as the word “Choppings.” Imagine chopping up some yummy celery or carrots into little bits for your soup. Atoms are like that; the little bits that make up the molecules in cells, just like celery and carrots make up the soup.

Of particular importance in living organisms are the ‘organic’ elements CHOPNS; carbon, hydrogen, oxygen, phosphorous, nitrogen, and sulfur.