Lesson 1.2 · Unit 1 · CED topic 1.2
Elements of life
About 90 elements occur naturally, yet living things are built mostly from six of them. Everything a cell makes, from a sugar to a strand of DNA, is assembled from these atoms, and which of them a molecule contains tells you a great deal about what kind of molecule it is. That is more than trivia: scientists have used it as a tool, as you will see in the Hershey–Chase experiment below.
Describe the composition of the macromolecules required by living organisms: which elements make up carbohydrates, lipids, proteins, and nucleic acids.
Six elements do almost everything
| Element | Symbol | Approximate share of human body mass |
|---|---|---|
| Oxygen | O | 65% |
| Carbon | C | 18% |
| Hydrogen | H | 10% |
| Nitrogen | N | 3% |
| Phosphorus | P | 1% |
| Sulfur | S | 0.25% |
These six, remembered as CHNOPS, add up to about 97 percent of body mass. Calcium (about 1.5 percent) is next, but it is found mainly in bones and teeth and is not a building block of the four classes of macromolecules. Oxygen and hydrogen rank so high mostly because the body is mostly water.
Which element goes into which molecule
Carbon, hydrogen, and oxygen are the most prevalent elements in the building of carbohydrates, lipids, proteins, and nucleic acids. Beyond those three: sulfur is used in building proteins, phosphorus in phospholipids and nucleic acids, and nitrogen in nucleic acids. (Every amino acid also has an amino group, so proteins contain nitrogen as well. The CED lists it with nucleic acids because that is where the nitrogenous bases are.)
| Macromolecule | C | H | O | N | P | S |
|---|---|---|---|---|---|---|
| Carbohydrates | ● | ● | ● | |||
| Fats and steroids | ● | ● | ● | |||
| Phospholipids | ● | ● | ● | some | ● | |
| Proteins | ● | ● | ● | ● | some | |
| Nucleic acids | ● | ● | ● | ● | ● |
In the table, “some” means that many phospholipid head groups contain nitrogen (choline, for example) and that only some amino acids contain sulfur (cysteine and methionine). Two elements are especially useful as markers. Phosphorus is in DNA and RNA (in every phosphate of the backbone) but not in proteins. Sulfur is in some proteins (the amino acids cysteine and methionine) but not in DNA.
Why carbon is the backbone
A carbon atom has four electrons in its outer shell, so it forms four covalent bonds. It can bond to hydrogen, oxygen, nitrogen, and other elements, and also to other carbon atoms. That last ability matters most: carbon atoms link into chains, branches, and rings of nearly any length, with single or double bonds between them. The bonds are strong and stable, which suits molecules that must last in a warm, watery cell. The carbon–carbon and carbon–hydrogen bonds are also nonpolar, which matters for lipids and for the cores of folded proteins.
Functional groups: what the atoms attached to the skeleton do
A bare carbon skeleton is not very reactive. What a molecule does depends largely on the functional groups attached to it: small clusters of atoms that behave the same way wherever they appear. A hydroxyl group makes a molecule polar and able to hydrogen-bond whether it sits on a sugar or on a nucleic acid.
- Hydroxyl (−OH): polar and hydrogen-bonding; on sugars and on the 3′ end of every nucleic acid (lesson 1.6).
- Carbonyl (C=O): polar; sugars are aldehydes or ketones (lesson 1.4).
- Carboxyl (−COOH): acidic; in every amino acid and fatty acid.
- Amino (−NH2): basic; in every amino acid, so every protein has one at its N-terminus (lesson 1.7).
- Sulfhydryl (−SH): two of these can join into a disulfide bridge that holds a protein's shape (lesson 1.7).
- Phosphate: negatively charged; in nucleotides, phospholipids, and ATP.
- Methyl (−CH3): nonpolar; makes a region hydrophobic.
The exam does not ask you to draw these structures. Know the names, the properties, and where each one shows up.
In 1952 Alfred Hershey and Martha Chase asked whether protein or DNA carries a virus's genetic information. A bacteriophage is a virus that infects bacteria: a protein coat around a DNA core. They grew one batch of phages with radioactive sulfur (35S) and a second batch with radioactive phosphorus (32P). Each batch infected bacteria. After a few minutes they agitated the mixture in a blender to shake the empty phage coats off the cells, then spun it in a centrifuge so the heavy bacterial cells formed a pellet and the lighter coats stayed in the liquid above (the supernatant).
Why those two elements? Protein contains sulfur but no phosphorus; DNA contains phosphorus but no sulfur. So 35S tags only protein and 32P tags only DNA. The independent variable is which radioactive element was used; the dependent variable is where the radioactivity was found afterward.
Result. In the 35S experiment most of the radioactivity stayed in the supernatant, with the empty protein coats. In the 32P experiment most of it was in the pellet, inside the bacteria. Conclusion. It was the phage DNA, not its protein, that entered the bacteria and directed the making of new viruses, so DNA is the genetic material. The reasoning depends entirely on a fact from this lesson: phosphorus marks nucleic acid and sulfur marks protein.
An analysis reports the composition by mass of one small molecule from each class of macromolecule. Name the class of each from the elements present, and explain how you can tell.
| Molecule | C | H | O | N | P | S |
|---|---|---|---|---|---|---|
| A | 40.0 | 6.7 | 53.3 | 0 | 0 | 0 |
| B | 75.9 | 12.2 | 11.9 | 0 | 0 | 0 |
| C | 29.7 | 5.8 | 26.4 | 11.6 | 0 | 26.5 |
| D | 36.3 | 4.3 | 29.0 | 21.1 | 9.4 | 0 |
D contains both nitrogen and phosphorus, and nucleic acids are the class that contains both along with C, H, and O, so D is a nucleotide. C contains nitrogen and sulfur, which together point to protein (an amino acid with sulfur, cysteine). A and B both contain only C, H, and O, so you need the proportions. In A the H to O atom ratio is 2 to 1 (6.7 / 1.008 : 53.3 / 15.999 is about 6.6 : 3.3), the same as in water, which is the signature of a carbohydrate; it is glucose. In B oxygen is only 11.9 percent and hydrogen 12.2 percent, so there are about 16 hydrogens for each oxygen. A molecule that is mostly carbon and hydrogen with very little oxygen is nonpolar and energy-rich: a fat (this one is tripalmitin).
Careful: phosphorus alone does not prove a nucleic acid, because phospholipids contain phosphorus too. A phospholipid would also show a very high carbon content and little nitrogen, so check the proportions.
- Learn the markers: P for nucleic acids (and phospholipids), S for proteins, N for proteins and nucleic acids, only C, H, O for carbohydrates and fats.
- A question that shows an elemental analysis or a radioactive-tracer experiment is asking you to match elements to molecules. Name the element, then name the macromolecule that contains it.
- When you describe an experiment like Hershey–Chase, always state what was measured (where the radioactivity ended up) and what the result means.
- Name the elements found in (a) a triglyceride, (b) a protein that contains cysteine, (c) a molecule of DNA, and (d) a phospholipid.
Show answer
(a) Carbon, hydrogen, oxygen only. (b) Carbon, hydrogen, oxygen, nitrogen, and sulfur (the sulfur comes from cysteine). (c) Carbon, hydrogen, oxygen, nitrogen, and phosphorus. (d) Carbon, hydrogen, oxygen, and phosphorus, plus nitrogen in many head groups such as choline.
- Bacteria are grown on a medium whose only phosphorus is radioactive 32P, then the cell contents are separated into classes of macromolecules. Which classes will contain radioactivity, and which will not? Justify your answer.
Show answer
Nucleic acids (DNA and RNA) and phospholipids will be radioactive, because both contain phosphorus (in the phosphate groups of the nucleotide backbone and in the phospholipid head). Proteins, carbohydrates, and fats will not, because they contain no phosphorus. Naming the element in each class is the justification.
- An unknown compound is 32.0% carbon, 6.7% hydrogen, 18.7% nitrogen, and 42.6% oxygen by mass, with no phosphorus or sulfur. Can it be a nucleic acid? Can it be a protein? Justify each answer.
Show answer
It cannot be a nucleic acid: nucleic acids contain phosphorus and this compound has none. It could be a protein (or an amino acid): it contains nitrogen as an amino group requires, and the missing sulfur does not rule out a protein, because only some amino acids contain sulfur. (The compound is glycine, C2H5NO2.)
- A carbon atom in the middle of a carbon chain is bonded to two neighboring carbons by single bonds. How many hydrogens can attach to it? What happens to that number if one of its bonds to a neighbor becomes a double bond? Explain.
Show answer
Two hydrogens, because carbon always forms four covalent bonds: two to carbon neighbors plus two to hydrogen. If one bond to a neighbor becomes a double bond, that carbon now has three bonds to carbon, leaving room for only one hydrogen. This is why a fatty acid with C=C double bonds holds fewer hydrogens than one with only single bonds (lesson 1.5).
- An amino acid has a carboxyl group and an amino group. Predict whether it can act as an acid, a base, or both, and justify your prediction.
Show answer
Both. The carboxyl group is acidic: it can release H+ and become −COO−. The amino group is basic: it can accept H+ and become −NH3+. Because one molecule carries a group of each kind, it can donate or accept a proton depending on the pH of its surroundings.