AP® Biology review sheet from Aim for Five (aimforfive.com/bio/units/1/1-2)
Unit 1 · Topic 1.2
1.2 Elements of Life
Living things are built mostly from carbon, hydrogen and oxygen, plus smaller but essential amounts of nitrogen, phosphorus and sulfur. Organisms can't make atoms, so they must take these elements in from their environment. Knowing which element shows up in which kind of molecule lets you predict what a nutrient shortage or a radioactive label will affect.
Key terms
- carbon
- nitrogen
- phosphorus
- sulfur
- organic molecule
Atoms come from the environment
Every new molecule a cell builds is made from atoms it got from somewhere else. Plants pull carbon out of the air as CO₂ during photosynthesis, take in water through their roots, and absorb nitrogen (as nitrate or ammonium) and phosphorus (as phosphate) from the soil. Animals get their atoms mainly by eating other organisms and breaking their molecules down into smaller pieces they can rebuild.
This is why a shortage of one element can limit growth. A plant in nitrogen-poor soil can have plenty of sunlight and water and still grow poorly, because it can't build enough proteins and nucleic acids.
Carbon is the backbone
Molecules built around carbon are called organic molecules. Carbon is special because each carbon atom can form four covalent bonds. That lets carbon atoms link into long chains, branched chains and rings, and bond to many other elements. This variety of shapes is why carbon can form the huge range of molecules life needs.
All four kinds of biological macromolecules (carbohydrates, lipids, proteins and nucleic acids) are built mostly from just three elements: carbon, hydrogen and oxygen.
Where nitrogen, phosphorus and sulfur show up
A useful shortcut: typical carbohydrates and fats (triglycerides) contain only C, H and O. Proteins add N, and almost all also contain some S (two amino acids contain sulfur). Nucleic acids add N and P. Phospholipids add P.
| Element | Main places you'll see it | Why it matters |
|---|---|---|
| Nitrogen (N) | Proteins (the amino group of every amino acid) and nucleic acids (the nitrogenous bases) | Without enough N, cells can't build proteins or DNA and RNA |
| Phosphorus (P) | Nucleic acids (the phosphate in each nucleotide), phospholipids and ATP | Needed for the DNA and RNA backbone, membranes and energy transfer |
| Sulfur (S) | Some amino acids, so some proteins | The sulfur in cysteine lets two cysteines form disulfide bridges that hold a protein's shape (1.7) |
Using elements as tracers
Because different molecules contain different elements, scientists can label one kind of molecule by feeding cells a radioactive or heavy version of an element. Radioactive sulfur (³⁵S) ends up in proteins but not in DNA. Radioactive phosphorus (³²P) ends up in DNA and RNA, but not in the amino acids of proteins. This trick shows up in Unit 6, where it helped show that DNA, not protein, is the genetic material.
On the exam, you may be asked to predict which molecules become labeled, or which would be affected if an element were missing. Reason from the table above.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Predicting a radioactive label
Bacteria are grown for several generations in a medium where the only phosphorus source contains radioactive ³²P. Which of these molecules in the bacteria will become radioactive: DNA, phospholipids, ATP, glucose, the amino acids in proteins?
Show the solutionHide the solution
- Step 1: Ask which molecules contain phosphorus. Each nucleotide in DNA has a phosphate group, so DNA will be labeled.
- Step 2: Phospholipids have a phosphate group in their head, so they will be labeled.
- Step 3: ATP (adenosine triphosphate) has three phosphate groups, so it will be labeled.
- Step 4: Glucose is made only of C, H and O, and amino acids contain C, H, O, N and sometimes S, but no P. So neither will carry the label in its own atoms.
Answer: DNA, phospholipids and ATP become radioactive; glucose and the amino acids do not. (Later in the course you'll see that phosphate groups can be attached to finished proteins, but the amino acids themselves contain no phosphorus.)
- Example 2
Nitrogen shortage (classic trap)
A farmer's corn plants are growing in soil with very little nitrogen. A student predicts the plants will have trouble making starch, because starch is the plant's main energy store. Evaluate this prediction.
Show the solutionHide the solution
- Step 1: List the elements in starch: starch is a polysaccharide made of glucose, which contains only C, H and O. Nitrogen is not part of starch.
- Step 2: List what does need nitrogen: proteins (every amino acid has an amino group) and nucleic acids (the bases contain N).
- Step 3: Connect to the plant: without enough proteins (including enzymes) and nucleic acids, cells can't grow and divide normally, so the whole plant grows poorly.
- Step 4: Conclude: starch production may drop as an indirect effect of poor growth, but nitrogen is not directly needed to build starch.
Answer: The prediction is wrong as stated. Nitrogen is needed to build proteins and nucleic acids, not starch; any drop in starch would be an indirect result of the plant's poor overall growth.
Common mistakes
- Saying proteins contain phosphorus or nucleic acids contain sulfur. Proteins use N and S; nucleic acids use N and P.
- Forgetting that carbon's ability to form four covalent bonds is the reason it can build chains, branches and rings.
- Assuming organisms can make elements they're missing. They can only rearrange atoms they take in from the environment.
On the exam
- Questions often give an element (or a labeled isotope) and ask which macromolecules are affected. A quick mental table of C-H-O-N-P-S for each molecule type answers most of them.
- Tie your answer to a function: say not just 'proteins need nitrogen' but what the organism loses, such as enzymes, without that element.
Connected topics
Videos
Check yourself
4 questions on 1.2 Elements of Life. Pick an answer to see if you got it, and why.
Bacteria are grown in a medium in which the only source of phosphorus is radioactive phosphate (³²P). After several generations, which of the following cell molecules would be expected to contain the radioactive label?
A plant is grown in soil that contains almost no usable nitrogen. The plant's ability to synthesize which of the following would be most directly limited?
A plant is grown for several weeks in air in which the only carbon dioxide contains radioactive carbon (¹⁴CO₂). Which of the plant's molecules would most likely contain ¹⁴C at the end of the experiment?
After fertilizer containing phosphate washes into a lake, the population of algae in the lake grows rapidly. Which of the following best explains why added phosphate can speed up the growth of algae?
0 of 4 answered