AP BiologyUnit 1: Chemistry of Life1.2
AP Biology · Unit 1 · Topic 1.2

Elements of Life

Play it as a 3-minute lesson → short cards, one idea at a time

The lesson3 min read

Nitrogen alone can't tell protein from DNA. Phosphorus can: DNA is rich in it, protein has almost none.

Cells cannot make elements. They can only rearrange atoms taken in from food, water, air, or soil. Each class of macromolecule is built from its own short list of elements, so knowing the lists turns element data into a lookup.

Key terms

  • Carbon — forms four covalent bonds, so it builds chains, branches, and rings. It is the frame of every macromolecule.
  • Hydrogen and oxygen — in every macromolecule. Carbohydrates and fats are built from carbon, hydrogen, and oxygen alone.
  • Nitrogen — in the amino group of every amino acid and in the bases of DNA and RNA.
  • Phosphorus — in phosphate groups: the backbone of DNA and RNA, the three phosphates of ATP, and every phospholipid's head.
  • Sulfur — in two of the 20 amino acids. Bonds between the sulfur atoms of two cysteines help some proteins hold their shape.

How it works

  1. Every atom a cell uses comes from outside. Plants take carbon from CO2 in the air and draw nitrate and phosphate from the soil.
  2. Animals get the same elements by eating. Digestion breaks food into small molecules, and cells rebuild them into their own molecules.
  3. Each kind of molecule needs its own elements. Proteins need N; nucleic acids need N and P; starch and fats need only C, H, and O.
  4. When one element runs short, the molecules that need it stall first. Low nitrogen → little protein, while starch piles up.

Why it matters

Element questions usually hand you a table of percentages. Read two columns at once. Carbon, hydrogen, and oxygen only → carbohydrate or fat, and a lot of oxygen points to carbohydrate. High nitrogen with trace phosphorus → protein. High nitrogen and high phosphorus → nucleic acid. Some phosphorus, little nitrogen, and lots of carbon and hydrogen → phospholipid.

Exam traps

TrapActually
High nitrogen means protein.Nucleic acids are nitrogen-rich too. Phosphorus tells them apart: DNA and RNA carry lots, proteins only a trace.
Every macromolecule contains phosphorus.Phosphorus goes into phosphate groups: nucleic acids, ATP, and phospholipids. Glucose, cellulose, fats, and amino acids contain none.
Linking small molecules into a big one adds new elements.Linking only removes water. Cellulose has the same elements as the glucose it is built from: carbon, hydrogen, and oxygen.
Carbon is central because cells contain more carbon than anything else.By mass, oxygen wins, mostly in water. Carbon is central for its four bonds, not its amount.
Worked example

Algae are grown for many generations with heavy nitrogen (nitrogen-15) as their only nitrogen source. Four molecules are then purified from the cells: starch, DNA, triglycerides, and protein. Which of them now contain nitrogen-15?

  1. Ask where nitrogen goes. It sits in the amino group of every amino acid and in the base of every nucleotide.
  2. Protein is built from amino acids, so it carries the label. DNA is built from nucleotides, so it does too.
  3. Starch is chains of glucose, and a triglyceride is glycerol plus three fatty acids. Both are C, H, and O only, so neither can carry the label.

Protein and DNA contain nitrogen-15. Starch and triglycerides do not.

A label can only land in a molecule whose structure includes that element. The near-miss is 'protein only,' from treating nitrogen as a protein signature. DNA's bases are nitrogen-rich too. To tell the two apart, trace phosphorus instead: it lands in DNA's backbone but not in amino acids.

Now try it on real exam-style questions

Ten questions on Elements of Life, each with a ten-second explanation. Free, no account.

Three questions to check yourself

Question 1 · 1.2 Elements of Life

Which kind of lipid contains phosphorus atoms?

  1. A The lipid that stores energy in fat cells
  2. B The lipid that forms steroid hormones
  3. C The lipid that waterproofs leaf surfaces
  4. D The lipid that forms membrane bilayers
Show the answer

D — Phospholipids, the lipids that form membrane bilayers, carry a phosphate group in their hydrophilic heads. The triglycerides that store energy in fat cells are made of only carbon, hydrogen, and oxygen.

Question 2 · 1.2 Elements of Life

A potato tuber stores the carbon its plant took in from the air mainly as which molecule?

  1. A Glycogen
  2. B Starch
  3. C Cellulose
  4. D Glucose
Show the answer

B — Plants store fixed carbon as starch, a polymer of glucose packed into storage organs like tubers. Glycogen is the storage polysaccharide of animals and fungi, and cellulose builds cell walls rather than storing carbon for later use.

Question 3 · 1.2 Elements of Life

Most plants obtain the nitrogen they use to make amino acids from which source?

  1. A Nitrate and ammonium ions in soil water
  2. B Nitrogen gas taken in through leaf pores
  3. C Carbon dioxide taken in through leaf pores
  4. D Energy from sunlight absorbed by leaves
Show the answer

A — Plant roots absorb nitrogen as nitrate and ammonium ions dissolved in soil water. Although air is mostly nitrogen gas, plants have no way to use that gas directly.