A rare element can still limit growth
- The idea
- Predict
- See it
- Why
- Check
The idea
“Carbon, hydrogen, oxygen and nitrogen are most of a cell, so a 1 percent element can't limit growth.”
Most of a cell is carbon, hydrogen, oxygen and nitrogen, so ranking by amount seems fair. But a cell must make every molecule it needs, and some use elements it barely has.
The short version
The mistake: Carbon, hydrogen, oxygen and nitrogen are most of a cell, so a 1 percent element can't limit growth.
What's actually true: An element matters for the molecules it builds. A scarce element the cell needs can limit growth.
- Carbon, hydrogen, oxygen and nitrogen are most of a cell's dry mass. Carbohydrates, lipids, proteins and nucleic acids are built mostly from them.
- Phosphorus is a small share, but it's in every nucleotide of DNA and RNA, in ATP, and in membrane phospholipids.
- A cell can't build a molecule without every element in it. If one runs short, growth slows or stalls. That scarce element is the limiting nutrient.
On the exam: Judge an element by the molecules it builds. A scarce one the cell requires can limit growth.