Tour of the cell
Every organism on Earth is either a single cell or a society of cells — and the differences between cell types explain almost every difference between the creatures they build.
The two great domains
All cells fall into two camps. Prokaryotes (bacteria and archaea) keep their DNA floating in a single compartment called the nucleoid, have no membrane-bound organelles, and are typically 1–10 µm across.
Eukaryotes (everything else — plants, animals, fungi, protists) wrap their DNA in a nucleus and divide their labor across organelles. They're usually 10–100 µm across, a thousand times bigger by volume than prokaryotes.
The eukaryotic organelle set
The nucleus stores DNA and runs the cell. Mitochondria make ATP via respiration. Ribosomes synthesize proteins. The endoplasmic reticulum (ER) folds and ships proteins; the Golgi apparatus packages and ships them. Lysosomes break down waste.
Plant cells add two more: chloroplasts for photosynthesis and a central vacuole for storage and structural rigidity. Animal cells lack both.
The membrane does the work
Every cell is wrapped in a phospholipid bilayer — two sheets of fat-like molecules with their water-loving heads facing out and their water-hating tails facing in. Proteins embedded in the bilayer act as gates, receptors, and pumps.
Because the membrane is selectively permeable, the cell controls what enters and leaves. That selectivity is what makes a cell a cell rather than a leaking bag of molecules.
Flashcard · Unit 1
A cell with no membrane-bound nucleus or organelles.
Check your understanding
Which of the following structures is found in plant cells but not in animal cells?