Assumed audience
General adult who has completed What Is Life.
Cell theory
All organisms are composed of cells; all cells come from preexisting cells.
Prokaryotes and eukaryotes
Prokaryotic cells (bacteria, archaea) lack a membrane-bound nucleus. Eukaryotic cells (plants, animals, fungi, protists) have a nucleus containing DNA, plus membrane-bound organelles (mitochondria, endoplasmic reticulum, and in plants, chloroplasts).
Cell components
DNA in the nucleus (or nucleoid) encodes the organism’s genetic information (DNA). Ribosomes translate genetic information into proteins. The cell membrane controls what enters and exits. Mitochondria generate energy through cellular respiration. Chloroplasts (in plants) capture light energy through photosynthesis.
Metabolism
The network of enzyme-catalyzed chemical reactions that sustain a cell. Metabolism has two broad categories: catabolism breaks down molecules to release energy; anabolism uses energy to build complex molecules. ATP (adenosine triphosphate) is the cell’s primary energy currency.
Photosynthesis and respiration
Photosynthesis captures light energy to build sugars from CO2 and water. Cellular respiration breaks down sugars to release energy as ATP. These are complementary processes — the products of one are the reactants of the other.
Why this matters
The cell is where biology’s abstract principles become concrete chemistry. Understanding cells and metabolism is the foundation for understanding how organisms work.