The cell is the basic structural and functional unit of all living organisms, often called the building block of life. The cell theory formalizes this idea with three core claims: every living thing is composed of cells, the cell is the basic unit of life, and all cells arise from pre-existing cells. This theory unites all biology under a common microscopic foundation, regardless of whether one studies bacteria, plants, or animals.
Cells fall into two broad categories. Prokaryotic cells, which include bacteria, are small and lack a true nucleus and other membrane-bound organelles. Eukaryotic cells, by contrast, contain a nucleus and a variety of organelles, each performing specialized roles. In eukaryotic cells, the cell membrane, a phospholipid bilayer embedded with proteins, regulates what enters and exits, maintains homeostasis, and allows communication with neighboring cells. Inside the membrane, the cytoplasm is a gel-like substance in which organelles are suspended and many metabolic reactions occur.
The nucleus houses the cell's DNA and directs cellular activities such as growth and reproduction, serving as the site of RNA synthesis. Mitochondria are the cell's powerhouses, producing ATP through cellular respiration, while chloroplasts, found in plants and some other organisms, carry out photosynthesis, capturing light energy to build glucose. Ribosomes, whether free in the cytoplasm or attached to the rough endoplasmic reticulum, synthesize proteins. The rough ER modifies those proteins for secretion or membrane insertion, whereas the smooth ER synthesizes lipids, detoxifies harmful substances, and stores calcium in muscle cells. The Golgi apparatus then further modifies, sorts, and packages proteins and lipids into vesicles. Lysosomes contain hydrolytic enzymes that digest waste and pathogens, and vacuoles store water, nutrients, and waste, with plant cells relying on a large central vacuole to maintain turgor pressure and rigidity.