The cell is the fundamental unit of life. All living organisms are composed of one or more cells. Cells are broadly classified into prokaryotic (no nucleus, like bacteria) and eukaryotic (with a nucleus, like plant and animal cells).
Eukaryotic cells contain membrane-bound organelles. The nucleus houses DNA and controls cell activities. Mitochondria generate ATP through cellular respiration. The endoplasmic reticulum and Golgi apparatus process and transport proteins. Lysosomes contain digestive enzymes.
The cell membrane is a phospholipid bilayer that regulates what enters and leaves the cell. It is selectively permeable, allowing small nonpolar molecules to pass freely while requiring transport proteins for larger or charged molecules.
Plant cells differ from animal cells by having a rigid cell wall, a large central vacuole, and chloroplasts for photosynthesis. These structures support the plant's upright structure and energy production.