The Cell and Cell Structure

Cells are the fundamental structural and functional units of all living organisms. There are two types: prokaryotic cells (bacteria) — small, no nucleus; and eukaryotic cells (plants, animals, fungi) — larger, with a membrane-bound nucleus. Animal cells contain: nucleus (controls cell activities, holds DNA), cell membrane (controls entry/exit), cytoplasm (fluid medium), mitochondria (ATP energy production), ribosomes (protein synthesis), and endoplasmic reticulum. Plant cells additionally have a cell wall (cellulose, for rigidity), chloroplasts (site of photosynthesis), and a large central vacuole (storage, turgor pressure). Cell division — mitosis (for growth and repair, producing 2 identical diploid cells) and meiosis (for gamete production, producing 4 haploid cells with genetic variation) — underlies development, reproduction, and evolution. The cell membrane is selectively permeable, controlling substance movement by diffusion, osmosis, and active transport.