CELLS

Cells are the basic units of life, capable of growth, metabolism, and reproduction. They store and transmit genetic information and coordinate the activities necessary to sustain the organism.

According to cell theory, all organisms are composed of one or more cells, the cell is the fundamental unit of structure and function, and new cells arise only from existing cells.

Cells are broadly categorized as prokaryotic or eukaryotic. Prokaryotic cells, such as bacteria and many archaea, lack a defined nucleus and membrane-bound organelles. They are usually smaller and have a nucleoid region, a cell membrane, and often a rigid cell wall.

Eukaryotic cells, found in animals, plants, fungi, and protists, contain a nucleus and multiple membrane-bound organelles, including mitochondria, endoplasmic reticulum, and Golgi apparatus. Plant and algal cells typically have chloroplasts, a large central vacuole, and cellulose-based cell walls; animal cells rely on extracellular matrix instead.

All cells share common features: a plasma membrane that controls material exchange; cytoplasm containing cytosol and organelles; and genetic material organized as DNA. Ribosomes synthesize proteins; mitochondria (and chloroplasts in plants) generate ATP.

Metabolic processes occur in specialized compartments. The endomembrane system processes and traffics proteins and lipids; lysosomes and peroxisomes digest macromolecules and detoxify harmful substances.

Cell division and gene expression are central to growth. Prokaryotes divide by binary fission; eukaryotes use mitosis for somatic cells and meiosis for forming gametes. Transcription of DNA produces RNA, which is translated by ribosomes into proteins.

Cells sense and respond to their environment through receptors and signaling pathways, and transport across membranes by diffusion, osmosis, and active transport. Understanding cells is fundamental to biology, medicine, and biotechnology.