All animals and plants are composed of cells and cell products. New cells come from division of preexisting cells, and the activity of a multicellular organism as a whole is the sum of activities and interactions of its constituent cells. The idea that a cell represents the basic structural and functional unit of life is an important unifying concept in biology.
Typically, eukaryotic cells are enclosed within a thin, selectively permeable plasma membrane. The most prominent organelle is a spherical or ovoid nucleus, enclosed within two membranes to form a double-layered nuclear envelope. Cellular material located between the cell membrane and nuclear envelope is collectively called cytoplasm. Within the cytoplasm are many organelles. In addition, plant cells typically contain plastids, some of which are photosynthetic organelles, and plant cells bear a cell wall containing cellulose outside the cell membrane. Nuclei contain linear chromosomes suspended in nucleoplasm. The chromosomes are normally loosely condensed, flexible strands of chromatin. The nucleolus is composed of specialized parts of certain chromosomes. After transcription from DNA, ribosomal RNA combines with protein to form the two subunits of ribosomes. The outer membrane of the nuclear envelope is continuous with a cytoplasmic endomembrane system composed of the endoplasmic reticulum (ER), the Golgi complex or apparatus, lysosomes, the plasma membrane, and the vesicles that pass between them. The ER membranes may be covered on their outer surfaces with ribosomes and are thus designated rough ER. When ER lacks a ribosomal covering, it is called smooth ER. Mitochondria are conspicuous organelles present in nearly all eukaryotic cells. Most of a cell’s ATP, the most important energy-transfer molecule of all cells, is produced in this organelle. Mitochondria are self-replicating and have their own genome. Eukaryotic cells characteristically have a system of tubules and filaments that form a cytoskeleton. Microfilaments are thin, linear structures made of a protein called actin. Microtubules are hollow, tubular structures composed of a protein called tubulin. Microtubules radiate from a microtubule organizing center, the centrosome, near the nucleus. Within centrosomes are found a pair of centrioles. Intermediate filaments are larger than microfilaments but smaller than microtubules. Specializations of cell surfaces include cilia, flagella, pseudopodia, tight junctions, adhesion junctions, gap junctions, and microvilli. Three principal ways that a substance may enter across a cell membrane are by diffusion, by a mediated transport system, and by endocytosis. As a rule, gases (such as oxygen and carbon dioxide), urea, and lipid-soluble solutes (such as fats, fatlike substances, and alcohol) are the only solutes that can diffuse through biological membranes with any degree of freedom. Sugars, water, many electrolytes, and macromolecules are moved across membranes by mediated transport systems. Ion diffusion through channels is the basis of signaling mechanisms in the nervous system and in muscles. Osmosis is the diffusion of water molecules across a membrane. Two distinctly different kinds of carrier-mediated transport mechanisms are recognized: facilitated diffusion (or facilitated transport) and active transport. Facilitated diffusion supports movement only in the downhill direction of a concentration gradient. In active transport, molecules are moved against the forces of passive diffusion and always involves an expenditure of ATP. As much as 10% to 40% of all energy produced by cells is consumed by the sodium-potassium pump. Endocytosis is the ingestion of material by cells involving pathways for specifically internalizing solid particles, small molecules and ions, and macromolecules; all require energy and thus are forms of active transport. Exocytosis is the extrusion of material by cells to the surrounding medium (e.g., to secrete substances such as hormones).
All cells arise from the division of preexisting cells. In most multicellular organisms, all cells originate from the division of a single cell, a zygote, which is the product of union (fertilization) between an egg and a sperm (gametes or germ cells). In the formation of body cells or somatic cells the process of nuclear division is called mitosis. Production of gametes in sexually reproducing organisms requires a special type of reductional division called meiosis. Every somatic cell has the same number of chromosomes. There are two distinct stages of cell division: nuclear division of chromosomes (mitosis) and division of cytoplasm (cytokinesis). Mitosis is artificially divided into four successive phases: prophase, metaphase, anaphase, and telophase. Cell division is important for growth, for replacement of cells lost to natural attrition, and for wound healing. When cells are not actively dividing, they are in interphase. Some cells continue to divide throughout the life of an animal to replace cells lost by attrition and wear, whereas others complete their division during early development and many do not divide again. Apoptosis is programmed cell death.
—May 2021
—June 2023
Lance Jones 