Cell Membrane
The cell membrane, also called the plasma membrane, is a thin, flexible bilayer of phospholipid molecules embedded with proteins that surrounds every cell and forms its outermost boundary. It is selectively permeable, meaning it regulates the passage of ions, nutrients, and waste products into and out of the cell through passive diffusion, facilitated diffusion, and active transport. The fluid mosaic model, proposed by Singer and Nicolson in 1972, describes its structure as a dynamic, two-dimensional fluid in which proteins float within or are attached to the phospholipid bilayer.
Components of the Cell Membrane and Their Functions
| Component | Type | Function |
|---|---|---|
| Phospholipid bilayer | Structural lipid | Forms the basic barrier; hydrophilic heads face outward |
| Integral proteins | Transmembrane protein | Transport channels, receptors, enzymes |
| Peripheral proteins | Surface protein | Cell signaling, structural support |
| Cholesterol | Lipid | Maintains membrane fluidity at varying temperatures |
| Glycoproteins | Glycosylated protein | Cell recognition, immune response, cell adhesion |
| Glycolipids | Glycosylated lipid | Cell identity markers, signal transduction |
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Related Terms
Cell Theory
Cell theory is the foundational scientific framework stating that all living organisms are composed of one or more cells, the cell is the basic structural and functional unit of life, and all cells arise from pre-existing cells. It was developed in the mid-19th century by Matthias Schleiden, Theodor Schwann, and Rudolf Virchow. This theory unified biology by providing a common framework for understanding how all living systems are organized and how life perpetuates itself.
Eukaryote
A eukaryote is an organism whose cells contain a membrane-bound nucleus housing the DNA, along with other membrane-enclosed organelles such as mitochondria, endoplasmic reticulum, and the Golgi apparatus. Eukaryotes include all plants, animals, fungi, and protists, and their cells are generally much larger and more structurally complex than prokaryotic cells. The compartmentalization of cellular functions within organelles allows eukaryotes to perform more specialized and regulated biochemical processes.
Endoplasmic Reticulum
The endoplasmic reticulum (ER) is an extensive network of interconnected membrane-bound tubules and flattened sacs (cisternae) that extends throughout the cytoplasm of eukaryotic cells and is continuous with the outer nuclear envelope. It exists in two forms: rough ER (RER), which is studded with ribosomes and specializes in the synthesis, folding, and quality control of proteins destined for secretion or membrane insertion; and smooth ER (SER), which lacks ribosomes and is involved in lipid synthesis, calcium storage, and detoxification of drugs and toxins. The ER works closely with the Golgi apparatus to process and sort proteins and lipids to their correct cellular destinations.
From Latin "membrana" meaning a thin skin or film, derived from "membrum" meaning limb or body part. The biological usage dates to the 17th century when early microscopists described the boundary layer of cells.