The internal membranes of eukaryotic cells are dynamic, constantly changing structures. The concept of the endomembrane system describes all internal cytoplasmic membranes, with the exception of mito-chondrial and plant plastid membranes, as a single continuum. In this model, the ER, generally the largest membrane system of eukaryotic cells, is the initial source of most other membranes. The ER is a network of interconnected, closed, membrane-bound vesicles that is contiguous with the nuclear envelope.
Vesicles from the ER carry proteins from the ER to the Golgi complex, fusing with its membranes. The Golgi complex can be described as a series of flattened membrane sacs, like a stack of hollow pancakes. The side closest to the nucleus receives vesicles from the ER, and the proteins inside these vesicles are processed and modified as they pass through the Golgi complex. Eventually, membrane vesicles containing the modified proteins will bud from the opposite surfaces of the Golgi complex and fuse with the cell membrane or the membranes of other organelles.
See also: Cell theory; Cell-to-cell communication; Cell wall; Chloroplasts and other plastids; Chroma tin; Cytoplasm; Cytoskeleton; Cytosol; DNA in plants; Endomembrane system and Golgi complex; Endoplasmic reticulum; Eukarya; Membrane structure; Microbodies; Mitochondria; Nuclear envelope; Nucleolus; Nucleoplasm; Nucleus; Oil bodies; Peroxisomes; Plasma membranes; Proteins and amino acids; Ribosomes; RNA; Vacuoles.
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