endoplasmic reticulum

Typical plant and animal cell. Plant and animal cells share many structures, such as ribosomes, mitochondria, and chromosomes, but they also have notable differences: plant cells have chloroplasts, a large vacuole, and a cellulose cell wall. Animal cells do not have a rigid cell wall but have an outside cell membrane only.

A membranous system of tubes, channels, and flattened sacs that form compartments within eukaryotic cells. It stores and transports proteins within cells and also carries various enzymes needed for the synthesis of fats. The ribosomes, or the organelles that carry out protein synthesis, are sometimes attached to parts of the ER.

Under the electron microscope, ER looks like a series of channels and vesicles, but it is in fact a large, sealed, baglike structure crumpled and folded into a convoluted mass. The interior of the ‘bag’, the ER lumen, stores various proteins needed elsewhere in the cell, then organizes them into transport vesicles formed by a small piece of ER membrane budding from the main membrane.