Changes

The endomembranes and plasmalemma form a continuous membrane system in the eukaryote cell. The cell contains a rich diversity of membranes, in addition to those found in the endosymbiotic organelles. The outer cell membrane or plasmalemma, controls what gets in and out of the cell by the use of pores, symports, antiports and pumps. It is composed of a fluid mosaic of proteins embedded in a phospholipid bilayer, as are all membranes. The plasmalemma also interacts with the extracellular space through receptors, gap junctions, plasmodesmata, and synapses.

The endomembranes and plasmalemma form a continuous membrane system in the eukaryote cell. The cell contains a rich diversity of membranes, in addition to those found in the endosymbiotic organelles. The outer cell membrane or plasmalemma, controls what gets in and out of the cell by the use of pores, symports, antiports and pumps. It is composed of a fluid mosaic of proteins embedded in a phospholipid bilayer, as are all membranes. The plasmalemma also interacts with the extracellular space through receptors, gap junctions, plasmodesmata, and synapses.

The endomembranes include the nuclear membrane, already mentioned, and the rough and smooth endoplasmic reticula (RER and SER). The lamellae of the rough endoplasmic reticulum are continuous throughout the cell, and also contiguous with the nuclear membrane. They are the site of synthesis for proteins destined for the endomembrane system and the extracellular space, with which its lumen (the cisternal space) is topologically identical. The tubules of the SER are responsible for steroid and lipid synthesis. After translation on ribosomes attached to the RER, proteins are carried in small transport vesicles to the Golgi bodies (dictyosomes), where they go undergo modification to the glycosylation they received as they were imported into the RER. After modification, some proteins are exported from the trans-Golgi network in export vesicles to the extracellular space. The ER is continuous with the nuclear envelope, and probably evolved at the same time, possibly by invaginations of the sort seen in the mesosomes of bacteria.

The endomembranes include the nuclear membrane, already mentioned, and the rough and smooth endoplasmic reticula (RER and SER). The lamellae of the rough endoplasmic reticulum are continuous throughout the cell, and also contiguous with the nuclear membrane. They are the site of synthesis for proteins destined for the endomembrane system and the extracellular space, with which its lumen (the cisternal space) is topologically identical. The tubules of the SER are responsible for steroid and lipid synthesis. After translation on ribosomes attached to the RER, proteins are carried in small transport vesicles to the Golgi bodies (dictyosomes), where they go undergo modification to the glycosylation they received as they were imported into the RER. After modification, some proteins are exported from the trans-Golgi network in export vesicles to the extracellular space.

[[Image:Endomembranes_evolution.png‎|center||Endomembranes probably evolved from invaginations of the plasmalemma, as seen in bacterial mesosomes, thylakoids and chromatophores.]]

[[Image:Endomembranes_evolution.png‎|center||Endomembranes probably evolved from invaginations of the plasmalemma, as seen in bacterial mesosomes, thylakoids and chromatophores.]]