Cellular Biology

Eukaryotic Membrane System



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The cell's endomembrane system consists of the rough and smooth ER, Golgi apparatus, vesicles and plasma membrane, all working together to transport cellular materials.

* Eukaryotic Cells *

Eukaryotes are organisms composed of cells that have their genetic material encased within a nuclear membrane, the nucleus. These eukaryotic cells also have a number of organelles, like little organs, that perform various cellular functions.

* Endomembrane System *

Some cellular organelles work together to transport materials into, out of, and within the cell. This is the eukaryotic cell's endomembrane system, a manufacturing and material transport network that enables the cell to make, move and break down cellular products.

The endomembrane system consists of the nuclear envelope, rough and smooth endoplasmic reticulum (ER), the Golgi apparatus as well as the cell's plasma membrane, and includes the vesicles that bud off these membranes for intracellular transport (moving stuff around inside the cell), exocytosis (stuff leaving the cell) and endocytosis (stuff coming into the cell).

* Plasma Membrane: The plasma membrane is the structure separating the inside from the outside of the cell. It consists of a phospholipid bilayer, a double membrane composed of a unique type of lipid that spontaneously organizes into two layers. The plasma membrane controls traffic of materials into and out of the cell.

* Rough Endoplasmic Reticulum: Folding out of the nuclear envelope, the rough ER is a network of continuous sacs, studded with ribosomes. The rough ER manufactures, processes, and prepares proteins for transport.

* Smooth Endoplasmic Reticulum: Similar in appearance to rough ER, the smooth endoplasmic reticulum is also a network of continuous sacs, but without ribosomes attached. This organelle is involved in the production of lipids, further processing of proteins, carbohydrate metabolism, and detoxification of drugs and poisons.

* Golgi Apparatus: The Golgi apparatus modifies the proteins and lipids made by the endoplasmic reticulum and prepares these materials for export from the cell. This organelle also encloses digestive enzymes into membranes to form vesicles called lysosomes.

* Vesicles: These are relatively small, enclosed compartments that have budded off from other components of the endomembrane system. The contents of a vesicle are separated from the cytosol (internal fluid of the cell) by at least one lipid bilayer. Vesicles are like the trucks of the endomembrane system. They function to store, transport, or digest cellular products and waste.

Many vesicles are made by the Golgi apparatus, the endoplasmic reticulum, or parts of the plasma membrane. Vesicles made from the plasma membrane form when the cell takes in (endocytosis) or discharges (exocytosis) materials.

* Lysosomes: If vesicles are the trucks of the endomembrane system, lysosomes are specifically the garbage (or recycling) trucks. These single membrane bound structures contain digestive enzymes that break down cellular waste, debris and nutrients for use by the cell. They are the sites of cellular digestion.

* Sources *

Bauman, R. (2005) Microbiology.
Park Talaro, K. (2008) Foundations in Microbiology.

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