BIO- Organelles of the Endomembrane System

One of the main functions of the smooth ER is in the synthesis of lipids. The smooth ER synthesizes phospholipids, the main component of biological membranes, as well as steroid hormones.

For this reason, cells that produce large quantities of such hormones, such as those of the female ovaries and male testes, contain large amounts of smooth ER.

A lysosome is an organelle that contains enzymes that break down and digest unneeded cellular components, such as a damaged organelle. A lysosome is similar to a wrecking crew that takes down old and unsound buildings in a neighborhood.) Autophagy "self-eating") is the process of a cell digesting its own structures.

Lysosomes are also important for breaking down foreign material.

Endoplasmic Reticulum ER) a) The ER is a winding network of thin membranous sacs found in close association with the cell nucleus. The smooth and rough endoplasmic reticula are very different in appearance and function (source: mouse tissue. (b)

Rough ER is studded with numerous ribosomes, which are sites of protein synthesis (source: mouse tissue. EM × 110,000. (c)

Golgi Apparatus (a) The Golgi apparatus manipulates products from the rough ER, and also produces new organelles called lysosomes. Proteins and other products of the ER are sent to the Golgi apparatus, which organizes, modifies, packages, and tags them.

Some of these products are transported to other areas of the cell and some are exported from the cell through exocytosis. Enzymatic proteins are packaged as new lysosomes (or packaged and sent for fusion with existing lysosomes). (b) An electron micrograph of the Golgi apparatus.

Endoplasmic Reticulum The endoplasmic reticulum ER) is a system of channels that is continuous with the nuclear membrane or "envelope") covering the nucleus and composed of the same lipid bilayer material. The ER can be thought of as a series of winding thoroughfares similar to the waterway canals in Venice.

The ER provides passages throughout much of the cell that function in transporting, synthesizing, and storing materials. The winding structure of the ER results in a large membranous surface area that supports its many functions

The Golgi apparatus is responsible for sorting, modifying, and shipping off the products that come from the rough ER, much like a post-office. The Golgi apparatus looks like stacked flattened discs, almost like stacks of oddly shaped pancakes. Like the ER, these discs are membranous.

The Golgi apparatus has two distinct sides, each with a different role. One side of the apparatus receives products in vesicles. These products are sorted through the apparatus, and then they are released from the opposite side after being repackaged into new vesicles.

This figure shows structure of the endoplasmic reticulum. The diagram highlights the rough and smooth endoplasmic reticulum and the nucleus is labeled.

Two micrographs show the structure of the endoplasmic reticulum in detail. The left micrograph shows the rough endoplasmic reticulum in a pancreatic cell and the right micrograph shows a smooth endoplasmic reticulum.

In contrast with the smooth ER, the primary job of the rough ER is the synthesis and modification of proteins destined for the cell membrane or for export from the cell. For this protein synthesis, many ribosomes attach to the ER giving it the studded appearance of rough ER).

Typically, a protein is synthesized within the ribosome and released inside the channel of the rough ER, where sugars can be added to it by a process called glycosylation) before it is transported within a vesicle to the next stage in the packaging and shipping process: the Golgi apparatus.

when certain immune defense cells white blood cells) phagocytize bacteria, the bacterial cell is transported into

a lysosome and digested by the enzymes inside. As one might imagine, such phagocytic defense cells contain large numbers of lysosomes.

Organelles of the Endomembrane System A set of three major organelles together form a system within the cell called the endomembrane system. These organelles work together to perform various cellular jobs,

including the task of producing, packaging and exporting certain cellular products. The organelles of the endomembrane system include the endoplasmic reticulum, Golgi apparatus, and vesicles.

Endoplasmic reticulum can exist in two forms: rough ER and smooth ER. These two types of ER perform some very different functions and can be found in very different amounts depending on the type of cell. Rough ER RER

is so-called because its membrane is dotted with embedded granules—organelles called ribosomes, giving the RER a bumpy appearance.

In addition to lipid synthesis, the smooth ER also sequesters stores) and regulates the concentration of cellular Ca++, a function extremely important in cells of the nervous system where Ca++

is the trigger for neurotransmitter release. The smooth ER additionally metabolizes some carbohydrates and performs a detoxification role, breaking down certain toxins.

This figure shows the structure of the Golgi apparatus. The diagram in the left panel shows the

location and structure of the Golgi apparatus. The right panel shows a micrograph showing the folds of the Golgi in detail.

Smooth ER synthesizes phospholipids, steroid hormones, regulates the concentration of cellular Ca++,

metabolizes some carbohydrates, and breaks down certain toxins

Lysosomes Some of the protein products packaged by the Golgi include digestive enzymes that are meant to

remain inside the cell for use in breaking down certain materials. The enzyme-containing vesicles released by the Golgi may form new lysosomes, or fuse with existing, lysosomes.

A ribosome is an organelle that serves as the site of protein synthesis. It is composed of two ribosomal RNA

subunits that wrap around mRNA to start the process of translation, followed by protein synthesis. Smooth ER SER) lacks these ribosomes.

Under certain circumstances, lysosomes perform a more grand and dire function. In the case of damaged or unhealthy cells, lysosomes can be triggered to open up and release their digestive enzymes into

the cytoplasm of the cell, killing the cell. This "self-destruct" mechanism is called autolysis, and makes the process of cell death controlled a mechanism called "apoptosis").

If the product is to be exported from the cell,

the vesicle migrates to the cell surface and fuses to the cell membrane, and the cargo is secreted