23 The Endomembrane System

The image shows the components of the endomembrane system inside a eukaryotic cell. Each of the structures described below is surrounded by a double line, representing the phospholipid bilayer. At the center is the nucleus, drawn as a large purple circle enclosed in a double membrane. Connected to the nucleus is the endoplasmic reticulum (ER), which appears as a network of folded membranes that look something like a maze. The rough ER has small red circles on it representing ribosomes. Vesicles, which are colored circles suurrounded by a line representing the membrane, are shown budding off from the ER and moving to the Golgi apparatus, which is drawn as a series of flattened, curved sacs. The Golgi further ships vesicles to other parts of the cell. Lysosomes and vacuoles are shown as separate membrane-bound sacs that receive and break down materials. The cell membrane forms the outer boundary of the cell, and arrows indicate the flow of materials between these structures. This diagram highlights how membranes and transport vesicles work together to move proteins and other molecules through the cell.

The endomembrane system is a collection of membrane-based organelles that work together to create, modify, and export cell products such as proteins and lipids. It consists of the nuclear envelope, the rough and smooth endoplasmic reticulum, the Golgi apparatus, and several types of vesicles.

Let’s start our tour with the endoplasmic reticulum. There are two types of endoplasmic reticulum, rough and smooth. The rough endoplasmic reticulum, or rough ER – is studded with ribosomes. These ribosomes are what give the rough ER its rough appearance. Ribosomes are responsible for producing proteins, which are packaged into transport vesicles and carried to the Golgi apparatus.

The smooth endoplasmic reticulum, or smooth ER, lacks ribosomes on its surface. This is why it’s called smooth. The smooth ER is mainly responsible for the detoxification of harmful chemicals. It’s also responsible for the production of lipids, which are also packaged into transport vesicles and sent to the Golgi apparatus.

The Golgi apparatus is made of a series of flattened sacs that look a little like pancakes. Its job is to package and distribute cell products that are either destined for a specific location within the cell, such as the plasma membrane or for secretion from the cell.

You can think of the Golgi apparatus as the “post office” of the cell, where packages are dropped off, and destinations are assigned. For example, some of the proteins from the rough ER are modified by the Golgi apparatus to become digestive enzymes, which are packaged into a special type of vesicle, called a lysosome. The job of this particular lysosome is to fuse with another vesicle coming in from the plasma membrane. The lysosome’s digestive enzymes then break down the contents of the vesicle, releasing nutrients for the cell.

Other cell materials leaving the Golgi apparatus may be destined for secretion from the cell. For example, some might be hormones that help with body regulation, while others might be proteins that become part of the material surrounding the cell. Before these products can be exported from the cell, they must first be packaged into a secretory vesicle. This vesicle then merges with the plasma membrane, releasing its contents into the external environment.

The endomembrane system can be modified slightly to meet the specific needs of each cell. For example, a muscle cell usually has more rough ER because of its need for protein, while a liver cell usually contains more smooth ER because of its role in detoxification.

Regardless of these modifications, the movement of materials within the endomembrane system is basically the same for all eukaryotic cells.