4.3 The Endomembrane System
KEY CONCEPTS
By the end of this section, you will be able to do the following:
- Compare the roles of vesicles and vacuoles within the endomembrane system
- Recognize the structure and describe the function of the endoplasmic reticulum (smooth and rough), Golgi apparatus, lysosomes, and the central vacuole of plants
- Explain which components of the endomembrane system are physically linked (shared a membrane) versus linked by vesicles
- Evaluate how different endomembrane system components work together to modify and transport macromolecules within and out of eukaryotic cells
- Give examples of how the abundance or structure of each endomembrane system component varies among different cell types
The endomembrane system (endo = “within”) is a group of membranes and organelles in eukaryotic cells that works together to synthesize, modify, package, store, transport, and digest macromolecules like lipids and proteins within the cell. Macromolecules can also be transported out of the cell via the endomembrane system. This system includes the nuclear envelope, the endoplasmic reticulum, Golgi apparatus, lysosomes, central vacuole, and vesicles, some of which are depicted in Figure 4.9. Although not technically within the cell, the plasma membrane is included in the endomembrane system because, as you will see, it interacts with the endomembranous organelles. The endomembrane system does not include mitochondria, chloroplasts, or peroxisomes.
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Vesicles and Vacuoles
Vesicles and vacuoles are membrane-bound sacs that function in storage and transport of materials within cells. Vesicles typically range from 20-50 μm in diameter, while vacuoles tend to be larger and can sometimes fill most of the cell’s volume. Other than the fact that vacuoles are somewhat larger than vesicles, there are very few distinctions between them. Vesicle membranes can fuse with either the plasma membrane or the membranes of various organelles in the endomembrane system. The term “vacuole” is used to describe a diverse array of both temporary structures (e.g., the “food vacuole” that results from phagocytosis) and organelles with lysosome-like function (e.g., lytic vacuole) or storage functions. The central vacuole in plant cells is the largest vacuole; it is a permanent structure and is described in further detail below. Generally, in this section, we use the term “vacuole” to refer to the central vacuole of plant cells.
The Endoplasmic Reticulum
The endoplasmic reticulum (ER; Figure 4.10 and 4.11) is a series of interconnected membranous sacs and tubules. The flattened membrane-bound sacs and tubes of the ER are called cisternae. Within the cisternae lies an internal space that is called the lumen or cisternal space. The ER’s membrane, which is a phospholipid bilayer embedded with proteins, is continuous with the outer membrane of the nuclear envelope (Figure 4.10). The size of ER can vary, but typically the lumen of the rough ER is 2