9.3 The Endomembrane System
The endomembrane system (endo = “within”) is a group of membranes and organelles in eukaryotic cells that works together to modify, package, and transport lipids and proteins. It includes the nuclear envelope, lysosomes, vesicles, endoplasmic reticulum, and Golgi apparatus. Although not technically within the cell, the plasma membrane is included in the endomembrane system because it interacts with the other endomembranous organelles. The endomembrane system does not include either mitochondria or chloroplast membranes.
The Nuclear Envelope
The nuclear envelope is a double-membrane structure that constitutes the nucleus’ outermost portion. Both the nuclear envelope’s inner and outer membranes are phospholipid bilayers. The nuclear envelope is punctuated with pores that control the passage of ions, molecules, and RNA between the nucleoplasm and cytoplasm. The nucleoplasm is the semi-solid fluid inside the nucleus, where we find the chromatin and the nucleolus. The nuclear envelope is continuous with the endoplasmic reticulum.
The Endoplasmic Reticulum
The endoplasmic reticulum (ER) is a series of interconnected membranous sacs and tubules that collectively modifies proteins and synthesizes lipids. However, these two functions take place in separate areas of the ER: the rough ER and the smooth ER, respectively.
We call the interior portion of the ER the lumen. The ER’s membrane, which is a phospholipid bilayer embedded with proteins, is continuous with the nuclear envelope.
Rough ER
Scientists have named the rough endoplasmic reticulum (RER) as such because the ribosomes attached to its cytoplasmic surface give it a studded appearance when viewing it through an electron microscope.
Ribosomes can be classified as either free or membrane-bound. Free ribosomes exist in the cytosol and are not associated with a membrane. Membrane-bound ribosomes are attached to the outside of the membrane of the endoplasmic reticulum. Free and membrane-bound ribosomes are identical in structure; they differ only in location. In fact, an individual ribosome may sometimes be free and sometimes be membrane-bound, depending on what type of protein it is producing.
Free ribosomes generally produce proteins that will be used within the cytoplasm. Membrane-bound ribosomes generally produce proteins that will either be embedded in a membrane, or secreted from the cell.
If the phospholipids or modified proteins are not destined to stay in the RER, they will reach their destinations via transport vesicles that bud from the RER’s membrane.
Since the RER is engaged in modifying proteins (such as enzymes, for example) that secrete from the cell, you would be correct in assuming that the RER is abundant in cells that secrete proteins. This is the case with liver cells, for example.
Smooth ER
The smooth endoplasmic reticulum (SER) is continuous with the RER but has few or no ribosomes on its cytoplasmic surface. SER functions include synthesis of carbohydrates, lipids, and steroid hormones; detoxification of medications and poisons; and storing calcium ions. In muscle cells, a specialized SER, the sarcoplasmic reticulum, is responsible for storing calcium ions that are needed to trigger the muscle cells’ coordinated contractions.
The Golgi Apparatus
We have already mentioned that vesicles can bud from the ER and transport their contents elsewhere, but where do the vesicles go? Before reaching their final destination, the lipids or proteins within the transport vesicles still need sorting, packaging, and tagging so that they end up in the right place. Sorting, tagging, packaging, and distributing lipids and proteins takes place in the Golgi apparatus (also called the Golgi body), a series of flattened membranous sacs.
The side of the Golgi apparatus that is closer to the ER is called the cis face. The opposite side is the trans face. The transport vesicles that formed from the ER travel to the cis face, fuse with it, and empty their contents into the Golgi apparatus’ lumen. As the proteins and lipids travel through the Golgi, they undergo further modifications that allow them to be sorted. The most frequent modification is adding short sugar molecule chains. These newly modified proteins and lipids are then tagged with phosphate groups or other small molecules in order to travel to their proper destinations.
Finally, the modified and tagged proteins are packaged into transport vesicles that bud from the Golgi’s trans face. While some of these vesicles deposit their contents into other cell parts where they will be used, other secretory vesicles fuse with the plasma membrane and release their contents outside the cell.
In another example of form following function, cells that engage in a great deal of secretory activity (such as salivary gland cells that secrete digestive enzymes or immune system cells that secrete antibodies) have an abundance of Golgi. In plant cells, the Golgi apparatus has the additional role of synthesizing polysaccharides, some of which are incorporated into the cell wall and some of which other cell parts use.
Lysosomes
Lysosomes are part of the endomembrane system. Lysosomes are found in animal cells. They contain hydrolytic enzymes that are used to digest macromolecules and worn-out organelles.
Lysosomes also use their hydrolytic enzymes to destroy pathogens (disease-causing organisms) that might enter the cell. A good example of this occurs in macrophages, a group of white blood cells which are part of your body’s immune system. In a process that scientists call phagocytosis or endocytosis, a section of the macrophage’s plasma membrane invaginates (folds in) and engulfs a pathogen. The invaginated section, with the pathogen inside, then pinches itself off from the plasma membrane and becomes a vesicle. The vesicle fuses with a lysosome. The lysosome’s hydrolytic enzymes then destroy the pathogen.
group of organelles and membranes in eukaryotic cells that work together modifying, packaging, and transporting lipids and proteins
double-membrane structure that surrounds the nucleus
series of interconnected membranous structures within eukaryotic cells that collectively modify proteins and synthesize lipids
region of the endoplasmic reticulum that is studded with ribosomes and engages in protein modification and phospholipid synthesis
region of the endoplasmic reticulum that has few or no ribosomes on its cytoplasmic surface and synthesizes carbohydrates, lipids, and steroid hormones; detoxifies certain chemicals (like pesticides, preservatives, medications, and environmental pollutants), and stores calcium ions
eukaryotic organelle comprised of a series of stacked membranes that sorts, tags, and packages lipids and proteins for distribution
organelle in an animal cell that functions as the cell’s digestive component; it breaks down proteins, polysaccharides, lipids, nucleic acids, and even worn-out organelles
