Supplemental Lecture (97/02/12 update) by Stephen T. Abedon (

  1. Chapter title: Endomembrane System
    1. A list of vocabulary words is found toward the end of this document
    2. Eucaryotic cells employ an elaborate series of both continuous and discontinuous membrane enclosed vessels that control and enhance the folding of proteins (e.g., rough endoplasmic reticulum; ER), effect the post-translatioal modification of certain proteins (e.g., addition of sugars in the Golgi apparatus), serve as reaction vessels for the synthesis of non-protein substances (e.g., lipids in the smooth ER), contain reactions that would otherwise be harmful to the rest of the cell (e.g., lysosomes) or are inefficiently performed in large volumes (e.g., smooth ER), place membrane proteins in lipid bilayers (e.g., rough ER), deliver membane and membrane proteins to membrane-bound organelles (e.g., Golgi), and secrete proteins and other substances into the extracellular environment (e.g., Golgi and secretory vesicles). These vessels are together refered to as the endomembrane system, and all of the just described components communicate with each other and with the plasma membrane and nuclear membrane via the formation, release, and then merging of membrane bound vesicles.
    3. The complexity of the eucaryote endomembrane system again starkly contrasts with that seen in procaryotic systems and the reason for this contrast lies in part with the greater volume of cytoplasm found in eucaryotic cells and also in part with the increased number of functions which eucaryotic cells perform relative to their eucaryotic counterparts (that is, ecuaryotes are much better at making complex structures than are procaryotes, while the latter is much better at evolving complex biochemistries than the former). By all rights this lecture on the above described endomembrane system deserves to be considered as simply an extension of the previous lecture on eucaryotic cellular anatomy, since the endomembrane system is no less critical to making a eucaryotic cell what it is than are any of the other structures described in that previous lecture. Here we describe the eucarote endomembrane system and the related functions called endocytosis, exocytosis, and secretion.
  2. Endomembrane system
    1. A series of closed membranes within eucaryotic cells that are either continuous with each other . . .
    2. . . . or communicate with one another via vesicles which are formed at one surface and move to a second where they are incorporated.
    3. Members of the endomembrane system include:
      1. endoplasmic reticulum
      2. Golgi apparatus
      3. various vesicles
      4. lysosomes
      5. microbodies
      6. the nuclear membrane
    4. In addition, the endomembrane system does a great deal of communicating, via vesicles, with the plasma membrane.
  3. Endoplasmic reticulum (ER)
    1. The endoplasmic reticulum is cytoplasmic a network of membranes.
    2. The ER is continuous with the membrane surrounding the nuclear membrane. Note, however, that nuclear pores open to the cytoplasm, not the ER.
    3. Non-cytoplasm protein synthesis:
      1. The ER is the sight of synthesis of proteins which are destined to be found some place other than the cell cytoplasm.
      2. This is the job of the Rough endoplasmic reticulum
    4. Non-protein synthesis:
      1. The ER is also the sight of synthesis of many non-proteinaceous substances such as lipids.
      2. This is the job of the Smooth endoplasmic reticulum.
  4. Rough endoplasmic reticulum (rough ER)
    1. Described as rough because its surface is studded with ribosomes.
    2. The rough ER serves as the site of membrane protein synthesis.
    3. The rough ER is also the site of the related work of synthesizing of some proteins destined to be located in membrane-bound organelles, or export to a location external to the cytoplasmic membrane.
    4. Equivalent compartments:
      1. Note that all three of these protein locations (lumen of ER, lumen of vesicles, and external to the plasma membrane) are equivalent in terms of how the protein molecule is synthesized.
      2. That is, you can derive what proteins are produced by the rough ER from an understanding of how proteins are synthesized at the rough ER.
  5. Movement and processing
    1. Extrusion into the lumen:
      1. Three categorically different protein locations in and out of the endomembrane system are each synthesized by a ribosome starting on the outside of the rough ER and are then extruded into (or at least toward) the lumen of the rough ER.
      2. These three different protein locations are (i) the lumen of the endomembrane system, (ii) the extracellular environment, and (iii) that location difined by an incomplete crossing of the ER lipid bilayer: membrane proteins.
    2. Membrane proteins are not completely extruded through the rough ER membrane (and thus retain a portion within the membrane--a membrane protein is born).
    3. Proteins destined to be found in membrane bound organelles are extruded all the way through the ER membrane and are then concentrated in a vesicle that is formed (via a number of steps that includes the Golgi) from the ER.
    4. For a protein to be exported from the cells the vesicle is simply delivered to the cell surface where its contents are released (exocytosis).
    5. Signaling sequences:
      1. Where a protein (more specifically, polypeptide) ends up is determined by the presence of specific amino acid signaling sequences found on the protein.
      2. For example, there are protein sequence primary structures which in effect signal (i.e., say):
        1. "Go through the membrane"
        2. "Don't go all the way through"
        3. "Direct me to a lysosome"
        4. "Export me"
        5. etc.
  6. Smooth endoplasmic reticulum (smooth ER)
    1. Endoplasmic reticulum lacking ribosomes and associated protein synthesis functions.
    2. The Smooth endoplasmic reticulum serves as membraned matrix within the cell upon which many membrane proteins having enzyme activity are found.
    3. Non-protein synthesis:
      1. A great deal of non-protein synthesis occurs in and about the smooth endoplasmic reticulum.
      2. Items synthesis include various lipids and carbohydrates.
      3. In addition, the smooth ER performs various detoxication functions, particularly in the smooth ER found in liver cells.
  7. Golgi (apparatus, body, complex)
    1. The Golgi is a series of discrete membranous compartments that "communicate" with the endoplasmic reticulum as well as other membranous structures including the plasma membrane.
    2. Additional protein folding, protein modification, and the delivery of both proteins and non-proteins occurs in or from the Golgi.
    3. Note that by communication I mean that ER and other membraned organelles exchange either vesicles with the Golgi (and thereby membrane as well) or fuse in whole with the Golgi.
    4. See text figure 1085.5.
  8. Vesicle
    1. Much smaller, animal cell, vacuole equivalent.
    2. Vesicles are products of endoplasmic reticulum via the Golgi.
  9. Microbody [peroxisome, glyoxysomes]
    1. Microbodies are traditionally referred to as peroxisomes in animals.
    2. Microbodies are traditionally referred to as glyoxysomes in plants.
    3. Enclosures of hazardous materials:
      1. Microbodies are a means by which enzymes and other materials (which would otherwise be reactive with normal cellular structures) are sequestered from the rest of the cell.
      2. That is, these materials are enclosed in membrane-bound vesicles.
      3. There these enzymes and other materials perform, particularly, degradative activities (e.g., see lysosome).
  10. Lysosome
    1. Lysosomes are microbodies which are employed in the degradation of a variety of typically worn-out cell components (lipids, carbohydrate, protein, nucleic acid).
    2. Lysosomes also are employed in the degradation of whole organelles such as mitochondria.
    3. Lysosomes also are employed in the degradation of normally extrinsic stuff such as engulfed bacterial cells.
    4. "It is not known what prevents lysosomes from digesting themselves, but the (protection) requires energy, which is the reason why metabolically inactive eukaryotic cells die. Without a constant input of energy, the hydrolytic enzymes of primary lysosomes digest the lysosomal membrane from within. When these membranes disintegrate, the digestive enzymes of the lysosomes pour out into the cytoplasm of the cell and destroy it." (page 95, Raven & Johnson, 1995)
  11. Secretion [exocytosis]
    1. Vesicle-plasma membrane fusion:
      1. The fusion of vesicles with the plasma membrane automatically empties their contents into the extracellular environment.
      2. This trick is employed by many eucaryotic cells as a means of transporting substances, proteins and non-proteins, out of cell.
    2. Secreted stuff:
      1. In multicellular organisms a variety of useful substances are secreted out of cells. These include proteins making up the extracellular mastices between cells (including but not limited to bone and cartilage), proteins of the blood, digestive enzymes, hormones, numerous glandular secretions, neurotransmitters, etc.
      2. Fungi make their livings particularly by secreting enzymes (exoenzymes) around them to digest plant carbohydrates, which they then absorb as monosaccharides.
    3. Exocytosis:
      1. The process of vesicle-associated secretion is called exocytosis and constitutes the means by which secretion is effected in eucaryotic cells.
      2. The converse process, the uptake of extracellular substances into vesicles, is called endocytosis and is discussed below.
  12. Generalized endocytosis
    1. Generalized endocytosis describes a variety of related processes including:
      1. phagocytosis
      2. pinocytosis
      3. receptor-mediated endocytosis
    2. Eucaryotes, but not Bacteria, utilize endocytosis to acquire nutrients .
    3. Eucaryotes also employ endocytosis as a means of removing unwanted substances and complexes from the extracellular environment (e.g., Bacteria).
    4. Endocytosis is roughly equivalent to exocytosis running reverse.
  13. Phagocytosis
    1. Movement of larger molecules and, particularly, big chunks of molecules (food particles), into cells is accomplished by a mechanism of cellular engulfment called phagocytosis.
    2. Note that phagocytosis and movement across a lipid bilayer are not synonymous. Instead, once engulfed a food particle must still be digested into small molecules and brought across the lipid bilayer as described above. See illustration below:
    3. Protozoa particularly are adapted to the procurement of nutrients via phagocytosis.
    4. Phagocytosis is employed by animal immune system cells (macrophages and neutrophils) to engulf unwanted extracellular material such as bacteria.
  14. Illustration, phagocytosis
  15. Pinocytosis
    1. Essentially phagocytosis except a solution rich in nutrients is engulfed rather than a chunk of suspended food.
    2. Egg cells (female germ-line cells) are nourished by nurse cells which secrete nutrients close to the egg cell plasma membrane, and those nutrients are internalized into the egg cell via the process of pinocytosis.
  16. Receptor-mediated endocytosis
    1. Essentially a specialized endocytosis in which specific molecules are recognized/bound on the membrane's extracellular surface, and this bining triggers their engulfment .
  17. Links
    1. Endomembrane System
  18. Vocabulary
    1. Endoplasmic reticulum
    2. ER
    3. Exocytosis
    4. Generalized endocytosis
    5. Golgi apparatus
    6. Lysosome
    7. Phagocytosis
    8. Phagocytosis, illustration
    9. Pinocytosis
    10. Receptor mediated endocytosis
    11. Rough ER
    12. Secretion
    13. Smooth ER
    14. See also Lecture: Eucaryote Cellular Anatomy.
  19. Practice questions
    1. A membrane protein in the endoplasmic reticulum is shaped like an arrow with its head present in the lumen of the endoplasmic reticulum (i.e., points toward the inside). Eventually that same protein is transported through the Golgi become a plasma membrane protein. Where is the point of the arrow now located? [PEEK]
    2. Which are most structurally similar in eucaryotes and procaryotes? (circle correct answer) [PEEK]
      1. rough endoplasmic reticulum
      2. plasma membrane
      3. flagella
      4. nucleus
    3. Part of the endoplasmic reticulum is labeled rough because it is studded with (circle correct answer) [PEEK]
      1. chloroplasts
      2. ribosomes
      3. vesicles
      4. chromosomes
      5. all of the above
      6. none of the above
    4. Within eukaryotic cells, an extensive system of membranes, often abbreviated _______, separates various regions of the cytoplasm from each other. [PEEK]
    5. A vesicle involved in within cell digestion is a (circle correct answer) [PEEK]
      1. nucleus
      2. golgi
      3. vacuole
      4. lysosome
      5. mitochondria
      6. all of the above
      7. none of the above
    6. Which of the following membrane-bound organelles are possessed by both humans and at least some bacteria? (circle as many as are applicable) [PEEK]
      1. mitochondria
      2. rough endoplasmic reticulum
      3. Golgi apparatus
      4. nuclear membrane
      5. lysosome
    7. Which of the following is not an organelle? [PEEK]
      1. endoplasmic reticulum
      2. cytoplasm
      3. mitochondria
      4. Golgi
      5. nucleus
      6. lysosome
  20. Practice question answers
    1. outside of the cell
    2. plasma membrane
    3. ii, ribosomes
    4. ER
    5. iv, lysosome
    6. none should be circled.
    7. ii, cytoplasm
  21. References
    1. Raven, P.H., Johnson, G.B. (1995). Biology (updated version). Third Edition. Wm. C. Brown publishers, Dubuque, Iowa. pp. 85-106.
    2. Tortora, G.J., Funke, B.R., Case, C.L. (1995). Microbiology. An Introduction. Fifth Edition. The Benjamin/Cummings Publishing, Co., Inc., Redwood City, CA, pp. 90-102.