{BKH Chapter 12} Endoplasmic Reticulum (ER), Golgi & Lysosomes

[A448, fig.14-1] [A449, Tab.14-1] [5BKH 324, fig. 12-1] See[5BKH 325, fig. 12-2]

I. Introduction

 A. Cell organelles divide the cell interior into compartments.

 B. The Endoplasmic reticulum, Golgi & lysosomes are a series of sacs, tubules and vesicles, which are structurally interconnected, either directly or by way of vesicles, which bud off one membrane and then fuse with a different membrane.

[A449, fig.14-2] [2KK257, fig7-2] [2KK258, fig7-4]

II. Endoplasmic reticulum, ER,

 A. Introduction

  1. The membranes are called cistera(e) and the space within the ER is the cisternal space (ER lumen) and the remainder of the cytoplasm is the cytosol. The ER is a series of membrane-bound structures, which can be adapted for a variety of tasks.

  2. The ER is further divided into the SER (Smooth or Agranular ER) and the RER (Rough or Granular ER).

  3. The RER consists of flattened membrane sacs, whereas the SER is made of interconnected membrane tubules.

   a. The RER has ribosomes on its outer surface but not the SER.

   b. The RER functions primarily in the synthesis of proteins for export and the SER functions mainly in the metabolism of lipids, drugs and toxic substances. The SER may also have a variety of other functions, such as establishing Ca²⁺ gradients.

{5BKH330+} [5BKH 332, fig. 12-3]

 B. The functions of the SER include the following:

  1. Glucose is stored as glycogen in the liver.

          Glycogen is converted to glucose-1-phosphate and then to glucose-6-phosphate.

          Then glucose-6-phosphatase, associated with the SER, converts glucose-6-phosphate to glucose, which is then released into the blood.

  2. The SER synthesizes triacylglycerols, which are storage fats found in adipocytes (fat cells).

  3. Steroids are synthesized in the SER. Therefore, cells involved in steroid synthesis (viz adrenal cortex cells) have abundant SER.

  4. The phospholipids, needed to make membranes, are synthesized by the SER.

  5. The SER oxidizes foreign substances (viz drugs, toxins and pollutants, like pesticides). The oxidation of these substances is necessary to make them more hydrophilic so they can be metabolized. Two cytochromes, P-450 and P-448, play key roles in two systems, which oxidizing foreign molecules.

[A459, fig.14-12] {2KK259+} [2KK266, fig7-10] [2KK265, tab7-2]

  C. The RER functions in protein synthesis, which can be:

          secretory (destined for export), luminal (destined to remain within the ER, Golgi or lysosomes) or integral (which remain within the cell membranes).

   - The proteins synthesized on the "free" ribosomes are released into the cytosol, where they will be incorporated into other organelles (viz mitochondria, chloroplasts, peroxisomes and the nucleus) or they can be inserted into the ER, posttranscriptionally.

   - Protein molecules contain amino acid sequences, targeting signals, which direct proteins to their correct destinations.

[A460, fig.14-13]  [2KK269, fig7-13]

  1. Synthesis of proteins has been explained by the signal hypothesis.

  2. Synthesis of Secretory Proteins:

   a. Protein synthesis is initiated when a mRNA binds to a free ribosome. 

          Note that the ribosomes are functionally the same; the mRNA dictates how the ribosome will function.

   b. Near the N-terminus of a forming peptide is an ER signal sequence, 16-26 amino acids, which attaches to a signal recognition protein(or particle) (SRP) (7S RNA + 6 peptides).

   c. The binding of the peptide to an SRP halts protein synthesis. The SRP binds to a docking protein, the SRP receptor, which is located on the ER.

   d. At this point there is a complex consisting of the ribosome, the signal sequence of the peptide, the SRP and the SRP receptor.

          As a result of the dephosphorylation of a GTP to GDP, the SRP and the SRP receptor disassociate from the complex. At the same time, the ribosome and its associated peptide connects with a protein translocation channel.

[A460, fig.14-14]

   e. Signal peptidase removes the signal sequence, as peptide synthesis resumes.

          The forming peptide is translocated through the ER membrane into the intracisternal space, cotransciptionally. Such proteins mainly will be exported from the cell.

[A461, fig.14-15]

   f. Single-pass integral proteins of membranes are synthesized in much the same way, except they are not translocated through the membrane but remain within the membrane. Translocation is halted by a stop-transfer sequence.

          These proteins are amphiphilic, with their hydrophobic part within the membrane (viz a-helices) and their hydrophilic parts interfacing the cytosol and the intracisternal space of the ER.

[A462, fig.14-16]

   g. Multiple-pass integral proteins of membranes conserve the ER signal sequence and the stop-transfer sequence. This leaves at least these two hydrophobic portions of the protein within the membrane.

   h. The ribosome dissociates from the ER membrane as peptide synthesis concludes.

[2KK272, fig7-16] [A468, fig.14-22] [5BKH 336, fig. 12-6] [5BKH 337, fig. 12-7]

  3. Glycosylation of Proteins Synthesized on the ER.

   a. In the ER, proteins are glycosylated on the N-terminus, where a sugar moiety is attached to a free amine. That is, they have a core oligosaccharide added to it.  These are branched chains of 14 monosaccharides, which attach to proteins at asparagine side chains.

{A468}     b. The same oligosaccharide is attached to different proteins.   As this core oligosaccharide is assembled from nucleoside-diphosphate sugar derivatives, it is attached to a large hydrophobic membrane molecule, dolichol phosphate, which is 80 to 100 carbons long.

   c. The completed core oligosaccharide is attached to the completed peptide.  Several of the 14 monosaccharides are then removed before the glycosylated proteins leave the ER.

[A470, fig.14-24] {2KK272+} [2KK273,4 fig 7-18,19] [5BKH 334, fig. 12-4]

III. Golgi (Golgi apparatus , G. complex, or G. substance)

 A. Introduction to Golgi

  1. In 1898, Camillo Golgi was the first to describe the Golgi apparatus, calling it an internal reticular apparatus. The Golgi is found in nearly all eucaryotic cells.

  2. Relatively clean preparations of Golgi are obtainable by mild homogenization, followed by differential and gradient centrifugation.

  3. The Golgi is primarily composed of membranes, which lack ribosomes; in fact, ribosomes are even lacking from the cytoplasm around the Golgi.

          Therefore, it is devoid of protein synthesis.

  4. In cells which exhibit polarity the Golgi is single, large, and occupies a position between the nucleus and the pole of the cell where secretion takes place.

          (Specific examples: goblet cells, exocrine pancreatic cells and thyroid cells).

  5. In other cells, the Golgi displays no polarity and is multiple (viz liver cells have ~50 Golgi complexes throughout the cytoplasm; plant cells have a dozen to 100's).

  6. Despite the diversity in form there are some uniform Golgi characteristics.

[2KK274, fig7-20]

 B. There are three membranous components of the Golgi:

  1. Cisternae are flattened sacs, arranged in parallel fashion and separated by a space of 20-30 nm.

   a. Usually, there are 3-7 cisternae in animals, but plants have 20 or more.

   b. There are two faces: the convex, cis and the concave, trans.

          The cis face is the forming face & is closer to the RER or the nuclear envelope. The trans face is the distal maturing face & is associated with vesicle formation.

  2. Tubules and vesicles

  3. Larger vacuoles

[A463, fig.14-17]

 C. Golgi functions are mainly involved with the synthesis of glycoproteins.

{2KK283}  1. Glycoprotein synthesis may be Constitutive (continuous) in some cells, such as hepatocytes and plasma cells, or Regulated (discontinuous), as in acinar cells of exocrine pancreas, where zymogen granules are stored.

  2. Proteins synthesized by the RER are processed and packaged in the Golgi.

          In actively secreting cells, proteins are synthesized on the RER and are then transported to the Golgi.

  3. The Golgi also glycosylates proteins, using glycosyl transferases (enzymes that transfer sugar residues to proteins).

   a. Recall that proteins are glycosylated in the RER, where they have a core oligosaccharide (of 14 monosaccharides) added to it.

   b. Because these oligosaccharides attach to proteins at asparagine side chains, this is called N-linked glycosylation.

   c. In addition to N-linked glycosylation, the Golgi does O-linked glycosylation, which attaches sugars to the oxygen atoms of the hydroxyls of the amino acids: serine, threonine and hydroxylysine.

[2KK279, fig7-25] [5BKH 336, fig. 12-6]

   d. The glycoproteins from the RER are then moved to the cis face of the Golgi, by way of vesicles from the RER to the Golgi.

   e. As the glycoproteins are passed through the Golgi, from the cis to the trans face, the oligosaccharide moiety is altered by enzymes, which are arranged in the Golgi according to the order they are used.

   f. The oligosaccharides mark the proteins, so they can be sorted.

[A471, fig.14-25] [2KK281,fig7-27] [5BKH 339, fig. 12-8]

  4.  Protein sorting occurs in the Golgi.

   a. RER proteins are sorted in the cis Golgi and returned to the ER. This is retrograde transport. Some of these proteins have a C-terminal signal sequence of lys-asp-glu-leu (KDEL).

   b. Secretory and lysosome proteins are sorted by the trans Golgi.

    (i) Secretory vesicles are moved by bulk flow to the membrane and are emptied by exocytosis, which also deposits proteins destined for the plasma membrane.

[5BKH 340, fig. 12-9]

    (ii) Proteins labelled with mannose-6-phosphate are packaged into 1E lysosomes, which bud off from the trans Golgi.

    (iii) In specialized cell types, substances (e.g. pigments, hormones, enzymes, yolk) are packaged into vesicles by the Golgi.

  5. Other Golgi Functions

{8DSD230,265}

   a. The Golgi glycosylates lipids which are synthesized on the SER. 

          These include gangliosides and glycosphingolipids.

{2KK279}

   b. In some cells, the Golgi synthesizes polysaccharides, which are not attached to proteins. These include hyaluronate in animal cells, as well as pectins and hemicellulose (but not cellulose) in plant cells.

{3SB459fig.18-14} [5BKH 339, fig. 12-8]

   c. Secretory vesicles fuse to the plasma membrane and release their contents to the outside via exocytosis, thus adding to the plasma membrane. 

          Endocytotic vesicles form and are transported back to Golgi. 

          Thus, there is membrane flow.

{3SB449 f.18-5} {3SB450 fig.18-6} {3SB460 Tab.18-1} [2KK684 fig15-13] (2N&H242)

  6. Specialized functions of Golgi exist in a variety of cells:

   a. When spermatids differentiate into spermatozoa, the Golgi makes the acrosome.

   b. Nematocysts of Hydra are made by Golgi

   c. The cell plate and cell wall in plant cells are made in part by the Golgi.

   d. In the gut, goblet cells secrete a protective layer of mucus.

   e. The Golgi of thyroid follicle cells is involved in thyroglobulin metabolism.

   f. In cartilage cells the Golgi synthesizes mucopolysaccharides, as well as glycoproteins.

          NOTE:  Acid Mucopolysaccharides are heteropolysaccharides, usually containing 2 types of alternating monosaccharide units, of which at least one has an acidic group (either COOH or sulfuric group).  When they occur as complexes with specific proteins they are called mucins or mucoproteins.

IV. LYSOSOMES (Greek: Lysis + Soma)

{2kk286+}A. General information about lysosomes:

  1. Lysosomes are small organelles, bound by a single membrane.

   a. They contain acid hydrolases, which are enzymes that have acid pH optima and accomplish hydrolysis:  A₁-A₂ + HOH à   A₁-H + A₂-OH

[A477, fig.14-31] [2KK289,tab 7-3] [5BKH 354, fig. 12-20]

   b. Acid phosphatases were first identified. Further research has identified classes of many other acid hydrolases, which can digest all types of macromolecules.

{3SB471,2} {3SB471,Tab.19-3}

  2. Lysosomes are common in animal cells, especially in epithelial tissue and phagocytic cells.

[2KK290,fig7-37]

  3. Many plant cells have been observed to have membrane-bound spherical granules (0.5-1.0µm in diameter) called spherosomes.

   a. Like lysosomes, spherosomes have acid phosphatase activity.

   b. Also, many plant cells have larger membrane-bound vacuoles. 

          These contain hydrolases, as well as lipids and are thought to be used for fat storage and retrieval. Other hydrolase-containing vacuoles are involved with recycling worn out cell components.

[5BKH 355, fig. 12-21]

  4. Lysosome enzymes are synthesized on RER ribosomes and then transported to the Golgi via the ER, where they are packaged into primary lysosomes.

  5. Lysosome enzymes are glycoproteins. 

   a. Their oligosaccharides contain a peculiar monosaccharide, mannose-6-PO₄ (M-6-P).

   b. Golgi membranes contain a receptor for M-6-P, which has a role in sorting and packaging enzymes.

[A477, fig.14-32] [2KK292,fig7-39]

 B. All lysosomes are involved with some form of intracellular digestion.

          The material to be digested may be from outside or inside the cell.

See[5BKH 356, fig. 12-22]

  1. In autophagy, the material is endogenous (intracellular) in origin.

   a. Macroautophagy is a normal process of organelle turnover. The organelle is engulfed in SER membranes to form an autophagic vacuole, which then fuses with a primary lysosome to form an autophagosome, where digestion takes place.

   b. Microautophagy is a related process involving pinocytosis of cytoplasmic proteins by the lysosome membrane.

  2. Autolysis is the internal destruction of the entire cell.

   a. This is part of apoptosis (programmed cell death).

   b. Worn out & diseased cells can be removed in this manner. This is also prevalent in tissues which are being reabsorbed. (e.g. tadpole tails and embryonic interdigit cells).

[2KK293,fig7-40] [2KK295,fig7-43]

  3. Heterophagy is the endocytosis (phagocytosis & pinocytosis) of exogenous (extracellular) material, which is then contained in endosomes (phagosomes) for subsequent digestion.

   a. Phagocytosis ingests particles to form membrane-bound phagosomes (phagocytic vacuole); these fuse with primary lysosomes to form secondary lysosomes. Phagocytosis is triggered when particles bind to specific plasma membrane receptors.  In mammalian phagocytic cells there are two main groups of these receptors: the complement system and antibodies.

   b. Pinocytosis ingests bulk liquid, forming endocytotic vesicles. This is done in a similar manner as phagocytosis, but specific receptors are not used.

 C. After digestion of the material has been completed the lysosome is referred to as a residual body, and may contain some undigested material.  [2KK296,fig7-45]

  1. Undigested material may be discharged from the cell or may remain indefinitely.

  2. Pathological conditions exist where a lack of enzymes causes an accumulation of residual bodies. For instance, in Pompe's disease, the liver lysosomes become engorged with glycogen because of a missing glycolytic enzyme.

{2KK 348+} [2KK349,fig8-40]

V. Microbodies are small membrane bound vesicles.

          Peroxisomes and glyoxysomes are microbodies with clearly defined contents.

 A. Peroxisomes are wide spread among organisms. See[5BKH 359-60, fig. 12-24,25]

  1. General characteristics of peroxisomes:

   a. They are ovoid granules, surrounded by a single membrane and range in diameter from 0.3 to 1.5 mm.

   b. They are more dense than lysosomes and mitochondria.

   c. In many tissues the peroxisomes have crystal like cores (or nucleoids), which contain urate oxidase. [2NH154,fig.II-57]

   d. Peroxisomes are probably connected forming a peroxisome reticulum (PR).

   e. New peroxisomes could form from fission of pre-existing peroxisomes or by budding from PR.

   f. Peroxisome enzymes are synthesized on free (cytosolic) ribosomes and are then translocated into peroxisomes.

{N&H 138} {DSD 492} {3SB 475} [2KK349,tab.8-4]

  2. General functions of peroxisomes:

   a. There are peroxisome enzymes related to H₂O₂ (hydrogen peroxide) metabolism.

    i. Enzymes involved in production of H₂O₂ include: urate oxidase,

                   D-amino acid oxidase and a-hydroxy acid oxidase.

   ii. The enzyme, catalase, destroys H₂O₂, because H₂O₂ is toxic to the cell.

   b. Liver peroxisomes contain enzymes for b-oxidation of fatty acids, which forms acetyl CoA; these are different enzymes than those found in mitochondria.

   c. The peroxisomes of the liver and kidney are also important in the degradation of purines (viz. adenine and guanine) and they have detoxification enzymes.

{DSD492} {N&H138-9} [2KK351,fig8-42]

 B. Glyoxysomes resemble peroxisomes structurally but have different enzymes.

  1. The glyoxylate cycle resembles the Krebs cycle and converts fats into carbohydrates. For instance, in the endosperm of seeds during germination, stored fats are converted into carbohydrates.

  2. They are used by plants to synthesize carbohydrates from lipids.

  3. They are a constant feature of plant cells, but can also be found in animal cells.

  4. Glyoxysome origin is thought to parallel that of peroxisomes.