Endoplasmic Reticulum and Golgi Apparatus Lecture Notes

Lecture Overview

This lecture focuses on the endomembrane system, its components, structure, and function within eukaryotic cells. It includes a review of important topics in preparation for an upcoming test and encourages active participation and questions from students.

1. Class Administration

  • Attendance is recorded through iClicker, which students must sign into at the start of class.

  • Students were reminded about a previously posted recording discussing photorespiration and the Hatch-Like cycle.

  • Upcoming test scheduled for next Friday, covering:

    • Chapter 8 (Membrane Transport)

    • Glycolysis and gluconeogenesis

    • Regulation of cellular respiration

    • Photosynthesis (with reduced coverage this year)

    • Topics covered during this lecture and the subsequent ones leading up to the exam.

2. Importance of Active Study Strategies

  • Encourages students to engage actively in their learning process by writing pathways out repeatedly.

  • Encourages use of physical activity (e.g., stretching, jumping jacks) if students feel tired during the lecture.

  • Reinforces the importance of attending office hours and asking questions for clarification.

3. Introduction to the Endomembrane System

  • The endomembrane system comprises multiple membranous organelles and is crucial for various cellular processes such as membrane trafficking and product sorting.

  • Key components of the endomembrane system include:

    • Endoplasmic reticulum (ER)

    • Golgi apparatus

    • Lysosomes

    • Endosomes (important for sorting within the endomembrane system)

    • Various vesicles involved in trafficking (secretory vesicles, transport vesicles)

    • Nuclear envelope

3.1 Endosomes

  • These organelles form during endocytosis and help sort and organize materials brought into the cell.

3.2 Paroxysomes

  • Previously not included in the endomembrane system, but research suggests they may begin biogenesis from ER-derived membranes.

4. Endoplasmic Reticulum (ER)

  • The ER is a vast network of flattened sacs (cisternae) and tubules that extends throughout the cell.

  • It accounts for 50-90% of the cell's membrane mass in mammalian cells.

4.1 Structure of the ER

  • The ER is depicted surrounding the nucleus and is categorized into two types:

    • Rough Endoplasmic Reticulum (RER)

    • Characterized by the presence of ribosomes, giving it a 'rough' appearance.

    • Functions primarily in the synthesis and processing of proteins, particularly:

      • Transmembrane proteins

      • Hydrolytic enzymes destined for lysosomes

      • Secreted proteins

    • Smooth Endoplasmic Reticulum (SER)

    • Lacks ribosomes, characterized by tubular structures.

    • Plays significant roles in:

      • Lipid synthesis, particularly steroids

      • Drug detoxification via cytochrome P450 enzymes

      • Glycogen metabolism (e.g., enzyme glucose-6-phosphatase)

      • Calcium storage crucial for muscle contraction (e.g., sarcoplasmic reticulum in muscle cells).

4.2 Functions of the Endoplasmic Reticulum

  • Biosynthesis of Membrane Lipids:

    • The ER synthesizes most phospholipids for cellular membranes.

    • Liver and hepatocytes are key in glycogen breakdown through enzymes found on the SER.

  • Protein Folding and Quality Control:

    • Molecular chaperones assist in proper protein folding in the RER.

    • The unfolded protein response (UPR) is activated when misfoledd or unfolded proteins accumulate, leading to:

    • Halting protein synthesis

    • Upregulation of chaperone production

    • Transport of misfolded proteins to the cytosol for degradation.

5. Golgi Apparatus

  • Structurally associated with the ER, involved in lipid and protein trafficking and processing.

5.1 Structure of the Golgi

  • Composed of stacked flattened sacs (cisternae), usually ranging from 3 to 8.

  • The Golgi has three distinct regions:

    • Cis Face: Facing the ER (CGN - Cis Golgi Network)

    • Medial Cisternae: Middle region with distinct enzymes

    • Trans Face: Facing the plasma membrane, where vesicles bud off for secretion.

5.2 Functions of the Golgi

  • Modifying proteins and lipids received from the ER before distribution.

  • Transport Mechanisms:

    • Movement through the Golgi takes place by either:

    1. Stationary Cisternae Model: Cisternae remain in place while vesicles transport materials between them.

    2. Cisternal Maturation Model: Cisternae themselves move through the Golgi, changing their functions as they mature.

  • Anterograde and Retrograde Transport:

    • Anterograde transport refers to movement from the ER to the plasma membrane, while retrograde transport ensures that excess membrane is recycled back to earlier compartments.

6. Conclusion

  • The ER and Golgi are dynamic organelles that play a crucial role in maintaining cellular function through effective lipid and protein processing.

  • As students prepare for the upcoming quiz, they should review the endomembrane system's components, transport mechanisms, and each organelle's specific functions.