CELS191 Lecture 4: Endomembrane System and Transport

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Dr. Rebecca Bird

• Anatomy specialist at the University of Otago.
• Research interests include Anatomical Science Education, First Year Experience, Student Success, and Educational Technology.

Lecture 4 Objectives

• Define the endomembrane system.
• Describe the function of the components of the endomembrane system.
• Outline bulk transport processes: endocytosis (phagocytosis, pinocytosis, receptor-mediated endocytosis) and exocytosis (constitutive and regulated).
• Outline the role of the lysosome.

What We Already Know

• Cells are surrounded by a membrane with embedded proteins.
• Organelles form compartments in the cell.
• Membrane transport occurs for small substances and volumes.
• Bulk movement is necessary for larger substances like proteins needing release, or transport into the cell.

The Endomembrane System

• Includes the nuclear envelope, endoplasmic reticulum, Golgi apparatus, vesicles, lysosomes, vacuoles, and the plasma membrane.
• Interconnected via direct physical contact or transfer by vesicles.

Moving Proteins Out of the Cell

• Synthesis occurs in the endoplasmic reticulum.
• Tagging and packaging takes place in the Golgi apparatus.
• Delivery involves vesicles transporting proteins.
• Vesicles can fuse with the membrane of organelles or the cell.

Endoplasmic Reticulum (ER)

• Composed of two distinct regions: Smooth ER (sER) and Rough ER (rER).

Smooth Endoplasmic Reticulum (sER) Functions

• Carbohydrate metabolism.
• Lipid synthesis for membranes.
• Detoxification of drugs and poisons.
• Storage of calcium ions, used as a signaling molecule in the cell.
• Found extensively in cells active in these processes.
• The amount can be adjusted to meet demand.

Rough Endoplasmic Reticulum (rER) Functions

• Rough appearance is due to ribosomes.
• Involved in protein synthesis.
• Secreted and membrane-bound proteins enter the lumen (interior).
• Processed by the rER and the rest of the endomembrane system.
• Release from the cell or stay on the cell membrane.
• Cytoplasmic proteins are synthesized on free ribosomes.

Golgi Complex

• Consists of membrane sacs and associated vesicles.
• Receives, modifies, sorts, and ships proteins that arrive from the rER.
• Exhibits polarity: Vesicles from the ER arrive at the cis face, and processed vesicles leave at the trans face.

Golgi Complex: Glycosylation

• The Golgi Complex is responsible for the addition or modification of carbohydrates to proteins.
• This process is important for secreted or cell surface proteins.
• It also produces various polysaccharides, some of which may be secreted (e.g., pectin, hemicellulose).

Golgi Complex: Sorting Proteins

• Molecular markers are added to direct proteins to the correct vesicles before budding from the trans face.
• Example: A phosphorylated sugar (mannose 6-phosphate) identifies proteins destined to become lysosomal enzymes.

Golgi Complex: Directing Vesicle Trafficking

• Molecular "tags" are added to vesicles leaving the trans face to direct them to targets.
• These tags are often short proteins on the vesicle surface.
• Vesicles are directed to lysosomes, secretory pathways, or the to add membrane proteins.
• Tags act as docking sites at the target location.

Types of Vesicles

• Membrane-bound organelles filled with cytoplasm/fluid.
• Classified by function: transport, secretory, and vacuoles.
• Artificial versions (liposomes) are used for drug and vaccine delivery.

Bulk Transport Across the Plasma Membrane

• Exocytosis: transports material (glycoproteins) out of a cell or delivers it to the cell surface.
  • Constitutive exocytosis: releases extracellular matrix proteins.
  • Regulated exocytosis: releases hormones and neurotransmitters.
• Endocytosis: The cell takes in molecules and particulate matter at the plasma membrane
  • Phagocytosis: aka Cell "eating"
    • Uptake of "food" particles.
    • Forms a phagocytic vacuole that is "digested" by lysosomes.
    • In humans, this occurs in macrophages.
  • Pinocytosis: aka Cell "drinking"
    • Uptake of extracellular fluid containing protein and sugars.
    • A coated protein aids the formation of an uptake vesicle.
    • Non-selective uptake.
  • Receptor-mediated endocytosis:
    • Specialized form of pinocytosis
    • Allows cells to selectively take up bulk quantities of specific substances with low concentrations in the extracellular fluid -Receptor proteins are used to selectively capture the required solute

Lysosomes

• Phagocytic vacuoles fuse with lysosomes.
• Membrane-bound organelles are created by the rER and Golgi body containing hydrolytic enzymes.
• An acidic interior is required for enzyme activity.
• Degrade proteins, lipids, carbohydrates, and nucleic acids and release the resulting products into the cell.
• Digest and recycle unwanted cellular materials.
• Autophagy is the process of cleaning up unwanted cellular materials and is important for cell health.
• Essential in programmed cell death.
• Defects in lysosomal enzymes can lead to lysosomal storage diseases.

Vacuoles

• Large vesicles from the rER and Golgi.
• Food vacuoles are involved in phagocytosis
• In plant cells, can perform lysosome-like functions.
• The large central vacuole absorbs water for growth without increasing cytoplasm.
• It is commonly the largest compartment in a plant cell.

Lecture 4 Summary

• The endomembrane system is a set of interconnected structures.
• Bulk transport involves synthesis, sorting, tagging, and delivery of proteins to the correct locations. Exocytosis: bulk transport out of the cell.
• Endocytosis: bulk transport into the cell, with the specific type depending on substances and cell needs.