13-The Endomembrane System: Structure, Function, and Protein Transport

The Endomembrane System

Overview

The endomembrane system is a network of interconnected membranes within eukaryotic cells that plays a key role in the synthesis, modification, and transport of proteins and lipids. This system compartmentalizes the cell, allowing for specific functions to occur in distinct environments.

• Prokaryotes do not possess an endomembrane system.

• Organelles are connected by phospholipid membranes and vesicular transport.

Components of the Endomembrane System

The endomembrane system includes several key organelles:

• Nuclear envelope

• Endoplasmic reticulum (ER) (rough and smooth)

• Golgi apparatus

• Vacuoles

• Transport vesicles

• Plasma membrane (not an endomembrane in physical location, but functionally related via vesicle transfer)

Why Do We Need an Endomembrane System?

Membranes provide compartmentalization, allowing different regions of the cell to maintain distinct environments. This enables the cell to build, break down, and store molecules in specialized areas, optimizing cellular function.

Protein Pathways in the Endomembrane System

Protein Synthesis: From DNA to Protein

Protein structure is determined by the cell's DNA, located in the nucleus.

• When a protein is needed, the relevant DNA is transcribed into mRNA within the nucleolus.

• The mRNA exits the nucleus and is translated by ribosomes in the cytoplasm, forming a polypeptide chain.

Key Steps in Protein Synthesis

1. Transcription: DNA is converted to mRNA in the nucleus. $\text{DNA} \xrightarrow{\text{transcription}} \text{mRNA}$

2. Translation: Ribosomes read mRNA and assemble amino acids into a polypeptide. $\text{mRNA} \xrightarrow{\text{translation}} \text{Polypeptide}$

Role of Ribosomes

• Ribosomes are the site of protein synthesis.

• They can be free in the cytoplasm or bound to the ER (focus here is on ER-bound ribosomes).

• Cells with high protein synthesis (e.g., pancreas) have many ribosomes.

Rough Endoplasmic Reticulum (Rough ER)

• Bound ribosomes synthesize polypeptides following mRNA instructions.

• Polypeptides enter the rough ER, where they fold into their three-dimensional shape.

• Short chains of sugars may be added, forming glycoproteins (glycosylation).

Transport Vesicles

• Proteins ready for export are packaged into transport vesicles that bud off from the ER.

• Vesicles move proteins to the next destination, such as the Golgi apparatus.

The Golgi Apparatus

Structure and Function

• The Golgi apparatus modifies, sorts, and ships cell products.

• Acts as a molecular warehouse and processing station for products from the ER.

• Consists of flattened sacs (cisternae) that are not connected like those in the ER.

• The receiving face (cis face) accepts transport vesicles from the ER.

• Products are modified as they progress through the Golgi stack.

• The shipping side (trans face) dispatches products in vesicles to other sites.

Processing of ER Proteins in the Golgi

• Golgi enzymes modify carbohydrate portions of glycoproteins (removing or substituting sugars).

• Molecular tags (e.g., phosphate groups) may be added for sorting.

• Finished products are packaged in transport vesicles for export or delivery to other organelles.

• Some products become part of the plasma membrane or organelles like lysosomes.

Vesicles: Structure and Types

Definition and Function

• A vesicle is a small, membrane-enclosed sac that stores or transports substances.

• Vesicles can store, transport, or digest cellular products and waste.

• Their membrane is similar to organelle membranes, allowing fusion with organelles or the plasma membrane.

Types of Vesicles

• Vacuoles

• Lysosomes/Peroxisomes

• Transport vesicles

• Secretory vesicles

Vesicle Transport in the Endomembrane System

Mechanisms

• Endomembrane compartments communicate via transport vesicles, which bud off from one membrane and fuse with another.

• Two main pathways:

  • Biosynthetic secretory pathway

  • Endocytic pathway (lysosomes and endosomes)

Transport of Macromolecules

• Vesicles transport large macromolecules out of the cell (exocytosis) and into the cell from external sources (endocytosis).

Exocytosis and Endocytosis

Exocytosis

Exocytosis is the process by which cells export bulky molecules, such as proteins or polysaccharides, by packaging them in vesicles that fuse with the plasma membrane.

• Initial event: membrane protein on vesicle binds to membrane protein on target site.

• Phospholipid regions of the two membranes merge, forming an opening to the outside.

• Vesicle contents are released; vesicle membrane is incorporated into the plasma membrane.

Endocytosis

Endocytosis is the process by which large molecules are internalized into the cell. The plasma membrane engulfs material, forming a vesicle containing the ingested substance.

• Three main subtypes:

  • Phagocytosis

  • Pinocytosis

  • Receptor-mediated endocytosis

Detailed Process of Endocytosis

• Plasma membrane folds inward around material, forming a pocket.

• Pocket deepens, forming a vesicle.

• Vesicle separates from the membrane and migrates into the cell interior.

Subtypes of Endocytosis

Type	Mechanism	Example
Phagocytosis	Engulfing large, solid particles (e.g., bacteria) via extensions called pseudopodia	Amoeba ingesting bacteria
Pinocytosis	Cell 'gulps' droplets of extracellular fluid in tiny vesicles; non-specific	Fluid uptake by animal cells
Receptor-mediated endocytosis	Specific molecules bind to receptor proteins in coated pits, triggering vesicle formation	LDL cholesterol uptake

Phagocytosis

• Highly specific; extensions of cytoplasm (pseudopodia) surround and enclose the target.

• Creates a vacuole or phagosome inside the cell membrane.

• Example: Amoeba ingesting bacteria.

Pinocytosis

• Non-specific uptake of extracellular fluid.

• Small vesicles form to bring fluid into the cell.

• Example: Animal cells taking in fluid droplets.

Receptor-Mediated Endocytosis

• Proteins with specific receptor sites exposed to extracellular fluid bind to ligands.

• Receptor proteins cluster in coated pits, which deepen and form coated vesicles.

• Example: Cholesterol (LDL) uptake via LDL receptors.

Summary Table: Components and Functions of the Endomembrane System

Organelle	Main Function
Nuclear envelope	Separates nucleus from cytoplasm; regulates transport of molecules
Endoplasmic reticulum (ER)	Synthesizes proteins (rough ER) and lipids (smooth ER)
Golgi apparatus	Modifies, sorts, and ships proteins and lipids
Vacuoles	Storage and transport of substances
Transport vesicles	Move molecules between organelles
Plasma membrane	Controls entry and exit of substances

Key Terms

• Endomembrane system: Network of membranes within eukaryotic cells for synthesis, modification, and transport.

• Vesicle: Membrane-bound sac for storage and transport.

• Exocytosis: Export of substances via vesicle fusion with plasma membrane.

• Endocytosis: Import of substances via vesicle formation from plasma membrane.

• Phagocytosis: Engulfment of large particles.

• Pinocytosis: Uptake of fluid.

• Receptor-mediated endocytosis: Specific uptake via receptor-ligand binding.

Additional info:

• Glycoprotein formation in the ER is a key step in protein maturation and targeting.

• Vesicle trafficking is essential for maintaining cellular organization and function.