Lysosomes and Endocytosis

Endosymbiotic Theory and Eukaryotic Organelles

Overview context: Some prokaryotes can perform photosynthesis. In a key event, a photosynthetic bacterium was engulfed by an assembling eukaryotic cell. Rather than being digested, it persisted inside and evolved into the chloroplast. This endosymbiotic event contributed to the evolution of chloroplasts in plants/algae and, alongside mitochondria, is foundational to eukaryotic complexity.
Endosymbiotic theory (endo = inside, symbiotic = mutual relationship): Explains how mitochondria and chloroplasts evolved from engulfed prokaryotes and became organelles with their own genomes.
Significance: This random, early event helped fuel the evolution of complex multicellular life (plants, animals) and major lineages in biology.

Eukaryotic Organelles: The Endomembrane System and Cytoskeleton

Rough Endoplasmic Reticulum (Rough ER)

Structure: Network of membranous sheets stemming from the nucleus; rough because ribosomes stud its surface.
Function: Protein synthesis. Ribosomes on the rough ER typically make proteins destined for secretion or export outside the cell (e.g., insulin from pancreatic cells).
Contrast: Free-floating ribosomes synthesize proteins that generally function inside the cell.
Insulin example: Insulin is synthesized by ribosomes on rough ER in pancreatic cells; it is secreted outside the cell to act elsewhere in the body.

Smooth Endoplasmic Reticulum (Smooth ER)

Structure: ER without ribosomes; appears smooth.
Function: Lipid synthesis, including oily/waxy lipids such as cholesterol and lipid-based hormones.

Golgi Apparatus

Description: Stacks of flattened membrane-bound sacs (often likened to flattened pita bread stacks).
Function: Modifies products from the ER (e.g., adds carbohydrate groups or CH3 groups); sorts and packages proteins and lipids for delivery outside the cell or to specific locations inside the cell.
Example pathway: Proteins made in Rough ER or lipids made in Smooth ER are sent to the Golgi for modification; then packaged and exocytosed.
Exocytosis: Process by which Golgi-packaged products are released outside the cell.

Lysosomes