Study sheet: Eukaryotic Cell Organelles

📋 Course Outline

1. Eukaryotic Organelle Classification
2. Endoplasmic Reticulum Structure
3. Endoplasmic Reticulum Functions
4. Ribosomes and Protein Targeting
5. Golgi Apparatus Organization
6. Golgi Processing and Trafficking
7. Lysosome Structure and Digestion
8. Lysosomal Pathways and Diseases

📖 1. Eukaryotic Organelle Classification

★ Must-know

📌 Eukaryotic cell organelles are classified as nonmembranous, one-membrane, or two-membrane structures.

• The endomembrane system includes:
  • the nuclear envelope
  • the endoplasmic reticulum
  • the Golgi apparatus
  • lysosomes
  • vesicles
  • the plasma membrane

Further detail

• Nonmembranous structures include:
  • ribosomes
  • microtubules
  • microfilaments
  • chromosomes

💡 Memory Hook

Membranous organelles are enclosed; nonmembranous organelles are not.

📖 2. Endoplasmic Reticulum Structure

🔑 Key Concepts & Definitions

• Endoplasmic reticulum : a convoluted membranous network extending from the nuclear surface through much of the cytoplasm and enclosing interconnected channels called cisternae

★ Must-know

📌 Rough endoplasmic reticulum has membrane-bound polyribosomes, whereas smooth endoplasmic reticulum lacks these attached ribosomes.

Further detail

• The endoplasmic reticulum has a membrane surface up to 30 times greater than that of the plasma membrane.

💡 Memory Hook

Rough ER carries ribosomes; smooth ER lacks them.

📖 3. Endoplasmic Reticulum Functions

📝 Essential Points

• The rough endoplasmic reticulum produces:

  • membrane-associated proteins
  • proteins of many membranous organelles
  • proteins secreted by exocytosis

• The smooth endoplasmic reticulum synthesizes phospholipids and steroids, detoxifies potentially harmful exogenous molecules, and sequesters and controls the release of Ca2+ ions.

💡 Memory Hook

SER functions: Lipids, Detoxification, Calcium (LDC).

📖 4. Ribosomes and Protein Targeting

🔑 Key Concepts & Definitions

• Ribosomes : complex structures containing many different proteins and at least three ribosomal RNAs that serve as sites of protein synthesis

★ Must-know

📌 Free ribosomes generally synthesize proteins used in the cytoplasm, whereas ribosomes bound to the endoplasmic reticulum synthesize proteins exported from the cell or incorporated into membranes.

• 🔄 Cotranslational sorting proceeds by: signal recognition particle detection of an amino-terminal sequence, targeting to the endoplasmic reticulum, transport to the Golgi complex, lysosomes, secretory vesicles, or plasma membrane

Further detail

• 🔄 ER-associated degradation involves:
  1. translocation back into the cytosol
  2. conjugation to ubiquitin
  3. degradation by proteasomes

💡 Memory Hook

Free ribosomes serve the cytoplasm; ER-bound ribosomes serve export and membranes.

📖 5. Golgi Apparatus Organization

🔑 Key Concepts & Definitions

• Golgi apparatus : a dynamic organelle composed of stacked membranous cisternae in which proteins from the rough endoplasmic reticulum are further processed and packaged

★ Must-know

• Golgi regions include:
  • the cis receiving face
  • the medial region
  • the trans exit and sorting face

Further detail

• The Golgi apparatus was discovered by Camillo Golgi in 1898.

💡 Memory Hook

Cis → medial → trans: receive, modify, release.

📖 6. Golgi Processing and Trafficking

★ Must-know

• Proteins enter the cis face in transport vesicles, move through medial cisternae for enzymatic modification, and leave in vesicles from the trans face.

• The Golgi apparatus modifies secretory products, sorts them, and forms transport vacuoles.

Further detail

• Important Golgi modifications include sulfation and many glycosylation reactions, and vesicle movement is guided by coat proteins such as COPII and COPI.

💡 Memory Hook

RER synthesis → Golgi modification and sorting → vesicular delivery.

📖 7. Lysosome Structure and Digestion

🔑 Key Concepts & Definitions

• Lysosomes : spherical membrane-enclosed vesicles that function as sites of intracellular digestion and are especially numerous in cells active in endocytosis

★ Must-know

• Lysosomal enzymes degrade:
  • proteins
  • nucleic acids
  • carbohydrates
  • lipids

📌 Lysosomal enzymes function optimally at an acidic pH of approximately 5.0, while the surrounding cytosol is less acidic, protecting cytosolic components from digestion.

Further detail

• A typical animal cell can contain up to 100 lysosomes, whereas phagocytic macrophages can contain up to 1000 lysosomes.

💡 Memory Hook

A membrane-bound acidic recycling compartment filled with digestive enzymes.

📖 8. Lysosomal Pathways and Diseases

🔑 Key Concepts & Definitions

• Autolysis : the digestion of cells caused by pathology or aging after lysosomal membranes break down

★ Must-know

📌 Heterophagy is the lysosomal breakdown of foreign substances absorbed by endocytosis, whereas autophagy is the breakdown of the cell's own storage nutrients, macromolecules, or nonfunctional organelles.

• Primary lysosomes emerge from the Golgi apparatus with inactive acid hydrolases, while secondary lysosomes form after fusion with endocytic vesicles containing material to be digested.

📌 Lysosomal diseases are inherited disorders caused by deficiency of a lysosomal enzyme, leading to accumulation of macromolecules, enlarged lysosomes, cellular dysfunction, and frequent central nervous system damage.

Further detail

• Digestion products from secondary lysosomes are released into the cytoplasm for reuse, while condensed vesicles containing indigestible molecules become residual bodies.

💡 Memory Hook

Heterophagy digests foreign material; autophagy digests the cell's own components.

📊 Synthesis Tables

Organelle Classification and Main Roles

Organelle or group	Membrane status	Main role
Endoplasmic reticulum	One membrane	Protein and lipid biosynthesis, detoxification, calcium handling
Ribosomes	Nonmembranous	Protein synthesis
Golgi apparatus	One membrane	Protein modification, sorting, and packaging
Lysosomes	One membrane	Intracellular digestion and recycling