Cell Biology Chapter: Organelles and Functions

The Compartmentalization of Cells

• Eukaryotic cells are subdivided into functionally distinct membrane-enclosed compartments called organelles.
• Each organelle houses a unique set of enzymes and molecules.
• To understand the eukaryotic cell, it is essential to know how these compartments are created, maintained, and how molecules move between them.
• The synthesis of proteins begins in the cytosol, the space of the cytoplasm outside the membrane-enclosed organelles.
• Each newly synthesized protein is then delivered to the organelle that requires it.
• The central theme for this chapter is the intracellular transport of proteins.

Understanding the Origins of Organelles

• The relationships between the compartments of the cell can be understood by considering how they may have evolved.

What do Organelles Do?

• Plasma Cells: Secrete their own weight in antibody molecules into the bloodstream every day. Contain large amounts of rough ER, found in large, flat sheets.
• Cells that specialize in lipid synthesis: Expand their ER to form a network of convoluted tubules.
• Golgi Apparatus: Located near the nucleus.
• ER Tubules: Extend from the nucleus throughout the entire cytosol.
• Microtubules: Support the localization of the ER and Golgi apparatus. If they are depolymerized, the Golgi apparatus fragments and disperses throughout the cell, and the ER network collapses.
• Size, Shape, Composition, and Location: All are regulated features of organelles that contribute to their function.

Function of the Main Organelles

• Nucleus: Contains the genome (excluding mitochondrial and chloroplast DNA). Principal site of DNA and RNA synthesis.
• Cytoplasm: Consists of the cytosol and cytoplasmic organelles.
• Cytosol: Main site of protein synthesis and degradation. Performs most of the cell’s intermediary metabolism.
• Endoplasmic Reticulum (ER): Occupies a large portion of the cell's membrane surface area. Responsible for protein synthesis, lipid production, and storage of Ca2+ ions.
  • Rough ER: Contains ribosomes bound to its cytosolic surface and synthesizes proteins destined for secretion or other organelles.
  • Smooth ER: Lacks bound ribosomes. Functions in lipid synthesis and Ca2+ storage.
• Golgi Apparatus: Receives proteins and lipids from the ER and dispatches them to various destinations, often modifying them en route.
• Mitochondria and Chloroplasts: Generate most of the cell’s ATP.
  • Chloroplasts: Function as the main sites of photosynthesis.
• Lysosomes: Contain digestive enzymes that degrade intracellular organelles, macromolecules, and particles taken up by endocytosis.
• Endosomes: Intermediate organelles between the plasma membrane and lysosomes.
• Peroxisomes: Small, vesicular compartments that contain enzymes involved in oxidative reactions.

Organelle Variations

• Each membrane-enclosed organelle performs the same set of basic functions in all cell types.
• Organelles vary in abundance and properties to meet the specialized functions of different cell types.
• In liver and pancreatic cells, the endoplasmic reticulum has a larger membrane surface area than the plasma membrane.
• The abundance and shape of membrane-enclosed organelles are regulated to meet the needs of the cell.