Chapter 03 Part C: Cells – The Living Units

Introduction

This section explores the structure and function of the cytoplasm and its organelles, which are essential components of eukaryotic cells. Understanding these structures is fundamental to cell biology and physiology.

Cytoplasm

Definition and Components

The cytoplasm is the cellular material located between the plasma membrane and the nucleus. It serves as the site for most cellular activities.

• Cytosol: A gel-like solution composed mainly of water and soluble molecules such as proteins, salts, and sugars. It provides a medium for chemical reactions.

• Inclusions: Insoluble molecules that vary with cell type. Examples include glycogen granules (energy storage in liver and muscle cells), pigments (such as melanin), lipid droplets (in fat cells), vacuoles, and crystals.

• Organelles: Specialized metabolic machinery structures within the cell, each with a specific function. Organelles can be membranous (surrounded by a membrane) or nonmembranous.

Cytoplasmic Organelles

Classification

Organelles are classified based on the presence or absence of surrounding membranes:

• Membranous Organelles:

  • Mitochondria

  • Endoplasmic reticulum (ER)

  • Golgi apparatus

  • Peroxisomes

  • Lysosomes

• Nonmembranous Organelles:

  • Ribosomes

  • Cytoskeleton

  • Centrioles

Membranes allow compartmentalization, which is crucial for cell functioning by separating different metabolic processes.

Mitochondria

Structure and Function

• Known as the "power plant" of the cell because they produce most of the cell's ATP (energy molecule) via aerobic (oxygen-requiring) cellular respiration.

• Composed of a double membrane:

  • Outer membrane: smooth

  • Inner membrane: folded into structures called cristae, which increase surface area for energy production and contain proteins involved in cellular respiration.

• Contain their own DNA, RNA, and ribosomes, allowing them to replicate independently of the cell (similar to bacteria).

• Capable of cell division by a process called fission.

Example: Muscle cells have many mitochondria due to high energy demands.

Ribosomes

Structure and Function

• Nonmembranous organelles composed of protein and ribosomal RNA (rRNA).

• Sites of protein synthesis in the cell.

• Exist in two forms:

  • Free ribosomes: Float freely in the cytosol; synthesize soluble proteins that function within the cytosol or other organelles.

  • Membrane-bound ribosomes: Attached to the endoplasmic reticulum (ER); synthesize proteins destined for membranes, lysosomes, or export from the cell.

Example: Pancreatic cells have abundant ribosomes to produce digestive enzymes.

Endoplasmic Reticulum (ER)

Structure and Types

The endoplasmic reticulum is a network of interconnected, flattened membranous tubes and sacs (cisterns) continuous with the outer nuclear membrane. It comes in two varieties:

• Rough ER: Studded with ribosomes; involved in protein synthesis and modification.

• Smooth ER: Lacks ribosomes; involved in lipid synthesis, detoxification, and calcium storage.

Functions of Rough ER

• Synthesizes proteins that will be secreted from the cell, incorporated into plasma membranes, or sent to lysosomes.

• Proteins enter the ER cisterns as they are synthesized, are modified, and then enclosed in vesicles for transport to the Golgi apparatus.

Functions of Smooth ER

• Lipid metabolism, including cholesterol and steroid hormone synthesis.

• Absorption, synthesis, and transport of fats.

• Detoxification of drugs and harmful chemicals.

• Conversion of glycogen to free glucose.

• Storage and release of calcium ions (especially in muscle cells, where it is called the sarcoplasmic reticulum).

Summary Table: Membranous vs. Nonmembranous Organelles

Key Terms and Concepts

• ATP (Adenosine Triphosphate): The primary energy carrier in cells.

• Cristae: Folds of the inner mitochondrial membrane that increase surface area for energy production.

• Fission: A type of cell division used by mitochondria, similar to bacterial division.

• rRNA (Ribosomal RNA): A type of RNA that, together with proteins, makes up ribosomes.

• Cisterns: Flattened membranous sacs in the ER and Golgi apparatus.

Additional info: The content above is foundational for understanding cell structure and function, which is essential for further study in biology, physiology, and related health sciences.