Cell Structure and Function: Cytoplasm and Organelles

Cells: The Living Units

Introduction

Cells are the fundamental units of life in all living organisms. Each cell contains specialized structures that perform distinct functions necessary for survival and homeostasis. This section focuses on the cytoplasm and the major organelles found within eukaryotic cells.

Cytoplasm

Components of the Cytoplasm

The cytoplasm is the cellular material located between the plasma membrane and the nucleus. It is composed of three main elements:

• Cytosol: A gel-like solution made up of water and soluble molecules such as proteins, salts, and sugars. It serves as the site for many metabolic reactions.

• Inclusions: Insoluble molecules that vary with cell type. Examples include glycogen granules (energy storage), pigments, lipid droplets, vacuoles, and crystals.

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

Cytoplasmic Organelles

Classification of Organelles

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

• Membranous Organelles: Mitochondria, Endoplasmic Reticulum (ER), Golgi Apparatus, Peroxisomes, Lysosomes

• Nonmembranous Organelles: Ribosomes, Cytoskeleton, Centrioles

Membranes allow compartmentalization, which is crucial for efficient cell functioning.

Mitochondria

Structure and Function

Mitochondria are often referred to as the "powerhouses" of the cell because they produce most of the cell's energy molecules, adenosine triphosphate (ATP), through aerobic (oxygen-requiring) cellular respiration.

• Composed of double membranes; the inner membrane has many folds called cristae, which increase surface area for ATP production.

• Mitochondria contain their own DNA, RNA, and ribosomes, allowing them to replicate and produce some of their own proteins.

Ribosomes

Structure and Function

Ribosomes are nonmembranous organelles that are the site of protein synthesis. They are made up of protein and ribosomal RNA (rRNA).

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

• Membrane-bound ribosomes: Attached to the membrane of the endoplasmic reticulum (ER); synthesize proteins to be incorporated into membranes, lysosomes, or exported from the cell.

Endoplasmic Reticulum (ER)

General Structure

The endoplasmic reticulum (ER) consists of a series of parallel, interconnected cisterns—flattened membranous tubes with fluid-filled interiors. The ER is continuous with the outer nuclear membrane and exists in two varieties:

• Rough ER

• Smooth ER

Rough Endoplasmic Reticulum (Rough ER)

The external surface of the rough ER appears rough because it is studded with attached ribosomes.

• Site of synthesis of proteins that will be secreted from the cell.

• Site of synthesis of many plasma membrane proteins and phospholipids.

• Proteins enter cisterns as they are synthesized and are modified as they wind through fluid-filled tubes.

• Final protein is enclosed in a vesicle and sent to the Golgi apparatus for further processing.

Smooth Endoplasmic Reticulum (Smooth ER)

The smooth ER is a network of looped tubules continuous with the rough ER. It lacks ribosomes and is involved in:

• Lipid metabolism, cholesterol and steroid-based hormone synthesis, and making lipids for lipoproteins.

• Absorption, synthesis, and transport of fats.

• Detoxification of certain chemicals (drugs, pesticides, etc.).

• Conversion of glycogen to free glucose.

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

Summary Table: Membranous vs. Nonmembranous Organelles

Membranous Organelles	Nonmembranous Organelles
Mitochondria	Ribosomes
Endoplasmic Reticulum (ER)	Cytoskeleton
Golgi Apparatus	Centrioles
Peroxisomes
Lysosomes

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.

• rRNA (Ribosomal RNA): A type of RNA that combines with proteins to form ribosomes.

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

Example: Protein Synthesis and Processing

Proteins destined for secretion are synthesized by ribosomes on the rough ER, modified as they pass through the ER, packaged into vesicles, and sent to the Golgi apparatus for further processing and sorting before being exported from the cell.

Additional info:

• The study notes above expand on the brief points in the slides, providing definitions, examples, and context for each organelle and process.