Cells: The Living Units – Structure and Function of Cellular Components

Chapter 3: Cells – The Living Units

Introduction

Cells are the fundamental units of life, forming the basis of structure and function in all living organisms. This chapter explores the composition, structure, and function of cellular components, with a focus on organelles, the cell cycle, and cellular processes essential for life.

Cytoplasm and Its Components

Cytoplasm

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 acts as the medium in which organelles and other inclusions are suspended.

• Inclusions: Insoluble molecules that vary with cell type, including glycogen granules, pigments, lipid droplets, vacuoles, and crystals.

• Organelles: Specialized metabolic machinery structures, each with distinct functions. Organelles can be membranous or nonmembranous.

Cytoplasmic Organelles

Classification of Organelles

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

Membranous Organelles	Nonmembranous Organelles
Mitochondria	Ribosomes
Endoplasmic reticulum	Cytoskeleton
Golgi apparatus	Centrioles
Peroxisomes
Lysosomes

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

Mitochondria

Structure and Function

Mitochondria are often called the "power plants" of the cell because they produce most of the cell's energy molecules, adenosine triphosphate (ATP), via aerobic (oxygen-requiring) cellular respiration.

• Contain their own DNA, RNA, and ribosomes.

• Resemble bacteria in structure and are capable of a type of cell division called fission.

• Key site for energy production through the process: $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP}$

Ribosomes

Structure and Function

Ribosomes are nonmembranous organelles that serve as the site of protein synthesis. They are composed 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 membranes of the endoplasmic reticulum (ER); synthesize proteins destined for incorporation into membranes, lysosomes, or for export from the cell.

Endoplasmic Reticulum (ER)

Types and Functions

• Rough ER: Studded with ribosomes; site of synthesis of proteins to be secreted from the cell, incorporated into membranes, or sent to lysosomes.

• Smooth ER: Lacks ribosomes; involved in lipid metabolism, cholesterol and steroid hormone synthesis, absorption and transport of fats, detoxification of chemicals, conversion of glycogen to glucose, and storage/release of calcium (notably as the sarcoplasmic reticulum in muscle cells).

Golgi Apparatus

Structure and Function

The Golgi apparatus consists of stacked and flattened membranous sacs. It modifies, concentrates, and packages proteins and lipids received from the ER for secretion or delivery to other organelles.

• Acts as the "shipping and receiving center" of the cell.

• Proteins are processed and sorted for their final destinations.

Peroxisomes

Structure and Function

Peroxisomes are membranous sacs containing enzymes that detoxify harmful substances and neutralize free radicals (toxic by-products of metabolism).

• Contain oxidases (convert toxins to hydrogen peroxide, H2O2) and catalase (converts H2O2 to water).

• Involved in the breakdown and synthesis of fatty acids.

Lysosomes

Structure and Function

Lysosomes are spherical membranous bags containing digestive enzymes (acid hydrolases). They are considered "safe" sites for intracellular digestion.

• Digest ingested bacteria, viruses, and toxins.

• Degrade nonfunctional organelles and cellular debris.

• Break down glycogen and release Ca2+ from bone.

• Involved in autolysis (self-digestion of injured or dying cells).

Clinical Example: Tay-Sachs disease is a lysosomal storage disorder caused by a missing enzyme needed to break down glycolipids in brain cells, leading to nervous system dysfunction.

Cytoskeleton and Centrosome

Cytoskeleton

The cytoskeleton is a network of protein filaments that provides structural support, facilitates intracellular transport, and enables cell movement.

• Includes microfilaments, intermediate filaments, and microtubules.

Centrosome and Centrioles

The centrosome is the cell's microtubule organizing center, containing a pair of centrioles arranged at right angles. It plays a key role in cell division and in forming the basis of cilia and flagella.

Cellular Extensions

Cilia, Flagella, and Microvilli

• Cilia: Short, hair-like projections that move substances across the cell surface.

• Flagella: Longer projections that propel the cell itself (e.g., sperm cell).

• Microvilli: Fingerlike extensions that increase the cell's surface area for absorption.

Nucleus

Structure and Function

The nucleus is the control center of the cell, containing genetic material (DNA) that directs protein synthesis and cellular activities.

• Most cells are uninucleate; some (e.g., skeletal muscle) are multinucleate; mature red blood cells are anucleate.

Cell Cycle

Phases of the Cell Cycle

The cell cycle is the series of changes a cell undergoes from formation to division. It consists of two major periods:

• Interphase: Cell grows and carries out normal functions. Subdivided into:

  • G1 (Gap 1): Vigorous growth and metabolism.

  • S (Synthesis): DNA replication occurs.

  • G2 (Gap 2): Preparation for division.

• Mitotic (M) Phase: Cell divides into two daughter cells. Includes:

  • Mitosis: Division of the nucleus.

  • Cytokinesis: Division of the cytoplasm.

Control of Cell Division

• "Go" signals: Surface-to-volume ratio, growth factors, hormones.

• "Stop" signals: Contact inhibition, lack of space, anchorage dependence.

Cell Death and Renewal

Autophagy and Apoptosis

• Autophagy: The process of disposing of nonfunctional organelles and cytoplasmic debris by forming autophagosomes, which are degraded by lysosomes.

• Apoptosis: Programmed cell death, involving activation of caspases that degrade DNA and cytoskeleton, leading to cell shrinkage and removal by macrophages.

• Proteasomes: Protein complexes that digest unneeded or damaged proteins.

Developmental Aspects and Aging

Cell Differentiation

All body cells contain the same DNA, but chemical signals during development activate specific genes, leading to cell specialization (differentiation).

Cell Division and Growth

• Hyperplasia: Increased cell number due to accelerated growth.

• Atrophy: Decrease in cell size due to loss of stimulation or use.

• Hypertrophy: Increase in cell size due to increased stimulation or use.

Aging Theories

• Wear and tear theory: Accumulated damage from chemical insults and free radicals.

• Mitochondrial theory: Free radicals in mitochondria reduce energy production.

• Immune system theory: Autoimmune responses and weakened immunity.

• Genetic theory: Aging and cessation of mitosis are programmed into genes.

Clinical Example: Progeria is a rare genetic disorder causing premature aging due to a defective protein in the nuclear lamina. Enhanced autophagy may help clear the abnormal protein.