A Tour of the Cell: Structure and Function of Eukaryotic and Prokaryotic Cells

The Fundamental Units of Life

Introduction to Cells

Cells are the basic units of life, forming the foundation of all living organisms. Despite their diversity, all cells share certain structural and functional features.

• All organisms are made of cells: Cells are the smallest units that can be considered alive.

• Descent from earlier cells: All cells originate from pre-existing cells, ensuring continuity of life.

• Common features: Despite differences, cells share fundamental characteristics such as genetic material, plasma membrane, and metabolic machinery.

Types of Cells: Prokaryotic vs. Eukaryotic

Classification and Key Differences

Cells are classified into two major types: prokaryotic and eukaryotic. This distinction is based on structural organization and complexity.

• Prokaryotic cells: Found in Bacteria and Archaea. Characterized by the absence of a nucleus and membrane-bound organelles.

• Eukaryotic cells: Found in Protists, Plants, Animals, and Fungi. Characterized by the presence of a nucleus and membrane-bound organelles.

Basic Features of All Cells

• Plasma membrane: Encloses the cell, controlling the movement of substances in and out.

• Cytosol: Semifluid substance within the cell where cellular processes occur.

• Chromosomes: Structures carrying genetic information (DNA).

• Ribosomes: Complexes that synthesize proteins.

Prokaryotic Cell Structure

• No nucleus: DNA is located in the nucleoid region.

• No membrane-bound organelles: Cellular processes occur in the cytoplasm.

• Cell wall: Provides structural support and protection.

• Other features: May include fimbriae, flagella, and glycocalyx.

Eukaryotic Cell Structure

• Nucleus: Contains DNA, surrounded by a double membrane (nuclear envelope).

• Membrane-bound organelles: Specialized compartments for distinct cellular functions (e.g., mitochondria, endoplasmic reticulum).

• Cytoplasm: Region between the plasma membrane and nucleus.

• Generally larger than prokaryotic cells.

Plasma Membrane Structure and Function

Selective Barrier and Composition

The plasma membrane regulates the passage of substances, maintaining cellular homeostasis.

• Phospholipid bilayer: Composed of hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails.

• Proteins: Embedded within the membrane, facilitating transport and communication.

• Carbohydrate side chains: Involved in cell recognition and signaling.

Surface Area to Volume Ratio

The efficiency of cellular processes depends on the surface area to volume ratio.

• As cells increase in size, their volume grows faster than their surface area, limiting the rate of exchange with the environment.

Cell Size	Total Surface Area	Total Volume	Surface-to-Volume Ratio
1 cube	6	1	6
150 cubes	150	125	1.2
750 cubes	750	125	6

Genetic Information: Nucleus and Ribosomes

Nucleus

The nucleus is the control center of eukaryotic cells, housing genetic material.

• Nuclear envelope: Double membrane separating the nucleus from the cytoplasm.

• Chromosomes: DNA organized into discrete units.

• Chromatin: DNA-protein complex within chromosomes.

• Nucleolus: Site of ribosomal RNA (rRNA) synthesis.

Ribosomes

• Structure: Composed of rRNA and proteins.

• Function: Protein synthesis.

• Locations: Free in cytosol or bound to endoplasmic reticulum/nuclear envelope.

The Endomembrane System

Components and Functions

The endomembrane system coordinates the synthesis, modification, and transport of cellular products.

• Nuclear envelope

• Endoplasmic reticulum (ER)

• Golgi apparatus

• Lysosomes

• Vacuoles

• Plasma membrane

Endoplasmic Reticulum (ER)

• Smooth ER: Synthesizes lipids, metabolizes carbohydrates, detoxifies drugs/poisons, stores calcium ions.

• Rough ER: Studded with ribosomes, synthesizes proteins, distributes transport vesicles, produces membranes.

Golgi Apparatus

• Structure: Flattened sacs called cisternae.

• Function: Modifies ER products, manufactures macromolecules, sorts/packages materials into vesicles.

Lysosomes

• Function: Membranous sacs containing hydrolytic enzymes for digestion of macromolecules.

• Processes: Phagocytosis and autophagy.

Vacuoles

• Types: Food vacuoles, contractile vacuoles, central vacuoles (in plants).

• Function: Storage, waste disposal, water balance, cell growth.

Mitochondria and Chloroplasts: Energy Conversion

Energy Transformation

Mitochondria and chloroplasts are specialized organelles responsible for energy conversion.

• Mitochondria: Sites of cellular respiration, generating ATP using oxygen.

• Chloroplasts: Sites of photosynthesis in plants and algae, converting light energy to chemical energy.

The Cytoskeleton: Structure and Motility

Organization and Function

The cytoskeleton is a dynamic network of fibers that provides structural support and enables cellular movement.

• Microtubules: Hollow rods made of tubulin; involved in cell shape, organelle movement, and chromosome separation.

• Microfilaments (Actin Filaments): Solid rods of actin; support cell shape, form microvilli, and enable movement.

• Intermediate Filaments: Fibers of intermediate diameter; maintain cell shape and anchor organelles.

Roles in Motility

• Motor proteins interact with cytoskeletal fibers to produce movement.

• Vesicles travel along cytoskeletal tracks within the cell.

Extracellular Components and Cell Connections

Coordination of Cellular Activities

Cells synthesize and secrete materials that form structures outside the plasma membrane, facilitating communication and structural integrity.

• Cell walls (plants): Provide protection and support; composed of cellulose.

• Extracellular matrix (ECM, animals): Made of glycoproteins such as collagen, proteoglycans, and fibronectin; supports and connects cells.

Cell Junctions in Animal Cells

• Tight junctions: Prevent fluid movement between cells.

• Desmosomes: Anchor cells together via intermediate filaments.

• Gap junctions: Allow ions and small molecules to pass between cells for communication.

Summary Table: Comparison of Cell Types

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent	Present
Membrane-bound organelles	Absent	Present
Size	Smaller	Larger
Examples	Bacteria, Archaea	Plants, Animals, Fungi, Protists