Inside the Cell: Structure and Function of Prokaryotic and Eukaryotic Cells

Inside the Cell

Fundamental Features of Cells

Cells are the basic unit of life, and all living organisms are composed of cells. Despite their diversity, all cells share certain key components that are essential for their function and survival.

• Proteins: Perform most cellular functions, including catalyzing reactions, providing structural support, and regulating processes.

• Nucleic acids: Store, transmit, and process genetic information (e.g., DNA and RNA).

• Carbohydrates: Provide chemical energy, carbon, structural support, and cellular identity.

• Plasma membrane: Serves as a selectively permeable barrier, controlling the movement of substances into and out of the cell.

Cell Types and Classification

Morphological and Phylogenetic Classification

Cells are classified based on their structure (morphology) and evolutionary relationships (phylogeny).

• Eukaryotes: Have a membrane-bound nucleus and numerous organelles.

• Prokaryotes: Lack a membrane-bound nucleus; genetic material is located in a nucleoid region.

Phylogenetically, all life is divided into three domains:

• Bacteria: Prokaryotic

• Archaea: Prokaryotic

• Eukarya: Eukaryotic

Comparison of Eukaryotic and Prokaryotic Cells

Eukaryotic Cells	Prokaryotic Cells
Generally much larger	Generally much smaller
Chromosomes enclosed in a nucleus	Chromosomes in a nucleoid region (no membrane)
Larger number of distinct organelles	Fewer distinct organelles

Prokaryotic Cell Structure

Overview of Prokaryotic Cell Components

• Prokaryotic DNA

• Ribosomes (protein synthesis)

• Photosynthetic membranes (in some species)

• Membrane-bound organelles (rare, specialized)

• Protein fibers and cytoskeleton

• Plasma membrane

• Cell wall (exoskeleton)

• External structures for movement and attachment

Prokaryotic DNA

• Single, circular chromosome located in the nucleoid region (not membrane-bound).

• Genes code for polypeptides (proteins).

• Small, circular DNA molecules called plasmids may be present, often carrying genes for antibiotic resistance or other functions.

• DNA is often supercoiled to fit within the cell.

Ribosomes

• Macromolecular machines composed of RNA and protein.

• Responsible for protein synthesis.

Photosynthetic Membranes

• Some prokaryotes (e.g., cyanobacteria) have internal membranes that convert sunlight to chemical energy.

• These membranes develop from folds of the plasma membrane.

Organelles in Prokaryotes

• Some bacteria possess membrane-bound compartments called organelles for specialized functions.

• Examples: Storage of calcium ions, magnetite crystals for navigation, organization of enzymes for biosynthesis.

Cytoskeleton

• Bacteria and archaea contain protein fibers forming a cytoskeleton.

• Functions: Assists in cell division and maintains cell shape.

Plasma Membrane

• Composed of a phospholipid bilayer with embedded proteins (integral and peripheral).

• Defines a distinct internal environment (cytoplasm).

Cell Wall

• Most prokaryotes have a tough, fibrous cell wall made of peptidoglycan.

• Provides structural support and protection.

• Many species have an additional outer layer composed of glycolipids.

External Structures

• Flagella: Long filaments that rotate to propel the cell.

• Fimbriae: Needlelike projections that promote attachment to surfaces or other cells.

Eukaryotic Cell Structure

Overview of Eukaryotic Cell Components

• Nucleus

• Ribosomes

• Endoplasmic Reticulum (ER)

• Golgi Apparatus

• Lysosomes

• Vacuoles

• Peroxisomes

• Mitochondria

• Chloroplasts (in plants and algae)

• Cytoskeleton

• Cell Wall (in fungi, algae, and plants)

Benefits of Organelles

• Eukaryotic cells have a smaller surface-to-volume ratio, making diffusion across the cell more difficult.

• Membrane-bound organelles compartmentalize the cell, allowing separation of incompatible chemical reactions and increasing the efficiency of cellular processes.

The Nucleus

• Surrounded by a double-membrane nuclear envelope with pore-like openings.

• Contains the nucleolus, where ribosomal RNA is synthesized and ribosome subunits are assembled.

• Stores and processes genetic information; each chromosome occupies a distinct area.

• DNA is densely packed at the periphery and loosely packed toward the interior.

Ribosomes

• Not membrane-bound; thus, not considered organelles.

• Some are free in the cytosol (manufacture proteins for use within the cell).

• Some are attached to the endoplasmic reticulum (manufacture proteins for export or for membranes).

Endoplasmic Reticulum (ER)

• Extension of the nuclear envelope; two types: rough and smooth.

Rough ER (RER)	Smooth ER (SER)
Studded with ribosomes	Lacks ribosomes
Proteins are folded and processed in the lumen	Contains enzymes for lipid synthesis and detoxification
Proteins may be exported, act as membrane transporters, or catalyze reactions	Reservoir for Ca2+

Golgi Apparatus

• Series of stacked, flat membranous sacs called cisternae.

• Has polarity: cis face (closest to nucleus) and trans face (oriented toward plasma membrane).

• Processes, sorts, and ships proteins synthesized in the ER.

• Membranous vesicles carry materials to and from the Golgi apparatus.

Lysosomes

• Contain hydrolytic enzymes for digestion of macromolecules.

• Maintain acidic pH (around 5) via proton pumps.

• Not found in plant cells.

Vacuoles

• Large, membrane-bound structures in plants and fungi.

• Store water, ions, proteins, pigments, and sometimes toxic compounds for protection.

Peroxisomes

• Carry out oxidation reactions, such as breakdown of fatty acids and detoxification.

• Contain catalase enzyme to neutralize hydrogen peroxide.

Mitochondria

• Double-membrane organelle; inner membrane folded into cristae.

• Site of ATP production via cellular respiration.

• Contain their own DNA and ribosomes; can divide independently.

Chloroplasts

• Found in plants and algae; site of photosynthesis.

• Contain their own DNA and ribosomes.

• Composed of outer, inner, and thylakoid membranes; thylakoids are arranged in stacks called grana.

Endosymbiosis Theory

• Suggests mitochondria and chloroplasts originated as free-living bacteria engulfed by ancestral eukaryotic cells.

• Evidence: Both organelles contain their own DNA, ribosomes, and can divide independently.

Cytoskeleton

• Network of protein fibers that gives the cell shape, structural stability, and aids in movement and transport.

• Organizes organelles and other cellular structures.

Cell Wall

• Present in fungi, algae, and plants.

• Composed of rods or fibers of carbohydrate (e.g., cellulose) in a matrix of other polysaccharides and proteins.

• Provides structural support.

Summary Table: Major Differences Between Prokaryotic and Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent (nucleoid region)	Present (membrane-bound)
Organelles	Few, rare, specialized	Many, membrane-bound
Size	Small (1-10 μm)	Larger (10-100 μm)
Cell Wall	Peptidoglycan (most)	Cellulose (plants), chitin (fungi), or absent
DNA	Circular, supercoiled, plasmids	Linear, associated with histones

Key Concepts and Examples

• Example: Cyanobacteria are prokaryotes with internal photosynthetic membranes that perform photosynthesis.

• Example: Muscle cells are packed with mitochondria to meet high energy demands.

• Additional info: If all DNA in a human cell were stretched end-to-end, it would reach approximately 6 feet, illustrating the high degree of DNA packaging within the nucleus.