Eukaryotic and Prokaryotic Cells: Structure, Function, and Organelles

Chapter 6: Cell Structure and Function

Organelles and Cell Types

Cells are the fundamental units of life, and they are classified into two main types: eukaryotic and prokaryotic cells. Eukaryotic cells contain membrane-bound organelles, including a nucleus, while prokaryotic cells (Bacteria and Archaea) do not.

• Plasma membrane: Selective barrier that controls the movement of substances in and out of the cell.

• Cytosol: Semifluid substance in which organelles are suspended.

• Chromosomes: Structures that carry genetic material (DNA).

• Ribosomes: Complexes that synthesize proteins according to genetic instructions.

Eukaryotic cells have a DNA-containing nucleus bound by a double membrane (specialists), while prokaryotic cells have DNA concentrated in a region called the nucleoid, which is not membrane-bound (generalists).

• Eukaryotic cells: Generally larger (10–100 μm in diameter)

• Prokaryotic cells: Smaller (1–5 μm in diameter)

Cellular Components and Their Functions

The interior of the cell is called the cytoplasm. Eukaryotic cells have a variety of organelles, each with specialized functions.

Nucleus

• Contains most of the cell's DNA.

• Surrounded by a double membrane called the nuclear envelope with nuclear pores for molecular exchange.

• DNA is organized into chromosomes; each chromosome is made of chromatin (DNA + proteins).

• The nucleolus is the site of ribosomal RNA (rRNA) synthesis and ribosome assembly.

Ribosomes

• Complexes of rRNA and protein that carry out protein synthesis.

• Can be free in the cytosol or bound to the endoplasmic reticulum (ER) or nuclear envelope.

• Free ribosomes synthesize proteins for use within the cytosol; bound ribosomes synthesize proteins for membranes or export.

Endomembrane System

The endomembrane system regulates protein traffic and metabolic functions. It includes:

• Endoplasmic reticulum (ER)

• Golgi apparatus

• Lysosomes

• Vacuoles

• Plasma membrane

Endoplasmic Reticulum (ER)

• Rough ER: Studded with ribosomes; synthesizes proteins and membranes.

• Smooth ER: Lacks ribosomes; synthesizes lipids, metabolizes carbohydrates, detoxifies drugs/poisons, stores calcium ions.

Golgi Apparatus

• Modifies, sorts, and packages proteins and lipids for storage or transport out of the cell.

• Consists of flattened membranous sacs called cisternae.

• Has a cis face (receiving side) and a trans face (shipping side).

Lysosomes

• Membranous sacs containing hydrolytic enzymes for digesting macromolecules.

• Assist in phagocytosis (cell eating) and autophagy (recycling of cell's own organelles).

Vacuoles

• Diverse maintenance compartments derived from ER and Golgi apparatus.

• Plant cells have a large central vacuole for storage and maintaining cell rigidity.

• Food vacuoles are formed by phagocytosis.

Mitochondria and Chloroplasts

These organelles are involved in energy conversion and have features similar to prokaryotes, supporting the endosymbiont theory.

• Mitochondria: Sites of cellular respiration; generate ATP by extracting energy from food.

• Chloroplasts: Found in plants and algae; sites of photosynthesis, converting solar energy to chemical energy.

• Both have a double membrane, their own DNA, and ribosomes.

• Both are thought to have evolved from prokaryotic cells engulfed by ancestors of eukaryotic cells.

Characteristics Table: Mitochondria vs. Chloroplasts

Feature	Mitochondria	Chloroplasts
Location	All eukaryotic cells	Plants and algae
Main Function	Cellular respiration (ATP production)	Photosynthesis (sugar production)
Membranes	Double; inner membrane has cristae	Double; contains thylakoids and grana
Genetic Material	Circular DNA, ribosomes	Circular DNA, ribosomes

Peroxisomes

• Specialized metabolic compartments bounded by a single membrane.

• Contain enzymes that remove hydrogen atoms and transfer them to oxygen, producing hydrogen peroxide ($H_2O_2$).

• Break down fatty acids and detoxify harmful substances.

Cytoskeleton

The cytoskeleton is a network of fibers extending throughout the cytoplasm, providing structural support, cell motility, and transport of materials.

• Microtubules: Thickest fibers; made of tubulin; maintain cell shape, guide organelle movement, separate chromosomes during cell division.

• Microfilaments: Thinnest fibers; made of actin; support cell shape, involved in muscle contraction and cell movement.

• Intermediate filaments: Middle diameter; provide mechanical support for the cell.

Comparison Table: Cytoskeletal Elements

Type	Protein Subunit	Main Functions
Microtubules	Tubulin	Cell shape, organelle movement, chromosome separation
Microfilaments	Actin	Cell shape, muscle contraction, cell motility
Intermediate Filaments	Various proteins (e.g., keratin)	Cell shape, anchorage of organelles

Summary of Key Terms

• Organelle: Specialized structure within a cell that performs a specific function.

• Endosymbiont theory: Theory that mitochondria and chloroplasts originated as prokaryotic cells engulfed by a host cell.

• Phagocytosis: Process by which a cell engulfs particles to form an internal compartment.

• Autophagy: Process by which cells recycle their own organelles and macromolecules.

Additional info: The notes reference a YouTube video for further review of the endomembrane system, which may provide visual reinforcement of these concepts.