Introduction to Eukaryotic Cells: Structure, Evolution, and Transport Mechanisms CHAPTER 4

Introduction to Eukaryotic Cells

Overview of Eukaryotes

Eukaryotic cells are a fundamental topic in microbiology, distinguished by their complex structure and evolutionary history. This section introduces the key features of eukaryotic cells, their evolutionary origins, and the main mechanisms by which they transport materials.

• Eukaryotic cells are found in plants, animals, protists, and fungi.

• They are generally larger and more complex than prokaryotic cells.

• Eukaryotes possess a defined nucleus and multiple membrane-bound organelles.

• Cell division in eukaryotes can occur via mitosis or meiosis.

How Eukaryotes Evolved

Endosymbiotic Theory

The endosymbiotic theory explains the origin of eukaryotic cells as a result of symbiotic relationships between ancient prokaryotes. This theory is supported by substantial molecular and structural evidence.

• Endosymbiosis: A symbiotic relationship where one organism lives inside another.

• Key events: An ancient eukaryotic ancestor engulfed certain prokaryotes, which then became organelles.

• Mitochondria evolved from engulfed non-photosynthetic prokaryotes.

• Chloroplasts evolved from engulfed photosynthetic prokaryotes (e.g., cyanobacteria).

• Prokaryotes evolved ~3.5 billion years ago; eukaryotes evolved ~2.5 billion years ago.

Evidence for Endosymbiotic Theory:

• Organelles (mitochondria and chloroplasts) have their own circular DNA.

• Presence of 70S ribosomes (similar to bacteria).

• Double-membrane structures.

• Similar size to bacteria.

• Ability to replicate by binary fission.

• Genes resembling certain bacterial genes.

Comparing Eukaryotic and Prokaryotic Cells

Structural and Genetic Differences

Eukaryotic and prokaryotic cells differ in several key aspects, including their size, complexity, and genetic organization.

Characteristic	Eukaryotic Cells	Prokaryotic Cells
Organisms	Plants, animals, protists, fungi	Bacteria, archaea
Cell size	Larger, more complex	Smaller, simpler
Genome	Larger, multiple linear chromosomes	Smaller, single circular chromosome
Nucleus	Present (defined)	Absent
Membrane-bound organelles	Present (e.g., mitochondria, chloroplasts)	Absent
Ribosomes	80S	70S

Membrane-Bound Organelles

Key Features of Eukaryotic Cells

Eukaryotic cells are characterized by the presence of a nucleus and various membrane-bound organelles, which compartmentalize cellular functions.

• Nucleus: Contains the cell's genetic material (DNA).

• Mitochondria: Site of cellular respiration and energy production.

• Chloroplasts (in plants and algae): Site of photosynthesis.

• Other organelles include the endoplasmic reticulum, Golgi apparatus, and lysosomes.

Cell Division in Eukaryotes

Modes of Reproduction

Eukaryotic cells can reproduce sexually or asexually. Cell division is more complex in eukaryotes due to their larger genome and organelles.

• Before division, the cell must replicate its genetic material.

• Cell division is longer in eukaryotes than in prokaryotes.

• Eukaryotes can undergo mitosis (asexual) or meiosis (sexual).

Mitosis

• Produces two genetically identical offspring from one parent cell.

• Offspring cells maintain the same number of chromosomes as the parent cell.

• Used for growth, repair, and asexual reproduction.

Meiosis

• Involved in sexual reproduction.

• Consists of two cell division stages.

• One parent cell produces four gametes (daughter cells).

• Gametes are haploid (contain half the chromosome number).

• Crossing over during meiosis allows for genetic recombination.

Comparison Table: Mitosis, Meiosis, Binary Fission

Process	Number of Divisions	Number of Daughter Cells	Genetic Identity	Chromosome Number
Mitosis	1	2	Identical to parent	Diploid (2n)
Meiosis	2	4	Genetically unique	Haploid (n)
Binary Fission	1	2	Identical to parent	Usually haploid (single chromosome)

Transport Mechanisms in Eukaryotic Cells

Endocytosis and Exocytosis

Eukaryotic cells use specialized transport mechanisms to move substances across their plasma membrane. These include endocytosis (import) and exocytosis (export).

• Endocytosis: Imports substances into the cell by engulfing them in vesicles.

• Exocytosis: Exports substances out of the cell by fusing vesicles with the plasma membrane.

Types of Endocytosis

• Pinocytosis ("cell drinking"): Endocytosis of dissolved substances in small vesicles.

• Phagocytosis ("cell eating"): Endocytosis of undissolved substances, such as particles or cells.

• Receptor-mediated endocytosis: Specific ligands bind to cell-surface receptors, triggering vesicle formation.

Phagocytosis Process

• Specialized immune cells (e.g., macrophages) engulf targets.

• Engulfed target is enclosed in a phagosome.

• Phagosome fuses with a lysosome containing hydrolytic enzymes.

• Fusion forms a phagolysosome, where enzymes digest the target.

• Waste products are expelled from the cell.

Note: Some pathogens can escape the phagolysosome or neutralize its enzymes, allowing them to survive and cause disease.

Receptor-Mediated Endocytosis

• Ligands (e.g., hormones, nutrients, pathogens) bind to specific cell-surface receptors.

• Inner surface of the plasma membrane is coated with clathrin.

• Clathrin polymerizes, forming a pit.

• Clathrin-coated pit pinches off, forming a clathrin-coated vesicle.

• Vesicle sheds its clathrin coat and fuses with an endosome.

• Altered pH separates ligand and receptor; both are sorted and delivered.

Key Terms and Definitions

• Eukaryote: An organism whose cells contain a nucleus and membrane-bound organelles.

• Prokaryote: An organism whose cells lack a nucleus and membrane-bound organelles.

• Endosymbiosis: A symbiotic relationship where one organism lives inside another.

• Mitosis: Cell division resulting in two genetically identical daughter cells.

• Meiosis: Cell division resulting in four genetically unique gametes.

• Endocytosis: Cellular process of importing substances by engulfing them in vesicles.

• Exocytosis: Cellular process of exporting substances by vesicle fusion with the plasma membrane.

• Phagocytosis: Endocytosis of large particles or cells.

• Pinocytosis: Endocytosis of dissolved substances.

• Receptor-mediated endocytosis: Endocytosis triggered by ligand-receptor binding.

Relevant Equations

• Binary Fission (Prokaryotes): $N_t = N_0 imes 2^n$ Where $N_t$ is the number of cells at time $t$, $N_0$ is the initial number of cells, and $n$ is the number of generations.

• Chromosome Number in Mitosis: $2n ightarrow 2n$ (Diploid parent produces diploid offspring)

• Chromosome Number in Meiosis: $2n ightarrow n$ (Diploid parent produces haploid gametes)

Example Applications

• Immune Response: Macrophages use phagocytosis to destroy pathogens.

• Hormone Uptake: Cells use receptor-mediated endocytosis to internalize hormones and nutrients.

• Cell Growth: Mitosis allows multicellular organisms to grow and repair tissues.

• Genetic Diversity: Meiosis and crossing over increase genetic variation in sexually reproducing organisms.

Additional info: Some context and definitions have been expanded for clarity and completeness.