Eukaryotic Microbial Cell Structure and Function

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Lecture 8: Eukaryotic Microbial Cells

Introduction

This study guide covers the structure and function of eukaryotic microbial cells, focusing on their organelles, cell division processes, motility structures, and the endosymbiotic hypothesis. Understanding these components is essential for appreciating the complexity and diversity of microbial eukaryotes in microbiology.

Eukaryotic Microbial Cell Structure

General Features of Eukaryotic Cells

Membrane-enclosed nucleus: Contains the cell's genetic material and is separated from the cytoplasm by a double membrane.
Organelles: Specialized structures including mitochondria, hydrogenosomes, chloroplasts (in phototrophic cells), Golgi complex, lysosomes, endoplasmic reticulum, microtubules, and microfilaments.
Motility structures: Some eukaryotic microbes possess flagella or cilia for movement.
Cell walls: Present in some eukaryotic microbes, providing structural support.
Sterols: Membrane lipids that lend structural strength to eukaryotic cell membranes.

Nucleus

Structure and Function

Nuclear envelope: Double membrane structure; the inner membrane interacts with the nucleoplasm, and the outer membrane with the cytoplasm.
Nuclear pores: Allow regulated transport of proteins and nucleic acids into and out of the nucleus.
Chromosomes: DNA is wound around histones for compaction and transcriptional regulation.
Nucleolus: Site of ribosomal RNA synthesis, located within the nucleus.
Ploidy: Eukaryotic microbes can be haploid or diploid; prokaryotes are generally haploid.

Eukaryotic Cell Division

Mitosis

Mitosis is the process by which a eukaryotic cell divides to produce two genetically identical daughter cells.

Chromosomes are replicated.
Nucleus is disassembled.
Chromosomes are segregated into two sets.
Nucleus is reassembled in each daughter cell.

Meiosis

Meiosis is a specialized form of nuclear division that reduces the chromosome number by half, producing four haploid cells from one diploid cell. This process is essential for sexual reproduction and the formation of gametes or spores.

Chromosomes are replicated.
First division segregates homologous chromosomes.
Second division resembles mitosis, producing haploid cells.

Comparison: Mitosis vs. Meiosis

Feature	Mitosis	Meiosis
Number of divisions	One	Two
Number of daughter cells	Two	Four
Genetic identity	Identical to parent	Genetically unique
Chromosome number	Same as parent	Half of parent

Energy-Related Organelles

Mitochondria

Mitochondria are the site of respiration and oxidative phosphorylation in aerobic eukaryotes.

Enclosed by a double membrane; outer membrane is permeable, inner membrane is less permeable and highly folded into cristae.
Cristae contain enzymes for respiration and ATP production.
Matrix contains enzymes for the citric acid cycle, a major pathway for the combustion of organic compounds to CO2.
Number per cell varies with cell type and size.

Hydrogenosomes

Hydrogenosomes are found in some anaerobic eukaryotes that lack mitochondria and utilize fermentative metabolism (e.g., Trichomonas).

Similar in size to mitochondria but lack cristae and citric acid cycle enzymes.
Function: Oxidation of pyruvate to H2, CO2, and acetate.
Acetate is excreted; hydrogenosomes cannot oxidize acetate further.
Some anaerobic eukaryotes host methanogenic Archaea that consume H2 and CO2 to produce methane (CH4).

Chloroplasts

Chloroplasts are chlorophyll-containing organelles found in phototrophic eukaryotes, responsible for photosynthesis.

Enclosed by a double membrane; outer membrane is permeable, inner membrane is less permeable.
Inner membrane surrounds the stroma, which contains large amounts of RubisCO (key enzyme of the Calvin cycle).
Thylakoids: Flattened membrane discs containing chlorophyll and other components for photosynthesis.
Thylakoid membrane is highly impermeable and forms a proton motive force for ATP synthesis.

Endosymbiotic Hypothesis

Origin of Mitochondria and Chloroplasts

The endosymbiotic hypothesis proposes that mitochondria and chloroplasts originated from free-living bacteria that established symbiotic relationships with ancestral eukaryotic cells.

Mitochondria descended from respiratory bacteria; chloroplasts from phototrophic bacteria.
Symbiosis provided new energy metabolism capabilities to the host and a stable environment for the bacteria.
Evidence: Both organelles contain their own circular DNA genomes and ribosomes similar to those of bacteria.

Other Eukaryotic Cell Structures

Endoplasmic Reticulum (ER)

Network of membranes continuous with the nuclear envelope.
Rough ER: Contains attached ribosomes; major producer of glycoproteins and membrane material.
Smooth ER: Lacks ribosomes; involved in lipid synthesis and carbohydrate metabolism.

Golgi Complex

Stacks of cisternae that function with the ER.
Chemically modifies (e.g., glycosylation) and sorts ER products for secretion or use in other membranous structures.

Lysosomes

Membrane-enclosed compartments containing digestive enzymes.
Hydrolyze food and recycle damaged cell components.
Separate lytic activities from the cytoplasm; nutrients released are used by cytoplasmic enzymes.

Cytoskeleton

Microtubules: Hollow tubes (25 nm diameter) composed of α- and β-tubulin; maintain cell shape, motility, and move chromosomes and organelles.
Microfilaments: Polymers of actin (7 nm diameter); maintain/change cell shape, involved in amoeboid motility and cell division.
Intermediate filaments: Keratin protein fibers (8-12 nm diameter); maintain cell shape and position organelles.

Motility Structures: Flagella and Cilia

Structure and Function

Present on the surface of many eukaryotic microbes; function as organelles of motility.
Cilia: Short flagella that beat in synchrony to propel the cell.
Eukaryotic flagella: Structurally distinct from prokaryotic flagella; do not rotate but use a whiplike motion.

Microtubule Arrangement

Both flagella and cilia contain a bundle of nine pairs of microtubules surrounding a central pair (9+2 arrangement).
Dynein: Motor protein attached to microtubules; uses ATP to drive motility via coordinated sliding of microtubules, resulting in a whiplike motion.

Comparison: Eukaryotic vs. Prokaryotic Motility Structures

Feature	Eukaryotic Flagella/Cilia	Prokaryotic Flagella
Structure	9+2 microtubule arrangement, dynein motor	Flagellin protein, helical filament
Movement	Whiplike motion	Rotation
Energy source	ATP hydrolysis	Proton motive force

Key Equations

Citric Acid Cycle (simplified):

Calvin Cycle (simplified):

Additional info: Some context and details were inferred to provide a complete, self-contained study guide suitable for college-level microbiology students.