Cell Ultrastructure

Overview of Eukaryotic Cell Components

The eukaryotic cell contains various specialized structures called organelles, each performing distinct functions necessary for cellular life. The cell is bounded by the plasma membrane and contains a centrally located nucleus surrounded by cytoplasm filled with organelles.

• Plasma Membrane: A phospholipid bilayer that controls the entry and exit of substances, maintaining cellular integrity.

• Nucleus: Contains genetic material (DNA) and is the control center for cellular activities.

• Cytosol: The fluid portion of the cytoplasm where metabolic reactions occur.

• Organelles: Specialized structures such as mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, and peroxisomes.

Cellular Organelles: Structure and Function

Nucleus

The nucleus is the largest organelle, responsible for storing genetic information and coordinating cellular activities.

• Nuclear Envelope: Double membrane with nuclear pores for exchange of materials.

• Chromatin: Unspooled DNA and associated proteins; condenses to form chromosomes during cell division.

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

Mitochondria

Mitochondria are the cell's powerhouses, specialized for ATP synthesis through aerobic respiration.

• Structure: Double membrane; inner membrane forms cristae, surrounding the matrix.

• Function: ATP production via oxidative phosphorylation.

• Mitochondrial DNA: Contains its own DNA, inherited maternally.

• Endosymbiotic Theory: Suggests mitochondria originated from prokaryotic cells engulfed by ancestral eukaryotes.

Ribosomes

Ribosomes are the sites of protein synthesis, composed of small and large subunits.

• Free Ribosomes: Float in cytosol; synthesize proteins for intracellular use.

• Bound Ribosomes: Attached to rough endoplasmic reticulum (RER); synthesize proteins for secretion or membrane insertion.

Endoplasmic Reticulum (ER)

The endoplasmic reticulum is a network of membranes involved in protein and lipid synthesis.

• Rough ER (RER): Studded with ribosomes; synthesizes and folds proteins.

• Smooth ER (SER): Lacks ribosomes; synthesizes lipids and detoxifies chemicals.

Golgi Apparatus

The Golgi apparatus modifies, sorts, and packages proteins and lipids for delivery to their destinations.

• Structure: Stack of flattened cisternae.

• Function: Processes and packages proteins/lipids from ER; synthesizes carbohydrates.

Lysosomes

Lysosomes are membrane-bound vesicles containing digestive enzymes (acid hydrolases).

• Function: Breakdown of waste, cellular debris, and worn-out organelles.

Peroxisomes

Peroxisomes contain enzymes for detoxification and lipid metabolism.

• Function: Neutralize toxic substances (e.g., hydrogen peroxide), break down fatty acids, and synthesize some phospholipids.

The Endomembrane System

Definition and Function

The endomembrane system is a group of membranes and organelles in eukaryotic cells that work together to modify, package, and transport lipids and proteins.

• Includes: Nuclear envelope, ER, Golgi apparatus, transport vesicles, lysosomes, endosomes, and plasma membrane.

• Functions as a series of compartments for packaging, labeling, and shipping proteins and molecules.

Key Components of the Endomembrane System

The Cytoskeleton

Structure and Function

The cytoskeleton is a network of protein filaments that provides structural support, determines cell shape, and enables movement.

• Microfilaments (Actin Filaments): Thin filaments involved in cell movement and shape; abundant in muscle cells.

• Intermediate Filaments: Provide mechanical strength; composition varies (e.g., keratin in skin, hair).

• Microtubules: Hollow tubes made of tubulin; involved in organelle movement, cell division, and forming cilia/flagella.

Role in Cellular Motion

• Allows movement of whole cells (e.g., muscle contraction, cell crawling).

• Facilitates specialized movement of organelles within cells (e.g., mitosis, vesicle transport).

• Motor proteins (e.g., kinesin, dynein) move along cytoskeletal tracks.

Cellular Extensions: Centrioles, Microvilli, Cilia, and Flagella

Centrioles

Centrioles are cylindrical structures composed of microtubules, located near the nucleus in the centrosome.

• Structure: Ring of nine groups of three microtubules.

• Function: Organize microtubules during cell division; form basal bodies for cilia and flagella.

Microvilli

Microvilli are finger-like extensions of the plasma membrane that increase surface area for absorption.

• Structure: Supported by actin filaments.

• Function: Enhance absorptive capacity, especially in intestinal and kidney tubule cells.

Cilia

Cilia are hair-like projections from the cell surface, involved in movement or sensory functions.

• Primary Cilia: Usually one per cell; sensory function (e.g., monitor fluid flow).

• Motile Cilia: Multiple per cell; beat in coordinated waves to move substances (e.g., mucus in respiratory tract).

• Structure: Axoneme with a 9+2 arrangement of microtubules.

Cystic Fibrosis: Caused by defective chloride channel protein, leading to thick mucus and impaired ciliary function.

Primary Ciliary Dyskinesia: Genetic disorder affecting ciliary movement, resulting in respiratory and reproductive issues.

Flagella

Flagella are long, whip-like extensions that propel cells (e.g., sperm cells).

• Structure: Similar axoneme to cilia (9+2 microtubule arrangement), but longer and stiffer.

• Function: Enables motility in certain cells.

Pseudopods

Pseudopods are temporary, actin-filled extensions used by cells like macrophages for movement and engulfing particles.

• Function: Cell crawling and phagocytosis.

Summary Table: Cellular Extensions

Key Equations and Concepts

• ATP Synthesis (Mitochondria):

• Endosymbiotic Theory: Mitochondria and chloroplasts originated from free-living prokaryotes engulfed by ancestral eukaryotic cells.

Additional info:

• Motor proteins such as kinesin and dynein are essential for intracellular transport along microtubules.

• Defects in cytoskeletal components can lead to diseases affecting cell motility, division, and structure.