Cytoskeleton, Mitochondria, Chloroplasts, Extracellular Matrix, and Peroxisomes

Introduction

This study guide covers the structure and function of key eukaryotic cell components, including the cytoskeleton, mitochondria, chloroplasts, extracellular matrix, and peroxisomes. Understanding these organelles and structures is essential for grasping how cells maintain organization, generate energy, and interact with their environment.

Vocabulary

• Polymers: Large molecules composed of repeating subunits (monomer, dimer, tetramer, etc.).

• Progeria: A genetic disorder characterized by accelerated aging.

• Centriole: Cylindrical cell structure involved in cell division and organization of microtubules.

• Centrosome: Organelle serving as the main microtubule organizing center in animal cells.

• Pseudopodia/Pseudopodium: Temporary, foot-like extensions of the cell membrane used for movement and feeding.

• Cilia: Short, hair-like structures that aid in cell movement or fluid movement across the cell surface.

• Flagella: Long, whip-like structures used for cell locomotion.

• Junctions: Specialized regions where cells connect to each other (including plasmodesmata in plants).

Learning Outcomes

• Define and illustrate the main structural and functional characteristics of cytoskeletal elements, mitochondria, chloroplasts, extracellular matrix, and peroxisomes.

• Describe the normal function and predict consequences of organelle malfunction.

• Recognize cytoskeletal elements (microtubules, microfilaments, intermediate filaments) and motor proteins; define basic intracellular traffic mechanisms and their regulation.

• Relate the structure of mitochondria, chloroplasts, and peroxisomes to their function.

• Illustrate extracellular matrix components, their origin, and how they crosstalk with cellular components.

Mitochondria and Chloroplasts

General Features

• Energy transformers of the cell: Mitochondria and chloroplasts convert energy into forms usable by the cell.

• Membrane-bound: Both are enclosed in membranes but are not part of the endomembrane system.

• Semiautonomous: They grow and reproduce within the cell and contain their own DNA.

• Dynamic location: Their position within the cell is not fixed; they move along cytoskeletal tracks.

Mitochondria

Mitochondria are present in all eukaryotic cells and are the primary site of ATP production through cellular respiration.

• Structure:

  • Double membrane: Outer and inner membranes.

  • Inner membrane folds (cristae) increase surface area for ATP production.

  • Matrix: Contains enzymes for the Krebs cycle.

  • Own circular DNA and ribosomes.

• Function:

  • Site of aerobic respiration: Converts glucose and oxygen into ATP.

  • Key enzymes for the Krebs cycle and oxidative phosphorylation are located in the matrix and inner membrane.

Equation for Cellular Respiration:

Chloroplasts

Chloroplasts are found in leaves and other green structures of plants and eukaryotic algae. They are the site of photosynthesis.

• Structure:

  • Double membrane: Outer and inner membranes.

  • Thylakoid membranes: Flattened sacs where light-dependent reactions occur.

  • Stroma: Fluid-filled space containing enzymes for the Calvin cycle.

  • Own circular DNA and ribosomes.

• Function:

  • Convert photons of light into sugar and oxygen via photosynthesis.

Equation for Photosynthesis:

The Endosymbiont Theory

The endosymbiont theory explains the origin of mitochondria and chloroplasts as formerly free-living prokaryotes that were engulfed by ancestral eukaryotic cells.

• Both organelles have double membranes, circular DNA, and ribosomes similar to prokaryotes.

• They reproduce independently within the cell by binary fission.

• They are not part of the endomembrane system.

Example: Photosynthetic cyanobacteria are thought to be the ancestors of chloroplasts.

Cytoskeletal Elements

Types and Functions

• Microfilaments (Actin Filaments):

  • Composed of actin protein.

  • Support the plasma membrane and are involved in cell movement and shape changes.

• Microtubules:

  • Hollow tubes made of tubulin protein.

  • Originate from the microtubule organizing center (MTOC), such as the centrosome.

  • Serve as tracks for motor proteins (e.g., kinesin, dynein) to transport organelles and vesicles.

• Intermediate Filaments:

  • Provide mechanical strength to cells.

  • Examples include keratin and vimentin.

Motor Proteins

• Kinesin and Dynein: Move along microtubules to transport cellular cargo.

• Myosin: Moves along actin filaments, important in muscle contraction and cell movement.

Extracellular Matrix (ECM)

Structure and Function

• Composed of proteins (e.g., collagen, elastin) and polysaccharides.

• Provides structural support to cells and tissues.

• Facilitates cell signaling and adhesion.

• ECM components are secreted by cells and interact with cell surface receptors.

Peroxisomes

Structure and Function

• Small, membrane-bound organelles.

• Contain enzymes for oxidation reactions, including breakdown of fatty acids and detoxification of hydrogen peroxide.

• Generate and degrade hydrogen peroxide () via catalase enzyme.

Comparison Table: Mitochondria vs. Chloroplasts

Summary

• Mitochondria and chloroplasts are essential for energy conversion in eukaryotic cells and have unique origins explained by the endosymbiont theory.

• The cytoskeleton provides structural support, enables movement, and facilitates intracellular transport.

• The extracellular matrix connects cells and tissues, influencing cell behavior and communication.

• Peroxisomes play a key role in cellular metabolism and detoxification.