Chapter 5: Cell Structure and Function

Lysosomes

Lysosomes are membrane-bound organelles found in eukaryotic cells that contain digestive enzymes. They play a crucial role in breaking down macromolecules and cellular debris.

• Primary lysosomes: Contain hydrolytic enzymes and are responsible for digesting macromolecules into monomers.

• Acidic environment: The interior of lysosomes is acidic, which is optimal for enzyme activity.

• Phagocytosis: Food particles are engulfed by the cell and fuse with secondary lysosomes for digestion.

• Dysfunction: If lysosomes stop working, undigested materials accumulate, leading to diseases such as Tay-Sachs disease.

Example: In Tay-Sachs disease, a genetic mutation leads to the absence of a specific lysosomal enzyme, causing harmful substances to build up in nerve cells.

Mitochondria

Mitochondria are known as the "powerhouses" of the cell, responsible for producing ATP through cellular respiration.

• Purpose: Harvest energy from organic molecules (e.g., glucose) via aerobic respiration.

• Structure: Consist of an outer membrane, highly folded inner membrane (cristae), and a matrix.

• Pathways: Involved in metabolic pathways such as the citric acid cycle and oxidative phosphorylation.

Example: The inner membrane's folds increase surface area for ATP production.

Peroxisomes

Peroxisomes are small organelles that contain enzymes for breaking down toxic substances.

• Function: Detoxify harmful compounds and break down hydrogen peroxide (H2O2) using the enzyme catalase.

• Importance: Protect cells from oxidative damage.

Example: Peroxisomes in liver cells help detoxify alcohol.

Cytoskeleton

The cytoskeleton is a dynamic network of protein filaments that provides structural support, maintains cell shape, and facilitates movement.

• Supports and maintains cell shape

• Holds organelles in position

• Movement of organelles

• Involved in cytoplasmic streaming

Components: Microfilaments, microtubules, intermediate filaments

Microtubules

• Structure: Made from alpha and beta tubulin dimers; have a "plus" (+) and "minus" (−) end; grow from the + end.

• Functions:

  • Maintenance of cell shape

  • Compression resistance

  • Cell motility (cilia and flagella)

  • Involved in cell division (spindle fibers)

  • Movement of vesicles within cells

Example: Microtubules form the mitotic spindle during cell division.

Microfilaments (Actin Filaments)

• Structure: Composed of actin protein; also have + and − ends.

• Functions: Maintain cell shape, enable movement, and are involved in cell motility (e.g., muscle contraction, amoeboid movement).

Example: Actin filaments are responsible for the formation of the contractile ring during cytokinesis.

Intermediate Filaments

• Structure: Tough, permanent fibers made of various proteins (e.g., keratin).

• Function: Provide mechanical strength and maintain cell shape.

Example: Intermediate filaments help maintain the integrity of skin cells.

Arrangement and Movement

• Cilia and Flagella: Microtubules are arranged in a "9+2" pattern; dynein motor proteins cause sliding, resulting in movement.

• Vesicle Transport: Motor proteins (e.g., kinesin, dynein) move vesicles along microtubules by attaching and detaching.

Extracellular Matrix (ECM) and Cell Junctions

The extracellular matrix is a network of proteins and carbohydrates outside the cell membrane that provides structural and biochemical support to surrounding cells.

• Functions: Contributes to the structure of skin, bones, and other tissues; regulates the passage of materials; involved in cell signaling.

Cell Junctions

• Tight junctions: Hold cells tightly together, preventing leakage of materials between cells.

• Desmosomes: Act as "rivets" to fasten cells into strong sheets.

• Gap junctions: Allow the passage of ions and small molecules between adjacent cells for communication.

Endosymbiosis Theory

The endosymbiosis theory proposes that mitochondria and chloroplasts originated from free-living prokaryotes that were engulfed by ancestral eukaryotic cells, leading to a symbiotic relationship.

• Evidence: Double membranes, their own DNA, and similarities to prokaryotic cells.

Example: Mitochondria replicate independently of the cell cycle, similar to bacteria.