Cell and Tissue Form: Structure, Function, and Interactions: Ch.10

Chapter 10: Cell and Tissue Form

10.1 Tissues and Organs

Tissues and organs are communities of cells that perform specific functions. The organization of cells into tissues and organs is essential for multicellular life, allowing for division of labor and specialization.

• Tissue: A group of cells that work together to perform a specific function (e.g., muscle tissue, nervous tissue).

• Organ: A structure composed of multiple tissue types working together (e.g., heart, skin).

• Example: The skin is an organ composed of epithelial tissue (epidermis), connective tissue (dermis), and other specialized cells.

The structure of skin relates to its function: The skin's layered structure provides a barrier to pathogens, prevents water loss, and supports thermoregulation. The epidermis consists of tightly packed epithelial cells, while the dermis contains connective tissue, blood vessels, and nerves.

10.2 The Cytoskeleton

The cytoskeleton is a network of protein filaments that provides structural support, determines cell shape, and enables movement within and by cells. It is composed of three main types of protein filaments:

• Microfilaments (Actin Filaments): Thin filaments involved in cell shape, movement, and muscle contraction.

• Intermediate Filaments: Provide mechanical strength and maintain cell integrity (e.g., keratin in skin cells).

• Microtubules: Hollow tubes that organize cell contents, separate chromosomes during cell division, and serve as tracks for motor proteins.

Microfilaments, intermediate filaments, and microtubules are polymers of protein subunits:

• Microfilaments are composed of actin monomers.

• Intermediate filaments are made of various proteins (e.g., keratins, lamins).

• Microtubules are composed of tubulin dimers (α- and β-tubulin).

Microfilaments and microtubules are dynamic structures: They can rapidly assemble and disassemble by adding or removing subunits, allowing cells to change shape and move.

Motor proteins associate with microfilaments and microtubules to cause movement: Motor proteins such as myosin (actin filaments), kinesin, and dynein (microtubules) convert chemical energy from ATP into mechanical work, moving vesicles, organelles, and even entire cells.

Cytoskeletal Element	Subunit	Major Function
Microfilament	Actin	Cell shape, movement, muscle contraction
Intermediate filament	Various (e.g., keratin)	Mechanical strength, cell integrity
Microtubule	α- and β-tubulin	Organelle movement, chromosome separation, cilia/flagella movement

10.3 Cell Junctions

Cell junctions are specialized structures that connect cells to one another and to the extracellular matrix, providing structural integrity and enabling communication.

• Cell adhesion molecules: Proteins that allow cells to attach to each other and to the extracellular matrix (e.g., cadherins, integrins).

• Anchoring junctions: Connect adjacent cells and are reinforced by the cytoskeleton (e.g., desmosomes, adherens junctions).

• Tight junctions: Prevent the movement of substances through the space between cells, maintaining distinct compartments (e.g., in epithelial layers).

• Communicating junctions (Gap junctions): Allow molecules and ions to pass directly between cells, enabling rapid communication.

Cell Junction	Main Component	Cytoskeletal Attachment	Primary Function
Anchoring	Cadherin, Integrin	Actin, Intermediate filaments	Cell-cell and cell-matrix adhesion
Tight	Claudin, Occludin	Actin	Barrier (epithelial boundary)
Gap	Connexin	None	Cell-cell communication

10.4 The Extracellular Matrix (ECM)

The extracellular matrix is a complex network of proteins and polysaccharides secreted by cells that provides structural support and mediates cell signaling.

• In plants: The ECM is primarily the cell wall, composed of cellulose, hemicellulose, and pectin, providing rigidity and protection.

• In animals: The ECM is abundant in connective tissues and is composed of proteins such as collagen, elastin, and proteoglycans. It provides tensile strength, elasticity, and mediates cell signaling.

Altered cell adhesion proteins allow cancer cells to spread throughout the body: Changes in cell adhesion molecules can enable cancer cells to detach from the primary tumor and invade other tissues (metastasis).

Extracellular matrix proteins influence cell shape and gene expression: The ECM can affect cell behavior by interacting with cell surface receptors, influencing gene expression and cell differentiation.

Summary Table: Types and Functions of Cell Junctions

Cell Junction	Main Component	Cytoskeletal Attachment	Primary Function
Anchoring	Cadherin, Integrin	Actin, Intermediate filaments	Cell-cell and cell-matrix adhesion
Tight	Claudin, Occludin	Actin	Barrier (epithelial boundary)
Gap	Connexin	None	Cell-cell communication

Key Equations and Concepts

• Polymerization of cytoskeletal filaments: $\text{n (monomer)} \rightarrow (\text{monomer})_n$

• Motor protein movement: $\text{ATP} \rightarrow \text{ADP} + \text{P}_i$ (energy released powers movement)

Self-Assessment Questions (Examples)

• What are the four tissue types in animals?

• How does the structure of the epidermis of the skin relate to its function?

• What are the three major cytoskeletal elements? For each, indicate the subunits they are composed of and their functions.

• What are the main types of cell junctions, and how do they contribute to tissue structure and function?

• What is the extracellular matrix, and how does it differ in plants and animals?

Additional info: This summary integrates and expands upon the provided material to ensure clarity and completeness for exam preparation.