Cellular Structures: Composition, Function, and Methods

Methods for Studying Cellular Composition and Function

Biochemical Methods: To understand the composition and function of cellular structures.

• Cell Fractionation: Homogenization and deep freezing.
• Chromatography and Electrophoresis: Used to separate macromolecules.

Isotopic Methods: Incorporating radioactive isotopes into molecules to track their evolution within the cell, followed by autoradiography. Fluorescent markers are also used.

Cytosol and Cytoplasmic Inclusions

Cytosol: The internal liquid medium composed of water, salts, organic molecules, and macromolecules.

Cytoplasmic Inclusions: Accumulations of substances in eukaryotic and prokaryotic cells. Functions include reserve storage (e.g., glycogen and fat in animal cells, vegetable oils and latex in plant cells), pigments, and precipitated proteins.

Ribosomes

Prokaryotes (70S):

• Large subunit (50S): 5S and 23S rRNA
• Small subunit (30S): 16S rRNA

Eukaryotes (80S):

• Large subunit (60S): 5S, 28S, and 5.8S rRNA
• Small subunit (40S): 18S rRNA

Extracellular Matrix

The extracellular matrix is a hydrated, gelatinous biological cement that holds cells together in animal tissues. It consists of an amorphous ground substance, adhesion proteins, and structural proteins.

Plant Cell Wall

The plant cell wall is a thick cover located on the plasma membrane, formed by cellulose fibers and a matrix of polysaccharides and proteins. Specialized cells may contain lignin, cutin, and suberin.

Layers of the Plant Cell Wall:

• Middle Lamella: Common to adjacent cells, formed by pectins.
• Primary Wall: Formed after the middle lamella, composed of cellulose and matrix. It is plastic, with fibers arranged in all directions.
• Secondary Wall: Found only in certain cells. It is very thick, rigid, and impermeable due to impregnation. This tissue is considered "dead."

Functions of the Plant Cell Wall:

• Mechanical protection
• Support for plant cells
• Osmotic protection to maintain cell turgor
• Skeletal function to keep plants upright
• Waterproofing to prevent water loss
• Orientation of cell and tissue growth
• Protection against insects, pathogens, and undesirable substances

Intracellular Junctions

1. Occluding Junctions: Transmembrane protein bands that contact adjacent cells.
2. Zonula Adherens: Connect actin filaments of adjacent cells using transmembrane proteins.
3. Desmosomes: Connect intermediate filaments of adjacent cells with transmembrane proteins (cadherins).
4. Hemidesmosomes: Join the basal part of the cell to the extracellular matrix.
5. Gap Junctions: Points of direct communication between adjacent cells.