BIOLOGY Lecture Notes - Golden Pages 2003

1. THE CELL

Non-Membranous Cell Structures

Class="center">Cytoskeleton

It is represented by microtubules and microfilaments, which form a system that ensures the mobility of cytoplasmic and intracellular membrane structures (Intracellular Transport and transcytosis), regulates Cell shape, and participates in mitosis.

Microtubules are fine cylinders measuring 20-30 nm in diameter with a wall thickness of 4.5-7.0 nm.

Microtubules are linear polymers composed of tubulin glycoprotein molecules existing as αβ-dimers. Tubulin molecules, arranged in a helical pattern, form longitudinal parallel protofilaments that make up the cylinder wall. Microtubules constantly undergo assembly and disassembly. Their formation occurs via self-assembly and requires GTP energy (derived from the Hydrolysis of guanosine triphosphate).

A crucial function of microtubules is The regulation of intracellular component translocation, such as chromosome movement during Cell Division. During division, the spindle fibers—bundles of microtubules—diverge toward opposite poles of The Cell.

The movement of newly synthesized material from the rough Endoplasmic reticulum to the Golgi apparatus is directed by microtubules, which are essential for coordinating transport during secretory processes.

Conversely, translocation from the periphery to the inner Regions of the cell during phagocytosis, pinocytosis, and cortical-type intracellular movement (axoplasmic flow) is governed by microfilaments.

In addition to fine filaments formed by the protein Actin and actin-binding Proteins (Myosin, spectrin, and dynein), the microfilament system also includes Intermediate filaments. The proteins of these filaments exhibit tissue-specific differences: epithelial Cells contain cytokeratins, Nerve Cells contain neurofilament proteins, Muscle cells contain desmin and skeletin, and glial cells contain glial fibrillary acidic protein.

Microfilament systems, together with microtubules, form numerous contacts with The cell membrane. The cytoskeletal systems within the cell direct the movement of glycoprotein molecules along the membrane while simultaneously restricting the mobility of Membrane Proteins. When a cell is treated with a microtubule inhibitor, membrane cell receptors cluster at a specific site on the membrane to form so-called "caps"; subsequent Treatment with a microfilament inhibitor causes these "caps" to disperse into diffuse "clusters." Similar phenomena are observed during the interaction of membrane Glycoproteins with Antigens, indicating a functional link between Membrane Receptors and the cell's cytoskeletal system.

The cytoskeleton determines cell shape, The ability to adhere to other cells, freedom of movement, and The transport of various substances into and out of the cell.

An analysis of various types of cell motility provides insight into how a cell's shape is determined by its movement pattern.

Many cells are capable of swimming. This type of movement is driven by flagella and cilia—outward cellular appendages containing a microtubule-based axoneme. The driving force is provided by the ATPase activity of the microtubule-associated protein dynein. The shape of swimming cells is maintained by submembranous microtubules radiating from a single point, such as a pair of basal bodies.

Leukocyte macrophages exhibit amoeboid movement. The shape of an amoeboid cell results from the local contractile activity of microfilaments, and its movement is similarly microfilament-dependent and regulated by the cell membrane.

Extracellular matrix fibroblasts perform fibroblast-like movement, which relies on microtubules. The fibroblast cytoskeleton maintains or alters cell shape, assists in cell spreading on a substrate, drives active movement and polarization, and generates cellular tension.

Mammalian erythrocytes maintain their shape through a submembranous cytoskeletal network composed of actin and spectrin proteins.

The platelet cytoskeleton participates in shape changes during rest and activation, and ensures their adhesion to various surfaces.

To date, a wealth of data has accumulated regarding the involvement of the cytoskeleton in mitosis, Gene Expression, and cellular transformation.