BIOLOGY Lecture Notes - Golden Pages 2003

1. THE CELL

Cellular Organelles

Class="center">Endoplasmic reticulum

Structure of The endoplasmic reticulum. Under a Light Microscope, the endoplasmic reticulum (endo — inside, reticulum — network) appears as a network.

The membranes of the endoplasmic reticulum consist primarily of Lipoproteins.

Within The Cell, the endoplasmic reticulum is composed of a system of membrane-bound channels represented by flattened "cisternae" and tubules communicating with each other. Upon tissue homogenization, the endoplasmic reticulum is disrupted, and the membranes reseal to form small vesicles known as microsomes.

Ribosomes may be attached to The surface of the endoplasmic reticulum. In this case, the endoplasmic reticulum is referred to as rough.

The surface of the rough endoplasmic reticulum membranes contains receptor Proteins for ribosomes, whose function is to bind ribosomes to the endoplasmic reticulum.

Membranes of the endoplasmic reticulum lacking ribosomes are termed smooth.

The rough endoplasmic reticulum consists of A large number of cisternae arranged parallel to one another and interconnected in such a way that the cell forms a communication system. Through this system, proteins synthesized on the membranes of the endoplasmic reticulum can be transported either to other PARTS OF THE cell or to the extracellular environment.

Upon completion of Protein Synthesis, the polypeptide chain exits the ribosome through pores formed by the receptor protein and enters the cisternae of the endoplasmic reticulum. There, the secondary and tertiary structures of the protein spontaneously form. The decision of whether the newly synthesized protein will remain in the Cytoplasm or enter the ER-Golgi system is made during protein synthesis (Translation), depending on the presence or absence of a "signal" Amino Acid Sequence within that protein.

The membranes of the smooth endoplasmic reticulum are also involved in the synthesis of various substances: they mediate the synthesis of Steroid Hormones and triglycerides, Glycogen METABOLISM, and the glucuronic acid cycle, which is associated with toxin degradation.

In the membrane of the smooth endoplasmic reticulum (for example, in Liver Cells), there is an electron transport system necessary for The oxidation of xenobiotics. During detoxification processes, the surface area of the smooth endoplasmic reticulum membrane increases tenfold. During detoxification, xenobiotics—typically poorly Water-soluble—are oxidized into water-soluble products that are easily eliminated from the Organism. This system contains cytochrome Enzymes B₅, P₄₅, and Flavoproteins.

One of the Functions of the smooth endoplasmic reticulum in Muscle cells, known as the sarcoplasmic reticulum, is the uptake of Calcium Ions, which are released into the cytoplasm (sarcoplasm) during muscle fiber contraction in response to a stimulus.

It should be noted that the smooth endoplasmic reticulum is highly developed in cells that transport ions (for example, in the glandular Cells of the gastric mucosa).

STRUCTURE OF THE Golgi Complex

A Golgi "stack" consists of flattened, membrane-bound structures called cisternae, associated with various types of vesicles (often described as a "stack of pancakes").

The number of Golgi apparatuses per cell depends on the degree of differentiation. Vesicles are formed at the cisternae by budding off from them. Since the cisternae have a curved (cup-like) shape, two distinct faces of the Golgi apparatus are recognized:

1) the concave, inner face (the trans or secreting pole) facing the cytoplasm;

2) the convex face (the cis or forming pole) facing the Endoplasmic reticulum and The Nucleus.

Functions of the Golgi Complex

1. At the forming pole, there is an uptake of proteins and Lipids synthesized in the rough and smooth endoplasmic reticulum and functionally associated with the vesicles of the Golgi apparatus.

2. Numerous studies of various Digestive System cells have shown that the Golgi apparatus plays an active role in incorporating carbohydrate moieties into proteins, leading to The formation of various important Glycoproteins (thyroglobulin, immunoglobulin, mucigen) and polysaccharide-Protein Complexes essential for building new cellular structures. The addition of terminal sugars to the protein molecule serves as the trigger for the exocytosis of secretory products.

3. The Golgi apparatus accumulates CARBOHYDRATES in the form of soluble oligomers and uronides, the polymers of which have a high Swelling capacity. These form part of the mucilages secreted by the Golgi apparatus.

4. The Golgi apparatus is involved in completing the formation of lipoproteins by encasing lipids in a membrane and delivering them to specific sites on The Plasma Membrane.

5. Evidence indicates that Golgi enzymes play an active role in the sulfation of secretory products (for example, the formation of mucopolysaccharides in gastrointestinal epithelial cells).

6. The Golgi apparatus is involved in milk secretion processes (incorporation of galactose into glycoproteins, accumulation of lactose).

7. According to some researchers, the Golgi apparatus plays a crucial role in the Synthesis and Secretion of carbohydrate-containing Components of the Extracellular matrix, as well as in The conversion of structural proteins, such as Collagen, into ordered structures.