Cell production system

Ribosomes

Ribosomes are small electron-dense particles measuring 20 x 30 nm.

• composed of a small (40S) and a large (60S) subunit
• from 4 types of rRNAs and about 80 different proteins
• two classes of ribosomes:

1. of prokaryotes, chloroplasts and mitochondrias, and
2. of eukaryote

• are formed in the nucleolus, their proteins are synthesized in the cytoplasm
• strongly basophilic → react with basic dyes
• occur either as individual granules or in clusters = polyribosomes (held together by mRNA)
• proteins formed for the cell's own use are synthesized on unbound polyribosomes
• the association of rRNA and ribosome occurs in the nucleus, complete ribosomal subunits travel out of the nucleus through nuclear pores

Endoplasmic reticulum

• the site of synthesis of lipids and saccharides
• segregation of proteins from the cytoplasm and initial posttranslational modification (preparation for specific functions) occurs here
• in fully differentiated cells there are 2 types of endoplasmic reticulum:

1. rough ER
2. smooth ER

Rough endoplasmic reticulum (GER)

• also granular; rought
• in cells specialized for protein secretion (pancreatic, fibroblasts, plasma cells)
• 'composed of' tubules and parallel clustered flat cisternae formed by membranes sometimes associated with the outer envelope of the nucleus
• the presence of ribosomes and polyribosomes on the cytoplasmic surface of the reticulum membrane, which give it a granular appearance
• Ribosomes are responsible for the basophilic properties
• the main task is the segregation of proteins for export or transfer to other cytoplasmic components
  • initial glycosylation of glycoproteins containing nitrogenous oligosaccharides, synthesis of phospholipids, post-translational modification of newly formed polypeptides
  • proteins synthesized in the GER have different destinations, may remain in the cytoplasm or be segregated from it and participate in different cellular activities

Smooth endoplasmic reticulum (HER)

• also agranular; smooth
• also forms the intracellular network
• without associated ribosomes
• cisternae are tubular; huge number of interconnected channels of various shapes and sizes
• membranes of HER arise from rough ER → often merge into each other
• synthesis of lipids, phospholipids, carbohydrates and steroid hormones
• sarcoplasmic reticulum: a specialized form of the HER that is involved in the process of muscle cell contraction (reservoir of calciums)
• often in steroid hormone synthesizing cells (ovary, testis, adrenal gland), and in hepatocytes (detoxification reaction)

Golgi complex (GK)

• determines post-translational modification of products synthesized by the cell, envelops them and labels them with the destination address
• of three distinct compartments bounded by a smooth membrane:

1. mildly curved sheathed cisternae,
2. a large number of small pouches and
3. several larger vacuoles located at one pole of the GC

• functional connection between the ER and the rest of the cell (difficult to determine its boundaries)
• ER → newly formed protein → GK cisternae (the nearest cisterna is called the production = convex = cis region), in the trans region (concave = matric) vesicles gather → condensing vacuoles (these structures bud from Golgi cisternae and split off as vesicles transporting proteins to various sites)
• plays an important role in glycosylation, sulfation, phosphorylation and selective proteolysis of proteins

Links

Literature used

• JUNQUIERA, L. Carlos – CARNEIRO, José – KELLEY, Robert O.. Fundamentals of Histology. 1. edition. Jinočany : H & H 1997, 1997. 502 pp. ISBN 80-85787-37-7.