Cytoplasm

Ribosomes

Ribosomes are composed of rRNA and protein. They consist of large (60S) and small (40S) sub-units. Ribosomes are assembled in the nucleus and transported to the cytoplasm through the nuclear pores. The large ribosomal subunits are synthesized in the nucleolus, whereas the small subunits are synthesized in the nucleus.

• Polysomes—Ribosomes often form polysomes, which consist of a single messenger RNA (mRNA) that is being translated by several ribosomes at the same time. The ribosomes move on the mRNA from the 5' end toward the 3' end. The two ribosomal subunits associate on the mRNA, with the small sub unit binding first.

Forms of ribosomes

Ribosomes exist in two forms:

• Free polysomes are the site of synthesis for proteins destined for the nucleus, peroxisomes, or mitochondria.

• Membrane-associated polysomes are the site of synthesis of secretory proteins, membrane proteins, and lysosomal enzymes.

Endoplasmic Reticulum

The endoplasmic reticulum exists in two forms, rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER).

Rough endoplasmic reticulum

RER is a single, lipid bilayer continuous with the outer nuclear membrane. It is organized into stacks of large flattened sacs called cisternae that are studded with ribosomes on the cytoplasmic side (Figure 1-1-2).

RER synthesizes proteins that are destined for the Golgi apparatus, secretion, the plasma membrane, and lysosomes. RER is very prominent in cells that are specialized in the synthesis of proteins destined for secretion (e.g., pancreatic acinar cells).

image copyright 1934 Uppmcott Williams & Wilkins. Used with permission.

Figure 1-1-3. Human Corpus Luteum of Pregnancy

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Figure 1-1-2. Rough Endoplasmic Reticulum

Copyright 2000 Gold Standard Multimedia, Inc. All rights reserved.

Figure 1-1-2. Rough Endoplasmic Reticulum

Smooth endoplasmic reticulum

SER is a network of membranous sacs, vesicles, and tubules continuous with the RER, but lack ing ribosomes (Figure 1-1-3).

SER contains enzymes involved m the biosynthesis of phospholipids, triglycerides, and sterols image copyright 1934 Uppmcott Williams & Wilkins. Used with permission.

Figure 1-1-3. Human Corpus Luteum of Pregnancy

6 médical

Cell Components

Functions of SER

Detoxification Reactions

These are reactions that make compounds water soluble so that they can be excreted. Two types of reactions that increase solubility are:

■ Steroid synthesis

• Hydroxylation reactions—by way of hydroxylase complexes containing cytochrome P450, a flavoprotein, and a nonheme iron protein

■ Conjugation reactions—the transfer of polar groups (i.e., glucuronic acid) from the active carrier UDPglucuronic acid to the toxic water-in soluble molecule

Glycogen Degradation and Gluconeogenesis

Removal of the phosphate group from glucose-6-phosphate by the enzyme glucose-6 phosphatase, an integral membrane protein of the SER. This controls the formation of free glucose from glycogen and via gluconeogenesis.

Reactions in Lipid Metabolism

Lipolysis begins in the SER with the release of a fatty acid from triglyceride. The SER is also the site where lipoprotein particles are assembled.

Sequestration and Release of Calcium Ions

In striated muscle the SER is known as the sarcoplasmic reticulum (SR). The sequestration and release of calcium ions takes place in the SR.

Figure 1-1-4. Cytoplasm

Mitochondria

Golgi

Do not confuse the Golgi apparatus with the Golgi tendon organs of the cell or any other factor bearing his name. Dr. Camillo Golgi was a prolific Italian histologist. Other structures or processes bearing his name include Golgi's silver stain for nerve cells, the cycle of Golgi for the development of the malaria parasite, the inhibitory Golgi cells of the cerebellum, and the acroblast, a part of the Golgi material of the spermatid known as the Golgi remnant

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