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Home , Endomembrane system

Cell of the Endomembrane System

Endomembrane System

• It is an integrated system of membranes suspended in the of eukaryotic cells. • These membranes form the functional compartments or the organelles within the . • It includes the (ER), vesicles, and ER associated . • The main function of this system is to synthesize, modify, package and transport and lipids in the cell.

The Endomembrane System Endoplasmic Reticulum

• The endoplasmic reticulum (ER) is a large network (reticulum) of a single membrane-bound tubes and sheets.

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• It appears as long tubules or round bags (vesicles). • The ER is continuous with the outer nuclear membrane. • The ER membrane is made up of and proteins. • Two types of ER are rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER). • RER looks rough under a microscope due to the presence of ribosomes on its surface. The ribosomes are the sites of manufacture. The ER then transports the proteins to other sites in the cell. • SER helps in the manufacture of fat molecules (lipids) which are important for cell function. These products help in membrane biogenesis i.e. building of the . • SER in muscle cells (also known as Sarcoplasmic reticulum) stores Ca2+ ions which play an important role in muscle contraction. Functions of ER

• The manufactured proteins and lipids function as enzymes and hormones. • ER serve as channels for the transport of materials (especially proteins) between various regions of the cytoplasm or between the cytoplasm and the nucleus. • The ER also functions as a cytoplasmic framework providing a surface for some of the biochemical activities of the cell. • In the cells of vertebrates, SER plays a crucial role in detoxifying many poisons and drugs.

• The Golgi apparatus was discovered by Camillo Golgi. • It consists of a pile/stack of membrane-bound vesicles called cisternae arranged approximately parallel to each other. • The membranes of Golgi Apparatus are single layered and often have connections with the membranes of ER. • Golgi apparatus modifies, packages and transports the materials, synthesized near the ER, in vesicles to sites inside or outside the cell. • Complex sugars may be made from simple sugars in the Golgi apparatus. • The Golgi apparatus is also involved in the formation of lysosomes.

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Lysosomes

• is a single membranous formed from the vesicles pinched off from Golgi Apparatus. • Lysosomes help to keep the cell clean by digesting any foreign material as well as worn-out cell organelles. Hence, acts as a waste disposal organelle. • Lysosomes contain powerful hydrolytic (digestive) enzymes, produced by the RER, which are capable of breaking down (lysis) all organic material. These enzymes work actively at an acidic pH of 5.6. • Any food or foreign material engulfed by the cell is broken down by the action of these lytic enzymes. • During the disturbance in cellular metabolism, for example, when the cell gets damaged, lysosomes may burst and the enzymes digest their own cell. Therefore, lysosomes are also known as the ‘suicide bags’ of a cell.

Ribosomes

• Ribosomes are non-membranous organelles found in both prokaryotic as well as eukaryotic cells. • Its structure comprises of two subunits, one larger than the other. • In , the ribosomes are found in free state within the cytoplasm and are of 70S type, which is smaller than the ones found in (80S) • Eukaryotic ribosomes can be found in free state as well as associated with the endoplasmic reticulum (RER). • 70S ribosomes are also found in certain eukaryotic cell organelles like Mitochondria and , which help them to produce some of their own proteins.

Their main function is to synthesize proteins. They have an important role in the translation process where the RNA (formed from DNA) is used as a template within the structure of ribosomes to add amino acids in sequence to form a protein chain.

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