BIOLOGY : C1. CELL STRUCTURE BIOLOGY : C1. CELL STRUCTURE ROUGH & SMOOTH ROUGH & SMOOTH

ENDOPLASMIC RETICULUMENDOPLASMIC RETICULUM

Endoplasmic Reticulum

The endoplasmic reticulum (ER) is an organelle of cells in eukaryotic organisms that forms an interconnected network of membrane vesicles. The lacy membranes of the endoplasmic reticulum were first seen in the year 1945.

According to the structure the endoplasmic reticulum is classified into two types, that is, rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER).

For extra, sarcoplasmic reticulum is the smooth ER found in smooth and striated muscle fibers. It stores and releases calcium ions.

The rough endoplasmic reticulum is studded with ribosomes on the cytosolic face. These are the sites of protein synthesis.The rough endoplasmic reticulum is predominantly found in hepatocytes where protein synthesis occurs actively.

The smooth endoplasmic reticulum is a smooth network without the ribosomes. The smooth endoplasmic reticulum is concerned with lipid metabolism , carbohydrate metabolism and detoxification. The smooth endoplasmic reticulum is abundantly found in mammalian testis.

Endoplasmic Reticulum

Smooth Endoplasmic Reticulum

Sarcoplasmic Reticulum Rough Endoplasmic Reticulum

The Structure of Smooth E.R

Smooth ER is more tubular than rough ER and forms a separate sealed interconnecting network. It is found fairly evenly distributed throughout

the cytoplasm. It is not studded with ribosomes hence 'smooth ER'.

Smooth ER is devoted almost exclusively to the manufacture of lipids and in some cases to the metabolism of them and associated products.

In liver cells for example smooth ER enables glycogen that is stored as granules on the external surface of smooth ER to be broken down to glucose.

Smooth ER is also involved in the production of steroid hormones in the adrenal cortex and endocrine glands.

Smooth ER: The Detox stop

Smooth ER also plays a large part in detoxifying a number of organic chemicals converting them to safer water-soluble products.

Large amounts of smooth ER are found in liver cells where one of its main functions is to detoxify products of natural metabolism and to endeavour to detoxify overloads of ethanol derived from excess alcoholic drinking and also barbiturates from drug overdose.

To assist with this, smooth ER can double its surface area within a few days, returning to its normal size when the assault has subsided.

The contraction of muscle cells is triggered by the orderly release of calcium ions. These ions are released from the smooth endoplasmic reticulum.

The Structure of Rough The Structure of Rough Endoplasmic ReticulumEndoplasmic Reticulum

This is an extensive organelle composed of a greatly convoluted but flattish sealed sac that is continuous with the nuclear membrane.

It is called 'rough' endoplasmic reticulum because it is studded on its outer surface (the surface in contact with the cytosol) with ribosomes.

These are called membrane bound ribosomes and are firmly attached to the outer cytosolic side of the ER About 13 million ribosomes are present on the RER in the average liver cell. Rough ER is found throughout the cell but the density is higher near the nucleus and the Golgi apparatus.

Ribosomes on the rough endoplasmic reticulum are called 'membrane bound' and are responsible for the assembly of many proteins. This process is called translation.

The rough ER working with membrane bound ribosomes takes polypeptides and amino acids from the cytosol and continues protein assembly including, at an early stage, recognising a 'destination label' attached to each of them.

Proteins are produced for the plasma membrane, Golgi apparatus, secretory vesicles, plant vacuoles, lysosomes, endosomes and the endoplasmic reticulum itself.

In the lumen some proteins have sugar groups added to them to form glycoproteins. Some have metal groups added to them. It is in the rough ER for example that four polypeptide chains are brought together to form haemoglobin.

PROTEIN FOLDING UNIT AND QUALITY CONTROL CENTER

• It is in the lumen of the rough ER that proteins are folded to produce the highly important biochemical architecture which will provide 'lock and key' and other recognition and linking sites.

• It is also in the lumen that an amazing process of quality control checking is carried out. Proteins are subjected to a quality control check and any that are found to be incorrectly formed or incorrectly folded are rejected.

• These rejects are stored in the lumen or sent for recycling for eventual breakdown to amino acids.

• A type of emphysema (a lung problem) is caused by the ER quality control section continually rejecting an incorrectly folded protein. The protein is wrongly folded as a result of receiving an altered genetic message.

• The required protein is never exported from the lumen of rough ER. Research into protein structure failures relating to HIV are also focusing on reactions in the ER

E.R RELATED DISORDERAdrenoleukodystrophy : A disorder of the peroxisomes due to the deposition of huge amounts of lipid material in the brain and spinal cord and the adrenal glands which results in dementia and adrenal failure. ER Storage diseases : A group of genetically based disorders in which mutant proteins fail to pass quality control.

SUMMARY Organelles called 'rough endoplasmic reticulum' and

'smooth endoplasmic reticulum' are separate networks of membrane enclosed flattened tubules found in eukaryotic animal and plant cells.

The rough ER, studded with millions of membrane bound ribosomes, is involved with the production, folding, quality control and despatch of some proteins.

Smooth ER is largely associated with lipid (fat) manufacture and metabolism and steroid production hormone production. It also has a detoxification function

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ELECTRON MICROGRAPH OF E.R

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Smooth and Rough Endoplasmic Reticulum - xLelly29x