Endoplasmic Reticulum Structure Functions and Types
- Endoplasmic means this organelle found in cytoplasm and reticulum means a type of network
- It was discovered and term coined by Porter
- Endoplasmic reticulum is often called system of membrane
- It occurs in all eukaryotic except mature erythrocytes.
- In muscles, endoplasmic reticulum is called sarcoplasmic reticulum.
- Resting and early embryonic cells contains least number of E.R.
- E.R. is less developed in spermatocyte.
- Metabolically active cells like plasma cells, pancreas cell contain more developed.
ER divides the intracellular space into two distinct compartments, i.e., luminal (inside ER) and extra luminal (cytoplasm) compartments.
- These are long, flattened, unbranched membrane bound structure
- They arrange parallel to form lamellae. It is well developed in RER.
- They are isolated rounded and oval, abundant in SER
- They are long isolated and branched. It is well developed in SER
- Membrane of ER is made of single unit membrane
- ER is similar to Golgi body but its constituent parts are separated. It has no fix place in cytoplasm like Golgi body
Type of ER
Rough Endoplasmic Reticulum
- Ribosome attached to ER with their larger unit by Ribophorin I and II (transmembrane glycoprotein). RER Found in liver, pancreas, and goblet cells. After translation polypeptide chain transferred to RER for protein folding and processing.
- RER is the site of protein folding, assembly of multi-subunit protein, di sulfide bond formation.
- RER is also part of a protein sorting pathway. The majority of ER resident proteins are retained in ER through a retention motif. This motif is composed of four amino acids at the end of protein sequence. The most common retention sequence is lys-asp-glu-leu (KDEL). However variation on KDEL does occur.
- This signal sequence is recognized by SRP consisting of six polypeptide and small cytoplasmic RNA. SRP binds to ribosomes as well as signal sequence
- The formation of di-sulfide bond between the side chains of cysteine residue is done in ER lumen by di-sulfide isomerase.
Ribosome and Ribophorin are absent. Found in Muscle and glycogen storing liver cells.
Major site of membrane lipid synthesis, steroid hormone synthesis
Modification of Endoplasmic Reticulum: –
- Sarcoplasmic reticulum: – they are modification of SER they store Ca2+, found in skeleton and cardiac muscle
- Glycosome: – In liver cells SER found rich in glycogen and can be observed as dense particles, called glycosomes,
- Myeloid Bodies: – they are specialization of SER present in pigmented epithelial cells of retina . these are light sensitive
- Ergastoplasm: – they are group of ribosome found in lamellae of ER (basophilic bodies, chromophilic substances or Nissl bodies)
- Microsome: – they are part of ER related to ribosome. Not found in living intact cells. Used in study of in vitro protein synthesis
- T-Tubules: – they are arranged transversely in skeleton and heart muscle cells. They stimulate and conduct contraction in muscle cells
Functions of SER
- Ascorbic acid is synthesized over SER. Retinal pigments are formed of Vitamin A over SER
- Takes parts in lipid synthesis and steroid hormone
- Detoxification of drug is carried out by a system of oxygen transferring enzymes (oxygenases) by help of Cyt P450. SER form Sphaerosomes in plants
Functions of RER
- Synthesis of Protein( secreted protein, integral membrane protein and soluble protein)
- Glycosylation occurs in RER
- Unfolded protein destroyed in proteasome by help of ER
- Makes SER. Enzyme of lysosome formed by RER
Enzymes of ER
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