Endoplasmic Reticulum Structure Functions and Types

Endoplasmic reticulum

  • 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.

Structure:-
ER divides the intracellular space into two distinct compartments, i.e., luminal (inside ER) and extra luminal (cytoplasm) compartments.

Picture

Source- wikimedia

Endoplasmic reticulum is a network of three types of elements –
1. Cisternae:-

  • These are long, flattened, unbranched membrane bound structure
  • They arrange parallel to form lamellae. It is well developed in RER.

2. Vesicle:-

  • They are isolated rounded and oval, abundant in SER

3. Tubule:-

  • 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.

Picture

Source- wikimedia

Smooth Endoplasmic Reticulum: – 
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: –

  1. Sarcoplasmic reticulum: – they are modification of SER they store Ca2+, found in skeleton and cardiac muscle
  2. Glycosome: – In liver cells SER found rich in glycogen and can be observed as dense particles, called glycosomes,
  3. Myeloid Bodies: – they are specialization of SER present in pigmented epithelial cells of retina . these are light sensitive
  4. Ergastoplasm: – they are group of ribosome found in lamellae of ER (basophilic bodies, chromophilic substances or Nissl bodies)
  5. Microsome: – they are part of ER related to ribosome. Not found in living intact cells. Used in study of in vitro protein synthesis
  6. 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

Picture

Source- Thorpe

References and source:-
Wikimedia
NCERT
Molecular Biology of the Cell-Alberts
Cell and Molecular Biology Concepts and Experiments.Geraldkarp
Cell Biology, Genetics, Molecular Biology, Evolution and Ecology by Verma, Agarwal

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