3. Endomembrane System
– It is a group of membranous organelles having coordinated functions. – They include endoplasmic reticulum (ER), Golgi complex, lysosomes and vacuoles.
Endoplasmic Reticulum (ER)
– These are a network of tiny tubular structures scattered in the cytoplasm.
– ER divides the intracellular space into 2 compartments: luminal (inside ER) & extra luminal (cytoplasm).
– Endoplasmic reticulum is 2 types:
a. Rough endoplasmic reticulum (RER):
– Bear ribosomes on their surface. RER is frequently observed in the cells actively involved in protein synthesis and secretion. They extend to the outer membrane of the nucleus.
b. Smooth endoplasmic reticulum (SER):
-Ribosomes are absent. SER is the major site for synthesis of lipid. In animal cells lipid-like steroidal hormones are synthesized in SER.
– Densely stained reticular structures near the nucleus.
– First observed by Camillo Golgi (1898).
– They consist of flat, disc-shaped sacs (cisternae) of 0.5micrometer 1.0micrometer diameter.
These are stacked parallelly.
– Cisternae are concentrically arranged with convex cis (forming) face and concave trans (maturing) face.
Cis & trans faces are totally different, but interconnected. Function of Golgi apparatus:
o Secretes materials to intra-cellular targets or outside the cell.
Materials to be packaged as vesicles from the ER fuse with the cis face and move towards the trans face. This is why Golgi apparatus remains in close association with the endoplasmic reticulum.
o Proteins synthesized by ribosomes on the ER are modified in the cisternae of Golgi apparatus before they are released from its trans face.
o Formation of glycoproteins and glycolipids.
– These are membrane bound vesicular structures formed by the process of packaging in the Golgi apparatus.
– Lysosomal vesicles contain almost all types of hydrolytic enzymes (hydrolases– lipases, proteases, carbohydrases).
They are active at acidic pH.
They digest carbohydrates, proteins, lipids and nucleic acids.
– These are the membrane-bound space found in the cytoplasm.
-It contains water, sap, excretory product and other materials not useful for the cell.
– Vacuole is bound by a single membrane called tonoplast.
– In plant cells, the vacuoles can occupy up to 90% of the volume of the cell.
– In plants, the tonoplast facilitates the transport of ions and other materials against concentration gradients into the vacuole.
-Hence their concentration is higher in the vacuole than in the cytoplasm.
– In Amoeba, the contractile vacuole helps for excretion. – In many cells (e.g. protists), food vacuoles are formed by engulfing the food particles.
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