$\displaystyle \small \bullet$ Membranous organelles are distinct in terms of structure and function.
$\displaystyle \small \bullet$ Many such organelles are considered together as an endomembrane system as their functions are coordinated.
$\displaystyle \small \bullet$ Endomembrane system of cell includes endoplasmic reticulum, golgi complex, lysosomes and vacuoles.
The Endoplasmic Reticulum (ER)
$\displaystyle \small \bullet$ ER is the network or reticulum of tiny tubular structures scattered in the cytoplasm
$\displaystyle \small \bullet$ It divides the intracellular space into two distinct compartments, luminal (inside the ER) and extra luminal (cytoplasm).
$\displaystyle \small \bullet$ Types of Endoplasmic Reticulum based on nature of its membranes
(i) Smooth Endoplasmic Reticulum (SER) These are smooth because they do not bear ribosomes in the form of granules on their surfaces. It is present in cells where they acts as a major site for the synthesis of lipid and also helps in synthesis of steroidal hormone in animal cells.
(ii) Rough Endoplasmic Reticulum (RER) They are found extensive and continuous with the outer membrane of nucleus. These have rough membrane because they bear ribosomes being attached to their surfaces. RER present in the cell actively involved in the protein synthesis.
Golgi Apparatus
$\displaystyle \small \bullet$ They consist of many flat, disc-shaped sacs or cisternae of 0.5µm to 1.0µm diameter.
$\displaystyle \small \bullet$ These are stacked parallel to each other. Varied number of cisternae are present in a Golgi complex.
$\displaystyle \small \bullet$ Golgi complex occurs in all cells except prokaryotes and some eukaryotes such as human RBCs, sieve tubes of plants, etc.
$\displaystyle \small \bullet$ The Golgi cisternae are concentrically arranged near the nucleus with distinct convex cis or the forming face and concave trans or the maturing face.
$\displaystyle \small \bullet$ Functions
(i) Involved in the formation of lysosomes, vesicles that contain proteins and remains within the cell.
(ii) Performs the function of packaging material.
(iii) It acts as an important site for the formation of glycoproteins and glycolipids.
(iv) Helps in the production of complex carbohydrates other than glycogen and starch.
(v) Helps in the formation of cell wall.
Lysosomes
$\displaystyle \small \bullet$ These are membrane bounded vesicles that are produced by the Golgi apparatus.
$\displaystyle \small \bullet$ Lysosomes were discovered by Cristian de Duve (1955), they are present only in animal cells.
$\displaystyle \small \bullet$ They are rich in several hydrolytic digestive enzymes.
$\displaystyle \small \bullet$ These enzymes are capable of digesting carbohydrates, proteins, lipids and nucleic acids.
$\displaystyle \small \bullet$ On the basis of morphology of content and function, they have been classified as
a) Primary lysosome: they are small bodies just cut off from Golgi apparatus and enclose enzymes.
b) Secondary lysosomes: the primary lysosomes fuse with the vacuole containing extracellular or intracellular material
c) Residual lysosome: lysosomes containing waste and unabsorbed material are called residual lysosome. They move to the surface and throw the content by exocytosis.
d) Autophagy lysosome: a lysosome contains a part of its own cell and digests it, called autophagy. It occurs more frequently during differentiation.
$\displaystyle \small \bullet$ Functions
$\displaystyle \small \circ$ Digest the extracellular particles
$\displaystyle \small \circ$ They can self-destruct by release of its content within cell hence they are called as suicidal bags.
$\displaystyle \small \circ$ They digest foreign materials that enter into cell.
Vacuoles
$\displaystyle \small \bullet$ Vacuoles are a large membranous space found in the cytoplasm.
$\displaystyle \small \bullet$ It contains water, sap, excretory product and other materials not useful for the cell.
$\displaystyle \small \bullet$ In plant cells the vacuoles can occupy up to 90% of the volume of the cell. In animals they are small or absent.
$\displaystyle \small \bullet$ The vacuole is bound by a single membrane called tonoplast.
$\displaystyle \small \bullet$ In Amoeba the contractile vacuole is important for excretion.
$\displaystyle \small \bullet$ In many cells, as in protists, food vacuoles are formed by engulfing the food particles.
0 Comments