Question

Write short notes on-

(i) Centrosome

(ii) Vacuoles

(iii) Microtubules

Answer 1

Vacuoles :

Dujardin (1941) discovered vacuoles in the plant cells. They may be one or more, small or large but always single membrane bounded round structures. The vacuoles that contains cell sap are called as sap vacuoles. They are bounded by a living membrane called tonoplast. In young cells, the vacuoles are small but their size increases with the increase in cell size. The cell sap indues sugar, amino acids, waste substances and mineral salts. They are called as Ergastic substances. The cell sap of fruits and flowers has dissolved anthocyanin pigments of blue, yellow, red etc. colours.

Types of Vacuoles:

On the basis of composition and functions, the vacuoles are of four types-

1. Sap vacuoles – They store ergastic substances.
2. Contractile vacuoles – They perform osmoregulation.
3. Food vacuoles – Digestion of food takes place in these vacuoles.
4.  Air vacuoles – They are found in some prokaryotes. They store metabolic air & provide buyaney.

Functions of vacuoles:

The vacuoles perform following functions-

1. They store and concentrate food materials and mineral ions.
2. The help to maintain osmotic pressure and turgidity.
3. They store waste products.
4. Their anthocyanin provides attractive colours to the flowers & fruits that help in pollination of flowers & dispersal of fruits.

Centrosome:

It was discovered by E. Van Benden (1883) and Later described and named by Theodor Boveri (1888). They are found in animal cells & cells of some algae & fungi. They are absent in plant cells. It is usually situated close to the nucleus.

Structurally, the centrosome consists of centrosphere and two centrioles. The centrosphere is an amorphous ground substance which is mainly made up of tubulin protein. The two centrioles remain embedded in the centrosphere and are situated at right angle to each other. The centriole is a cylendrical structure which is 300 to 500 nm in length and 150-250 nm in diameter.

Each centriole has a central tubule which is surrounded by nine peripheral tubules. Each peripheral tubule consists of three subtubules (triplete). These subtubules are made up of microtubules of tubulin protein. The peripheral tubules remain connected to the central tubule by 9 spokes and also remain interconnected.

The Centriole exhibits “Cart Wheel” structure Functions of Centrosome

1. It’s centrosphere part forms spindle fibres during cell division.
2. The centrioles form two opposite poles during cell division.
3. It forms cilia & flagella.
4. It forms tail in sperms.

Microtubules:

Microtubules were first seen by Porter (1963) in plants  ‘microtubule’ was given by Slautterback. They are unbranched and round tubes and their diameter is 25 nm. They are made up of tubulin protein. They are found in centriole, cilia, flagella, spindle fibres etc.

They are absent in Amoeba and prokaryotes. They consists of two subunits viz. – a and P subunits which arranged in spiral array. Every spiral array includes 13 subunits. The microtubules are formed by the polymerization of the sub units. Functions of Microtubules.

1. They provide cytoskeleton to the cells.
2. They form centrioles.
3. They form spindle fibres during cell division.
4. They help in the division of plant cells.
5. They form cilia & flagella.