(4)

Nucleolus is naked (devoid of membrane), round or slightly irregular structure which is attached to the chromatin at a specific region called nucleolar organiser region (NOR). Nucleolus is a site for rRNA synthesis. Nucleolus disappears when the cell undergoes division and reappears after its completion.

(2)

Golgi complex is a complex cytoplasmic structure made up of smooth membrane saccules or cisternae, a network of tubules with vesicles and vacuoles, which takes part in membrane transformation, secretion and production of complex biochemicals. One face of Golgi apparatus is convex (forming, cis) face while the other is concave (maturing, trans) face. Cis face receives contents from endoplasmic reticulum which then reaches trans face and bud off as secretory vesicles.

(4)

Smooth endoplasmic reticulum is involved in the production of lipids and sterols. Complexing of lipids and proteins to form lipoproteins occurs in SER. RER is involved in protein folding, glycosylation of protein and cleavage of signal peptide.

(4)

Polytene chromosomes are giant chromosomes formed by somatic pairing between homologous chromosomes and repeated replication (endomitosis) of their chromonemata. They are quite common in salivary glands of insects and are therefore, popularly known as salivary chromosomes.

(3)

Mitochondria are miniature biochemical factories where food stuffs or respiratory substrates are completely oxidised to carbon dioxide and water. The energy liberated in the process is initially stored in the form of reduced coenzymes and reduced prosthetic groups. The latter soon undergo oxidation and form energy rich ATP. ATP comes out of mitochondria and helps perform various energy requiring processes of the cell like muscle contraction, nerve impulse conduction, biosynthesis, membrane transport, cell division, movement, etc. Because of the formation of ATP, the mitochondria are called power houses of the cell.

(2)

Large central vacuoles are the characteristic of plant cell, not animal cell which may have many small scattered vacuoles.

(1)

Lysosomes are small vesicles which are bounded by a single membrane and contain hydrolytic enzymes in the form of minute crystalline or semicrystalline granules of 5–8 nm. About 50 enzymes have been recorded to occur in them. All the enzymes do not occur in the same lysosome but there are different sets of enzymes in different types of lysosomes. The important enzymes are acid phosphatases, sulphatases, proteases, peptidases, nucleases, lipases and carbohydrases. They are also called acid hydrolases because these digestive enzymes usually function in acidic medium or pH of 4–5.

(1)

Both mitochondria and chloroplast are semi-autonomous organelles. They have their own DNA which produces its own mRNA, tRNA and rRNA. These organelles also possess their own ribosomes and hence are able to synthesise some of their proteins.

(4)

Microtubules are unbranched hollow submicroscopic tubules of protein tubulin which develop on specific nucleating regions. It can undergo quick growth or dissolution at their ends by assembly or disassembly of monomers. They are present in the cytoplasm as well as in specialised structures like centrioles, basal bodies, cilia or flagella, sensory hair, equatorial ring of thrombocytes, spindle apparatus, chromosome fibres, nerve processes, sperm tails, axostyle of parasitic flagellates, fibre system of Stentor, cytopharyngeal basket of Nassula, etc.

(1)

Lysosomes are small vesicles bounded by a single membrane and contain hydrolytic enzymes. Nucleus, mitochondria and chloroplasts are double membrane bound cell organelles.