NEET Test 57

The activating nature of –CH₃ group linked to benzene ring can be explained with the help of inductive effect and hypercongugation. 

Inductive effect explains the how methyl group is an electron releasing group. 

Hyperconjugation explains how alkyl groups can be electron releasing in a resonance manner. Hyperconjugation explains the mild activating effect of an alkyl group in electrophilic aromatic substitution and its ortho-para directing effect.

The minimum and maximum oxidation number of S are –2 and +6 respectively. Since the oxidation number of S in SO₂ is +4, therefore it can be either increased or decreased. Therefore SO₂ behaves both as an oxidising as well as reducing agent. 
Hence, option A is correct.

• DDT is a non-biodegradable pollutant i.e., once it is absorbed by an organism, it cannot be metabolized and broken down or excreted out.
• It is generally accumulated in the fat containing tissues of the organism.
• High concentration of DDT disturbs calcium metabolism in birds, which causes thinning of egg shell and their premature breaking, eventually causing the decline in bird populations.

• Good ozone is found in the upper part of the atmosphere called the Stratosphere and it acts as a shield absorbing the ultraviolet radiations of the sun.
• Dobson unit i.e., 1 DU=1ppb is used for measuring the thickness of the ozone in a column of air from the ground to the top of atmosphere.
• Ozone gas is formed continuously by the action of UV rays on molecular oxygen and degrades into molecular oxygen into stratosphere.

• Compound epithelium consists of more than one layer of cells.
• Compound epithelia being multilayered have little role in secretion or absorption, but they provide protection to underlying tissues against mechanical, chemical, thermal or osmotic stresses.
• Compound epithelia are of two types-

(a) Stratified epithelium- consists of many layers of epithelial cells. 
(b) Transitional epithelium- more stretchable and much thinner than stratified epithelium. 

• Cuboidal epithelium consists of a single layer of cube-like cells lying on the basement membrane.
• Columnar epithelium consists of a single layer of tall and slender cells, lying on the basement membrane.
• If the columnar or cuboidal cells bear cilia on their free surface, they are called ciliated epithelium.

• Macula adherens also called as Desmosomes perform cementing to keep the neighboring cells together.
• These are like Zonula adherens or intermediate junctions but are thicker and stronger and are disc like junctions.
• They have inter-cellular proteins present in them.
• The protein plates or plaque-like structures are much thicker. Desmosomes are also involved in anchoring.
• Zonula occludens (tight junctions) help to stop substances from leaking across the tissue.
• Zona adherens occur just below tight junction.
• Gap junction facilitates the cells to communicate with each other by connecting the cytoplasm of adjoining cells for rapid transfer of ions, small molecules and sometimes big molecules.