(a) What are Pesticides? Explain their different types; why persistent pesticides are more lethal for mankind? (b) What are carbohydrates? Classify and give detail of each class along with examples. (c) Discuss different methods of Food preservation. (d) Where and how Fiber Optics are used? Also write down their advantages and disadvantages.

(a) Pesticides

Pesticides are substances used to kill, repel or control pests that damage crops, spread disease or destroy stored goods.

Types by target: insecticides (insects), herbicides (weeds), fungicides (fungi), rodenticides (rats and mice), nematicides (roundworms) and bactericides (bacteria).

Types by chemical group: organochlorines (e.g. DDT), organophosphates (e.g. malathion), carbamates (e.g. carbaryl) and pyrethroids, which are synthetic forms of natural plant compounds.

Why persistent pesticides are more lethal: persistent pesticides, chiefly organochlorines, break down very slowly, so they remain in soil, water and food for years. Being fat-soluble, they build up in the fatty tissue of organisms (bioaccumulation), and their concentration increases at each step of the food chain (biomagnification). Humans, near the top of many food chains, can therefore receive high doses. They have been linked with cancer, nerve and reproductive damage, and hormone disruption, and can pass to infants through breast milk. They can also travel long distances. For these reasons, the Stockholm Convention (2001) restricts or bans many of them.

(b) Carbohydrates

Carbohydrates are organic compounds of carbon, hydrogen and oxygen, often following the general formula Cₙ(H₂O)ₘ. They provide energy and also serve structural functions.

Monosaccharides: simple sugars that cannot be broken into smaller sugars, e.g. glucose, fructose and galactose. Disaccharides: two monosaccharide units joined together, e.g. sucrose (glucose + fructose), lactose (glucose + galactose) and maltose (glucose + glucose). Oligosaccharides: short chains of about 3 to 10 units, e.g. raffinose in beans. Polysaccharides: long chains of many units. Starch stores energy in plants, glycogen stores it in animals, and cellulose forms plant cell walls.

(c) Methods of food preservation

Preservation slows or stops spoilage by microbes, enzymes and oxidation.

Heat: pasteurisation, sterilisation and canning kill microbes. Low temperature: refrigeration slows growth, and freezing nearly stops it. Drying: removing water, by sun drying or dehydration, prevents microbial growth. Salting and sugaring: high concentrations draw water out of microbes, as in pickles and jams. Fermentation and pickling: the acid produced in yoghurt or added as vinegar inhibits harmful microbes. Chemical preservatives: sodium benzoate, sorbates, nitrites and antioxidants are used within legal limits. Smoking: it dries food and adds antimicrobial compounds. Packaging: vacuum and modified-atmosphere packs reduce the available oxygen. Irradiation: controlled doses of radiation kill insects and microbes and delay sprouting.

(d) Fibre optics

How they work: light pulses carrying data travel along a thin glass core through total internal reflection from the surrounding cladding. A transmitter converts electrical signals to light, and a receiver converts them back.

Where they are used: in internet backbones and undersea cables, telephone and mobile networks, cable television, computer networks in data centres, medical endoscopes, sensors, and aircraft and military communication.

Advantages: very high bandwidth; low signal loss over long distances; immunity to electromagnetic interference; better security; light weight and small size; and no risk of electric sparks.

Disadvantages: high initial installation cost; fragile glass that can break if bent too sharply; difficult joining (splicing) and repair needing skilled workers and special equipment; and a need for devices to convert between light and electrical signals.