Why does hair not grow on scars?

Skin consists of two principal layers — epidermis and dermis. The epidermis is a continuously self renewing layer. Keratinocyte (a cell producing a protein found in tissues like hair, nails, scales and skin.) is the chief cell in this layer. It also consists of melanocyte — the pigment forming cell.

Deeper to the epidermis, is the dermis, which is composed of collagenous (Greek; Kolla: glue) and elastic fibre network in an amorphous ground substance. It accommodates nerves, blood vessels and pilo-sebaceous unit.

A pilo-sebaceous unit consists of hair and its follicle, associated muscle and a secretory gland called sebaceous gland. An injury to the skin results in a scar.

A scar (Greek; eschara: a hearth, fire place, scab) is defined as a mark consisting of fibrous material formed in the healing process of a wound.

The scar forms in three overlapping phases — an immediate response to the injury, when the hemostasis (arrest of bleeding) is achieved and the germs are engulfed, followed by formation of new blood vessels and new cells required for the regeneration of epidermis and finally replacement of these newly formed blood vessels and cells by fibres, i.e., the scar.

The scar contains few cells and blood vessels and no pilo-sebaceous unit and nerves as they never regenerate, hence no hair growth.

Experimentally, addition of dermal papillae fibroblasts to a wound can induce hair follicle formation. Scars bleed despite a few blood vessels, because they lack the elastic and contracting properties of normal skin.

Source: The Hindu

Why are frogs not seen in seas?

Frogs and toads belong to the class amphibia (they live in water and on land).One of the biggest problems faced by these animals is Osmoregulation. When they are in water they face the same problem as freshwater fishes; they have to expel excess water from their body. When on land they have to conserve water.

They do not replace water by intentional drinking nor do they have the impermeable skin characteristic of other tetapods. Above all their kidneys are not capable of producing hypertonic urine.

Amphibians limit their water loss by their behaviour that reduces exposure to desiccating conditions. When they come to land foraging during night, they lose water and this is compensated by water uptake across the skin. Paradoxically, the skin, the most important source for dehydration is also the most important structure for rehydration.

The frogs and toads have specialized patches of skin called seat patches on their abdomen and thighs which can absorb water very quickly allowing them to rapidly rehydrate. They flatten their body on moist surfaces and absorb water through seat patches.

Neurohypophyseal(ADH) and adrenocortical hormones have special roles in regulating water and salt transfer across epithelial membranes of tetrapods. Such effects are seen in the skin and urinary bladder of frogs and toads.

Amphibians on the whole are intolerant of saline condition. Their kidneys are not capable of producing hypertonic urine. Only amphibians and mammals excrete urea. Had the amphibians not inherited the Ornithine-urea cycle from their crossopterygian ancestors, they would not have succeeded in the movement onto dry land.

Source: The Hindu

How are flies able to change their flying direction quickly?

A fly, Musca domestica, has four wings joined to its body in the thorax region where the legs are also joined.

The flies fly by flapping their wings at a very fast rate ranging from 50 to more than 1000 flaps per second.

While the articulation of the two forewings actually propels the fly, rear wings are rather redundant or vestigial because they are not used for propelling.

But they also vibrate along with the forewings, however, in the vertical direction. Thus, they act as gyroscopes to inform the fly about the turns and tilts so that the forewings can effectively thrust the insect forward on a steady course.

The left and right forewings are highly synchronised for a steady flight. A fly, in flight, can make very sharp 90 degree turns in less than a millisecond, i.e. 1/1000 of a second. This is faster than the speed of a blink of a human eye. Such a manoeuvre is called a saccade and for this the fly has to generate enough of torque to overcome both the inertia of motion of itself and the viscous drag of the air.

Detailed high speed photography has revealed that during a saccade, a fly has also to negotiate the necessary banking or tilting to the side it turns.

This is accomplished by the rear wings while the fore wings actually manage the turn by differential beating. If, for example, the fly has to turn to left, the left wings momentarily stop with the right wings continuing to flap. This provides the necessary twisting torque in a very short time. It is now clear that the air friction is not very important and the inertia alone governs the fantastic feat of near instantaneous turning in air.

Source: The Hindu

Why does a fan make noise while running on an inverter and not on normal electric supply?

Inverter is an electronic device for converting the direct current (DC) from storage batteries or solar cells into alternating current (AC) to match the domestic supply which is basically 220 volt and 50 Hz (cycles/sec).

It also forms the basic part of a UPS device that supports the computers when domestic current fails. The current variation of the good domestic AC supply is represented by a smooth sine wave, similar to a smooth wave on a still water surface.

The sine-wave form AC alone generates uniformly rotating magnetic field in the induction motor of a fan for noiseless rotation. But inverters can generate AC of any shape current variation. A well designed inverter alone supplies sine-wave AC.

The cheap inverters can generate, for example, square wave current variation. The square wave is composed of superposition of many harmonic sine waves (Fourier’s theorem — multiples of basic frequency).

This square wave AC, when connected to a fan, creates jerkily revolving magnetic field inside, making the rotor to make jerky motion and hence noise. Also these unfiltered harmonics in the AC create vibrations in the coil, generating humming noise.

These types of inverters are used for thermal and lighting applications only where the shape of AC change does not matter. Sensitive equipment like computers and motors need sine-wave producing inverters.

Source: The Hindu

Why and how do milk and water get separated when lime juice is added?

Milk contains an average of 13 per cent solids and 87 per cent water. Milk solids include fat, proteins, carbohydrates, minerals and vitamins.

In milk, fat is in emulsion form, proteins in colloidal form and carbohydrates and minerals in true solution. The milk protein, casein, is present in milk as calcium caseinate in combination with calcium.

These calcium caseinate particles are in equilibrium with milk serum in which they are suspended. Thus any change in ionic concentration of milk serum will disturb the colloidal stability of calcium caseinate particles.

When lime juice is added to milk, the citric acid present in lime reduces the pH value and changes the ion concentration of milk serum which alters the equilibrium. When pH value becomes 4.7 which is the iso-electric point, the protein (calcium caseinate) gets precipitated and separated from milk solution leaving the watery portion.

This precipitated protein mass is called channa/cakka and the watery portion which contains dissolved carbohydrates and water soluble vitamins is called whey.

Source: The Hindu

Why does the ice tray not stick to the freezer of the fridge when the ice tray is kept on rock salt?

The temperature of the freezer box is around -18 degrees Celsius. At this temperature ordinary water would freeze. The ice tray usually sticks to the freezer box because the layer of water between the tray and the bottom of the freezer gets converted to ice.

But the tray would not stick to the freezer if the water between them can be prevented from freezing. This is what happens when the tray is placed on rock salt.

Rock salt is a mineral called halite and chemically it is sodium chloride — the same as common salt. It has physical and chemical properties same as that of common salt which can be used to make an effective freezing mixture which lowers the freezing point to far below zero degrees Celsius.

When the tray is placed on rock salt, the intermediate water layer dissolves a part of the salt and remains in liquid form even after the water in the tray becomes ice.

Source: The Hindu

Why do earthworms die when salt is sprinkled on them?

Earthworms are grouped under the animal phylum Annelida, which was derived from the Latin word, anellus meaning ‘little rings.’ As this name implies, the body of an earthworm seems to be made up of several rings which is known as segmented body.

The skin of the earthworm secretes mucous. This mucous keeps the skin moist always, which is vital for the survival of earthworms. Because earthworms have no respiratory organ and hence the respiration takes place through the moist skin.

The oxygen from the environment diffuses passively across the moist skin and it is carried by the circulatory system (blood) to the cells. Similarly, carbon dioxide from the cells will also be carried by the circulatory system and diffuses out through the moist skin. Hence, the earthworms will die if their skin dries out.

Salinity (saltiness) of the soil in which an earthworm lives is an important factor that decides its survival because earthworms are highly sensitive to salt stress. The salinity may reduce their growth at low salt concentrations or cause mortality at high salt concentrations.

They are unable to tolerate high ionic strength, because high salt concentrations destroy their sensitive skin, and the earthworms cannot have control over the osmotic regulation. In addition, the neurosecretory cells in earthworms play a vital role in water balance as well as ionic and osmotic regulation.

The salt stress and desiccation significantly interfere with the functions of these neurosecretory cells. Hence, salts like sodium chloride are extremely toxic to most of the earthworm species.

Sodium chloride could cause mortality at a concentration of 0.5 per cent itself, whereas other salts such as potassium chloride are toxic at a concentration of more than 1 per cent. However, the salts at lower (sublethal) concentrations may not cause mortality in earthworms, although they would substantially reduce the reproduction, especially production of cocoons (egg cases).

Source: The Hindu

How does the ant move up a wall, like the lizard?

Almost all insects have three pairs of legs, and each leg is segmented. The terminal segment of the leg is known as pre-tarsus. The pre-tarsus of insects bears a pair of claws, which are used to walk on rough surfaces.

However, the claws do not provide sufficient grip when the surface is too smooth, such as that of a leaf or a wall.

Hence, many insects, including ants, possess a median lobe between their claws, known as arolium which acts as an adhesive organ.

It is a flexible cuticular lobe, which can be folded and unfolded while walking. In general, the arolium is made up of dense tubular hairs whose tips are filled with glandular secretions.

These secretions contribute to the ant movement on smooth surfaces. Thus, the presence of the arolium on their legs enables ants to move on any smooth surface without falling. In some insects, the arolium is also believed to act as a vacuum cup or suction pad.

Source: The Hindu

Why do flightless birds such as ostrich and emu have wings?

Flightless birds are grouped under Ratitae. Generally ratites are large which have independently lost the ability to fly. These birds are found in South America, South Africa, New Zealand and Australia.

They are quoted as example for discontinuous distribution. This discontinuity is believed to have happened millions of years ago due to continental drift.

It is believed by scientists that the most flightless birds evolved in the absence of predators on islands and lost the power of flight, because they had few enemies, although this is likely not the case for ostrich, emu and cassowary as all have claws on their feet/wings to use as a weapon against predators.

Many ratites live on isolated oceanic islands where there are no predators. Structurally these birds’ sternum (breast bone) is without keel to which flight muscles could be anchored.

All species of ratites are thus unable to fly. They have more feathers than flying birds. The wings of these birds may be considered as the vestigial organs like pinna andappendices of human beings. New Zealand has more species of ratites like kiwi and penguin. One reasonis until the arrival of humans about 1,000 years ago there were no largeland predators in this island.

But yet we are not able to answer this question as ratites are also found in South America and South Africa where predators are common.

Studies by different authors on the phylogeny and biogeography of ratites inferred from DNA sequences do not reveal any clue. Alistair Dawson put forth a theory that these ratites are neotenous descendents of flying birds as Axolotyl larva of Salamander ( Amphibia).

That is, the young birds of flying birds ( Neognathae) that have attained sexual maturity even before the body growth is completed. Why these ratitese volved in this way is open to speculation.

Source: The Hindu

Why do people have red eyes eyes in flash photographs?

Red eye reflex is seen in some flash photographs as flash photographs are taken in dull light conditions. The pupils of the subject are dilated as the ambient light is dull and the subject is looking straight at the camera. The light from the flash enters the eye through the pupil and is reflected out. This reflected light is red in colour as the retina being highly vascular is red in colour and is what is seen as the red reflex of flash photography.

This can be avoided by the subject not looking directly into the camera. A pre-flash strobe from the camera flash will cause the pupil to contract and the red eye reflex will not be seen. This is now available in most modern cameras as ‘red eye reduction’ feature.

Red eye reflex is seen more in children as they naturally have larger pupils — certain animals have a highly reflective surface under the retina giving them greater red eye reflex, that is why red eye reflex is sometimes called ‘cat eye reflex’.

Source: The Hindu