HOW DO ANIMALS SEE IN THE DARK?

Photoreception is activation of a biological process by light. Most organisms including man respond to light. Some animals react to light waves not perceived by man. Vertebrates have two types of photosensitive cells, rods and cones, so called because of their shape. The rods, which are long and fat, contain large amounts of visual pigment and they mediate vision under dim illumination. The cone cells, which are relatively small, mediate daylight vision and colour sensation. The retinas of animals active both day and night, as are those of humans, contain both rods and cones. In parts of human retina, the rods and cones are intermingled and the nervous system provides a switching mechanism that permits adjustment for light conditions. In nocturnal animals, the optical arrangement of the eyes suggests that resolution is scarified for high light-gathering power. For example, in animals such as dog, the lens is large, that is, it has a short focal length. In diurnal animals, the lens is smaller and the front surface is flatter. Thus the focal length is longer and so the image on the retina is larger and dimmer than in the nocturnal eye. As a result, the resolution is improved. The retinas of diurnal animals, have localized areas having a high density of cone cells. In nocturnal animals, the retina is mainly made of rod cells. Rhodopsin, a photosensitive pigment, present in rods is declourised by photons and slowly regenerated in the dark. This ensures better vision for them in dim light.

WHAT CAUSES COOLING WHEN GLUCOSE IS DISSOLVED IN WATER?

Formation of a solution is a physico-chemical process. When two substances mix to form a solution, heat is either absorbed or released. This depends on various interactions taking place between the solvent and the solute at the molecular level. Glucose exists in a crystalline form. When dissolved in water, the crystal structure is broken. To break the bonds in the crystal energy is required. This is obtained from the water itself and so its temperature is reduced. Chemists call this an endothermic process. But considering a similar reaction, the dissolution of salt in water. Though this is also an endothermic process the heat transfer involved is very less. Strong exothermic effects are observed in certain cases where the substances interact strongly with water molecules. For example, dissolution of washing soda or sodium hydroxide.

WHY DO PLASTICS BECOME BRITTLE WHEN EXPOSED TO THE SUN FOR A LONG PERIOD?

Plastics are made of polymers which are giant molecules having long chains of repeating units derived from short molecules. These long chains are entangled with one another and held together by weak interatomic forces such as Vander Waals force. These weak bonds can be easily broken up by sunlight and so long exposures to sunlight makes them brittle.

WHY ARE SOAP BUBBLES AND OIL LAYERS COLOURED?

Coloured bands on soap bubbles and oil layers are caused by interference of light waves with themselves. Sunlight is a composite of seven colours each of which lies in a specific wavelength range. For ex. Violet light has a wavelength of 380 nanometres and red, above 600nm. Light waves falling on any thin film are partially reflected from the top and bottom surfaces of the film. A wave reflected from the bottom surface has to travel a longer distance than the one reflected from the top. This difference in distance traveled by the waves, called path difference, leads to a difference in the phases of the light waves. This phase difference depends on the thickness of the film at the point of reflection and angle of viewing. If the waves are ‘out of phase’, then they cancel each other and if they are ‘ in phase’ they add up. Thus the regions of the spectrum, which interfere constructively can be seen whereas those which interfere destructively are lost. Even if there is a slight variation in the thickness of the film, it will be coloured differently.

WHY DO RUNNERS RUN IN ANTI-CLOCKWISE DIRECTION?

As the heart is on the left side, for humans and animals, running anticlockwise makes the centrifugal force in the body to act from left to right. Whereas it is from right to left for clockwise running. Superior venecava takes blood to the heart aided by heart suction. This vein carries blood from left to right. Centrifugal force due to anticlockwise running helps this suction. If we run clockwise, the centrifugal force impedes suction. That is why, in olden days, health officers ensured that all carnival merri-go-rounds were run only in the anti-clockwise direction. Racing tracks, animal shows in circuses, bullock-drawn pelt on wheels, all mostly have only left turns. Stairways in temple towers have only left turns for going up. Clockwise running tires people, especially children easily.

WHY DOES THE TOUCH-ME-NOT PLANT SHRINK WHEN TOUCHED?

The touch-me-not plant shrinks within a few minutes of being touched. This is due to the loss of turgidity by cells within the pulvini-specialised motor organs at leaf joints. Upon stimulation the leaf cells lose potassium ions which causes water to leave the cells by osmosis. It takes about 10 minutes for the cells to regain turgidity and the leaflets to open out.

WHY DO VEGETABLES SUCH AS CUCUMBER, SNAKE GOURD AND BOTTLE GOURD SOMETIMES TASTE BITTER?

Bitterness in cucumber and other cucurbitaceae vegetables is due to the presence of compounds called cucurbitacins. Chemically these are tetracyclic triterpenes having high oxidative levels. They occur in nature as free glucosidesor as complicated mixtures, at high concentrations, in fruits and roots, for example in a wild variety of cucumber called cucumis hardwikii. High temperature above 92 degrees has been implicated in the increase of bitterness in fruits, although there is no evidence to support this. Conversely more bitter cucumbers are seen growing during the cooler growing season.