Category Science

Why does apple turn brown on being cut?

     Apple contains an enzyme known as polyphenol oxidase (it is a copper containing enzyme).

            When the fruit is cut, this enzyme becomes reactive as it comes into contact with air. It reacts with the sugar present in the fruit and results in the formation of brown colour on the cut surface. If cut apple is dipped in an ascorbic acid solution browning of the cut surface can be prevented as the acid inhibits activity of the enzyme.

            Apple contains iron in the form of ferrous ions. These ferrous ions easily oxidize into ferric ions. This ion in the ferric state is brown in colour. When the apple is cut open the ferrous ions on the cut surface are exposed to the air. Air oxidizes them and the resulting ferric ions turn the surface brown.

Why does the lotus leaf not get wet?

            Lotus leaf does not get wet due to our layers of cells in the epidermal layer of leaves. They contain cellulose, which get converted to cutin by the process of cutinization and form an impermeable membrane on the cell wall known as cuticle.

            Cuticle is a layer of wax-like substances which are simple lipids containing one molecule of fatty acids esterifies with one molecule of long-chain alcohols instead of glycerol. A molecule of wax consists of odd number of carbon atoms ranging from C25 to C35. These are highly insoluble in water and chemically inert because these do not have double bonds in their hydrocarbon chains. Hence waxes form a protective covering. The formation of wax will be more in lotus leaf and hence being impermeable, it won’t allow the leaf to get wet.

            The epidermal cells of the aerial parts of the plant very often deposited with a layer of fatty material called cutin on their exposed wall surface. It is actually made up of two layers namely inner cutinized layer, a layer of cellulose encrusted with cutin and outer cuticularised layer, a layer consisting of cutin ad crusted on the cell wall. When the cuticle is of considerable thickness, its chemical nature often varies in different plants, at least proportionally and may include cutin and wax. Since this layer is resistant to decay, and to microbes, in all the land plants it may have protective function and also prevent surface evaporation. In the case of aquatic plants like lotus, in the outer surface of the upper epidermis of floating leaves there is a conspicuous deposit of wax.

            It is found as a thick layer on the surface of the cuticle. It is this wax that gives the bloom to these glaucous leaves and also resists wetting. So when water is spilled over this surface it will roll down and will not form a film over the leaf surface. If this unwetting property is not found, this film of water when happens to cover the leaf surface will close all the stomata, which will affect the gaseous exchange in this floating leafed plant, which has stomata only on the upper surface of the leaves. It is one kind of Hydrophytic adaption.

            The unwettability prevents the retention of water on the leaf surface and thereby reduce the water weight which otherwise cause the submerging of the leaf instead of floating.

 

Why do flowers have different colours?

The flavonoids are fifteen-carbon compounds that are generally distributed throughout the plant kingdom.

The most common basic flavonoid skeletron, shown below, is usually modified in such a way that more double bonds are present, causing the compounds to absorb visible light and thus giving them colour.

The two carbon rings at the left and right ends of the molecule are designed the A and B rings respectively.

Three widely distributed groups of flavonoids are: anthocyanins, flavonols, and flavones. The anthocyanins are coloured pigments most commonly seen in the red, purple and blue flowers. They are also present in various other plant parts, such as certain fruits, stems, leaves, and even roots.

Most fruits and flowers owe their colours to anthocyanins, although some, such as tomato fruits and several yellow flowers, are coloured by carotenoids.

Several different anthocyanins exist in plants, and often more than one is present in a particular flower. These molecules differ only in the number of hydroxy1 groups attached to the B ring of the basic flavonoid structure.

The exact colour of the anthocyanins depends first upon the substituent groups present on the B ring.

When methy1 groups are present they cause a reddening effect. Secondly, the anthocyanins are sometimes associated with other phenoic types of compounds, and this seems to cause them to become bluer. Finally, the pH of the cell sap has a strong controlling influence upon their colour.

The flavonols and flavones are closely related to the anthocyanins, except that they differ in the central oxygen-containing ring structure of the flavonoid. Naturally occurring flavonols and flavones are hydroxylated in various positions on both A and B rings.

Most of the flavones and flavonols are yellowish or ivory coloured pigments and, like the anthocyanins, they often contribute to the yellow, cream, ivory and white colour of flowers.

            Sometimes they do not appear coloured to the human eye, but they are apparent to bees or other insects that are attracted to flowers containing them. This is because the eyes of the insects are sensitive t ultraviolet wavelengths that give these compounds their colours.

                        

Why does jasmine bloom only in the night?

Nastic movement is responsible for blossoming of flowers. Usually this movement takes place in a flat plant part oriented relative to the plant body and produced by diffuse stimuli causing disproportionate growth or increased turgor pressue in the tissues of one surface. It normally occurs in leaves and petals which are bilaterally symmetrical.

          Changes in the environment send a signal to the plant and result in a differential growth between the upper and lower surfaces of petals resulting in blossoming of flowers and different conditions.

            In case of jasmine, this response occurs due to stimulus caused by the change over from brightness to darkness.

            As there is more growth on the upper sides (epinastic movement) of the petals, the flower opens. If there is more growth on the lower side of petals the flower closes (hyponastic movement).

Why and how does the sunflower always face the Sun?

Certain flowers such as sunflower are attracted to the sun strongly. They begin the day facing east and then follow the sun. This is because of a phenomenon called phototropism.

         Phototropism is a growth-mediated response of a plant to stimulation by visible light. The response is stimulated by a hormone called auxin present in the stem.

   Auxins promote lengthwise growth of plants. The auxin, beta-indylacetic acid (IAA), is formed either from the amino acid, tryptophan, or from the breakdown of carbohydrates known as glycosides.

They promote growth by acting on the chemical bonds of carbohydrates on the cell wall. In positively phototropic plants when one side of the plant is shaded, greater quantities of auxin are produced on the darker side. This causes that side of the plant to grow fast. In the case of sunflower, the phenomenon is pronounced so as to make the flower turn towards the sun.