My Science School

   Ripening may be regarded as a special case of sequence. During ripening, a number of enzyme-assisted reactions take place inside the fruits. The list includes softening of tissues, hydrolysis, changes in pigmentation, flavour and respiration rate, and conversion of carbohydrates and organic acids into fruit sugars. These changes are induced by ethylene which is also called as ripening hormone.

            It has been found that during ripening, ethylene production goes up. In fleshly fruits like mango significant amount of ethylene may be present some time before ripening, but the fruit’s response to ethylene is inhibited till the fruit is harvested. In banana, a presumably effective ethylene concentration may be present in the unripe fruit, but the fruit is insensitive to that concentration at that stage. Only as it matures, it becomes sensitive and begins to ripen.

            Generally, an ethylene-forming mechanism and breaking of the insensitiveness to ethylene are attained only fruits reach a certain physiological age.

            When unripe fruits are kept inside a sack or tin of rice, the time needed to attain this critical physiological age is shortened. It could be that the fruit is totally cut off from light which promotes yellowing. (It is not known whether there is any increase in the temperature of the fruit.) The ethylene produced in the fruit also diffuses rapidly through the fruit’s tissues.

            If the fruits are placed in an airy place, this ethylene may be immediately lost in the air. When confined in rice or sack, its flow is restricted and there is always a layer of ethylene surrounding the fruit which accelerates ripening.

 

"Temperature changes can delay or hasten the ripening of banana. Banana is a tropical fruit, adapted to ripen quickly at a certain stage of its development and at a particular temperature and humidity. It continues to ripen after harvest, with more and more of its starch converted into sugars by the action of enzymes. When harvested, a banana contains about 20 per cent starch and only 1 percent sugar. By the time the fruit is ripe, the proportions are reversed.

            Banana also releases comparatively large quantities of ethylene gas to help itself ripen; the gas will even ripen other fruit put in a bag with a ripening banana.

            Bananas are usually harvested when still green, cutting off the supply of nutrients at the stem, and then shipped at a temperature low enough to slow the action of the enzymes of ripening. Later, the bananas are brought back up to a temperature and humidity that let the enzymes become active again.

            To high a temperature destroys the enzymes, and too low a temperature can break down the cell walls of the fruit so the contents mix and the fruit oxidizes browns and soften abnormally. The optimum temperature and humidity conditions for ripening are about 20 degree C and 90-95 per cent relative humidity. Storage temperatures should be about 13 degree C.

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The details can also be taken from NYT article: http://www.nytimes.com/1998/02/10/science/q-a-490229.html as originally this article has been published as link submitted above.

         

 

 

 Ripening of fruits is associated with the process of senescence or aging in plants. It involves change in colour, texture, flavour, sugar content and acidity, and is influenced by the ripening hormone ethylene.

            Mr. T. Nagendra Pillai of Guruvayoor, Kerala, writes: As ripening begins, there is a climacteric increase in respiration, which is followed by increased ethylene production. It triggers a series of biochemical changes such as lateral growth, loosening of cell walls resulting in more intercellular spaces, conversion of starch and organic acids into sugars, hydrolysis of stored materials, softening by enzymatic changes of pectin substances, decrease in chlorophyll content with corresponding increase in anthocyanins (colouring) pigments and emission of characteristic volatile oils. 

            Mr. S. Palaniappan of Pudukkottai, TN, adds that ethylene production is increased more than 100-fold during climatic rise.

            Mrs. P.S. Dheenadayalan, Cimbatore, says: Colour changes occur due to synthesis f carotenoids (yellow and red) and phenolic compounds like anthocyanins (red and blue).

            Changes in texture occur by limited degradation of cell walls followed by an increase in poly-galaturonase and pectin-esterase activity. In banana and apple, the enzyme phosphorylase and in mangoes, amylase, break the starch into glucose and sucrose leading to their sweet taste. Volatile compounds such as ethyl 2- methyl1 but rate (in apple) elicit a sweet smell.

            He adds, acidity of fruits is due to the presence of malic acid (in apple, apricot, banana, cherry and plum), citric acid (in gooseberry, tomato, and peaches), and malic acid and tartaric acid (in grapes).

            Ripening is a pre-requisite for the development of embryos after fertilization for better dispersal of seeds for survival.

     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.

         

 

 

 

 

 

 

  Lime juice contains 6-10 per cent of citric acid. Cement is a complex mixture of calcium silicates and calcium aluminates. When drops of lime juice fall on the floor, a chemical change takes place. One of the products is calcium citrate which gives a white colour on these spots.

            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.

 

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.

                        

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).

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.

Before flowering, a plant must attain the ripe-to-flower condition. To attain this condition the plant must complete a period of vegetative growth. Attainment of this condition does not automatically lead to the initiation of flower primordial.

            Certain environmental condition must follow. Temperature and the duration of light and dark periods within the 24 hour cycle are the two important environmental factors that influence the initiation of flower primordial in a plant that has attained ripe-to-flower condition. The response of a plant to this aspect of light is called photoperiodism.

            When appropriate photo period is given to a plant that has attained the ripe-to-flower condition, this metabolism is altered. This results in the formation of flower stimulus, which may be a hormone (florigen).

            When this flower stimulus is translocated to the shoot apex, the vegetative shoot apex is transformed to reproductive shoot apex, which results in the initiation of flower primordium.

            In natural conditions, the period taken to attain the ripe-to-flower condition and the period taken to obtain appropriate photoperiod differ widely among different plant species. Kurinji (Strobilanthes kunthianus) needs a period of 12 years for having to be subjected to a cycle conducive to flowering.

 

 

 

 

     

      On March 3, 1998, a joint patent (US patent No: 5723765) has been granted to United States Department of Agriculture (USDA) and the Delta and Pine Land Company, Mississippi in the name of ‘Control of Plant Gene Expression’. Mr. Hope Shand, Research Director, Rural Advancement Foundation International (RAFI), christened it as ‘Terminator technology’ as the hybrid seeds containing it do not germinate after one generation.

            The terminator technology is an extremely complex technology in which two gene systems are brought together to stop the normal process of embryo development, leading to the failure of seed germination. The gene systems are: Gene System I (Gene A) and Gene System II (Gene B and C).

            The gene system I consist of a gene ‘A’ which produces the ribosome inactivating protein (RIP), which is lethal to the growing embryo. Gene ‘A’ is linked to a transistently active LEA Promoter, ‘PA’, through a blocking sequence. A recombinase specific excision sequence (LOX sequence) flanks the blocking sequence on either side.

            The gene system II consists of a gene B linked to a promoter, PB the gene B encodes for a recombinase which is specific to the LOX sequence of the gene system I. A third gene C produces a repressor protein which blinds to the promoter PB and prevents the expression of gene B. The gene B can be depressed by exogenous application of tetracycline.

            To develop a variety of seeds with functional terminator system, two cells of the same crop are transferred with the gene system I and II separately. As a result, one transgenic is obtained with unexpressed gene A due to the presence of blocking sequence between gene A and its promoter PA and another transgenic is obtained with gene system II having repressor of gene B. To recombine these two systems into one, the obtained transgenic are hybridized and normal hybrid seeds are obtained. Since the gene A does not express, the seeds obtained remain viable. Upon treatment of the seeds with tetracycline, the antibiotic is absorbed by the seeding tissue. Since tetracycline acts as an inducer of gene B, it depresses the gene and recombinase is produced. The recombinase removes the intervening blocking sequence between gene A and its promoter PA. Thus PA comes in proper orientation with gene A and the gene ready for expression. The promoter specifically expresses during early embryo development. As a result, the seeds germinate normally in that generation and give rise to normal crop and seeds. But the seeds obtained do not germinate as the embryo gets aborted due to expression of gene A. so long as gene B remains repressed in absence of tetracycline, gene A is not expressed leading to production of viable seed.

            There are reports that this technology is presently being incorporated into two crops viz., tobacco and cotton. But it is a matter of time that it can be incorporated into other crops as well. This technology is not yet introduced into our country.

            It is premature to predict its impact on our farmers. However, we can visualize its utility in curbing the spurious practice of selling F2 seeds of a hybrid variety as F1 seed.

 

            Milk is a fine suspension of fat and protein globules in a watery liquid containing milk sugar (lactose) and other dissolved substances. Normally the fat is dispersed uniformly in the form of fine droplets and hence remains as an emulsion. But when milk is heated these fine droplets tend to raise up and at temperature of about  form a thick layer of cream on the surface. When milk comes to boiling, bubbles of steam formed at the bottom of the vessel rise up and force the thick creamy layer upwards causing the milk to spill.

                                    

            There is a close link between the sense of taste and the sense of smell. Much of what we think of as taste has a big smell component. The taste buds in the tongue monitor relatively crude sensation of the four basic tastes – sweet, salty, sour and bitter. Faint vapours of whatever we eat drift into the nasal cavity where the smell receptors add more detail to the information given by the taste buds.

When we have a cold our nose becomes blocked and the sensation of smell ceases. The gas and vapours from the food cannot be sensed by the smell receptors; consequently we cannot perceive the smell or aroma of the food, which in turn makes the food appear tasteless.

            Spoilage of foodstuffs is caused by microorganisms which decompose them to produce acid, alcohol, etc. Bacteria are always present in the atmosphere. When freshly prepared food is kept in the open for long periods it gets contaminated. The bacteria then act upon the foodcomponents such as proteins, carbohydrates and fats and break them down to products which due to their off-odours or bad-taste are not suitable for consumption. Factors such as high temperature and humidity help the bacteria to grow.

    Spoilage of food can be prevented by treatments like heat sterilization, refrigeration, deep-freezing and desiccation.

                                    

  Flowering is a natural mechanism by which the majority of plants perpetuate their species through the production of seeds. All the parts of a flower take part in one way or another in the reproductive process. The stamens consist of thread-like stalks supporting sac-like anthers which produce the pollen. The central pistil contains the ovary and a sticky stigma. The colourful petals serve as signals to attract insects and other pollinators. When pollinating insects or birds visit the flowers for nectar, they help transfer pollen to the stigma which leads to fertilization of the egg cells in the ovary. The ovary then grows into a fruit containing seeds that would, if planted, grow into new plants thus perpetuating the species.