NCERT Solutions Curiosity Chapter 10 Chapter exercises — Let Us Enhance Our Learning

Life on the earth would collapse. Photosynthesising organisms — green plants, algae and some bacteria — are the only ones that make food from simple substances. Everything else depends on them.

• Food would run out. No new food would be produced anywhere. Herbivores would starve first; then the carnivores that eat herbivores; finally every animal, including human beings.
• Oxygen would fall. Oxygen is added to the air mainly by photosynthesis. Respiration by all living things, and burning, would keep using it up without any being replaced.
• Carbon dioxide would build up in the air, because nothing would be removing it.
• Food chains and food webs would break down, and with them the whole balance of nature.
• There would be no grains, fruits, vegetables, cotton, timber or fodder, so farming, clothing and much of industry would end too.

Where the starch comes from: it comes from the food the potato plant made in its own leaves by photosynthesis. It was not taken from the soil.

Where the food is synthesised: in the leaves, the primary site of photosynthesis. Carbon dioxide from the air and water from the roots are combined, in the presence of sunlight and chlorophyll, into glucose.

How it reaches the potato: the glucose is loaded into the phloem and carried down from the leaves to the underground stems. There the extra food is converted into starch and stored, and those swollen underground stems are the potatoes we eat.

Yes, it does. The broad, flat shape suits every requirement of photosynthesis at once.

• Large surface for sunlight. A wide flat blade held out to the sky catches far more light than a narrow or rolled leaf of the same amount of material.
• Thin, so light reaches all the chlorophyll. In a thick leaf the deeper cells would be shaded by the ones above them.
• More stomata exposed to the air, so carbon dioxide can enter quickly and oxygen can leave.
• Short distance for gases inside the leaf. Because the blade is thin, carbon dioxide has only a tiny distance to travel from a stoma to the cells that use it.
• A branching network of veins spread across the flat blade brings water everywhere in the leaf and carries food away.

The equation is the word equation for respiration:

Comparing term by term:

(i) The idea she is testing: that sunlight is essential for plants to grow and to make food (starch). Both pots are of the same size with similar plants and, we assume, the same water and soil; only the light is different. So whatever difference appears must be due to sunlight.

(ii) The visible differences (as seen in Fig. 10.10):

(iii) Which leaves confirm the iodine test: only the leaves of the plant kept in sunlight turn blue-black, confirming the presence of starch. The leaves of the plant kept in the dark show no colour change, because with no sunlight no photosynthesis took place and no starch was made.

Photosynthesis needs all four of sunlight, chlorophyll, water and carbon dioxide. Each plant in Fig. 10.11 has enough water and is green, so only sunlight and carbon dioxide decide the result.

(i) Starch will be formed — only in plant (a), kept in sunlight with carbon dioxide.

(ii) Starch will not be formed — in plants (b), (c) and (d).

(iii) Oxygen will be generated — only in plant (a), since oxygen is released only during photosynthesis.

(iv) Oxygen will not be generated — in plants (b), (c) and (d).

What she wants to find out: how living things change the amount of carbon dioxide in the water around them — that is, that an animal (the snail) adds carbon dioxide by respiring, a green plant uses up carbon dioxide by photosynthesising, and the two together can balance each other. Test tube D, with water only, is her control.

How she will know she is correct: by comparing the colours after 2–3 hours with the initial colour she recorded, and with test tube D.

• If D shows no change, then any change in A, B or C was caused by the living organism inside and not by the water or the indicator itself.
• If A shows more carbon dioxide, B shows less, and C shows almost none of either change, her idea is supported.
• To be surer still, she can repeat the whole set in the dark. Then B should also show an increase, because the plant can only respire — showing that the fall in B was really due to photosynthesis.

Question being tested: does water move up a plant faster in warm conditions than in cold ones?

Materials: two similar tender twigs with white flowers (white sadabahar or balsam), two identical glass tumblers, water, red ink, a marker pen, a scale, a clock, and a warm place and a cool place (or a bowl of warm water and a bowl of cold water to stand the tumblers in).

Method:

1. Label the tumblers W (warm) and C (cold) and fill each one-third with water.
2. Add the same number of drops of red ink to both, and mark the starting water level on each tumbler.
3. Cut both twigs obliquely at the base while keeping them under water, and put one twig in each tumbler at once.
4. Keep tumbler W in a warm place (in sunlight, or standing in a bowl of warm water at about 35–40 °C) and tumbler C in a cool, shaded place (or standing in cold water). Everything else must be the same — same twigs, same ink, same amount of water, same light if possible.
5. After 1 hour, 2 hours and 3 hours, note in each twig: how far up the stem the red colour has risen (measure with the scale), and whether the leaf veins and flowers have begun to turn pink.
6. Also note how much the water level in each tumbler has fallen.

Observation table:

Expected result: the red colour rises higher and faster in the warm tumbler, its flowers turn pink sooner, and more water disappears from it. So water transportation in plants is quicker in warm conditions.

The two processes are opposite to each other, and that is precisely why they keep nature in balance. What one uses, the other produces.

How the balance works

• Balance of gases: respiration (and burning) removes oxygen and adds carbon dioxide to the air; photosynthesis removes carbon dioxide and adds oxygen. So the proportion of these gases in the air stays roughly steady.
• Balance of food and energy: photosynthesis locks the sun's energy into glucose; respiration unlocks it wherever it is needed. Without photosynthesis there would be no food to respire; without respiration the food could not be used.
• Balance of matter: the carbon dioxide breathed out by a cow feeds the grass, and the grass feeds the cow. Carbon and water keep going round and round.

What happens if the balance is upset: cutting down forests reduces photosynthesis while respiration and the burning of fuels go on, so carbon dioxide in the air rises and oxygen falls. Planting trees restores the balance — which is why forests are called the lungs of the earth.