SELINA Solutions for Class 10 Biology Chapter 6 - Photosynthesis

(a) Chlorophyll : Magnesium :: Haemoglobin : Iron

(b) Light reaction : Granum :: Dark reaction : Stroma

(c) Producers : Autotrophs :: Consumers : Heterotrophs

(d) Respiration : Carbon dioxide :: Photosynthesis : Oxygen

(e) Water and minerals : Xylem :: Prepared food : Phloem

(a) Photosynthesis: Photosynthesis is the process by which living plant cells, containing chlorophyll, produce food substances (glucose and starch), from carbon dioxide and water, by using light energy and release oxygen as a by-product.

(b) Thylakoids: Closely packed flattened sacs arranged in piles in the interior of chloroplasts are called thylakoids.

(c) Chloroplast: Chloroplasts are minute oval bodies bounded by a double membrane which contains thylakoids arranged in piles called grana lying in a colourless ground substance called stroma.

(d) Photolysis of water: Photolysis of water is defined as the splitting of H₂O molecules into hydrogen ions and oxygen in the presence of light.

(e) Polymerisation: Polymerisation is the process in which several glucose molecules are transformed to produce one molecule of starch.

(a) Guard cells: They regulate the opening and closing of stomata and thus regulate the entry of carbon dioxide through the stomata.

(b) Cuticle: Cuticle is transparent and water proof due to which light can penetrate this later easily.

(c) Chlorophyll: Chlorophyll is vital for photosynthesis, which allows plants to absorb energy from light. Light energy is converted into chemical energy. With photosynthesis, chlorophyll absorbs energy and then transforms water and carbon dioxide into oxygen and carbohydrates.

(d) Stomata: The main function of stoma is to let in carbon dioxide from the atmosphere for photosynthesis. Also most of the oxygen produced during photosynthesis diffuses out into the atmosphere through the stomata.

(e) Xylem Tissue in the Leaf Veins: Water is essential for photosynthesis to occur. Water is taken up by the roots from the soil, sent up through the stem and finally brought to the leaves (site of photosynthesis) through the xylem tissue. The water is then distributed in the mesophyll tissue.

(f) Phloem Tissue in the Leaf Veins: The prepared food is transported from leaves to all parts of the plant by the phloem tissue. The glucose is converted into insoluble starch and later into soluble sugar i.e. sucrose, which is transported in solution through the phloem in the veins of the leaf and down through the phloem of the stem. 

(a) The presence of starch is regarded as evidence of photosynthesis. Hence before starting an experiment on photosynthesis, the plant should be placed in the dark for 24-48 hours to destarch the leaves. During this period, all the starch from the leaves will be sent to the storage organs and the leaves will not show the presence of starch. So the various experiments on photosynthesis can be carried out effectively.

(b) If a green plant is kept in bright light, it tends to use up all the CO₂ produced during respiration, for photosynthesis. Thus, the release of CO₂ cannot be demonstrated. Hence, it is difficult to demonstrate respiration as these two processes occur simultaneously.

(c) The chloroplasts are concentrated in the upper layers of the leaf which helps cells to trap the sunlight quickly. Also the epidermis is covered by a waxy, waterproof layer of cuticle. This layer is thicker on the upper surface than the lower one. Hence most leaves have the upper surface more green and shiny than the lower one.

(d) During the starch test,

(1) The leaf is boiled in water to destroy enzymes so that further chemical changes do not take place in the leaf.

(2) The leaf is boiled in methylated spirit to dissolve chlorophyll.

(a) Differences between light reaction and dark reaction (end products):

(b) Differences between producers and consumers (organisms):

(c) Differences between grass and grasshopper (mode of nutrition):

(d) Differences between stoma and stroma (structure):

(a) The above experiment is conducted to show that chlorophyll is necessary for photosynthesis.

(b) The plant was placed in the dark before beginning the experiment to destarch the leaves.

(c) Balanced chemical equation for photosynthesis:

(d) After the starch test on leaf A, only the green parts of the leaf turn bluish, showing the presence of starch.

(e) Chloroplast:

a) continue to live, but will not be able to store food

(a) Producers / Autotrophs

(b) Chloroplasts

(c) ATP (Adenosine triphosphate)

(d) Glucose

(e) Carbon dioxide dissolved in water

(f) Stroma

(a) False

Correct Statement: Dark reaction of photosynthesis is independent of light and occurs simultaneously with light reaction.

(b) True

(c) False

Correct Statement: Green plants are producers.

(d) False

Correct Statement: Respiration results in loss of dry weight of the plants.

(e) False

Correct Statement: Photosynthesis stops at a temperature of above 40^oC.

(f) True

(g) True

(h) True

(a) grana

(b) iodine solution

(c) chloroplast

(d) Calvin cycle

(e) Sucrose

(a) False

Photosynthesis increases with the light intensity up to a certain limit only and then it gets stabilized.

(b) False

The atmospheric temperature is an important external factor affecting photosynthesis. The rate of photosynthesis increases up to the temperature 35^oC after which the rate falls and the photosynthesis stops after 40^oC.

(c) False

Ice cold water will hamper the process of photosynthesis in the immersed leaf, even if there is sufficient sunshine because the temperature is an important factor for the rate of photosynthesis.

(d) False

For destarching, the potted plant can kept in a dark room for 24-48 hours.

(e) False

There is no start point or end point in the carbon cycle, the carbon is constantly circulated between the atmosphere and the living organisms.

(f) False

If a plant is kept in bright light all the 24 hours for a few days, the dark reaction (biosynthetic phase) will continue to occur because the dark reaction is independent of light and it occurs simultaneously with the light dependent reaction.

(g) True

Photons, grana, water molecules, hydrogen and hydroxyl ions, oxygen

(Any 4)

Oxygen is released during photosynthesis. Some of this oxygen may be used in respiration in the leaf cells, but the major portion of it is not required and it diffuses out into the atmosphere through the stomata. However, in a sense, even this oxygen is not a waste because all organisms require it for their existence including the plants.

(i) Light Reaction:

The light reaction occurs in two main steps:

(1) Activation of chlorophyll - On exposure to light energy, chlorophyll becomes activated by absorbing photons.

(2) Splitting of water - The absorbed energy is used in splitting the water molecule into hydrogen and oxygen, releasing energy. This reaction is known as photolysis of water.

2H₂O  4H⁺ + 4e^- + O₂

The fate of H⁺, e^- and (O) component are as follows:

• The hydrogen ions (H⁺) obtained from above are picked up by a compound NADP (Nicotinamide adenine dinucleotide phosphate) to form NADPH.

NADP⁺ + e^- + H⁺  NADPH

• The oxygen (O) component is given out as molecular oxygen (O₂).

2O  O₂

• The electrons (e^-) are used in converting ADP into energy rich ATP by adding one inorganic phosphate group P_i.
  
  ADP + P_i ATP
  
  This process is called photophosphorylation.
  
  (ii) Dark reaction: The reactions in this phase does not require light energy and occur simultaneously with the light reaction. The time gap between the light and dark reaction is less than one thousandth of a second. In the dark reaction, ATP and NADPH molecules (produced during light reaction) are used to produce glucose (C₆H₁₂O₆) from carbon dioxide. Fixation and reduction of carbon dioxide occurs in the stroma of the chloroplast through a series of reactions. The glucose produced is either immediately used up by the cells or stored in the form of starch.
  
  
  
  

Complete the following food chains by writing the names of appropriate organisms in the blanks:

(i) Grass → Rabbit. → Snake → Hawk

(ii) Grass/Corn → Mouse → Snake → Peacock

Non-green plants such as fungi and bacteria obtain their nourishment from decaying organic matter in their environment. This matter comes from dead animals and plants. Fungi and bacteria break down the organic matter to obtain the nourishment and they release carbon dioxide back in the atmosphere.

Chlorophyll is the foundation site for the photosynthesis in green plants. The initiation of photosynthesis takes place when the chlorophyll molecule traps the light energy. The light energy is then converted into chemical energy in the form of glucose using carbon dioxide (CO₂) from the atmosphere, and water (H₂O) from the soil. All other organisms, directly or indirectly depend on this food for their survival. The starting point of any food chain is always a plant. If green plants were to suddenly disappear, then so would virtually all life on Earth. Thus, we can say that all life owes its existence to chlorophyll.

To test the leaf for starch, the leaf is boiled in water to kill the cells. It is next boiled in methylated spirit to remove chlorophyll. The leaf is placed in warm water to soften it. It is then placed in a dish and iodine solution in added. The region, which contains starch, turns blue-black and the region, which does not contain starch, turns brown.

a.  

1 - Sunlight

2 - Oxygen

3 - Glucose

4 - Xylem

b. A - Transpiration

B - Translocation

a. Food chain

b. Hawk, eagle

c. Photosynthesis

d. Carbon

Test to determine the presence of starch in a leaf:

• Dip a leaf in boiling water for a minute to kill the cells.

• Boil the leaf in methylated spirit in a water bath to remove the chlorophyll, till the leaf turns pale blue and becomes hard and brittle.

• Now place the leaf in hot water to soften it.

• Place the leaf in a Petri dish and pour iodine solution over it.

• The appearance of a blue-black colour on the leaf is indicative of the presence of starch.

• The absence of starch is indicated by a brown colouration.

a. To demonstrate the importance of carbon dioxide in photosynthesis

b. No, the experiment will not work satisfactorily, as the beaker contains lime water and not potassium hydroxide to absorb CO­₂.

c. Place potassium hydroxide in the beaker instead of lime water

d. Before starting the experiment, it is necessary to destarch the leaves of the plant by keeping the plant in complete darkness for 48 hours. This is because if the plant is not destarched, then the experiment will give false results because starch stored previously may be detected in the leaf placed in the beaker even if no starch is produced during the experiment.

(a) Light is required for photosynthesis.

(b) Before beginning the experiment, the plant was kept in dark in order to destarch it, i.e. to remove the pre-existing starch from the storage organs.

(c)

   (1) The leaf was boiled in water to destroy enzymes so that further chemical changes do not take place in the leaf.

   (2) The leaf was boiled in methylated spirit to dissolve chlorophyll.

(d) Chemical equation for the process of photosynthesis:

(e) Chloroplast:

(a) Photosynthesis releases bubbles of oxygen.

(b) Photosynthesis is a physiological process by which plant cells containing chlorophyll produce food in the form of carbohydrates by using carbon dioxide, water and light energy. Oxygen is released as a by-product.

(c) Carbon dioxide released by the snail during respiration is used by the plant for photosynthesis. This increases the rate of photosynthesis in the plant placed in test tube B. This also suggests that both respiration and photosynthesis are complementary processes to maintain the concentration of oxygen and carbon dioxide in the atmosphere.

(d) A plant and a snail are kept in test tube B. The plant in test tube B has more concentration of CO₂ available because the snail releases CO₂ during respiration. This increases the rate of photosynthesis in the plant placed in test tube B which leads to the release of more amount of oxygen.

(e) Hydrilla

(f) Chemical equation for photosynthesis: