Class 10 Science Notes Chapter 6 (Chapter 6) – Lab Manual (English) Book

Detailed Notes with MCQs of Chapter 6 from your Science Lab Manual. These experiments are crucial not just for your practical exams but also form the basis for many conceptual questions in competitive government exams. We'll break down the key experiments, their procedures, observations, and the underlying principles.

Chapter 6: Life Processes - Experimental Study

This chapter deals with verifying some fundamental concepts of life processes like photosynthesis and respiration through hands-on experiments.

1. Experiment: To prepare a temporary mount of a leaf peel to show stomata.

• Aim: To observe stomata in a leaf peel under a microscope.
• Materials: Fresh leaf (e.g., Bryophyllum, Tradescantia, or any common broadleaf plant), forceps, watch glass, slide, coverslip, safranin stain, glycerine, dropper, brush, blotting paper, microscope.
• Procedure:
  1. Take a fresh leaf and tear it obliquely (usually from the lower surface, as stomata are more numerous there).
  2. Carefully pull out a thin, transparent peel using forceps.
  3. Place the peel in a watch glass containing water immediately to prevent drying.
  4. Add a few drops of safranin stain to the watch glass and leave the peel for 1-2 minutes. Safranin stains the cell walls and nuclei, making them more visible.
  5. Wash off excess stain by dipping the peel in another watch glass with clean water.
  6. Place the stained peel flat in the centre of a clean glass slide using a brush.
  7. Add a drop of glycerine over the peel. Glycerine prevents the peel from drying out quickly and provides a clear medium.
  8. Carefully lower a coverslip onto the peel using a needle or forceps, avoiding air bubbles.
  9. Remove excess glycerine from the sides using blotting paper.
  10. Observe first under low power and then under high power of the microscope.
• Observation: You will see irregular-shaped epidermal cells. Scattered among them are tiny pore-like structures called stomata (singular: stoma). Each stoma is surrounded by two bean-shaped cells called guard cells, which contain chloroplasts. The central opening is the stomatal pore.
• Conclusion: Stomata are present on the leaf epidermis, primarily on the lower surface. They are surrounded by guard cells that regulate their opening and closing.
• Precautions:
  • The peel should be thin and transparent.
  • Avoid folding the peel.
  • Do not overstain or understain.
  • Avoid air bubbles while placing the coverslip.
  • Use glycerine to prevent drying.

2. Experiment: To show experimentally that carbon dioxide is necessary for photosynthesis. (Often referred to as Moll's Half-Leaf Experiment)

• Aim: To demonstrate that CO2 is required for the process of photosynthesis (starch production).
• Materials: A healthy potted plant (preferably with broad leaves), wide-mouthed bottle with a split cork, potassium hydroxide solution (KOH), beaker, alcohol, water bath, iodine solution, grease/Vaseline.
• Procedure:
  1. Destarch the plant: Keep the potted plant in complete darkness for 48-72 hours. This ensures any previously stored starch is used up.
  2. Take some concentrated KOH solution in the wide-mouthed bottle. KOH absorbs carbon dioxide.
  3. Select a healthy leaf. Insert half of this leaf into the bottle through the split cork. Ensure the bottle is airtight by applying grease. The other half of the leaf remains outside in the open air.
  4. Place the entire setup in bright sunlight for 4-6 hours.
  5. Starch Test: Detach the leaf from the plant. Remove it carefully from the bottle.
  6. Boil the leaf in water for a few minutes to kill the cells and make them permeable to iodine.
  7. Boil the leaf in alcohol using a water bath (direct heating of alcohol is dangerous as it's flammable). This removes chlorophyll (decolorizes the leaf).
  8. Wash the decolorized leaf with warm water to soften it.
  9. Spread the leaf in a petri dish and add a few drops of iodine solution.
• Observation: The part of the leaf that was inside the bottle (deprived of CO2 due to KOH) does not turn blue-black, or shows a very light yellowish-brown colour (iodine's original colour). The part of the leaf that was outside the bottle (exposed to atmospheric CO2) turns blue-black.
• Conclusion: This experiment shows that carbon dioxide is essential for photosynthesis, as starch (indicated by the blue-black colour with iodine) was only produced in the part of the leaf that had access to CO2.
• Precautions:
  • The plant must be properly destarched.
  • The bottle must be airtight to ensure KOH effectively absorbs all internal CO2.
  • Use a water bath for boiling the leaf in alcohol.
  • Handle KOH solution carefully as it is corrosive.

3. Experiment: To show experimentally that light is necessary for photosynthesis.

• Aim: To demonstrate that light is required for photosynthesis.
• Materials: A destarched potted plant, black paper strips (or Ganong's light screen), paper clips, beaker, alcohol, water bath, iodine solution.
• Procedure:
  1. Destarch the plant: Keep the plant in darkness for 48-72 hours.
  2. Select a healthy leaf. Cover a portion of it from both sides using strips of black paper, securing them tightly with paper clips. The black paper prevents sunlight from reaching that part.
  3. Place the plant in bright sunlight for 4-6 hours.
  4. Starch Test: Detach the covered leaf. Remove the black paper strips. Perform the starch test as described in the previous experiment (boil in water, then alcohol, wash, add iodine).
• Observation: The part of the leaf that was covered with black paper does not turn blue-black with iodine solution. The parts of the leaf that were exposed to sunlight turn blue-black.
• Conclusion: Light is necessary for photosynthesis, as starch was only synthesized in the parts of the leaf exposed to sunlight.
• Precautions:
  • Ensure the plant is properly destarched.
  • The black paper should cover the leaf portion completely, blocking all light.
  • Perform the starch test carefully.

4. Experiment: To show experimentally that chlorophyll is necessary for photosynthesis.

• Aim: To demonstrate that chlorophyll is essential for photosynthesis.
• Materials: A potted plant with variegated leaves (e.g., Coleus or Croton), tracing paper, pencil, beaker, alcohol, water bath, iodine solution. (Variegated leaves have green and non-green patches).
• Procedure:
  1. Destarch the plant: Keep the variegated plant in darkness for 48-72 hours.
  2. Place the plant in bright sunlight for 4-6 hours.
  3. Detach a variegated leaf. Place it on a sheet of paper and carefully trace the outlines of the green and non-green areas.
  4. Starch Test: Perform the starch test on this leaf (boil in water, then alcohol, wash, add iodine).
• Observation: After adding iodine solution, only the parts of the leaf that were originally green (containing chlorophyll) turn blue-black. The non-green parts (lacking chlorophyll) do not turn blue-black and remain yellowish-brown. Compare this with the tracing made earlier.
• Conclusion: Chlorophyll is necessary for photosynthesis, as starch is produced only in the green parts of the leaf containing chlorophyll.
• Precautions:
  • Use a plant with distinct green and non-green areas.
  • Destarch the plant properly.
  • Trace the patches accurately before the starch test.
  • Decolorize the leaf completely with alcohol.

5. Experiment: To show that carbon dioxide is produced during respiration.

• Aim: To demonstrate that germinating seeds (or exhaled air) produce carbon dioxide during respiration.
• Materials: Germinating seeds (like gram or moong), conical flask, test tube, bent glass tube (delivery tube), freshly prepared lime water [Calcium Hydroxide solution, Ca(OH)2], thread, beaker with water, cotton wool/cork. (Alternatively: test tube, straw, lime water for exhaled air).
• Procedure (using germinating seeds):
  1. Take some germinating seeds in a conical flask. Add a little water to keep them moist.
  2. Fit the flask with a cork through which a bent delivery tube is passed.
  3. The other end of the delivery tube should dip into a test tube containing freshly prepared lime water.
  4. Seal the flask to make it airtight.
  5. Observe the lime water after some time (usually an hour or more).
• Procedure (using exhaled air):
  1. Take freshly prepared lime water in a test tube.
  2. Use a straw or delivery tube to blow air from your mouth (exhaled air) gently through the lime water.
• Observation: In both cases, the lime water turns milky or cloudy white.
• Conclusion: Germinating seeds respire and produce carbon dioxide. Exhaled air contains carbon dioxide. The CO2 reacts with lime water (Calcium Hydroxide) to form a white precipitate of Calcium Carbonate (CaCO3).
  • Reaction: Ca(OH)2 (aq) + CO2 (g) → CaCO3 (s) + H2O (l)
• Precautions:
  • Use freshly prepared lime water, as old lime water might already have reacted with atmospheric CO2.
  • The setup with seeds must be airtight to prevent atmospheric CO2 from interfering.
  • Use actively germinating seeds. A control setup with boiled (dead) seeds can be used for comparison (lime water won't turn milky).
  • When blowing exhaled air, do it gently to avoid splashing the lime water out.

6. Experiment: To identify the different parts of an embryo of a dicot seed (Pea, gram or red kidney bean).

• Aim: To study the structure of a dicot embryo.
• Materials: Dicot seeds (pea, gram, bean), beaker, water, forceps, hand lens/dissecting microscope.
• Procedure:
  1. Soak the seeds in water overnight. This makes them soft and easy to dissect.
  2. Carefully remove the outer covering, which is the seed coat.
  3. Gently split the seed open into its two halves. These fleshy halves are the cotyledons.
  4. Observe the structure attached between the two cotyledons using a hand lens. This is the embryo axis or tigellum.
• Observation:
  • Seed Coat: The protective outer layer.
  • Cotyledons (2): Thick and fleshy structures that store food for the embryo.
  • Embryo Axis: This is the main embryonic plant. It consists of:
    • Plumule: The upper part which develops into the shoot (stem, leaves).
    • Radicle: The lower part which develops into the root.
    • The point of attachment of cotyledons to the axis is the cotyledonary node.
• Conclusion: A dicot seed consists of a seed coat enclosing an embryo. The embryo comprises two cotyledons (food storage) and an embryo axis (plumule and radicle).

Multiple Choice Questions (MCQs):

1. In the experiment to show that CO2 is necessary for photosynthesis, why is KOH solution used?
   a) To provide CO2
   b) To absorb CO2
   c) To provide O2
   d) To absorb O2

2. While preparing a temporary mount of a leaf peel, glycerine is used to:
   a) Stain the cells
   b) Prevent the peel from drying
   c) Remove chlorophyll
   d) Absorb excess water

3. In the experiment to show that light is necessary for photosynthesis, the part of the leaf covered with black paper does not turn blue-black with iodine because:
   a) Chlorophyll was removed from that part.
   b) Stomata were blocked by the paper.
   c) Starch was not synthesized due to lack of light.
   d) CO2 could not reach that part.

4. Which stain is commonly used to observe stomata in a temporary leaf peel mount?
   a) Methylene blue
   b) Iodine solution
   c) Safranin
   d) Eosin

5. Lime water turns milky in the presence of:
   a) Oxygen
   b) Nitrogen
   c) Water vapour
   d) Carbon dioxide

6. Destarching a plant before photosynthesis experiments is done by:
   a) Keeping it in sunlight for 24 hours.
   b) Keeping it in complete darkness for 48-72 hours.
   c) Watering it excessively.
   d) Removing all its leaves.

7. In the experiment demonstrating that chlorophyll is necessary for photosynthesis using a variegated leaf, which part turns blue-black after the starch test?
   a) The non-green parts
   b) The entire leaf
   c) The green parts
   d) The parts exposed to maximum sunlight

8. The function of guard cells in stomata is to:
   a) Absorb water for the leaf
   b) Synthesize starch
   c) Regulate the opening and closing of the stomatal pore
   d) Provide structural support to the epidermis

9. In the structure of a dicot embryo, the part that develops into the root is called:
   a) Plumule
   b) Cotyledon
   c) Radicle
   d) Seed coat

10. Why is the leaf boiled in alcohol during the starch test for photosynthesis?
    a) To kill the cells
    b) To remove the chlorophyll
    c) To make the leaf permeable to iodine
    d) To soften the leaf

Answer Key for MCQs:

1. b) To absorb CO2
2. b) Prevent the peel from drying
3. c) Starch was not synthesized due to lack of light.
4. c) Safranin
5. d) Carbon dioxide
6. b) Keeping it in complete darkness for 48-72 hours.
7. c) The green parts
8. c) Regulate the opening and closing of the stomatal pore
9. c) Radicle
10. b) To remove the chlorophyll

Remember to understand the 'why' behind each step and observation, as that's crucial for tackling application-based questions in exams. Good luck with your preparation!