6. LIFE PROCESSES
Events in photosynthesis:
- Absorption of light energy by chlorophyll.
- Conversion of light energy to chemical energy and splitting of water molecules into hydrogen & oxygen.
- Reduction of CO2 to carbohydrates.
Experiment to show that chlorophyll is essential for photosynthesis:
- Take a potted plant with variegated leaves (e.g. money plant or crotons).
- Keep it in a dark room for 3 days to use up all the starch (destarch).
- Keep the plant in sunlight for about 6 hours.
- Pluck a leaf and mark the green areas in it and trace them on a paper.
- Dip the leaf in boiling water for a few minutes.
- Then immerse it in a beaker containing alcohol. Place the beaker in a water-bath and heat to boil the alcohol.
- The leaf becomes colourless. Chlorophyll is dissolved in alcohol and the alcohol turns green.
- Dip the leaf in a dilute iodine solution for a few minutes.
- The green areas of leaf turn dark blue. It indicates the presence of starch. Colourless part of leaf shows no formation of starch.
Experiment to show that CO2 is essential for photosynthesis:
- Take 2 healthy potted plants having nearly same size.
- Keep them in a dark room for 3 days.
- Now place each plant on separate glass plates. In one, place a watch-glass containing potassium hydroxide (KOH). KOH is used to absorb CO2.
- Cover both plants with separate bell-jars.
- Using Vaseline, seal the bottom of the jars to the glass plates to make it air-tight.
- Keep the plants in sunlight for about two hours.
- Test the leaves from both plants using iodine.
- Leaf of plant kept without KOH turn blue. It indicates the presence of starch. Plant with KOH does not turn blue.
Experiment to show that sunlight is essential for photosynthesis:
- Keep a plant in dark room for 3 days to destarch leaves.
- Cover a leaf partially with a black paper.
- Expose the plant to sunlight for 3-4 hours.
- Remove chlorophyll from the leaf and perform a starch test with iodine.
- Covered leaf part shows brown colour. Exposed leaf turns dark blue due to the presence of starch.
- Some organisms break down food outside the body and absorb it. E.g. fungi like bread moulds, yeast and mushrooms.
- Some take in food and break down inside the body.
- Some organisms take food from organisms without killing them (parasitism). E.g., Cuscuta (amar-bel), ticks, lice, leeches and tape-worms.
Human alimentary canal is a long tube extending from the mouth to the anus.
It is the breakdown of complex food with the help of enzymes into smaller absorbable molecules.
- Take 1 mL starch solution (1%) in test tubes A & B.
- Add 1 mL saliva to test tube A and leave both test tubes undisturbed for 20-30 minutes. Now add a few drops of dilute iodine solution to the test tubes.
- In test tube B, blue colour develops. It indicates presence of starch. In test-tube A, no colour change occurs because starch is digested by salivary amylase.
- Herbivores have a longer small intestine for cellulose digestion.
- Meat is easier to digest. So carnivores have a shorter small intestine.
- Emulsifies fat. It is the breakdown of large fat globules into the smaller globules with the help of bile salts. It increases the efficiency of enzyme action. This is similar to emulsifying action of soaps on dirt.
- Makes the acidic food (from stomach) alkaline.
- Proteins → amino acids.
- Complex carbohydrates → glucose.
- Fats → fatty acids & glycerol.
Dental caries (Tooth decay):
It is the gradual softening & demineralisation of enamel & dentine due to the production of acids by bacteria. They convert sugary foods to acids.
Bacteria invade the pulp causing inflammation & infection.
Masses of bacterial cells together with food particles stick to the teeth to form dental plaque. It prevents saliva reaching the tooth surface to neutralise the acid.
Brushing the teeth after eating can remove plaque.