Bio 11 – Botany – Exercise 12 – Photosynthesis

A. Chlorophyll as a Factor in Photosynthesis

1. Examine young variegated leaves of coleus or caricature plant, Graptophyllum pictum (L.) Griff, and note the distribution of chlorophyll as indicated by the green area.

2. place the leaf in boiling water for a few minutes. Immerse it in a test tube containing 95% ethyl alcohol. Place the test tube in a warm bath until the pigments are extracted.

2.1 What is the purpose of boiling the leaf?

Burst the chloroplasts

3. Rinse the bleached leaf with water and place in a petri dis. Test for the presence of starch using dilute IKI solution. The presence of starch as shown by a bluish-black to purple coloration indicates the occurence of photosynthesis.

3.2 How does it compare with your sketch in 1.1?

Green ares – photosynthetic

Non-green areas – non-photosynthetic

3.3 Do the non-green areas of the leaf contain starch?

Yes although minimal

3.4 What can you conclude from this experiment?

Only those with chlorophyll can undergo photosynthesis

B. Light as a Factor in Photosynthesis

4. Place a potted coleus or bean plant in the dark. After 48-75 hours, warp one of the leaves with carbon paper or aluminum foil. Expose the plant to light for 6 to 10 hours. Detach the covered and one uncovered leaf of the same size and test for presence of starch as previously done in section A above.

4.1 Why was it necessary to place the plants in the dark for 48-72 hours?

Consume starch reserves in plant

4.2 Why was carbon paper or aluminum foil used?

Prevent light from reaching leaves

4.3 What areas in the covered and uncovered leaves showed a positive reaction to the starch test?

Uncovered areas contained starch

4.4 Can plants carry out photosynthesis under artificial light as well as natural or day light?

Yes

4.5 What can you conclude from this experiment?

Light is needed for photosynthesis

5. Immerse a hydrilla shoot with the cut end of the stem turned up in a clean test tube with tap water. To enrich the CO2 content of the water, add 10-15 drops of 1 percent NaHCO3. place the tube under dim light and count the number of bubbles released at 1 min intervals for 10 mins. Repeat the experiment under bright light.

5.2 How would you correlate the rate of gas evolution with the rate of photosynthesis?

directly proportional

5.3 What is the effect of light intensity on the rate of photosynthesis?

More intense light, higher rate of photosynthesis

C. Carbon Dioxide as a Factor in Photosynthesis

Half-fill each of three test tubes with water that has previously been boiled to remove dissolved air and cooled to room temperature. Add several drops of phenol red to each tube and shake gently. Phenol red is an indicator of p H. Indirectly it is an indicator of CO2 concentration since CO2 in water forms carbonic acid which lowers the pH of water. At low pH, the color of phenol red changes to yellow. With the aid of a soda straw, blow into the water in test tubes 1 and 2 until the solution just shows a change in color from red to yellow. Test tube 3 serves as control.

6.1 Account for any change in color in test tubes 1 and 2

Increase in acidity

7. Using the set-up prepared in section 6, place a shoot of hydrilla in test tube 1. place the 3 tubes in very bright light. After 20 to 30 minutes, note the color changes.

7.1 Which tubes show a change in color? Why?

Color in test tube 1 goes back to red since CO2 is being used for photosynthesis by the hydrilla plant. With less CO2, the water’s acidity goes back to normal level.

D. Separation of Pigments

8. Secure 5 mature leaves of papaya or 10 of hibiscus. Cut the lamina into small pieces and grind in a mortar to secure a deep green liquid extract and collect this into a small test tube. Place two or three drops of the leaf extract at about 1 to 2cm fromthe base of a strip of filter paper the width of which should not touch the sides of the test tubes. Carefully hang the strip on the lower or basal end of a cork with the use of a aper clip or adhesive and lower this into a clean dry test tube to which has been added 3-5ml of a solvent consisting of a mixture of 95 parts petroleum ether and 5 parts of acetone. The part of the strip containing the leaf extract should not be submerged in the solvent. Cork the test tube and observe the separation of the pigments. The development of the chromatogram is stopped when the solvent has travelled about 1 to 2 cm from the top of the strip. The chromatogram is stopped when the solvent has travelled about 1 to 2 cm from the top of the strip. The chromatogram should be observed at frequent intervals because if the separation continues for too long some of the pigments may be superimposed on each other near the top of the strip.

8.1 What colors are indicated in the chromatogramand what are the corresponding pigments they represent?

orange – carotene

yellow – xantophyll

yellow green/ light green – chlorophyll b

green / blue green – chlorophyll a

Note:

Some answers differ for each class/group.

Table 12.1 is based on your group’s data.

For 8.1, it is possible for the chromatogram to not show an orange color.

For “8.2. Which pigment is the least soluble or moves the slowest along the strip? The most soluble and travelling farthest up the strip?” Least soluble would be the color at the bottom and the most soluble would be the one at the topmost.

NaHCO3 was used to supply CO2 in order to drive photosynthesis.