Thursday, October 13, 2011

PHOTOSYNTHESIS


               PHOTOSYNTHESIS

Experiment 1. Production of gas by pond-weed


(a) Fill a beaker or glass jar with tap-water and add about 5 cm3 (20 mm in a test- tube) saturated sodium hydrogencarbonate solution.

(b) Select a pond-weed shoot between 5 and 10 cm long
.
(c) Take a small paper clip, prize it slightly open and slide it over the pond-weed shoot a
 short distance behind the growing point. This is to hold the weed down in the water.

(d) Place the pond-weed in the jar so that it floats vertically with the cut end of the stem      uppermost but still entirely below the water.

(e) Switch on the bench lamp and bring it close to the jar.

(f) After a minute or two, bubbles should appear from the cut end of the stem. If they do  not, obtain a fresh piece of plant material and try again.

(g) When the bubbles are appearing with regularity, switch the lamp off and observe any changes in the production of bubbles.

(h) Switch the lamp on and place it about 25 cm from the pond-weed. Try to count the number of bubbles appearing in a minute. Now move the lamp to about 10 cm from the plant and again try to count the number of bubbles.



paper-clip
 
 bubbles appear from cut end of stem
 
..\..\My Pictures\Oxygen production.jpg





Experiment 1. Discussion


I Although the bubbles are seen to escape from the cut end of the stem, which parts of the pond-weed could be producing the gas ?

2 In what way does the production of gas appear to be related to the intensity of light reaching the plant?

3 Suppose that putting the lamp at 10 cm caused twice as many bubbles to appear per minute as when it was at 25 cm.  Why would you not be justified in saying that the production of gas by the plant had doubled ?

4 (a) Is there any evidence to suggest what gas or gases might be present in the bubbles?

   (b) Bearing in mind the composition of atmospheric air and the composition of water, what gases might be present in the bubbles ?

5 What was the point of adding sodium hydrogencarbonate to the water at the beginning of the experiment ?

Experiment 1. Discussion - answers


I Assuming some continuity between leaves and stem, the leaves could be producing the gas.

2 An increase in light intensity appears to increase the rate of production of gas.

3 Although twice as many bubbles appear per minute, the bubbles might be smaller so that the total volume of gas produced has not doubled.

4 (a) There is no experimental evidence to indicate the composition of the gas in the bubbles.

   (b) Oxygen, nitrogen, hydrogen and carbon dioxide are present in free or combined form in air or water. All or any of these might be present in the bubbles.

5 The sodium hydrogencarbonate provides carbon dioxide for photosynthesis.

Experiment 1. Production of gas by pond-weed - preparation


Outline Shoots of Elodea are held upside down in a beaker of water. Bubbles of gas appear from the stems when the plants are illuminated.

Prior knowledge The gases present in air and water. Sodium hydrogencarbonate is a source of carbon dioxide.

Advance preparation and materials-per group


Elodea shoots about 80 mm long              
 5 cm3 10% sodium hydrogencarbonate solution

The Elodea should be collected before the experiment, the shoots cut and placed in a large container of fresh tap-water. Just before the experiment the shoots are illuminated strongly with the fluorescent tube (or sunlight) so that the students can select those which are bubbling most strongly. Branched shoots with abundant leaves are likely to prove most satisfactory and the pond-weed will still function after several days in the laboratory.
Ceratophyllum will work but less effectively than Elodea.
Potamogeton crispus has worked well but other species have not been tried.

Apparatus-per group


beaker (250 cm3 or larger) or jam jar                                   bench lamp (60 watts)
test-tube (for collecting hydrogencarbonate solution)          paper clip

-per class
clock

NOTE. Some razor blades should be available for cutting the stems cleanly if they fail to bubble satisfactorily but their use must be supervised.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 




Experiment 2. Testing a leaf for starch


(a) Half fill a 250 cm3 beaker or tin can with water (hot water if available) and place it on a tripod over a Bunsen burner. Heat the water till it boils and then turn down the Bunsen flame sufficiently to keep the water at boiling point.

(b) Hold the leaf in forceps and plunge it into the boiling water for 5 seconds. This will kill the cells, arrest all chemical reactions and make the leaf permeable to alcohol and iodine solution later on.

(c) With the forceps, push the leaf carefully to the bottom of a test-tube and cover it with methylated spirit.

(d) TURN OUT THE BUNSEN BURNER.

(e) Place the test-tube in the hot water and leave it for 5 minutes. The alcohol will boil and dissolve out the chlorophyll in the leaf (See Figure on p.2.02).

(f) Use a test-tube holder to remove the test-tube from the water bath and tip the green alcoholic solution into the receptacle for waste alcohol but take care not to tip the leaf out as well.
If the leaf is white or very pale green, go on to (g).
If there is still a good deal of chlorophyll left in the leaf, boil it for a further 5 minutes with a fresh supply of alcohol, using the hot water bath. If it is necessary to relight the Bunsen to heat the water to boiling point, remove the test-tube and do not replace it until the Bunsen flame is extinguished.

(g) Fill the test-tube with cold water and the leaf will probably float to the top.
     I. Tough leaves (e.g. Tradescantia). Hold the leaf stalk with forceps and dip it into the
     hot water in the water bath for 2-3 seconds. Spread it flat on a tile or Petri dish lid with
     the aid of a little cold water.
    2. Soft leaves (e.g. Busy Lizzie). Use forceps to place the leaf on a tile or back of a       
    Petri dish lid and, holding the leaf stalk firmly against the tile or lid, let a fine trickle of   
    water from the cold tap run over it to wash away the alcohol.

(h) If necessary, use the forceps to spread the leaf quite flat on the tile or lid. Using a dropping pipette cover the leaf with iodine solution for one minute.

(i) Take the leaf to the sink and holding it on the tile or lid, wash away the iodine solution with a fine trickle of cold water .


  

Experiment 2. Discussion


 I What is the reason for extracting the chlorophyll from the leaf?

2 If one is testing a leaf for the products of photosynthesis, why is it necessary to arrest all chemical reactions after detaching it ?

3 For what substance is iodine a test ? What result do you see if this substance is present ?

4 What was the colour of the leaf (a) immediately before adding iodine, (b) after adding iodine?

5 How do you interpret this change ?

6 In subsequent experiments we are going to claim that the presence of starch in a leaf is strong evidence that photosynthesis has taken place. Why is the result in the present experiment not good evidence of this ?

7 What products of photosynthesis might be present which are not revealed by this test?

8 Assuming that the leaf has, in fact, been photosynthesizing and has produced starch, do your results indicate whether starch is the first product of photosynthesis to be formed?



Extracting chlorophyll from a leaf

 
hot water
 
leaf
 
boiling alcohol
 
BUNSEN OUT
 
inflammable vapour
 
..\..\My Pictures\Chlorophyll extraction.jpg




 

Experiment 2. Discussion - answers


I If the leaf is not decolourized it is difficult to decide whether the final colour is due to starch or to the effect of brown iodine staining a green leaf.

2 If the chemical reactions are not arrested at once, the products might change, e.g. the starch made during photosynthesis might be changed to something else so that the results of a starch test prove negative. (Clearly, this is not likely to happen in
experiments such as this but the principle is important.)

3 Iodine is a test for starch and should produce a blue colour.

4 (a) Before adding iodine the leaf should be white or very pale green or yellow.
   (b) After adding iodine the leaf should be black, blue or blue-grey according to the  
    amount of starch.

5 A blue colour indicates that starch is present in the leaf.

6 In this case there is nothing to indicate whether starch is permanently present in a leaf or whether it accumulates solely as a result of photosynthesis.

7 Fructose, glucose and sucrose are products of photosynthesis which are not revealed by the iodine test.

8 The results do not indicate whether starch is the first, the final or an intermediate product of photosynthesis.


Experiment 2. Testing a leaf for starch - preparation


Outline Iodine solution is added to a leaf after killing it and extracting the chlorophyll with
alcohol.

Prior knowledge Chlorophyll is the green pigment in a leaf. Alcohol boils at a lower temperature than does water. Alcohol is inflammable. Methylated spirit, 'meths', ethyl alcohol, ethanol are the various names applied to the alcohol used in this experiment. Photosynthesis involves the formation of carbohydrates in a green plant. Iodine test for starch.

CAUTION. The alcohol sometimes becomes super-heated and shoots out of the tube.
 If it ignites at the same time it can cover a nearby hand or face with burning alcohol.
The Bunsen burner must be extinguished when the alcohol is being heated. Super-heating can be reduced by putting a few grains of coarse sand or unglazed porcelain in the test-tube.

It is best to dispense the alcohol from two or three points about the laboratory, away from the students' burners.

Advance preparation and materials

(a)  This experiment is the test used in experiments 3-5 and, if time is short, can be
         used directly in one of these without making a separate experiment of it. If it is
         done separately, the plants should be exposed to sunlight or artificial light for
         4-6 hours before the leaves are tested.
(b)  Thick leaves with waxy cuticles are not suitable. Pelargonium, nasturtium, busy
       Lizzie, Tradescantia, Coleus are all suitable plants. The plants should have
       received 2-3 hours of sunlight or illumination from the fluorescent lamp.
(c)  Industrial methylated spirit or the blue, mineralized 'meths' are suitable for 
        decolourizing the leaf. Allow 25 cm3 per group, per leaf.
(d)  Iodine solution. Grind 1 g resublimed iodine and 1 g potassium iodide in a mortar
while adding distilled water. Pour the solution into a measuring cylinder and dilute with distilled water to 300 cm3. Do not store the solution in polythene bottles or it will be decolourized.
        Allow 5 cm3 per group.

Apparatus-per group

tripod                                                            gauze
heat mat                                                       Bunsen burner
forceps                                                         dropping pipette, if not incorporated in bottles
iodine solution                                                                                          
beaker (250 cm3) or tin can for water bath             
tile (a glass or plastic Petri dish will serve as well as a tile and has the advantage that the leaf can be held up to the light to see the starch distribution if the colours are faint)

-per class
flasks for dispensing the alcohol and collecting waste alcohol

Experiment 3. The need for light in photosynthesis


You are provided with a potted plant which has been kept in darkness for at least 48 hours so that any starch in its leaves has been converted to sugar and removed.

(a) Water the plant if the soil appears to be dry

(b) DO NOT REMOVE ANY LEAVES but select one at the top of the shoot, preferably one held out from the stem nearly horizontally.

(c) Round this leaf wrap a strip of aluminium foil. Press it close to the surface so that this part of the leaf cannot receive any light at all. Put your initials on the foil with a spirit marker.

(d) Place the plant in direct sunlight or under the light source so that the leaf is directly under the fluorescent tube. When the whole class has prepared the plants, the tube can be lowered to within a centimetre or two of the top leaves.

(e) After a period of time (40-50 minutes is the minimum), detach the leaf with the foil and test it for starch as described in Experiment 2.

press foil closely to leaf to exclude light
 
aluminium foil
 
..\..\My Pictures\Light, need for 600dpi.jpg

Experiment 3. Discussion

 

.I Describe the appearance of the leaf after it had been tested for starch.

2 (a) What is the significance of the colours and their distribution in the leaf ?
   (b) How could this be explained in terms of photosynthesis and light ?
   (c) Suggest at least one other way in which the results could be explained.

3 (a) Apart from cutting off the light from part of the leaf, what other effect might the
        aluminium foil have had on the leaf which could influence the results ?
   (b) What control experiments could be conducted which would check whether these
        effects had influenced the results ?

 4 In this experiment, the plant was not tested at the beginning to ensure that its leaves were free from starch. Why was this precaution omitted?
       
5 Explain how the method used for destarching (not decolourizing) the plant tends to assume the results of this experiment.?

   .

Experiment 3. Discussion - answers


I The leaf should be some shade of blue except where the foil has excluded light. This area will
be yellow from the iodine stain.

2 (a) The significance of the blue colour is that it indicates the presence of starch. The
        distribution of blue colour suggests that starch has been formed only in those parts of the leaf
        which received light.
   (b) Some stages of carbohydrate production in photosynthesis need a supply of energy from
         light. If this energy is lacking, starch (at least) cannot be produced.
   (c) The leaf could be producing sugar everywhere but light is needed to change sugar to starch.
        Alternatively, in darkness the starch might be removed as fast as it is. formed, i.e. light could
        inhibit the conversion of starch to sugar .

3 (a) The aluminium foil will also interfere with the free passage of carbon dioxide into the leaf.
        A shortage of carbon dioxide could reduce photosynthesis as could the absence of light.
   (b) A strip of transparent tape could be applied to the leaf. This will interfere with the passage
        of gases but not light. If the leaf produces starch in the area covered by the tape we can
       assume that the aluminium foil did not prevent gaseous exchange.

4 If the area of leaf under the foil remains unaffected by iodine, this is evidence that the leaf was
free from starch at the beginning of the experiment.

5 One assumes that during 3 days of darkness, starch is removed from the leaves and is not re-
formed.


Experiment 3. The need for light in photosynthesis  - preparation


Outline An aluminium foil strip is wrapped round a leaf of a destarched potted plant. After exposure to light, the leaf is tested for starch.

Prior knowledge Starch is a product of photosynthesis. During darkness starch is removed from the leaves so that they are; starch-free at the beginning of the experiment.
The iodine test for starch. Method of testing a leaf for starch. (This information can be
obtained by following the instructions for Experiment 2. p.2.01)

Advance preparation and materials

(a)  Potted plants. Impatiens (busy Lizzie), Tradescantia, Pelargonium, French bean will all give good results after about 4-6 hours exposure to light. For results within a 
     double lesson, Impatiens is recommended. Allow one plant per group.
(b)  Destarching. Water the plants and leave them in a dark cupboard for 3 days
     (72 hours) if possible. Most of the plants will be starch-free after this time.
(c)  Starch test on leaf. (See Teachers' Notes, p.2.04.)

Apparatus-per group
1 strip of aluminium foil about 10 X 60 mm
apparatus for starch test as listed in Teachers' Notes p.2.04.
spirit marker

-per class
one or more fluorescent tubes

NOTE. To obtain results with only 45-60 minutes exposure to light, the fluorescent tube must be nearly touching the leaves.

Experiment 4. The need for chlorophyll


You are provided with a potted plant having variegated leaves. This plant has been in darkness for 2 days so that starch has been removed from the leaves.

(a)   Detach a leaf from near the top of the plant and test it for starch as described in Experiment 2 on p.2.01 If any blue colour appears, the plant is unsuitable for the experiment.

(b) Water the plant if the soil looks dry.

(c)   Place the plant in a situation where the leaf can receive sunlight or arrange the plant
 with the leaf nearly touching a fluorescent tube and leave it for at least 45 minutes.

(d) Detach a leaf from near the top of the plant and make a drawing in your notebook to show the outline and the areas which contain chlorophyll.

(e) Test this leaf for starch as described in Experiment 2,  p.2.01.

(f) Alongside your first drawing of the leaf, make a second drawing after testing the leaf with iodine, to show its outline and the distribution of the blue colour due to starch and the brown colour due simply to staining with iodine.


Experiment 4. Discussion


I What was the relationship between the distribution of chlorophyll in your leaf and the distribution of starch revealed by iodine ?

2 How can you explain this distribution of starch in terms of photosynthesis ?

3 What alternative explanations could there be for any correspondence between the distribution of chlorophyll and the distribution of starch ?

4 Why was it necessary to draw the leaf before testing it for starch ?

5 Why was there no need for a separate control experiment ?

6 How do you know that there was no starch present in the leaf at the beginning of the experiment ?
Are you satisfied with this evidence ?

Experiment 4. Discussion - answers


I There should be an exact correspondence between the previous distribution of chlorophyll and the distribution of starch in the leaf as revealed by the iodine test.

2 Starch has been produced only in the green areas presumably because chlorophyll is necessary for photosynthesis.

3 (a) Perhaps all parts of the leaf produce sugar by photosynthesis but this is converted
         to starch only in the green parts.
   (b) The white parts of the leaf might lack not only chlorophyll but various enzymes
         which are normally essential for photosynthesis.
   (c) The white parts of the leaf might be non-living.

4 When the leaf is fully decolourized it will not be possible to remember exactly where the green parts were in order to compare the pattern of starch distribution with the pattern of chlorophyll distribution.

5 The control in this case is the presence of chlorophyll in the leaf so that a variegated leaf acts as both experiment and control.

6 A similar leaf from the same plant will have given a negative result with the starch test at the beginning of the experiment. This does not prove, however, that the leaf used for the experiment was similarly starch-free. One way of overcoming this difficulty is to cut off the top half of the leaf from a destarched plant and show that it contains no starch. The bottom half of the leaf, still attached to the plant, is exposed to light for an hour or so.

Experiment 4. The need for chlorophyll - preparation


Outline A destarched variegated potted plant is exposed to light and one of its leaves tested for starch.

Prior knowledge Starch is a product of photosynthesis. During darkness starch is removed from the leaves so that they are starch-free at the beginning of the experiment. The iodine test for starch. Method of testing a leaf for starch. (This information can be obtained by following the instructions for Experiment 2, p.2.01).

Advance preparation and materials

(a) Potted plant. Variegated Tradescantia and variegated Impatiens (busy Lizzie) give adequate results in 45-60 minutes but can also be left for a week.
Allow one plant per group.

(b) Destarching. See  ‘Introduction’).

(c) Starch test. See Experiment 2, p.2.04).

Apparatus

fluorescent tube
apparatus and materials for starch test; see p.2.04)

NOTE. In a double lesson, adequate results can be obtained in 45-60 minutes if the leaves are nearly touching the fluorescent tube, but in this case the test to see if the leaves are starch-free to begin with should be conducted concurrently with the exposure of the rest of the plants to light in the hope that they will turn out to be destarched. Exposures of 4 hours or more give very good results.

Experiment 5. The need for carbon dioxide


You are provided with two potted plants which have been in darkness for 48 hours so that any starch present in their leaves has been removed.

(a) Label the pots A and B.

(b) Detach a leaf from near the top of each plant and test each one as described in Experiment 2, p.2.01,  to ensure that no starch is present. If either leaf gives a blue colour the plant should not be used for the experiment.

(c) Water the plant if the soil seems dry.

(d) In a small container place about 20 g soda lime. In a similar container pour about
10 cm3 (40 mm in a test-tube) saturated sodium hydrogencarbonate solution. Place the container of soda-Iime on the soil in pot A and the sodium bicarbonate container in pot B (See Figure on page 5.02).

(e) Cover each plant with a transparent plastic bag, after checking it for holes. Secure the bag round the pots with elastic bands and mark the pots with your initials.

(f) Place both plants where they can receive daylight or artificial light for several hours

(g) Label two test-tubes A and B.

(h) Copy the table on p.5.02 into your notebook

(i) After several hours of illumination remove the plastic bags from the plants and detach a leaf from near the top of each plant. Place the leaf in the correct test-tube to identify it.

(j) Test each leaf for starch as described in Experiment 2, p.2.01.

(k) Record your results in the table

NOTE. Soda lime absorbs carbon dioxide; sodium hydrogencarbonate solution decomposes slowly to give carbon dioxide.

Experiment 5.  Discussion


1 Results



A (No CO2)
B (Control)

Colour

Interpretation

Colour

Interpretation
Leaf tested for
starch at beginning of experiment





Leaf tested for starch after ...hours light





2 Assuming that the accumulation of starch in a leaf is evidence of photosynthesis make a general statement about photosynthesis and carbon dioxide.

3 In this experiment, what abnormal conditions other than removal of carbon dioxide might have affected the plant ?

4 How far did the control (a) succeed, (b) fail, in showing that these other effects were not influencing photosynthesis ?


plastic bag secured
with elastic band
 
soda-lime or
sodium hydrogencarbonate
solution
 
..\..\My Pictures\Carbon dioxide need 600dpi.jpg

Supplying or removing carbon dioxide

Experiment 5. Discussion - answers


l One would expect that leaves from both destarched plants would give only a yellow colour when tested with iodine solution, showing that starch is absent. After exposure to light it is expected that the leaf from plant B which had carbon dioxide provided, will give a blue colour with iodine showing the presence of starch while a leaf from plant A which lacks carbon dioxide will give only a yellow colour, indicating that starch has not been formed.

2 These results suggest that photosynthesis takes place only if carbon dioxide is available.

3 The plastic bag might (a) affect the quality of the light reaching the plant, (b) cause a rise in temperature round the plant, (c) increase the humidity and so slow down transpiration.
The soda-lime might have effects other than just removing carbon dioxide.

4 The fact that the control plant succeeded in making starch rules out a, b and c as possible causes of plant A 's failure to produce starch. The possibility that soda lime adversely affects the plant is not ruled out by the control.

Experiment 5. The need for carbon dioxide - preparation


Outline The shoots of destarched potted plants are enclosed in plastic bags in which carbon dioxide is removed by soda-lime or supplemented by sodium hydrogencarbonate. The plants are exposed to light for several hours and the leaves tested for starch.

Prior knowledge Starch is a product of photosynthesis. During darkness starch is removed from the leaves so that they are starch-free at the beginning of the experiment. The iodine test for starch. Method of testing a leaf for starch. (This information can be obtained by following the instructions for Experiment 2, p.2.01).

Advance preparation and materials


(a) Potted plants. Impatiens (busy Lizzie) seems to be the most suitable for this experiment.
Allow 2 plants per group.

(b) Destarching. See ‘Introduction’..

(c) Starch test. See Experiment 2, p.2.04).

soda lime, allow 20g per group
sodium bicarbonate, allow 10 cm3 10% solution per group

Apparatus-per group

fluorescent tubes, 2 sets if possible
2 plastic bags, about 400 X 250 mm according to size of plant
2 elastic bands to fit round flower pot
2 containers for soda-lime or sodium hydrogencarbonate solution, e.g. 50 mm plastic Petri dishes or 75 X 25 mm specimen tubes
apparatus for the starch test; (see p.2.04).
spirit marker

NOTE. The experiment does not give reliable results during a period of 50-60 minutes but can be left for 1-7 days. Provided the plants are illuminated for several hours (e.g. overnight) before the class needs to test them, the results should be satisfactory.



Experiment 6. Collecting the gas evolved by pond-weed


You are provided with a glass jar having a lid through which a test-tube can pass, a rubber band and a cork with a wide hole in it.

(a) Pass the test-tube up through the hole in the lid and secure it with the elastic band.

(b) Write your initials on the jar and fill it nearly to the top with tap-water. Add about
5 cm3 sodium hydrogencarbonate solution (about 20 mm in a test-tube).

(c) Collect about 10 pieces of pond-weed up to 10 cm long. Arrange the shoots parallel to each other and trim the ends of the stems with a razor blade.

(d) Fill the test-tube with water to the top of the cork. Push the cut ends of the pond-weed stems through the hole in the cork so that they are held firmly but without crushing them (Figure 1, p.6.02).

(e) Hold the test-tube horizontally with the cork over the edge of the jar (Figure 2, p.6.02) and then turn it upside down so that the pond weed enters the jar, the lid fits over the opening and the test-tube is held by the elastic band (Figure 3, p.6.02).

(f) Little or no air should enter during this operation but if, for some reason, more than about 10 mm air gets into the tube, the experiment should be set up again. Make sure that the cork is under the water in the jar.

(g) Repeat the whole operation from (a) to set up an identical experiment but cover the jar with aluminium foil or some opaque material to exclude all light. Place both jars in a position where they can receive maximum daylight or artificial light.

TESTING THE GAS. READ ALL THIS SECTION BEFORE PROCEEDING.

(h) When either of the test-tubes is more than half full of gas, remove the elastic band and lid, lift the tube and weed out of the jar and turn the tube the right way up. Remove the cork and pond-weed and quickly close the mouth of the tube with your thumb (Figure 4, p.6.02).

(i) Light a wooden splint and when it is burning well, blow it out so that the end continues to glow red.

(j) Remove your thumb from the test-tube and at the same time insert the glowing end of the splint into the tube. (By closing the tube again, blowing out the splint and re-inserting it, you may be able to repeat the test several times.)

Experiment 6. Discussion


I What happened to the glowing splint when it was placed in the test-tube?

2 What gas usually causes this reaction ?

3 The test does not prove that only this gas is present in the test-tube. What other gases might be present?

4 What evidence do you have which suggests that the production of this gas depends on light reaching the plant ?

5 What evidence is there to suggest that the gas in the test-tube came from the pond-weed and not directly from the water in the jar ?

6 How could you eliminate this last possibility ?

Figure 1

 
 elastic band
 
  lid
 
Push the pond-weed stems into the hole in the cork
 

Figure 3 333
 
Turn the test-tube upside down and fit the lid on the jar
 

Hold the test-tube
horizontally with the cork over the edge of the jar
 
..\..\My Pictures\tilted tube.jpg

Figure 4

 
close the mouth of the tube with your thumb
 

Figure 2

 




Experiment 6. Discussion - answers


I The glowing splint should burst into flames when placed in the test-tube.

2 This reaction is caused characteristically by oxygen.

3 Nitrogen and carbon dioxide might also be present. Hydrogen is not likely to be present since it would have caused explosive combustion in the test-tube.

4 The plants in the jar from which light was excluded will have produced little or no gas.

5 The only evidence against the water as a direct source of the gas is the failure of the control to produce any gas but since this water was not exposed to light, it is not a fair comparison. If the students have done experiment 1 they will have evidence that the gas can be seen escaping from the cut stems. of the plant.

6 A further control should be set up with a jar containing sodium hydrogencarbonate solution as in the experiment but without the pond-weed. If, on illumination, little or no gas collects it is unlikely that the gas in the experiment comes from the water independently of the plant.

Experiment 6. Collecting the gas from pond-weed - preparation


Outline Several cut shoots of Elodea are illuminated in a jar of water. The gas from the cut stems is collected in an inverted test-tube supported by the lid of the jar. The gas is tested with a glowing splint.

Prior knowledge Oxygen relights a glowing splint.

Advance preparation and materials-per group

10 shoots of Elodea 5 cm3 10% sodium bicarbonate solution
The pond-weed should be collected before the experiment and placed in fresh tap-water where it will remain healthy for several days.
Jam jars (300-350 cm3) with screw-on or clip-on lids should be collected and a hole cut in the centre of the lid with a pair of sharp-pointed scissors. The hole should be large enough to admit a test-tube but too small to allow the lip to pass through.
Corks to fit the test-tube should be bored with a 10 mm hole (No.5 cork borer).

Apparatus-per group

2 jam jars (250-300cm3) with lids. The lids to have a central hole
sheet of aluminium foil (or similar opaque material), about 100 X 250 mm, to cover
one jar
2 test-tubes)
2 elastic bands                                               
2 corks with holes bored
splint and access to flame (later)

-per class
4 or 5 spirit markers for students to identify their jars
fluorescent tube as light source (one 4 ft tube will illuminate 15 jars)

NOTE. If results are needed by the following day the fluorescent light should be placed on its side and the jars arranged close to the tube. If a week is to elapse before testing the gas, the jars can be left on a window sill in daylight and exposed to the fluorescent light as necessary before the next lesson. The gas production does tend to fall off as time progresses and it is better to accumulate a tube full of gas as soon as possible even if it has to stand for several days.
The experiment can be varied to compare hydrogencarbonate solution with tap-water  which has been boiled to expel carbon dioxide.
Experiment 7 Gaseous exchange in leaves

The experiment depends on the use of hydrogencarbonate indicator, a pH indicator, containing the dyes cresol red and thymol blue in a solution of sodium hydrogencarbonate. This pH indicator is in equilibrium with atmospheric carbon dioxide, i.e. its orange colour when you receive it indicates the acidity of the atmosphere due to carbon dioxide. Carbon dioxide is an acid gas.
Increase in acidity (fall in pH) turns the indicator yellow while decrease in acidity (rise in pH) turns it first red and eventually purple.

(a) Wash three test-tubes in tap-water. Rinse them with distilled water and finally rinse them
with the bicarbonate indicator itself.

(b) Label the tubes 1-3.

(c) Use a graduated pipette or syringe to place 2 cm3 hydrogencarbonate indicator in each tube.

(d) Roll the leaf longitudinally, its upper surface outwards and slide it into tube 1 so that it is held against the wall of the test-tube and does not touch the indicator solution (Fig. 1). Do
the same with tube 2.

(e) Close each tube with a rubber bung.

(f) Cover tube 1 with aluminium foil to exclude light and place the three tubes a few centimetres
away from a bench lamp (or in direct sunlight if possible). Switch on the lamp and leave the
apparatus for 40 minutes.

(g) Copy the table given below into your notebook.

(h) At the end of 40 minutes, hold all three test-tubes against a white background to compare
the colours of the indicator solutions and record these colours in your table. 

Tube
Conditions
Colour of indicator
Change in pH
1
Leaf in darkness


2
Leaf in light


3
No leaf


  

Experiment 7 Discussion

Read the introductory paragraphs to the experiment once again.

1 Although the hydrogencarbonate indicator solution is a pH indicator, i.e. its colour depends on its acidity, this experiment assumes that its changes of colour depend entirely on changes in the
carbon dioxide content of the air. Explain why this is a reasonable assumption.

 2 (a) What change in pH is suggested by the indicator becoming more yellow?
    (b) What change in the composition of the air in the test-tube is most likely to cause such a
           change of pH?

3 (a) What change in pH is suggested by the indicator becoming more red or purple?
   (b) What change in the composition of the air in the test-tube is most likely to cause such a
         change in pH? .

4 What was the purpose of setting up tube 3?

5 What effect did the presence or absence of light have on the concentration of carbon dioxide in tubes 1 and 2?

6 If you know something about the processes in leaves which lead to gaseous exchange, explain
the difference in results in test-tubes 1 and 2.


 

..\Photosynthesis pictures\Bicarb indicator.jpg
Experiment 7. Discussion - answers

1 If the leaf does not touch the liquid, it must be a change in the composition of the gas in the
test-tube which affects the indicator. Of the atmospheric gases, only carbon dioxide is significantly acid. Unless the leaf gives out a different acid or alkaline gas, variations in carbon dioxide concentration seem to be the most likely cause of pH changes.

2 (a) A change in the indicator from red to yellow indicates a fall in pH, i.e. increased acidity.
   (b) For the reasons outlined above, carbon dioxide is likely to have caused a change in pH,
         in this case by a rise in the carbon dioxide concentration.

3 (a) A change in the indicator from orange red to deep red or purple suggests a rise in pH, i.e.
        reduced acidity.
   (b) This is probably caused by a fall in the carbon dioxide concentration in the test-tube.

4 Tube 3 is a control
   (a) to provide a means of deciding by comparison whether there has been a 
        significant change in colour in the indicator in tubes 1 and 2,
   (b) to show that it is the presence of a leaf which causes the change and not just exposure of 
         the indicator to light or darkness.

5 In the presence of light, carbon dioxide in the air in the test-tube seems to diminish. In darkness, carbon dioxide seems to accumulate.

6 In tube 1, no photosynthesis occurs and so carbon dioxide will accumulate as an end-product of respiration. In tube 2, photosynthesis will proceed rapidly enough for carbon dioxide to be used faster than it is produced in respiration.
(If the experiment is used as a preliminary investigation, leading up to ideas about photosynthesis, this question should be ignored.)


Experiment 7. Gaseous exchange in leaves - preparation

Outline Detached leaves are placed in closed test-tubes and illuminated or darkened. The rise of carbon dioxide concentration resulting from respiration, or its fall as a result of photosynthesis,
changes the colour of the pH indicator in the bottom of the test-tube.

Prior knowledge Air contains oxygen, carbon dioxide and nitrogen but only carbon dioxide
is an acid gas. What a pH indicator does. What 'equilibrium' means in the context of this
experiment.

Advance preparation and materials

Leaves or leaf portions from deadnettle, iris, rose, plantain, dandelion, dock, forsythia have
given satisfactory results in 30-45 minutes. Allow two leaves per group.

Hydrogencarbonate indicator. Dissolve 0.2 thymol blue and 0.1 g cresol red powders in 20 cm3 ethanol. Dissolve 0.84 g  sodium hydrogencarbonate  (analytical quality) in 900 cm3 distilled water. Add the alcoholic solution to the hydrogencarbonate solution and make the volume up to 1 litre with distilled water.
Shortly before use, dilute the appropriate amount of this solution 10 times, i.e. add 9 times its
own volume of distilled water.
To bring the solution into equilibrium with atmospheric air; bubble air from outside the
laboratory through the diluted indicator using a filter pump or aquarium pump. After about
10 minutes, the dye should be red.
Allow 10 cm3 per group,

Apparatus-per group

Note that the glassware and bungs must be clean. Any trace of acid or alkali will affect the indicator.

3 test-tubes and rubber bungs
1 bench lamp (in the absence of sunlight)
1 test-tube rack
1 piece of aluminium foil, about 120 x 140 mm
1 graduated pipette or syringe, 5 or 10 cm3
 3 labels or a spirit marker
1 pair forceps

NOTE Experiment 8 is identical to Experiment 7 except that an aquatic plant is used. It may be expedient to have half the class doing Experiment 7 and half doing Experiment 8



Experiment 8. Gaseous exchange in pond-weed

The experiment depends on the use of hydrogencarbonate indicator, a pH indicator containing the dyes cresol red and thymol blue in a solution of sodium hydrogencarbonate. This pH indicator is in equilibrium with atmospheric carbon dioxide, i.e. its orange colour when you receive it indicates the acidity of the atmosphere due to carbon dioxide.
Increase in acidity (fall in pH) turns the indicator yellow while decrease in acidity (rise in pH) turns it first red and eventually purple.

(a) Wash three test-tubes in tap-water. Rinse them with distilled water and finally rinse them with the hydrogencarbonate indicator itself.

(b) Label the tubes 1-3.

(c) Select 4 equivalent shoots of pond-weed about 50 mm long and place 2 shoots in each of tubes 1 and 2.

(d) Pour hydrogencarbonate indicator into all three tubes to the same level and sufficient to cover the pond-weed in tubes 1 and 2 (Fig. 1).

(e) Close each tube with a rubber bung.

(f) Cover tube 1 with aluminium foil to exclude light and place the three tubes a few centimetres away from a light source or in direct sunlight if possible. Leave the tubes in the light for 40 minutes.

(g) Copy the table given below into your notebook.

(h) At the end of 40 minutes, hold all three test-tubes against a white background to compare
the colours of the indicator solutions and record these colours in your table.


Tube
Condition
Colour of indicator
Change in pH
1
Pond-weed in darkness


2
Pond-weed in light


3
No pond-weed



Experiment 8  Discussion

Read the introductory paragraphs to the experiment once again.

1 Although the hydrogencarbonate indicator solution is a pH indicator, i.e. its colour depends on its acidity, this experiment assumes that its changes of colour depend entirely on changes in the carbon dioxide content of the air or water. Explain why this is a reasonable assumption.

2 (a) What change in pH is suggested by the indicator becoming more yellow?
   (b) What change in the composition of the water in the test-tube is most likely to cause such a
         change of pH?

3 (a) What change in pH is suggested by the indicator becoming more red or purple?
   (b) What change in the composition of the water in the test-tube is most likely to cause such a
         change in pH?

4 What was the purpose of setting up tube 3?

5 What effect did the presence or absence of light have on the concentration of carbon dioxide in tubes 1 and 2?

6 If you know something about the processes in pond-weed which lead to gaseous exchange,
explain the difference in results in test-tubes 1 and 2.




..\Photosynthesis pictures\pondweed in cresol red.jpg
 
 


























Experiment 8 Discussion - answers

1 Water contains dissolved oxygen and carbon dioxide. Only the carbon dioxide is significantly acid so changes in this will affect the pH of the solution. Nevertheless, the pond-weed could be
giving out acid or alkaline chemicals.

2 (a) A change from red to yellow in the indicator suggests a fall in pH, i.e. increased acidity.
   (b) If carbon dioxide only is responsible for the pH changes, the colour indicates an increase
        in the carbon dioxide concentration. The plant could, however, be releasing acid   
        metabolites into the water.

3 (a) A change to deep red or purple in the indicator suggests a rise in pH, i.e. a fall in acidity.       
   (b) If carbon dioxide only is affecting the pH then this change could be due to a fall in the
         carbon dioxide concentration.. The plant, however, could be releasing alkaline products        
         into the water.

4 Tube 3 is a control
     (a) to provide a means of deciding by comparison whether there has been a significant change   
           in colour in the indicator in tubes 1 and 2,
     (b) to show that it is the presence of a plant which causes the change and not just exposure of     
           the indicator to light or darkness.

5 In the presence of light, carbon dioxide in the water in the test-tube seems to diminish. In darkness, carbon dioxide seems to accumulate.

6 In tube 1, no photosynthesis occurs and so carbon dioxide will accumulate as an end-product of respiration. In tube 2, photosynthesis will proceed rapidly enough for carbon dioxide to be used faster than it is produced in respiration.

 (If the experiment is used as a preliminary investigation, leading up to ideas about photosynthesis, this question should be ignored.) 



Experiment 8. Gaseous exchange in pond-weed - preparation

Outline Shoots of Elodea are submerged in test-tubes of hydrogencarbonate indicator and darkened or exposed to light. The intake or output of carbon dioxide as a result of photosynthesis or respiration causes a change in the colour of the dye.

Prior knowledge Air contains oxygen, carbon dioxide and nitrogen but only carbon dioxide is an acid gas. Atmospheric gases are more or less soluble in water. What a pH indicator does.
What 'equilibrium' means in the context of this experiment.

Advance preparation and materials

Elodea canadensis works well but Ceratophyllum, Potamogeton crispus and other submerged
plants would probably be satisfactory. Wash the pond-weed in tap-water before giving it to the students.
Allow 4 shoots, about 50 mm long for each group
hydrogencarbonate indicator. See p. 7.04. Allow 80 cm3 per group

Apparatus-per group

Note that the glassware and bungs must be clean. Any trace of acid or alkali will affect the
indicator.

3 test-tubes and rubber bungs
1 piece of aluminium foil, about 120 x 140 mm
1 test-tube rack
3 labels or a spirit marker
1 bench lamp (in the absence of sunlight)

Experiment 9. The need for mineral elements

(a) Label four test-tubes as follows: +, Ca, N, --.
(b) Fill each tube to within about 20 mm of the top, with the appropriate water culture.
     (+) Solution containing all mineral elements thought to be needed for healthy growth
     (Ca) Solution containing all mineral elements as in + except for calcium
     (N) Solution as in + but lacking nitrate
     (--) Distilled water, i.e. no mineral elements at all
(c) Select four seedlings which appear to be at the same stage of development.
(d) If there is a wide difference between the development of the root systems, reduce all systems
to the same number and approximate length of root as in the least developed.
(e) Leaving the shoot and roots free, roll a strip of cotton wool round the grain to hold the seedling lightly but firmly in place in the mouth of the: test-tube (Fig. 1). Place a seedling in each test-tube so that the root is well covered by the culture solution.
(f) Mark your initials and the date on the rack or container provided and place the four tubes in a position where they can receive daylight or artificial illumination. Leave the seedlings to
grow for two weeks.
(g) During this period of time the levels of the solutions will fall in the test-tubes. They need to be inspected every two days and the level restored if necessary. Carefully remove the cotton wool and top up the test-tubes with DISTILLED WATER from a wash-bottle.
(h) After two weeks transfer the tubes to a rack so that the seedlings can be compared side by side.
(i) Draw up a table similar to the one below.
(j) Study the whole group of seedlings and note in your table  any which show abnormalities of
leaf colour or shape, e.g. dead areas, discoloured patches, pale green colour.
(k) Remove the seedling from the full culture (+), unwind and discard the cotton wool and cut off the leaves as shown in Fig. 2. By placing the leaves end to end along a ruler, measure and
record their total length.
(l) Cut the root system just below the grain (Fig. 2) and use forceps to separate the main roots, working from the top. Place these end to end along a ruler to measure their total length. Ignore the lateral roots for this purpose (unless you have plenty of time and patience).
(m) When you have made the measurements, place the roots and leaves in the container labelled
'+' so that their dry. weight can be found later.
(n) Repeat the measurements for each of the seedlings in turn, placing the leaves and roots in
the appropriate container afterwards.
(o) Plot histograms (Fig. 3) of the root and shoot lengths for each seedling.

Culture solution
+
Ca
N
--
Leaf colour




Total leaf length




Total root length




Dry weight*




* Whole class
Experiment 9. Discussion

1 Which solution of mineral salts provided (a) the best and (b) the worst medium for the growth
of the seedlings as judged by leaf length?

2 From your knowledge of plant nutrition, explain why nitrates (source of nitrogen) and calcium
should be so important to a green plant.

3 Why would it be difficult to judge whether the lack of a particular element did more harm to
the root growth than the shoot growth or vice versa?

4 Why do you think a small-seeded plant, rather than a runner bean, was used for this experiment?

 5 Why do you think the solutions were topped up with distilled water rather than with the
appropriate culture solution? .

6 In what ways is this experiment unrepresentative of natural conditions?

7 Why was a culture solution lacking carbon not included, bearing in mind that plants need carbon for making their carbohydrates?


 

































Experiment 9. Discussion - answers

1Theoretically, the best growth should be in the full culture and the worst in distilled water.
Failure to achieve these results may be attributed to, e.g. genetic variability in the seedlings, disease, impurities in the solutions, damage to seedlings in setting up the experiment or
topping-up the tubes.

2 Calcium pectate, by its contribution to the middle lamella, affects the adhesion of cells. In the
absence of calcium, the growing points of shoots and roots may become disorganized. Calcium  deficiency sometimes causes a harmfully high level of magnesium absorption, which may explain why some seedlings in the calcium-deficient solution grow less well than those in distilled water. Nitrates are essential as the only source of nitrogen for making amino acids and proteins.

3 If the lack of a particular element resulted in poor root growth, the development of the shoot
would be retarded through inadequate water and mineral supply from a deficient rooting system,
irrespective of any specific effect on the shoot of the lack of that element. Similarly, poor development of the leaves could limit root growth as a result of inadequate food supply to the roots from the leaves.

4 Large seeds such as runner bean have a considerable supply of nutriment in their cotyledons or endosperm. It takes a week or two to exhaust this supply so that the effect of the deficient cultures is delayed.

5 The rates of water uptake and salt uptake are not necessarily the same. If during the course of transpiration the plant removes more water from the test-tube than it removes salts, topping up
with culture solution may increase the concentration of salts too much.

6 The proportion of minerals in the solution may be different from that in the soil. The ions of the elements selected may not be the same as those in the soil. Mineral particles are lacking. The experiment is confined to only the early stages of growth; the plant may have different
requirements as it matures.

7 The carbon needed for making carbohydrates comes from carbon dioxide in the air.


Experiment 9 The need for mineral elements - preparation

Outline Wheat seedlings are grown in water cultures some of which lack essential elements
Measurements are made on the seedlings to find the relative effect of these elements.

Prior knowledge The connection between the element and its salt, i.e. nitrate contains nitrogen. Some idea of the role of mineral elements in metabolism.

Apparatus - per group

test-tube rack and 4 test-tubes                           wash bottle of distilled water for 'topping up'
4 labels or spirit marker                                     graph paper
1 can or box to hold 4 tubes                               labelled receptacle for collecting seedlings from
4 strips cotton wool about 20 x 100 mm              each solution for determining dry weight

Advance preparation and materials

Wheat seedlings. Seven days before the experiment, soak the wheat for 24 hours. On day 1 pour oft the water but leave the wheat covered in the container. On day 2 select those grains which show signs of germinating and roll them in moist blotting paper or newsprint with the seedlings about 20 mm from the top of the roll and the embryo directed downwards. Space the fruits about15 mm apart so that the roots do not get entangled. Place the rolls upright in a plastic bag and allow the wheat to germinate in darkness. Four seedlings are needed per group but germinate twice as many to allow for failures. If several rolls are prepared, the students can select their own seedlings from them.

Water cultures. Either purchase the Sach's water culture tablets or prepare the solutions as in Table 1 (p.9.05), using chemicals of the highest purity available (analytical reagents) and freshly distilled or deionized water. Store the stock solutions in stoppered borosilicate flasks and mix and dilute them when required as described in Table 2. The three solutions selected have been found to give the most consistent and reliable results with wheat,

TABLE 1
Solution
Compound
Formula
Mass
(g)
Volume of water (cm3)
A
calcium nitrate
Ca(NO3)2.4H2O
9.5
100*
B
calcium chloride
CaCl2.2H2o
4.5
100
C
magnesium sulphate
MgSO4.7H2O
15.5
300
D
sodium nitrate
NaNO3
6.9
100
E
potassium dihydrogen phosphate
KH2PO4
8.5
300
F
iron (II) chloride
FeCl2.4H20
1
200
























*To make up 500 cm of each culture solution (enough for about 16 experiments) add the solutions in the volumes indicated in Table 2 to 485 cm3 distilled water. If the mixed culture solutions are to be kept for more than a week, add a further 0.5 cm3 iron (II) chloride solution to each  500 cm3 just before use. Set out the culture solutions in labelled containers and also a
container of distilled water.  Allow 30 cm3 of each solution per experiment

TABLE 2  CULTURE SOLUTIONS

Full culture
Lacking calcium
Lacking nitrogen
Solution A
calcium nitrate
5 cm3
no calcium
no nitrate
Solution B
calcium chloride


5cm3
Solution C
magnesium sulphate
5 cm3
5 cm3
5 cm3
Solution D
sodium nitrate

5cm3

Solution E
potassium phosphate
5 cm3
5cm3
5 cm3
Solution F
iron (II) chloride
0.5 cm3
0.5cm3
0.5 cm3

These tables have been selected from Wellington Hydroponics Solutions compiled by
D.J. Angwin of Wellington  College








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