Voorkant

Photosynthesis

Subject: Biology 2

Date: 5 - 5 -‘03

Class: AG5BI2D

NAALDWIJK - NL

Table of contents

Table of contents………………………………………………………………………. 2

Introduction……………………………………………………………………………… 3

Theory of photosynthesis……………………………………………………………… 4

Photosynthesis in a coloured leaf……………………………………………………. 6

The making of oxygen ………………………………………………………………….8

Chromatography from leafs…………………………………………………………… 10

Conclusion………………………………………………………………………………. 13

Acknowledgement……………………………………………………………………… 14

Vocabulary……………………………………………………………………………….15

Enclosure 1………………………………………………………………………………16

Enclosure 2………………………………………………………………………………19

Introduction

This year we had an exchange with Danish students. The theme of the week in Denmark was the sun. That’s why we got an assignment about photosynthesis in Holland. We did some practical work to prove the things we learned about photosynthesis. Now we would like to teach YOU more about photosynthesis, because this process is essential to live.

At first we will give an introduction in the theory of photosynthesis. After that we describe the practical work we’ve done and give some conclusions about it.

Of course we could only give you the theory, but the practical work proves that the theory is right. We’ve done the practical works, because you shouldn’t believe everything that’s written in the books.

Theory of photosynthesis

Plants are autotroph, it means that they can make organic materials (like proteins, carbohydrates and lipids), which they need to live, out of only anorganic materials (like oxygen, water, carbon dioxide, and salts). They don’t need other organisms to feed themselves.

Only organisms that have chlorophyll are photo autotroph, so only plants and some bacteria are autotroph. Chlorophyll is a photosynthesis pigment. You can find chlorophyll in the thylakoids, in the thylakoids occurs the photosynthesis.

Sunlight consists of different colours. The colours have a different wavelength. At a wavelength of 440 nm and 670 nm is the activity of the photosynthesis highest. You can see the peak in the graphics.

Chlorophyll absorbs blue and red light, the carotenoids (another pigment in the plant) absorbs blue-green light. Photosynthesis pigments in the plant do not effectively absorb the green and yellow light. So leafs reflect green and yellow. We can see the reflected colours, that’s why leafs are green. Photosynthesis occurs only in the green parts of the plant, because that’s the place were the chlorophyll is.

In the graphics you can see that the green light isn’t absorbed well. That’s why a leave is green.

The energy of the absorbed light is used for the photosynthesis. Other materials you need for the photosynthesis are carbon dioxide and water. The equation of the net reaction of photosynthesis is:

6CO₂ (g) + 6H₂O (l) + light energy C₆H₁₂O₆ (s) + 6O₂ (g)

The equation of the net reaction shows that CO₂, H₂O and light energy are essential for the photosynthesis. These three things affect the intensity of the photosynthesis. The intensity of the photosynthesis is the velocity with which glucose and O₂ is formed. The intensity also depends on the temperature and the quantity of chlorophyll.

The photosynthesis is separated in two parts: the light and dark reaction. At first we will explain the light reaction.

The light reaction is divided in two systems. Photo system 1 (PS I) and photo system 2 (PS I I). In the light reaction is O₂ (g) and H⁺ (g) formed out of water and light energy. The O₂ goes into the air, so we can breathe it. The H⁺ is full of energy and it transmits its energy to the NADP and ADP. NADP becomes NADPH and ADP becomes ATP, both molecules are energyrich now. With that energy and the CO₂ from the air, there can be created glucose in the dark reaction. A part of this glucose will immediately transposed into starch.

Glucose is the main nutrition for a plant, the starch is a reserve for the plant. If there is no light for a long time, it will use the starch to survive.

The dark reaction is called dark reaction, because it doesn’t need any light in contradiction to the light reaction. But the dark reaction needs the reaction products from the light reaction (NADPH and ATP) and it goes straight after the light reaction. So when a plant is in a dark room, the light reaction cannot occur. When there is no light reaction, there are no reaction products and therefore no dark reaction.

Photosynthesis in a coloured leaf

In this practical work, we will prove starch in a coloured leaf. We used a leaf from a scindapsis aureus plant. The leaf wouldn’t die, because it was still at the plant.

We covered a part of the leaf with styrofoam and made a drawing from the leaf. You can find the drawings in enclosure one. After that we put the plant in the window-sill for two days (49 hours and 40 minutes to be exact).

Then we took the leaf of the plant.

You can show the starch with a solution of iodine. The solution of iodine makes the starch black. But first we boiled the leaf, so it became weak. We put the leaf in a petri plate with some alcohol. Then we did the petri plate with alcohol in water of 80 degrees.

Scindapsis aureus

The leaf should be uncoloured now, but that didn’t really happen. After these steps we put the leaf for the last time in boiling water. Now we did the solution of iodine on the leaf. There were some black spots on the

plant. And the stem also became black. We made a

drawing of this beautiful leaf with black spots, you can find

the drawing in enclosure two.

After the practical work, we studied the plant and discussed about our experiences.

We found out that there was no difference between the covered part and uncovered part of the leaf. This was an unexpected result. We thought that there couldn’t be any starch at the covered part, because there was no light and therefore no photosynthesis in that part of the leaf. We have an explanation for it.

The leaf should have been in the dark for more than 24 hours, before we covered a part of the leaf. The starch should be totally used, than we would have known for sure, that there was no starch left in the leaf. We start with a blank plant and than the photosynthesis would only take place in the uncovered part of the leaf.

We have another explanation for our unoptimal result. The outside of our leaf was leathery, so the alcohol couldn’t reach the inside of our leaf. Now the alcohol couldn’t break the structure of the leaf, so the solution of iodine couldn’t show the starch.

For better results we should have put the plant for 24 hours in the light. The intensity of the photosynthesis would be larger, because there was more light. The results of our practical work would be better, because there would have been formed more glucose. So there would be also more starch. Our leave wasn’t 24 hours in the light, there was a night between the two days. So in that night the leaf used it starch. The results can be affected.

For better comparison we should have used three variants: a totally covered leaf, a totally uncovered leaf and a partly covered leaf. If we used these variants, we could have seen the difference between covered and uncovered better.

Text Box: Our leaf was spotted, with light and dark spots. The photosynthesis only takes place in the dark spots, because there was more chlorophyll. You can see it in our leaf over

Our leaf after the practical work

here.

Now we proved you that there really starch originate, by photosynthesis. And that was our target of this practical work. So unless our failed practical work, we have our proof that we wanted.

The making of oxygen

In our theory of photosynthesis, we told you that oxygen comes into being with photosynthesis. In this practical work, we will prove that it’s true.

We used an elodea Canadensis (Canadian Waterweed) for this experiment. It were two pieces of 10 cm. We made the experimental set-up as follows.

At first we filled the glass with some water out of the tap. We put the plant into the glass, and set the funnel over it. The test tube filled with water was set over the funnel. We made two of these experimental set-ups and set one of them in a dark room, and the other one in the window-sill.

Experimental set-up

Test tube

Funnel

Glass

Water

Canadian Waterweed

After two and five days, we looked at the quantity of air in the test tube. You can see our results in the following table.

Height (mm)

Day 1

Height (mm)

Day 3

Height (mm)

Day 6

Light

Dark

The more days, the more air as you can see. But now we want to know what kind of air it is in the test tube. We think it’s oxygen, we will find out.

After the six days, we got the test tube off the funnel. To see if the air really is oxygen, we put a glowing piece of wood in the test tube. Oxygen should set the piece of wood in fire. But that didn’t really happened.

We have an explanation for the fact that the piece of wood didn’t burn. The concentration of the oxygen was to low, because it was mixed with air. A solution for this problem is to get the oxygen out of the test tube in another way. For example we could place a connection in the test tube, with on the end a syringe.

Than we can empty the test tube and spray the oxygen on the piece of wood. The oxygen is less in contact with the air.

We showed you that oxygen is produced in the photosynthesis. We can also investigate whether the temperature affects the photosynthesis or show you that CO₂ is needed for it.

At first we will describe an experiment to investigate the influence of the temperature of the surroundings on photosynthesis.

We can use the experimental set-ups (one in a dark room, one in a light room) for this practical work. But now we will put two set-ups in a fridge, two in a hot-house and two in a classroom. We use two fridges, one with light, and one without light. We do the same in a hot-house and a classroom. Then we can compare the results. We compare the heights of air in the test tubes.

In the water we have used CO₂ is present, in air it is about 0.03% CO₂. In this experiment CO₂ was present, so we can’t show with our practical work that CO₂ is really necessary. To show that, we have to omit the CO₂. That’s possible on the following way. Instead of the normal water, we use demi-water (H₂O). That is water without CO₂. We will use the same experimental set-up, but now we have to take care that our set-up is not in contact with air. To realize that, we put all this in a vacuum surrounding. So we have used a set-up with water with CO₂, and a set-up with water without CO₂.

If there isn’t formed any air in the test tube from the water without CO₂, we will know that there was no photosynthesis. Than we can conclude that CO₂ is needed for photosynthesis.

Canadese waterpest

Chromatography from leafs

The most important pigments in a plant are chlorophyll a, chlorophyll b, carotene and xanthophylls. These pigments can absorb light at different wavelengths. This is important, because now there can be photosynthesis at almost all wavelengths. To look if these pigments are really in a plant, we did an experiment.

We used a leaf from a liguster (Ligustrum Vulgare L.) and a bishop’s weed (Aegopodium Podagraria). We made these leafs in very little pieces with a pair of scissors. With the mortar and pestle we made it even smaller. We did some acetone with our little leafs and made a concentrated solution of it.

This solution is put on a filtering paper and we hang it in a liquid (8% acetone, 92% petroleum ether). We started our chromatography experiment.

Chromatography is a way to find out what pigments are in a solution. It’s a separation competence. There are different kinds of chromatography, and we used the paper chromatography. This chromatography is based on difference in solvability and absorption. It works as follows:

A colour on a filtering paper is set in a liquid. The liquid is pulled into the paper and it takes the pigments of the colour with it. The pigments have a different solvability in the liquid and a different absorption on the paper. So some pigments will be pulled further than other pigments. Every pigment ends on its own height. So after this experiment the pigments are separated and you can see the different pigments in a colour.

We used the solution we’ve made from our leaf instead of a colour. So after this practical work, we can see which pigments are in our leafs.

The explanation for the liquid we’ve used is that the chlorophyll is well soluble. And the other materials in a plant are not well soluble in this liquid.

On the next page you can see the chromatography of our liguster and bishop’s weed. We will explain both, but we start with the liguster.

As you can see, there are four colours. The colours are the four photosynthesis pigments. The upper colour is gone, but it was yellow. That means it is carotene.

The second colour is also gone and was also yellow, but it was xanthophylls. The third colour is green; it’s chlorophyll a. The fourth colour is also green and that’s chlorophyll b. So the leaf has al the four pigments.

The upper horizontal line is the limit where the liquid came.

The bishop’s weed shows two colours. Both colours are green, they are chlorophyll a and chlorophyll b. The two other pigments we saw in the liguster are also present in the bishop’s weed, these colours are in the dark spot at the top.

These colours are in a spot, because the colours couldn’t go any further while the colours from below came up. They ended all in the same spot.

compared our liguster with ligusters from our classmates. All ligusters contain the four pigments. We measured the length of the colours from the middle of the spot below till the top of the colour. You can see the results in the following table. The numbers are in centimetres.

	Chlorophyll b	Chlorophyll a	Xanthophylls	Carotene
Liguster 1 (our liguster)	3,8	5,3	6,2	7,0
Liguster 2	6,8	7,5	7,8	8,4
Liguster 3	3,4	4,5	5,6	8,1

These results should be all the same, because it’s the same plant. But they aren’t, we can try to find an explanation for it.

How far the colours rise, depends on the solvability and absorption. We used all the same paper, so there couldn’t be a difference in absorption. The difference must be in the solvability. Maybe some people used more acetone than others, so the concentration of the solution differs.

The top of the chromatography wasn’t at the same level at the end of the practical work. That’s because of the time the papers hang in the liquid. When a paper hangs longer in the liquid, the top is higher.

After we compared the ligusters, we also compared the liguster and the bishop’s weed with the elder (Sambucus Nigra) and the horse chestnut (Aesculus hippocastanum L.).

It was difficult to compare, because the bishop’s weed, elder and the horse chestnut had a spot at the top. So we could only compare the green colours, but we saw some yellow colours in the spot, so we know it was there.

The quantity of the green colour is the same at all plants.

Now we compared four plants and in every plant were four colours. So we can conclude that every plant has the four photosynthesis pigments.

Iiguster bishop’s weed horse chestnut elder

(Ligrustrum Vulgare L.) (Aegopodium podagraria) (Aesculus hippocastanum L.) (Sambucus Nigra)

Conclusion

We hope that we have hit our target; to teach you something about photosynthesis.

With our practical works we have proved that what we’ve learned in the theory is correct. Now we know for sure that a plant produces starch and oxygen with photosynthesis. We also know that every plant has the four photosynthesis pigments.

Despite the practical work didn’t passed off spotless sometimes, we know that the theory is right. Because we can argue why the experiments failed.

We did the practical work and this report with a lot of fun. We made this report in English, but that was more difficult than we thought it would be. But overall it was a challenge and we think it worked out as a success.

Acknowledgement