The effect of light intensity on the amount of chlorophyll in “Cicer arietinum”

Extended Essay

Biology (SL)

“The effect of light intensity on the amount of chlorophyll in “Cicer arietinum”

 

 

Word count: 4 413 words

Content

Abstract ……………………………………………………………………………… 2

Introduction ………………………………………………………………………….. 3

Hypothesis …………………………………………………………………………… 3

Method:

Description ..………………………………………………………………………….. 8

Results ……………………………………………………………………………….. 10

Discussion ……………………………………………………………………..…….. 14

Conclusion ………………………………………………………………………..….. 14

Evaluation of the method ………………………………………………………..…… 15

Bibliography …………………………………………………………………………. 16

Abstract.

Plants, growing on the shaded area has less concentrated green color and are much longer and thinner than plants growing on the sun areas as they are dark green, short and thick. Research question was: “How does the amount of chlorophyll-a and chlorophyll-b, gram per gram of plant, depends on the light intensity in which plants are placed?”

Hypothesis suggests that there are several inner and outer factors that affect the amount of chlorophylls a and b in plants and that with the increase of light intensity the amount of chlorophyll will also increase until light intensity exceeds the value when the amount of destructed chlorophylls is greater than formatted thus decreasing the total amount of chlorophylls in a plant.

The seeds of Cicer arietinum were divided into seven groups and placed into various places with different values of light intensities. Light intensities were measured with digital colorimeter. After three weeks length was measured. Then plants were cut and quickly dried. Their biomass was also measured. Three plants from each group were grinded and the ethanol extract of pigments was prepared. The amount of chlorophylls was measured using method of titration and different formulas.

The investigation showed that plants growing on the lowest light intensity equal 0 lux contained no chlorophyll and had the longest length. The amount of chlorophyll quickly increased and length decreased with the increase of light intensity from 0 lux to 1200 lux. The amount of chlorophyll in plants unpredictably decreased during light intensity equal to 142 lux and than continued increasing and didn’t start decreasing reaching very high value (1200 lux).

The sudden decrease happened due to mighty existence of some inner genetical damages of seeds which prevented them from normal chlorophyll synthesis and predicted decrease didn’t decrease because extremely high light intensity was not exceeded.

Word count: 300 words

I. Introduction.

This theme seemed to be attractive for me because I could see that results of my investigation could find application in real life.

While walking in the forest in summer I saw lots of plants of different shades of green color: some of them were dark green, some were light green and some even very-very light green with yellow shades, hence I became very interested in this situation and wanted to know why it happens to be so. I also saw that those plants that were growing on sunny parts of forest, where trees were not very high, had dark green color and those, that were growing in shady parts of the same forest had very light green color. They also had difference in their length and thickness – those, that were growing on light were very short, but thick and strong, and those, growing in shady regions were very thin and fragile.

Hence I became very interested in finding scientifical description of my observations.

The aim of my project is to find out how does the changes in light intensity affect balance of chlorophyll in Cicer arietinum.

 

II. Hypothesis.

There are several factors that affect the development of chlorophyll in plants.[1]

Inner factors. The most important one is – genetical potential of a plant, because sometimes happen mutations that follow in inability of chlorophyll formation. But most of the times it happens that the process of chlorophyll synthesis is broken only partly, revealing in absence of chlorophyll only in several parts of the plant or in general low rate of chlorophyll. Therefore plants obtain yellowish color. Lots of genes participate in the process of chlorophyll synthesis, therefore different anomalies are widely represented. Development of chloroplasts depends on nuclear and plastid DNA and also on cytoplasmatic and chloroplastic ribosomes.

Full provision of carbohydrates seem to be essential for chlorophyll formation, and those plants that suffer from deficit of soluble carbohydrates may not become green even if all other conditions are perfect. Such leaves, placed into sugar solution normally start to form chlorophyll. Very often it happens that different viruses prevent chlorophyll formation, causing yellow color of leaves.

Outside factors. The most important outside factors, affecting the formation of chlorophyll are: light intensity, temperature, pH of soil, provision of minerals, water and oxygen. Synthesis of chlorophyll is very sensitive to all the factors, disturbing metabolic processes in plants.

Light. Light is very important for the chlorophyll formation, though some plants are able to produce chlorophyll in absolute darkness. Relatively low light intensity is rather effective for initialization and speeding of chlorophyll development. Green plants grown in darkness have yellow color and contain protochlorophyll – predecessor of chlorophyll а, which needs lite to restore until chlorophyll а. Very high light intensity causes the destruction of chlorophyll. Hence chlorophyll is synthesized and destructed both at the same time. In the condition of very high light intensity balance is set during lower chlorophyll concentration, than in condition of low light intensity.

Temperature. Chlorophyll synthesis seems to happen during rather broad temperature intervals. Lots of plants of умеренной зоны synthesize chlorophyll from very low temperatures till very high temperatures in the mid of the summer. Many pine trees loose some chlorophyll during winters and therefore loose some of their green color. It may happen because the destruction of chlorophyll exceeds its formation during very low temperatures.

Provision with minerals. One of the most common reason for shortage of chlorophyll is absence of some important chemical elements. Shortage of nitrogen is the most common reason for lack of chlorophyll in old leaves. Another one is shortage of ferrum, mostly in young leaves and plants. And ferrum is important element for chlorophyll synthesis. And magnesium is a component of chlorophyll therefore its shortage causes lack of production of chlorophyll.

Water. Relatively low water stress lowers speed of chlorophyll synthesis and high dehydration of plants tissues not only disturbs synthesis of chlorophyll, but even causes destruction of already existing molecules.

Oxygen. With the absence of oxygen plants do not produce chlorophyll even on high light intensity. This shows that aerobic respiration is essential for chlorophyll synthesis.

Chlorophyll.[2] The synthesis of chlorophyll is induced by light. With light, a gene can be transcripted and translated in a protein.

The plants are naturally blocked in the conversion of protochlorophyllide to chlorophyllide. In normal plants these results in accumulation of a small amount of protochlorophyllide which is attached to holochrome protein. In vivo at least two types of protochlorophyllide holochrome are present. One, absorbing maximally at approximately 650 nm, is immediately convertible to chlorophyllide on exposure to light. If ALA is given to plant tissue in the dark, it feeds through all the way to protochlorophyllide, but no further. This is because POR, the enzyme that converts protochlorophyllide to chlorophyllide, needs light to carry out its reaction. POR is a very actively researched enzyme worldwide and a lot is known about the chemistry and molecular biology of its operation and regulation. Much less is known about how POR works in natural leaf development.

ALA Portoporphyrine

 

Protochlorophyllide

 

Chlophyllide

 

 

Chlorophyll b Chlorophyll a

 

Chlorophyll[3] is a green compound found in leaves and green stems of plants. Initially, it was assumed that chlorophyll was a single compound but in 1864 Stokes showed by spectroscopy that chlorophyll was a mixture. If dried leaves are powdered and digested with ethanol, after concentration of the solvent, 'crystalline' chlorophyll is obtained, but if ether or aqueous acetone is used instead of ethanol, the product is 'amorphous' chlorophyll.

In 1912, Willstatter et al. (1) showed that chlorophyll was a mixture of two compounds, chlorophyll-a and chlorophyll-b:

Chlorophyll-a (C₅₅H₇₂MgN₄O₅, mol. wt.: 893.49). The methyl group marked with an asterisk is replaced by an aldehyde in chlorophyll-b (C₅₅H₇₀MgN₄O₆, mol. wt.: 906.51).

The two components were separated by shaking a light petroleum solution of chlorophyll with aqueous methanol: chlorophyll-a remains in the light petroleum but chlorophyll-b is transferred into the aqueous methanol. Cholorophyll-a is a bluish-black solid and cholorophyll-b is a dark green solid, both giving a green solution in organic solutions. In natural chlorophyll there is a ratio of 3 to 1 (of a to b) of the two components.

The intense green colour of chlorophyll is due to its strong absorbencies in the red and blue regions of the spectrum, shown in fig. 1. (2) Because of these absorbencies the light it reflects and transmits appears green.

Fig. 1 - The uv/visible adsorption spectrum for chlorophyll.

Due to the green colour of chlorophyll, it has many uses as dyes and pigments. It is used in colouring soaps, oils, waxes and confectionary.

Chlorophyll's most important use, however, is in nature, in photosynthesis. It is capable of channelling the energy of sunlight into chemical energy through the process of photosynthesis. In this process the energy absorbed by chlorophyll transforms carbon dioxide and water into carbohydrates and oxygen:

CO₂ + H₂O (CH₂O) + O₂

Note: CH₂O is the empirical formula of carbohydrates.

The chemical energy stored by photosynthesis in carbohydrates drives biochemical reactions in nearly all living organisms.

In the photosynthetic reaction electrons are transferred from water to carbon dioxide, that is carbon dioxide is reduced by water. Chlorophyll assists this transfer as when chlorophyll absorbs light energy, an electron in chlorophyll is excited from a lower energy state to a higher energy state. In this higher energy state, this electron is more readily transferred to another molecule. This starts a chain of electron-transfer steps, which ends with an electron being transferred to carbon dioxide. Meanwhile, the chlorophyll which gave up an electron can accept an electron from another molecule. This is the end of a process which starts with the removal of an electron from water. Thus, chlorophyll is at the centre of the photosynthetic oxidation-reduction reaction between carbon dioxide and water.

Treatment of cholorophyll-a with acid removes the magnesium ion replacing it with two hydrogen atoms giving an olive-brown solid, phaeophytin-a. Hydrolysis of this (reverse of esterification) splits off phytol and gives phaeophorbide-a. Similar compounds are obtained if chlorophyll-b is used.

Chlorophyll can also be reacted with a base which yields a series of phyllins, magnesium porphyrin compounds. Treatment of phyllins with acid gives porphyrins.

 

Now knowing all these factors affecting the synthesis and destruction of chlorophyll I propose that the amount of chlorophyll in plant depends on light intensity in the following way: with the increase of light intensity the amount of chlorophyll increases, but then it starts decreasing because light intensity exceed the point when there is more chlorophyll destructed than formed.

 

 

Diagram 1. The predicted change of amount of chlorophyll in leaves of depending on light intensity

plateau

max

Light intensity, lux

Chlorophyll, gram per gram of plant.

Variables.

 

Independent:

Light intensity, lux

Constant:

pH of soil water supply, ml temperature, t^o C

Dependent:

length, cm amount of chlorophyll in gram of a plant, gram per gram

III. Method.

 

Apparatus:

·     seeds of Cicer arietinum

·     28 plastic pots

·     water

·     scissors

·     ruler (20 cm ± 0.05 cm)

·     CaCO₃

·     soil (adopted for home plants)

·     digital luxmeter (± 0.05 lux)

·     test tubes

·     H₂SO₄ (0.01 M solution)

·     Pipette (5 cm³ ± 0.05 cm³)

·     mortar and pestle

·     burette

·     ethanol (C₂H₅OH), 98%

·     beakers

Firstly I went to the shop and bought germinated seeds of Cicer arietinum. Then sorted seeds and chose the strongest ones. After that I prepared soil for them and put them in it.

As the aim of this project is to investigate the dependence of mass of chlorophyll in plants during different light intensities it was needed to create those various conditions. Pots with seeds were placed into the following places: in the wardrobe with doors (light intensity is o lux), under the sink (light intensity is 20,5 lux), in the shell of bookcase (light intensity is 27,5 lux), above the bookcase (light intensity is 89,5 lux), above the extractor (light intensity is 142 lux), beyond the curtains (light intensity is 680 lux) and on the open sun (light intensity is 1220 lux). Light intensity was measured with the help of digital luxmeter. It was measured four times each day: morning, midday, afternoon, evening. During those four periods four measurements were done and the maximum value was taken into consideration and written down. Those measurements lasted for three weeks of the experiment as the whole time of the experiment was three weeks. The luxmeter’s sensitive part was placed on the plants (so it was just lying on them) in order to measure light intensity flowing directly on plant bodies, then two minutes were left in order to get stabilized value of light intensity and the same procedure was repeated. All those actions were done in order to get more accurate results of light intensity.

Growing plants were provided with the same amount of water (15 ml, once a day in the morning) and they were situated in the same room temperature (20^o C), pH of soil was definitely the same because all the plants were put in the same soil (special soil for room flowers).

After three weeks past the length of plants was measured with the help of ruler. Firstly the plants were not cut, so their length had to be measured while they were in the pots. The ruler was placed into the pot and plants were carefully stretched on it. The action was repeated three times and only maximum value was taken into consideration. After that plants were cut. Then those already cut plants were put into the dark place and quickly dried.

Titration.

I have chosen three plants from each light intensity group and measured their weight. . In order to obtain the pigments, three plants were cut into small pieces and placed in a mortar. Calcium carbonate was then added, together with a little ethanol (2 cm³). The leaf was grinded using a pestle until no large pieces of leaf tissue were left, and the remaining ethanol was poured into the mortar (3 cm³). Then 1 ml of obtained solution was placed into the test tube and this 1 ml of solution was then titrated against 0.01 M solution of sulfuric acid, through the use of a pipette. The titration was complete when the green solution turned dark olive-green[4]. This solution obtained from the first action was stored as the etalon for the following ones. The settled olive-green coloring meant that all chlorophyll had reacted with H₂SO₄. So the process of titration was repeated 7 times for all light intensity groups.

The solution is titrated until the dark olive-green color because it is known that when the reaction between chlorophyll and sulfuric acid happens, chlorophyll turns into phaeophetin which has grey color (see table 1), therefore when the solution is olive-green, than the reaction has succeeded. But while searching in the internet and books I found out that there are several opinions about the color of phaeophytin – in the book written by Viktorov it is ssaid to have grey color, but in the internet link http://www.ch.ic.ac.uk/local/projects/steer/chloro.htm it is said to have brown olive-green color. Also I made chromatography in order to investigate the color of phaeophytin and the result was that it has grey color. It can be proposed that olive-green color is obtained because grey phaeophetyn is mixed with other plant pigments.

 So titration is one of the visual methods that can be used in order to find the mass of chlorophyll in plants.

All the measurements and even chromatography were done three times and the mean value was taken, for chromatography grey color was confirmed.

Table 1. Plant pigments.

Name of the pigment	Color of the pigment
Chlorophylls ( a and b )	Green
Carotene	Orange
Xanitophyll	Yellow
Phaeophytin-a	OLIVE BROUN or GREY

IV. Results.

Table 2. Raw data.

Number of plant	Light intensity (lux)
	0,0	20,5	27,5	89,5	142,0	680,0	1220,0
1	23	35	20	1	30	2	15
2	30	36	33	4	31	20	16
3	38	37	35	8	34	21	16
4	39	37	36	9	35	21	16
5	44	38	37	9	38	21	17
6	46	39	40	12	38	22	17
7	50	39	40	12	38	22	19
8	52	40	43	13	39	23	20
9	55	40	43	15	39	25	21
10		40		18	40	27	22
11		42		20	41	29	26
12		42		22	41	30
13		42		22	41	31
14		42		24	42	33
15		43		25	42	34
16		43		25	43	34
17		44		25	43	35
18		44		25	43	35
19		45		26	45	37
20		45		26	45	38
21		45		26	46	38
22		45		26	46	41
23		46		27	48	41
24		46		29	48	44
25		49		32	49
26				34	49
Mean	41,888889	41,76	36,33333	19,80769	41,30769	29,33333	18,63636
Median	44	42	37	23	41,5	30,5	17
St. deviation	10,50529	2,928	4,740741	7,467456	4	7,472222	2,694215

Table 3. Frequency of lengths of 3-weeks-old plants depending on light intensity.

	Light intensity, lux
Plant length, cm (class)	0,0	20,5	27,5	89,5	142,0	680,0	1220,0
0.0-10.0	0	0	0	5	0	1	0
10.1-20.0	0	0	1	6	0	1	8
20.1-30.0	2	0	0	13	1	10	3
30.1-40.0	2	9	6	2	9	9	0
40.1-50.0	3	15	2	0	16	3	0
50.1-60.0	2	0	0	0	0	0	0
Total	9	24	9	26	26	24	11

Table 3 (alternative) Frequency of length of 3-weeks-old plants depending on light intensity.

	Light intensity, lux
Plant length (Class)	0,0	20,5	27,5	89,5	142,0	680,0	1220,0
0.0-10.0	0	0	0	19,23%	0	4,17%	0
10.1-20.0	0	0	11,10%	23,08%	0	4,17%	72,72%
20.1-30.0	0	0	0	50%	3,85%	41,62%	27,28%
30.1-40.0	0	37,50%	66,60%	7,69%	34,62%	37,52%	0
40.1-50.0	0	62,50%	22,30%	0	61,53%	12,52%	0
50.1-60.0	100%	0	0	0	0	0	0
Total	1	1	1	1	1	1	1

Calculation of the mean length of plants.

For light intensity equal to 20,50 lux:

The sum of lengths of all plants in this group is 45cm + 37cm + 39cm + 49cm + 46cm + 44cm + 45cm + 44cm + 42cm + 37cm + 40cm + 40cm + 39cm + 43cm + 42cm + 42cm + 36cm + 45cm + 38cm + 45cm + 46cm + 40cm + 35cm + 42cm + 43cm = 1044cm

Hence mean length is 1044cm : 25 plants = 41,76cm

Table 4.

Light intensity, lux	Mean wet biomass, g	Mean dry biomass, g	% of water	Mean length, cm	Mass of chl. In 1 g
0	0,273	0,041	84,98	41,89	0,0000
20,5	0,579	0,056	90,33	41,76	0,0496
27,5	0,332	0,033	90,06	36,33	0,1462
89,5	0,181	0,018	90,06	19,81	0,1769
142	0,511	0,047	90,80	41,33	0,0697
680	0,338	0,043	87,28	29,33	0,1557
1220	0,301	0,034	88,70	18,64	0,1939

 

Calculation of amount of chlorophyll in plants basing on the results of titration

H₂ SO₄ + C₅₆ O₅ N₄ Mg => C₅₆ O₅ N₄ H + MgSO₄

Concentration of H₂SO₄ is 0,01 M

C – concentration

V – volume

n – quantity of substancy

m – mass

Mr – molar mass

For light intensity equal to 20,5 lux.

n = V (in dm³) ∙ C

2 ∙ 10^-3 ∙ 0,01 = 2 ∙ 10^-5

n = m / Mr => m = n ∙ Mr

m = 2 ∙ 10^-5 ∙ 832 = 1,664 ∙ 10^-2 grams

mass of plant mass of chlorophyll

1,68 grams - 0,08335 grams of chlorophyll