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

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:

[pic]

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.

[pic]

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 [pic](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.

[pic]

Chlorophyll can also be reacted with a base which yields a series of

phyllins, magnesium porphyrin compounds. Treatment of phyllins with acid

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