Light Intensity As A Main Factor Affecting The Rate Of Photosynthesis

Karla T. Balino

Group 3 Sec. I-1L

August 28, 2012

____________________A scientific paper submitted in partial fulfillment of the requirements in General Biology I laboratory

under Sir Kevin Labrador, 1st sem., 2012-2013

ABSTRACT

The factors affecting photosynthesis were determined using two experiments. The first one was through determining the carbon dioxide uptake of a plant. The test tubes A and C that were wrapped with carbon paper and exposed to light did not produced a blue color unlike test tube B which produced a blue color due to the plant (hydrilla), carbon dioxide, and sunlight present during photosynthesis. The second experiment was determining the starch production with the use of I2KI solution Two mongo seedlings grown at the same time, temperature, soil type and water conditions except that one is grown in the dark and the other is left under natural light condition were heated and added with two drops of I2KI solution. A black color appeared on the mongo leaf that was left under light condition resulting from the reaction between the starch present and the said solution while a greenish color appeared on the mongo leaf left on dark condition. Therefore, the less the light intensity is, the less rapidly the plant photosynthesizes.

INTRODUCTION

Photosynthesis is the major source of energy for practically all plants living in sunlight. It

is the process that plants go through to convert energy from sunlight to produce sugar that is then

later transformed into ATP by cellular respiration. Green plants obtain all their energy by

photosynthesis, the process in which light is converted to chemical energy. The light reactions

are the ‘photo’ part of photosynthesis, while the so-called ‘dark’ reactions constitute the

‘synthesis’ part of the process, which is also called carbon fixation. The light and dark reaction

are intimately linked and are summarized by the left-to-right reaction. (Hong, 2005)

Light is one of the main factors that affects the rate of photosynthesis, which literally means

using light to create something new. Plants use energy absorbed from the sun or another light source

as fuel for the photosynthesis process. When light intensity increases, the rate at which

photosynthesis occurs increases as well. Outdoor plants photosynthesize faster on a sunny day as

opposed to a cloudy day. (Lee, 2003)

Temperature is another factor that affects the rate at which photosynthesis occurs.

Photosynthesis is a chemical reaction, and higher temperatures speed up chemical reactions. This is

one of the reasons people grow plants in greenhouses; the greenhouses convert light from the sun

into warmth and trap it inside the greenhouse, creating a warmer natural environment for plants,

which then photosynthesize faster. Plants also photosynthesize faster during the summer than the

winter because of the warmer temperatures. (Wiley, 1937)

Carbon dioxide is one of the main elements in the photosynthesis scientific formula. It is

converted into oxygen during photosynthesis. When levels of carbon dioxide are increased, the rate

at which photosynthesis occurs increases also, up to a point. More carbon dioxide leads to faster

photosynthesis and more oxygen produced in a shorter amount of time than with less carbon dioxide.

Water is one of the main ingredients required in the process of photosynthesis and it's

important for a plant or any other photoautotroph to maintain or receive sufficient supply of

water during the process of photosynthesis. Water has a direct effect on the stomata which is

responsible for the diffusion of carbon dioxide.

In photosynthesis, plants use light energy to produce glucose from carbon dioxide. The

glucose is stored mainly in the form of starch granules, in plastids such as chloroplasts and

especially amyloplasts. Toward the end of the growing season, starch accumulates in twigs of

trees near the buds. Fruit, seeds, rhizomes, and tubers store starch to prepare for the next growing

season.

Glucose is soluble in water, hydrophilic, binds much water and then takes up much space;

glucose in the form of starch, on the other hand, is not soluble and can be stored much more

compactly.

Glucose molecules are bound in starch by the easily hydrolyzed alpha bonds. The same

type of bond is found in the animal reserve polysaccharide glycogen. This is in contrast to many

structural polysaccharides such as chitin, cellulose and peptidoglycan, which are bound by beta

bonds and are much more resistant to hydrolysis. (Sone, 1937)

The study aimed to discuss the factors affecting photosynthesis. The objectives

was/were:

1. to determine the main factor affecting photosynthesis.

MATERIALS AND METHODS

In showing the rate of photosynthesis, two experiments were done.

The first experiment was determining the carbon dioxide uptake of Hydrilla sprigs. A 250

ml beaker filled with 150 ml distilled water was obtained and placed an equal volume of

bromthymol indicator. This will help keep track the amount of carbon dissolved in water since

carbon dioxide is a colorless gas. It was stirred well and CO2 was introduced to the solution. A

volunteer was asked to blow through the straw until a color changed from blue to yellow

occurred.

Four large test tubes were poured with equal amounts of the solution. The test tubes were

then labeled A-D to avoid confusion. Sprigs of Hydrilla with the same developmental stage,

length and leaf size were added to test tubes A and B only. Test tubes A and C was wrapped with

carbon paper and placed on a test tube rack. The tubes were left under the bright sunlight for

about one hour. The test tubes were then unwrapped afterwards and observation for any color

change in the set-ups was done.

Solution after introducing to CO2 Test tubes with carbon paper

For the second experiment which is the starch production, two mongo seedlings grown at

the same time, temperature, soil type and water conditions were obtained. One is grown in the

dark while the other is left under natural light condition. Four leaves from each seedlings were

picked and placed in separate beakers. Each of the beakers was added by 40 ml of 70% ethyl

alcohol and were heated for 3 to 5 minutes while gently shaking.

After heating, the heated leaves were then transferred to separate petri dishes. In each

leaf, two drops of I2KI solution was added.

Mongo leaves w/ ethyl alcohol after heating Mongo leaves in separate petri dishes

Addition of I2KI solution

RESULTS AND DISCUSSIONS

In the first experiment, the results showed that photosynthesis can only take place when

light, chlorophyll and carbon dioxide are present. The test tubes A and C that were wrapped with

carbon paper and exposed to light did not produced a blue color unlike test tube B which

produced a blue color due to the plant (hydrilla), carbon dioxide, and sunlight present during

photosynthesis. Test tube D having exposed to sunlight but no amount of plant placed also did

not produce a blue color.

Furthermore, carbon dioxide wasn’t utilized by the plant covered with carbon paper (Test

tube A) since photosynthesis takes place mainly in leaves and depends on an important green

pigment called chlorophyll, which is found in chloroplasts. To obtain the most sunlight as

possible, leaves have a large surface area and the more sunlight the plants receives, the better it

can photosynthesize.

In the second experiment, testing for the presence of starch using iodine potassium iodide

(I2KI) showed that plants use light energy to produce glucose from carbon dioxide and stored as

starch. When the heated mongo leaves were added with two drops of I2KI solution, a black color

appeared on the mongo leaf that was left under light condition resulting from the reaction

between the starch present and the said solution while a greenish color appeared on the mongo

leaf left on dark condition. This is because plants need light energy to produce glucose from

carbon dioxide and stored as starch.

Test tubes after exposing to light for one hour Production of starch after adding I2KI solution

SUMMARY AND CONCLUSION

The factors affecting photosynthesis were determined using two experiments. The first

one was through determining the carbon dioxide uptake of a plant. The test tubes A and C that

were wrapped with carbon paper and exposed to light did not produced a blue color unlike test

tube B which produced a blue color due to the plant (hydrilla), carbon dioxide, and sunlight

present during photosynthesis. Test tube D having exposed to sunlight but no amount of plant

placed also did not produce a blue color.

The second experiment was determining the starch production with the use of I 2KI

solution since it reacts with starch. Two mongo seedlings grown at the same time, temperature,

soil type and water conditions except that one is grown in the dark and the other is left under

natural light condition were obtained. When the heated mongo leaves were added with two drops

of I2KI solution, a black color appeared on the mongo leaf that was left under light condition

resulting from the reaction between the starch present and the said solution while a greenish

color appeared on the mongo leaf left on dark condition.

Therefore, the less the light intensity is, the less rapidly the plant photosynthesizes.

LITERATURE CITED

Hong, B. T. (2005) Siri Teks Referens Biologi SPM. Selangor: Pearson Malaysia Sdn. Bhd.

Lee Soon Ching, L. S. (2003) Success in Biology for STPM Volume 1. Selangor, Fajar Bakb dn. Bhd.

Belle Lowe Experimental Cookery From The Chemical And Physical Standpoint. John Wiley & Sone Year 1937, Copyright 1937. Belle Lowe