Contents

Sub-Title Page

Introduction to Photosynthesis

- History of Photosynthesis

- What is Photosynthesis?

1-2

Structure of A Leaf

- A Two-dimensional Cross Section of The Leaf

- A Three-dimensional Cross Section of A leaf

3-4

Adaptation of Leaves for Optimal Photosynthesis

- Features of A Leaf

- Adaptation of Plants from Different Habitats for

Photosynthesis

5-6

Mechanism of Photosynthesis

- Light Reaction

- The Dark Reaction

7-8

Factors That Affect Photosynthesis

- Light Intensity

- Concentration of Carbon Dioxide

- Temperature

- Water

9-11

Experiment to Determine The Effect of Light Intensity on The

Rate of Photosynthesis.12-13

Examples of Past Year SPM Questions about Photosynthesis

14

References

15-19

1

Introduction to Photosynthesis

Discovery of Photosynthesis  

In 1649, Jan Baptista Van Helmont did the first biological experiment in which the ingredients were

measured accurately and all changes noted precisely. Van Helmont began by transplanting the shoot of a

young willow tree into a large bucket of soil. He weighed the willow and then the soil separately. If the willow

tree formed its tissues by absorbing the nutrients from the soil then the soil should lose weight as the plant

grew. Van Helmont carefully kept the soil covered so that absolutely nothing could interfere with his

experiment.

Naturally, Van Helmont had to water the willow tree or else it wouldn’t grow. He concluded that the

water he was adding helped carry the nutrients to the tree and then simply evaporated into the air. For five

years, Van Helmont waited patiently, watching the tree grow until finally he removed it from the pot, shook

off all the soil and weighed the plant. In five years the willow tree had added 164 pounds to its original

weight. Then, for the second part of the experiment, Van Helmont dried and weighed the soil. It had only lost

2 ounces. From this, Van Helmont concluded that the willow tree drew its nutrients, not from the soil but from

water. Accidentally, he made a mistake and said that the material that made up the bark, wood, roots and

leaves came from the water he had added over the five years!

The next big important step in the understanding of photosynthesis came in the early 1770’s. Joseph

Preistly, the British man who received the recognition of discovering oxygen, found that a piece from a mint

plants could restore the air in a container with a burning candle, so that it <the candle> could be used again.

Accidentally, one day, Joseph Preistly placed the candle in a dark corner of his laboratory. Since the mint

plant could not photosynthesize, the candle’s flame extinguished.  Unfortunately, Mr. Preistly never did really

understand that great role which light played in his experiment.

Several years later, in 1979, a Dutch physician, Jan Ingenhousz, wanted to find out whether flowers

really did help cure illnesses. After many different tests, he finally concluded that only the green parts of

plants cleaned the air and only when placed in strong light. Flowers and other non-green parts of plants

used up oxygen just like animals. Ingenhousz suggested that this process of photosynthesis causes carbon

dioxide to split into carbon and oxygen. Then the oxygen is released as a gas.

Later, other scientists discovered that sugars contain carbon, hydrogen and oxygen atoms in a ratio

of one carbon molecule per molecule of water (CH2O). This is where the word carbohydrate comes

from, carbo- for “carbon” and hydrate for “water”. Carbohydrates are a family of chemical compounds

including sugars and starches, which are made up of large numbers of sugar units linked together. In 1804,

the Swiss scientists, Nicholas Theodore de Saussure repeated Van Helmont’s experiment but carefully

measured the amounts of carbon dioxide and water that were given to the plant. He showed that the carbon

in the plants came from carbon dioxide and the hydrogen from water. Then, forty years later, a German

scientist, Robert Mayer, showed that the energy of sunlight is captured in photosynthesis.   

2

What is Photosynthesis?

Photosynthesis originates from the Greek word photo, meaning "light," and synthesis, from the

Greek work syntithenai, which means "to combine". Photosynthesis is basically the conversion of the

energy from the sun in to a usable chemical energy. Photosynthesis occurs in plants, algae and many

species of Bacteria. Photosynthetic organisms are called photoautotrophs, since it allows them to create

their own food. The photosynthesis process uses carbon dioxide and water, releasing oxygen as a waste

product.

It is also defined as a biochemical process through which light energy is absorbed by chlorophyll

and is used to fuel the synthesis of sugar molecules (i.e.; glucose). Photosynthesis is vital for life on Earth.

As well as maintaining the normal level of oxygen in the atmosphere, nearly all life either depends on it

directly as a source of energy, or indirectly as the ultimate source of the energy in their food. Photosynthesis

can be represented using a chemical equation. The overall balanced equation is:

CO2 : carbon dioxide 

H2O : water

C6H12O6 : glucose

O2 : oxygen

3

6CO2 + 6H2O C6H12O6 + 6O2

sunlight

chlorophyll

Structure of A Leaf

A Two-dimensional Cross Section of The Leaf

A

leaf typically consists of the following tissues:

1. An epidermis that covers the upper and lower surfaces

The epidermis is the outer multi-layered group of cells covering the leaf. It forms the boundary

separating the plant's inner cells from the external world. The epidermis serves several functions as for

protection against water loss, regulation of gas exchange, secretion of metabolic compounds, and

absorption of water. The epidermis is usually transparent and coated on the outer side with a

waxy cuticle called sebum that prevents water loss. The cuticle is in some cases thinner on the lower

epidermis than on the upper epidermis, and is thicker on leaves from dry climates as compared with those

from wet climates.

The epidermis is covered with pores called stomata, which are surrounded on each side guard cells

containing chloroplasts that controls the size of the stomata opening. The stoma complex regulates the

exchange of gases and water vapor between the outside air and the interior of the leaf. Typically, the

stomata are more numerous over the lower epidermis than the upper epidermis.

2. An interior chlorenchyma called the mesophyll

Most of the interior of the leaf between the upper and lower layers of epidermis is

a  chlorenchyma tissue called the mesophyll (Greek for "middle leaf"). This assimilation tissue is the primary

location of photosynthesis in the plant. The products of photosynthesis are called "assimilates". The

mesopyhll cells are divided into two:

Palisade mesopyhll: Arranged tightly near the upper surface of the leaves to receive maximum amount

of light. Have high density of chloroplasts.

4

Spongy mesophyll : Consists of cells that have irregular shapes and fewer chloroplasts than palisade

cells. Loosely arranged and have air spaces between them that allows water and carbon dioxide to

diffuse easily through the leaves. The irregular shape also increase the internal surface area for

gaseous exchange.

3. An arrangement of veins (the vascular tissue).

The veins are the vascular tissue of the leaf and are located in the spongy layer of the mesophyll. The

veins are made up of:

Xylem : Tubes that brings water and minerals from the roots into the leaf.

Phloem: Tubes that usually moves sap, with dissolved sucrose, produced by photosynthesis in the leaf,

out of the leaf.

A Three-dimensional Cross Section of A leaf

5

Palisade mesophyll

Spongy mesophyll

Cuticle

Upper Epidermis

Lower Epidermis

Vascular bundle

Stomata Guard cells

Adaptation of Leaves for Optimal Photosynthesis

Features of A Leaf

The leaf is the main site for photosynthesis in plants and it has several special features which helps

it carry out this role.

The leaves of most plants grow arranged so that they do not overlap each other as to receive as much

as sunlight as possible. This arrangement is called a leaf mosaic and functions to:

Enable leaves to receive as much light as possible.

Plant can also detect the direction of the light so that their leaves are always held in the best position

to absorb maximum amount of light.

A leaf consist of a flat, thin lamina which is joined to the stem by a petiole holds the leaf in the best

position to receive the maximum amount of light. From the petiole, a main vein leads down the leaf

and branches out into side veins which support the lamina.

Moreover, leaves have a flat, thin lamina (layer) joined by the stem by a petiole (The slender stem that

supports the blade of a leaf) that holds the leaf in the best position to receive the maximum amount of sunlight.

Adaptation of Plants from Different Habitats for Photosynthesis

Features of a leaf

Flattened shape Increases surface

area

Thin Gases can diffuse

quickly

A vascular systemTo supply water &

take away the products

Stomata To allow gas

exchange

Chloroplast To capture light

energy

The distributionof stomata

The distributionof chloroplasts

Plants from different habitats are adapted to carry out photosynthesis with two main aspects

6

The distribution of stomata and chloroplasts differ according to environmental conditions. Plants

show various adaptations to carry out photosynthesis in different types of habitats. To photosynthesise

optimally, plants need a method for gaseous exchange and an efficient means of absorbing light energy.

There are three types of plants:

Mesophytes : A land plant that grows and adapted in an environment having a moderate amount

of moisture.

Xerophytes : A plant adapted to living in a dry habitat (a desert plant).

Hydrophytes : A plant adapted to grow in water.

TYPE OF PLANT HABITAT

ADAPTATION

EXAMPLEDistribution of

stomata

Distribution of

chloroplasts

Mesophyte Normal terrestrial

or land plants that

live in areas where

water is readily

available.

More stomata at the

lower epidermis of the

leaf to reduce loss of

water through

transpiration.

Many chloroplasts

in the palisade

mesophyll cells,

spongy mesophyll

cells and guard

cells to carry out

photosynthesis.

Hibiscus

Balsam

(Impatiens

sp.)

Maize (Zea

Mays)

Xerophyte Plants that live

under arid or dry

conditions.

Sunken stoma on

the stems

The leaves are

modified to form

thorns with no

stomata to reduce

the problem of

transpiration.

The stem have

many chloroplasts.

Photosynthesis

only occurs at the

stems which have

chlorophyll and

stomata.

Cactus

Opuntia

Hydrophyte Water surface

(lower epidermis of

leaf on the surface

of water)

Stomata are found on

the upper surface of

leaf only to allow

exchange of gases.

Many chloroplasts

in the palisade

mesophyll cells of

leaf.

Water lily

(Nymphea sp.)

with large flat

leaves floating on

water surface.

In the water. No stomata

Diffusion of gases

occurs throughout

the whole surface

of the plant.

Many chloroplasts

in all parts of the

plant. Every part of

the plant carries

out

photosynthesis.

Hydrilla sp. which

is fully

submerged in

water.

7

Mechanism of Photosynthesis

Light Reaction

Structure of Chloroplast

Photosynthesis is a two stage process. The first process is the Light Dependent Process or light

reaction. It is the photosynthetic process in which solar energy is harvested and transferred into the

chemical bonds of ATP (Adenosine Triphosphate). The light reaction only occurs in the presence of light and

occurs in the Grana of the chlorophyll.

In the light reaction, light strikes chlorophyll in such a way as to excite electrons to a higher energy

state. In a series of reactions the energy is converted into ATP. Water is split in the process into hydrogen

ions (H+) and hydroxyl ions (OH-), releasing oxygen as a by-product of the reaction. This reaction is known

as photolysis of water.

The hydrogen ions then receives electrons from the photolysis process to become hydrogen atoms.

Oil droplet

RibosomesThylakoid membranes

DNA

Starch grain Intergranal membrane

Stroma

Granum(plural; Grana)

24H2O 24H+ + 24OH-

water molecule

light

chlorophyllhydrogen ions hydroxyl ions

24H+ 24e- 24H

hydrogen atoms

8

The hydroxyl ions then loses an electron to form a hydroxyl group.

The hydroxyl groups then combines to form water and gaseous oxygen. The oxygen is released into

the atmosphere through the stomata.

The ATP molecules synthesised and hydrogen atoms move to the stroma of the chlorophyll to

provide energy and reducing power for the second process of photosynthesis which is the dark reaction.

The Dark Reaction

The Light Independent Process (or dark reaction) occurs when the products of the light reaction are

used to form covalent bonds of carbohydrates. It is also known as the Calvin Cycle or Carbon Fixation. The

dark reactions usually occur in the dark, if the energy carriers (ATP molecules and hydrogen atoms) from the

light process are present. The dark reaction occurs in the Stroma of the chlorophyll.

In this reaction, carbon dioxide is converted to sugar using ATP and hydrogen atoms. The reduction

of carbon dioxide into one CH2O (carbon sugar), which is the basic unit of glucose. 6-carbon sugars

combines to form one molecule of glucose. The glucose monomers are then condensed to form starch

granules that are then temporarily stored in the chloroplasts.

24OH- 24OH + 24e-

Hydroxyl groups

24OH 12H2O + 6O2

water oxygen gas

9

Factors That Affect Photosynthesis

Light Intensity

Light is essential during the light reaction of photosynthesis. When the concentration of carbon

dioxide and temperature are controlled at constant level, the rate of photosynthesis is directly proportional to

light intensity up to a certain point. Beyond the maximum point, an increase in light intensity will no longer

affect the rate of photosynthesis as it has already reached its optimum light intensity. At a very high light

intensity, the rate of photosynthesis slows down because the pigment chlorophyll is damaged by ultra-violet

rays.

Factors Affecting Photosynthesis

Light Intensity

Concentration Of Carbon Dioxide

Temperature

Water Supply

Light intensity

Rate

of p

hoto

synt

hesis

AA

BB CCDD

10

A: Light intensity increases, rate of photosynthesis increases.

B: Other factors become limiting.

C: Light intensity is no longer limiting factor.

D: Maximum rate of photosynthesis.

Concentration of Carbon Dioxide

Carbon dioxide makes up 0.035% of the atmosphere. It is needed in the dark reaction as a raw

material used in the synthesis of glucose. If there is no other factors limiting photosynthesis, an increase in

the concentration of carbon dioxide results in an increase in the rate of photosynthesis.

Temperature

Temperature is critical for the dark reaction as it is enzyme driven. an increase of 10 degree Celsius

in the surrounding temperature will doubled the rate of photosynthesis. The optimum temperature for most of

the plants are between 25-30 degree Celsius. However, when the temperature is too high the photosynthetic

enzyme are destroyed and photosynthesis stops altogether.

Light intensity

Rate

of p

hoto

synt

hesis

0.035% CO2

1% CO2

Light intensity

Rate

of p

hoto

synt

hesis

0.035% CO2

1% CO2

Rate

of p

hoto

synt

hesis

Temperature

25ºCOptimum temperature

Enzymes denature

Rate

of p

hoto

synt

hesis

Temperature

25ºCOptimum temperature

Enzymes denature

11

Water Supply

Water is needed for photosynthesis, however water is rarely the limiting factor in photosynthesis

because the amount of water required is small. If water is not supplied, wilting occurs and the stomata is

closed. This prevents the diffusion of carbon dioxide into the leaves. As a result the rate of photosynthesis

decreases as the lower concentration of carbon dioxide becomes the limiting factor.

12

Experiment to Determine The Effect of

Light Intensity on The Rate of Photosynthesis.

Aim of Experiment

To determine how does light intensity affect the rate of photosynthesis.

Problem statement

How does light intensity affect the rate of photosynthesis?

Hypothesis

The rate of photosynthesis increases with the increase of light intensity.

Variables

Manipulated variable : Distance between light source and plant/Intensity of light.

Responding variable : Number of bubbles released in five minutes/Rate of photosynthesis

Fixed variable(s) : Type of plant, percentage of sodium hydrogen carbonate solution (concentration

of carbon dioxide)and voltage of bulb.

Materials

A few sprigs of hydrilla sp., 1% sodium hydrogen carbonate solution, distilled water and plasticine.

Apparatus

60W bulb as light source, 500ml beaker, a test tube, a glass funnel, a stopwatch, a thermometer and a

meter rule.

Technique

Observing and counting the air bubbles released by the plants.

Procedure

1. A segment of plant of approximately 8cm was cut with scissors.

2. The end of the stem at the site of incision was gently crushed.

3. The apparatus as shown in the diagram below is set up.

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4. The plant was fully submerged into a test tube filled with 20ml of distilled water that is maintained at

28۫۫C (room temperature) and 5ml of sodium hydrogen carbonate to supply carbon dioxide to the

plant.

5. The plant is secured with some plasticine to prevent it from floating upwards.

6. A light source was placed 50 cm away facing the test tube.

7. The light source is turned on and the stopwatch is started as soon as the plants starts to release air

bubbles.

8. The number of bubbles released by the plant in 5 minutes of counted and recorded. This step is

repeated twice to and the average number of gas bubbles per minute is calculate to obtain an

accurate count.

9. Steps one to eight were repeated with different distances from the light source on each consecutive

experiment. On the second trial the light source was placed at 40 cm from the test tube with the

plant. On the third trial the light source was 30 cm away. On the fourth trial the test tube was

20cm away. On the fifth trial the light source was placed 10 cm away from the test tube. This is to

obtain different light intensities.

10. The results obtained are recorded and tabulated.

11. Analysation of data is done by plotting the following graphs:

i. Number of gas bubbles released per minute against the distance from the light source.

ii. Rate of photosynthesis versus light intensity.

Results

Tabulation of Data

Distance from light

Source, d(cm)

Light Intensity (1/d) Number of bubbles

formed in 5 minutes

Number of bubbles

formed per minute

10 0.1 105 21

20 0.05 80 16

30 0.033 55 11

40 0.025 35 7

50 0.02 15 3

14

Analysation of Data

Distance from light source (cm)

Num

ber

of g

as b

ubbl

es r

eles

ed p

er m

inut

e

10 20 30 40 50

3

21

18

15

12

9

6

24

Distance from light source (cm)

Num

ber

of g

as b

ubbl

es r

eles

ed p

er m

inut

e

10 20 30 40 50

3

21

18

15

12

9

6

24N

umbe

r of

gas

bub

bles

rel

esed

per

min

ute

10 20 30 40 50

3

21

18

15

12

9

6

24

Light Intensity

Ra

te o

f oh

oto

syn

the

sis

10 20 30 40 50

3

21

18

15

12

9

6

24

Light Intensity

Ra

te o

f oh

oto

syn

the

sis

10 20 30 40 50

3

21

18

15

12

9

6

24

Number of gas bubbles released per minute against the distance from the light

source.

Rate of photosynthesis versus light intensity source

15

Discussion

To keep this experiment controlled, the same amount and concentration of sodium hydrogen

carbonate is used for each trials. The temperature is also kept constant at 28 ۫C so that temperature will not

affect the rate of photosynthesis and cause this experiment not accurate. The rate of photosynthesis os

equivalent to the rate of oxygen released. Hence, the number of gas bubbles or oxygen released per minute

can be taken as the rate of photosynthesis. Varying the distances from the light source to the plant will give

different light intensities. The reciprocal of the distance of light source (1/d) is taken as the light intensity. A

farther distance will cause the light intensity to decrease.

Conclusion

The rate of photosynthesis increases with the increase of light intensity. The hypothesis is accepted.

16

Example of Past Year SPM Questions about Photosynthesis

Biology SPM 2007 (Paper 1)

Question 13

Soalan 13

Which of the following is needed during the light dependent reaction of photosynthesis?

Antara yang berikut, yang manakah diperlukan semasa tindak balas cahaya fotosintesis?

A. ATP

ATP

B. Water

Air

C. Hydrogen atom

Atom Hidrogen

D. Carbon Dioxide

Karbon Dioksida

Question 14

Soalan 14

Diagram 8 is a graph showing the effect of light intensity on the rate of photosynthesis.

Rajah 8 ialah graf yang. Menunjukkan kesan keamatan cahaya ke atas kadar fotosintesis.

A. It is not influenced by the concentration of carbon dioxide

Tidak dipengaruhi oleh kepekatan karbon dioksida

B It is limited by the concentration of carbon dioxide

Dihadkan oleh kepekatan karbon dioksida

C. It is limited by the light intensity

Dihadkan oleh keamatan cahaya

D It is not influenced by the temperature

Tidak dipengaruhi oleh suhu

Explanation

Water is needed in the light reaction whereas

hydrogen and ATP is needed as energy in the

dark reaction. Carbon dioxide however is

broken down and then combined in the dark

reaction to form glucose

Which of the following can be concluded about

the rate of photosynthesis between the curves

J and K?

Antara yang berikut, yang manakah boleh

dirumuskan tentang kadar fotosintesis di

antara lengkung J dan K?

Explanation

Beyond the maximum point, an

increase in light intensity will no

longer affect the rate of

photosynthesis as it has already

reached its optimum light intensity.

Other factors such as the

concentration of carbon dioxide will

become the limiting factor.

J K

17

References

1. Internet (Web Address)

i. http://www.biologie.uni-hamburg.de/b-online/e24/24a.htm

ii. http://www.answers.com/topic/hydrophyte

iii. http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=2107959

iv. http://wiki.answers.com/Q/What_is_the_adaptation_of_leaf_for_photosynthesis

v. http://www.slideshare.net/chalkie28/leaf-structure-and-function

vi. http://dictionary.reference.com/browse/photosynthesis

vii. http://www.juliantrubin.com/bigten/photosynthesisexperiments.html

viii. http://www.agrium.com/print_version/in_the_community/education_centre.jsp

2. Magazines

i. Pamela Maitland, April 24th Edition, Biological Sciences Review Volume 6,

Number 4, Phillip Allan Updates.

3. Books

i. Biology Form 4 Textbook, Gan Wan Yeat; Manohoran a/l Subramaniam; Azmah

Binti Rajion, Bakaprep Sdn. Bhd. 2005.

ii. SPM Biology Quick Revision, Nalini T. Balachandran; Sia Chwee Khim; Kee Bee

Suan, Pelangi Sdn. Bhd. 2008.

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