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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.
13
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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