PHOTOSYNTHESIS
MODULE - 2
Photosynthesis
Forms and Functions of
Plants and animals
11
Notes
PHOTOSYNTHESIS
Photosynthesis (Photo = light; synthesis = to join) is the single most important
process on earth on which depends the existence of human beings and almost all
other living organisms. It is a process by which green plants, algae and chlorophyll
containing bacteria utilize the energy of sunlight to synthesize their own food
(organic matter) from simple inorganic molecules. Innumerable number of organic
molecules which compose the living world are derived directly or indirectly from
the photosynthetic organic matter. The oxidation of organic compounds releases
stored energy to be utilized by the living organisms to carry out essential metabolic
processes. It is important to note that photosynthesis is the only natural process
which liberates oxygen to be used by all living forms for the process of aerobic
respiration.
You have studied in lesson 4, that chloroplasts are the organelles that carry out
photosynthesis or in other words they act as solar cells producing carbohydrates.
In this lesson you will learn how green plants carry out photosynthesis.
OBJECTIVES
After completing this lesson, you will be able to :
z
z
z
z
z
z
z
z
242
define photosynthesis;
name the different pigments found in chloroplasts;
explain the main aspects of the process of photosynthesis;
enumerate the steps involved in the light and dark reactions of photosynthesis;
define the terms absorption spectrum, action spectrum, electron acceptor and
photophosphorylation;
distinguish between, absorption spectrum and action spectrum; light and dark
reactions, cyclic and non-cyclic photo-phosphorylation, C 3 and C4
photosynthesis;
list the environmental variables and internal factors affecting photosynthesis;
describe the principle of limiting factor giving suitable graphs.
BIOLOGY
MODULE - 2
Photosynthesis
Forms and Functions of
Plants and animals
11.1 PHOTOSYNTHESIS
11.1 Let us look into the significance of the process
Significance
1. Green plants possess the green pigment, chlorophyll which can capture,
transform, translocate and store energy which is readily available for all forms
of life on this planet.
Notes
2. Photosynthesis is a process in which light energy is converted into chemical
energy.
3. Except green plants, no other organism can directly utilise solar energy to
synthesize food, hence they are dependent on green plants for their survival.
4. Green plants which can prepare organic food from simple inorganic elements
are called autotrophic while all other organisms which cannot prepare their own
food are called heterotrophic.
5. During photosynthesis, oxygen liberated into the atmosphere makes the
environment livable for all aerobic organisms.
6. Simple carbohydrates produced in photosynthesis are transformed into lipids,
proteins, nucleic acids and other organic molecules.
7. Plants and plant products are the major food sources of almost all organisms
on the earth.
8. Fossil fuels like coal, gas, and oil represent the photosynthetic products of the
plants belonging to early geological periods.
11.1.1 What is photosynthesis?
Photosynthesis is the process by which green plants, in the presence of light combine
water and carbon dioxide to form carbohydrates. Oxygen is released as a by product
of photosynthesis. Current knowledge of photosynthesis has resulted from discoveries
made over 300 years of work. Some landmark experiments are given in the box
below.
z
Joseph Priestley (1772) and later Jan Ingenhousz (1779) showed that plants
have the ability to take up CO2 from the atmosphere and release O2.
z Ingenhousz also discovered that release of O2 by plants was possible only
in presence of sunlight and by the green parts of the plant.
z Robert Hill (1939) demonstrated that isolated chloroplasts evolve O2 when
they are illuminated in the presence of electron acceptor which gets
reduced. This reaction called Hill reaction accounts for the use of water
as a source of electrons and protons for CO2 fixation and release of O2
as bye-product.
Photosynthesis is represented by the following overall chemical equation:
Chlorophyll
Sunlight
6CO2 + 12H2O ? ????¡ú C6H12O6 + 6H2O + 6O2
BIOLOGY
243
MODULE - 2
Forms and Functions of
Plants and animals
Photosynthesis
In photosynthesis, CO2 is fixed (or reduced) to carbohydrates (glucose C6H12O6).
Water is split in the presence of light (called photolysis of water) to release O2. Note
that O2 released comes from the water molecule and not from CO2.
11.1.2 Where does photosynthesis occur?
Photosynthesis occurs in green parts of the plant, mostly the leaves, sometimes the
Notes green stems and floral buds. The leaves contain specialised cells called mesophyll
cells which contain the chloroplast¨C the pigment containing organelle. These are
the actual sites for photosynthesis.
Look at the figure 11.1 that shows leaf Cell Structure and Function.
Sunlight
Upper epidermis
Palisade cell
Cell wall
Cytoplasm
Vacuole
Water Water passes
into cell
from vessel
by osmosis
In the chloroplast
carbon dioxide and
water combine to
make sugar
Carbon dioxide
Nucleus
Carbon dioxide enters leaf
through a stoma (pore)
Vessels carrying
water
Cells carrying
food made in leaf
Carbon dioxide
diffuses through
air spaces to
reach cells
Fig. 11.1 Diagram to show structure of leaf cells
11.2 PHOTOSYNTHETIC PIGMENTS
The thylakoids of the chloroplast contain the pigments which absorb light of different
wavelengths and carry out the photochemical reaction of photosynthesis.
The role of the pigments is to absorb light energy, thereby converting it to chemical
energy. These pigments are located on the thylakoid membranes and the chloroplasts
are usually so arranged within the cells that the membranes are at right angles to
the light source for maximum absorption. The photosynthetic pigments of higher
plants fall into two classes the chlorophyll and carotenoids.
The photosynthetic pigment chlorophyll is the principle pigment involved in
photosynthesis. It is a large molecule and absorbs light maximally in the violet blue
and in the red region of the visible spectrum and reflects green light and thus leaves
appear green in colour. Carotenoids (carotene and xanthophyll) absorb light in the
regions of the spectrum not absorbed by the chlorophylls and transfer that energy
to chlorophyll to be used in photosynthesis.
244
BIOLOGY
MODULE - 2
Photosynthesis
Forms and Functions of
Plants and animals
Chlorophyll-a (a special type of chlorophyll) is the main pigment that traps solar
energy and converts it into chemical energy. Chlorophyll-a is present in all
autotrophic plants except photosynthetic bacteria. Thus Chl-a is called the essential
photosynthetic pigment responsible for representing the reaction centre.
All other pigments such as chlorophyll b and carotenoids are collectively called
accessory pigments since they pass on the absorbed light energy to chlorophyll a
(Chl-a) molecule to be utilized for photosynthesis. These pigments, that is the
Notes
reaction centres (Chl-a) and the accessory pigments (harvesting centre) are packed
into functional clusters called photosystems. Photosystems are of two types PSI
and PSII.
About 250-400 Chl-a molecules constitute a single photosystem. Two different
photosystems contain different forms of chlorophyll a in their reaction centres. In
photosystem I (PSI), chlorophyll¨C a with maximum absorption at 700 nm (P700)
and in photosystem II (PSII), chlorophyll¨C a with peak absorption at 680 nm (P680),
act as reaction centres. (P stands for pigment). The primary function of the two
photosystems, which interact with each other is to trap the solar energy and convert
it into the chemical energy also called assimilatory power (ATP and NADPH2).
The differences between them are given in the following Table 11.1.
Table 11.1 Differences between Photosystem I and Photosystem II
Photosystem I
Photosystem II
z
PS I has a reaction centre of
chlorophyll ¡®a¡¯ molecule with
maximum light absorption at 700 nm
wavelength. This reaction centre is
referred to as P700.
z
PS II has a reaction centre of
chlorophyll ¡®a¡¯ molecule with maximum
light absorption at 680 nm. This
reaction centre is also referred to
to as P680.
z
Primary electron acceptor is an iron
protein (Fe-S-protein)
z
z
A set of electron carriers are
plastocyanin, ferredoxin and
cytochrome
z
Primary electron acceptor, pheophytin,
is a modified chlorophyll-a molecule with 2
hydrogen atoms in place of magnesium ion.
A set of electron carriers are pheophytin
plastoquinone, cytochromes.
11.3 ROLE OF SUNLIGHT IN PHOTOSYNTHESIS
Light consists of small particles or packages of energy called ¡°photons¡±. A single
photon is also called quantum. What does the chlorophyll do? It absorbs light
energy.
Chlorophyll molecules absorb light energy and get into an excited state and
lose an electron to the outer orbit. No substance can remain in an excited
state for long, so the energised and excited chlorophyll molecule comes
down to a low energy state known as ground state and releases the extra
amount of energy. This energy can be lost as heat, or as light (fluorescence)
or can do some work. In photosynthesis, it works by splitting water
moelcule to produce H+ and OH¨C ions.
BIOLOGY
245
MODULE - 2
Photosynthesis
Forms and Functions of
Plants and animals
Carotene is orange-yellow pigment present along with chlorophylls in the
thylakoid membrane. A carotene molecule breaks down into the vitamin A
molecules. It is this pigment which gives carrot its colour.
Absorption
Rate of photosynthesis
Absorption and Action Spectra
For investigating a process such as photosynthesis that is activated by light, it is
Notes important to establish the action spectrum for the process and to use this to identify
the pigments involved. An action spectrum is a graph showing the effectiveness
of different wavelengths (VIBGYOR) of light in stimulating the process of
photosynthesis, where the response could be measured in terms of oxygen produced
at different wavelengths of light. An absorption spectrum is a graph representing
the relative absorbance of different wavelengths of light by a pigment. An action
spectrum for photosynthesis is shown in Fig. 11.2 together with an absorption
spectrum for the combined photosynthetic pigments. Note the close similarity, which
indicates that the pigments, chlorophyll-a in particular, are responsible for absorption
of light used in photosynthesis.
All wavelengths of light are not equally effective in photosynthesis i.e. the rate of
photosynthesis is more in some and less in others.
Action spectrum
Absorption spectrum
Chlorophyll b
Chlorophyll a
Fig. 11.2 Absorption Spectra of electromagnetic radiation B. Action Spectrum
Photosynthesis occurs maximum in blue and red region of spectra.
Photosynthesis is very little in green and yellow light, because these rays
are reflected back from the leaf.
INTEXT QUESTIONS 11.1
1.
(i) Define photosynthesis
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246
BIOLOGY
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