PHOTOSYNTHESIS IN HIGHER PLANTS
Photosynthesis in higher plants class 11 notes
Definition of Photosynthesis
Photosynthesis is a process used by plants in which energy from sunlight is used to convert carbon dioxide and water into molecules needed for growth.
• These
molecules include sugars, enzymes and chlorophyll. Light energy is absorbed by
the green chemical chlorophyll.”All animals and human beings are dependent on
plants for food and these plants synthesize the food via physio – chemical
process called Photosynthesis. This process is important because:
o
It is the primary source of food.
o
It results in the release of oxygen in
atmosphere.
Early
experiment for Photosynthesis
·
Priestly observed that the candle burning in closed
space, i.e. a bell jar, extinguishes after some time. This concluded that both
candle and mouse require air, but somehow damaged it. But when the mint plant
was placed in the jar, “Plants restore to the air whatever breathing animals
and burning candles remove.”
·
Using the similar set up used by Priestly, Jan Ingenhousz (1730 – 1799) carried
out an experiment to show the importance of sunlight to plants, which somehow
purifies the air fouled by breathing animals or burning candles. He took
aquatic plants into observation and showed that in the presence of bright
sunlight, small bubbles were formed around the green parts, while there were
not bubbles during night. Later, he concluded that the bubbles were of oxygen
and only the green parts are able to release oxygen.
·
In the year 1854, Julius Von Sachs provided evidence of the production of glucose
during the growth of plants. This glucose is stored as starch and later, he
concluded that the green substances are located in special bodies within plant
cells. He also concluded that glucose is made in the green part of the plant
and is stored as Starch.
·
T.W.
Engelmann (1843 – 1909) also carried out an interesting experiment.
He used prism and split the light in several components and illuminated green
algae, called Cladophora, placed in the suspension of aerobic bacteria. These
bacteria helped in detecting the site of evolution of oxygen. During the
experiment, he observed that the bacteria accumulated mainly in the region of
red and blue light of the split spectrum. This was the first time when
photosynthesis was described and it resembled the absorption of spectra of
chlorophyll.
·
By the middle 19th century, the were features
about the photosynthesis were known and following empirical equation was
introduced that represented the entire process of photosynthesis:
· Then, Cornelius van Niel (1897 – 1985) added that photosynthesis is a light dependent reaction in which hydrogen form an oxidisable compound and reduces carbon dioxide to carbohydrates. The entire reaction is represented as follows:
·
He also concluded that O2 evolved from green
plants comes from H2O and not from CO2. This was later proved via radio
isotopic techniques. The correct equation that represented the entire
photosynthesis process is:
Where,
C6H12O6 is glucose and O2 is released from water.
Photosynthesis in Higher Plants NCERT PDF
Where
Photosynthesis does takes place?
·
Photosynthesis includes series of chemical
reactions which are carried out in chloroplast,i.e.The specialized structures
found on cells of plants.
·
In these series of reaction, water and carbon
dioxide are converted into glucose and in this reaction energy from sunlight is
used.
·
Because, it is an endothermic reaction, the
entire process requires input of energy. Photosynthesis is also classified as
oxidation – reduction reaction as it includes loss of electrons by water and
gain of electrons by carbon dioxide.
·
The process of photosynthesis takes place in
Mesophyll Cells and the carbon dioxide required by the process enters the
process via stomata, i.e. the small holes present on the outer layer of leaves.
The water required for the process is transported via roots through the
vascular tissues.
·
The chloroplast contains membranous system
(shown in image below) consisting of the stroma lamellae, grana and the fluid
stroma. The membrane system traps the light energy and helps in synthesizing
ATP and NADPH. Following diagram shows the electron micrograph of a section of
chloroplast:
Structure
of Mesophyll Cells
Following figure shows the structure of Mesophyll Cells. It
includes outer cell wall, cell membrane, cytoplasm, chloroplast, vacuole and
nucleus.
Role
of different Part of Mesophyll Cells:
• Cell Wall – It provides mechanical and structural
support, determine and maintain the shape of cell, protect cells against
pathogens and control the direction of growth.
• Cytoplasm – It is the platform of different chemical
processes and is controlled by enzymes.
• Cell Membrane – It acts as a barrier and helps in
controlling the movement of substances in and out of cell.
• Chloroplasts – It contains chlorophyll and green
substance that absorb light energy.
• Vacuole – It holds moisture and keeps the plant turgid.
• Nucleus – It contains DNA and controls the activities of
cell.
Photosynthesis in higher plants Class 11 NCERT
How
many pigments are involved in Photosynthesis?
·
“Pigments are the substances that possess the
ability to absorb light at specific wavelength.”
·
Leaves of plants have four types of pigments,
i.e. Chlorophyll a (bright or blue green in chromatogram), Chlorophyll b
(yellow green), Carotenoids (yellow to yellow – orange) and Xanthophylls
(yellow).
·
Photosynthesis takes place in red and blue
regions of spectrum and some photosynthesis also takes place at other
wavelengths.
·
Chlorophyll is the major pigment that traps the
light energy and other pigments are referred as accessory pigment which traps
light and transfer the energy to chlorophyll a.
Types
of Photosynthetic Reactions
Photosynthetic Reactions are of two types, i.e.
• Light Dependent Reaction – In
these reactions, the energy from sunlight is absorbed by chlorophyll and
transformed into chemical energy in the form of ATP and NADPH (electron carrier
molecule).
• Light Independent Reaction – This
reaction is also referred as Calvin Cycle. In this reaction, the energized
electron from light dependent reactions provides energy to form carbohydrates
from CO2 molecules.
Photosynthesis in higher plants class 11 notes
What
is Light Reaction?
·
Light Reaction is also called Photochemical
Phase. It includes absorption of light, splitting of water, oxygen release
followed by release of high energy chemical intermediates, NADPH and ATP.
·
The pigments in light reaction are organized in
light harvesting complexes (LHC) within PSI and PSII (Photosystem 1 and
Photosystem 2). Both these photo chemicals are named in the sequence of
discovery.
·
LHC is formed from hundreds of pigment
molecules bound to proteins (except chlorophyll a) forming a light harvest
system called Antennae. These pigments make the process of photosynthesis more
efficient.
·
The single molecule of chlorophyll a form the
Reaction Center and this center are different in both photosystems first and
second. The following diagram shows the light harvesting complex:
The
Electron Transport
·
In PS II, the reaction center chlorophyll a
absorbs 680 nm wavelengths of light. This result the electrons to be excited
and jump in an orbit. These electrons are captured by electron acceptor which
passed to electron transport system of cytochromes.
·
These electrons are not used as they pass
through the entire electron transport chain but are passed onto the pigment of
PS I.
·
At the same time, electrons at PS I reaction
center are also excited when they receive red light of wavelength 700 nm.
·
These electrons are transferred to another
acceptor molecule with greater redox potential. In this electron transport
chain, electrons are not used rather they are passed to the pigments of b.
·
At the same time, electrons present in reaction
center of PS I also gets excited after receiving red light of wavelength 700
nm. Then, these electrons are transferred to another acceptor molecule with
greater redox potential.
·
These
electrons, then move downhill and this time to energy rich molecule i.e. NADP+,
whose addition reduces NADP+ to NADPH + H+.
·
This entire scheme of transfer of electrons
starting from PS II to uphill then down the electron chain to PS I, excitation
of electrons, transferring to another acceptor and ultimately downhill to NADP+
resulting in formation of NADPH + H+ is referred as Z Scheme.
Following diagram
shows the entire process of Electron Transport in detail, as discussed above:
Cyclic
and Non – Cyclic Photo – Phosphorylation
·
In bacterial photosynthesis, a single PS (Photo
System) is involved. When an electron is energized by absorption of light, it
is ejected from the PS reaction center.
·
This electron then passes through an electron
transport system and finally back to reaction center. “The energy released
during the electron transport is used to produce ATP.
·
Since the excited electron returns to the
reaction center, this mechanism of making b is called Cyclic Photophosphorylation.”
·
Following diagram shows the above explained
process of cyclic photophosphorylation. No reducing power, required for
biosynthesis is generated in this process. In this process, the energy released
during the electron transport is used to produce ATP and the excited electron
returns to the reaction center:
·
Cyanobacteria and Plants use two PS which
simultaneously work to produce energy and reduce power.
·
Primarily, a photon of light ejects a high
energy light from PS II. This electron travels from excited reaction center of
PS II down the chain and enters in PS I.
·
This electron transport system generates a
proton motive force that is used to produce ATP. Since the excited electron
does not return to PS II, this mechanism for making ATP is called Non – Cyclic Photophosphorylation.”
·
When PS I absorb a photon of light, it releases
high energy electron that is used to drive the formation of reducing power in
the form of NADPH. This ejected electron is replaced by an electron of PS II.
·
The following diagram shows the entire process
of noncyclic photophosphorylation as
discussed above. In this process the excited electron does not return to PS II
and therefore, this entire mechanism for making ATP is referred as Non – Cyclic
Photophosphorylation:
Chemiosmotic
Hypothesis
·
“The chemiosmotic hypothesis suggests that the
action of ATP synthase is coupled with that of a proton gradient. It is the
action of the proton gradient that causes a proton motive force that allows ATP
synthase to phosphorylate ADP and inorganic phosphate to ATP.”
·
Peter
Mitchell in the year 1961 postulated this hypothesis which explains
the mechanism of synthesis of ATP during photosynthesis, in chloroplast.
·
During light reaction or photochemical phase,
ATP and NADP are generated and these are the key components used in dark
reaction for production of sugar molecule.
·
According to chemiosmotic hypothesis, ATP
production is the outcome of photon gradient across the membrane of thylakoids.
·
The essential components required in this
process are proton gradient, proton pump and ATP synthase (enzyme that helps
ATP synthesis).
·
Following diagram explains the entire process
included in chemiosmotic hypothesis. In this process, protons are pumped across
the membrane as electron flows through the respiratory chain:
Importance
of Proton Gradient
·
The Proton Gradient is important in this
process because it is the breakdown of this gradient result in release of
energy.
·
This gradient is broken down due to the
movement of proton across membrane via transmembrane channel of Fo of ATPase.
·
This ATPase comprise of Fo and F1, whereby Fo
is embedded in membrane and forms transmembrane channel and protrudes F1 on
outer membrane of thylakoid membrane.
Where
are the ATP and NADPH used?
·
We are aware of the products of light reaction,
i.e. ATP, NADPH and O2. Amongst these O2 diffuses out of chloroplast and ATP
and NADPH helps in driving the process leading to food synthesis and forming
sugars.
·
It is also referred as biosynthetic phase of photosynthesis
and this process does not depend on the presence of light, rather it depends on
the product of light reaction. It is carried out in stroma of chloroplast.
Photosynthesis in higher plants class 11 notes
Definition
of Biosynthetic Phase
“Biosynthetic Phase is the process by which carbon dioxide
is reduced to carbohydrates and the process is termed as carbon fixation.
It makes use of the ATP and NADPH produced in the light
phase. This process occurs in the stroma of chloroplasts with the help of
series of enzyme-catalysed reactions.”
How ATP
and NADPH are used in Biosynthetic Phase?
·
We are aware of the fact that CO2 is combined
with H2O to produce sugar. Scientist were very eager to find out how this
reaction proceeded and just after Second World War, the use of radioisotope 14C
led to the discovery that the first CO2 fixation product was 3 – Carbon Organic
Acid.
·
Melvin Calvin contributed in answering this and
therefore, the complete biosynthetic pathway is named as Calvin Cycle. The first identified product was PGA, i.e. 3 –
Phosphoglyeric Acid.
·
Scientist also worked hard to understand if all
plants have PGA as a first product of CO2 fixation, or some other product is
found in plants.
·
In this direction, several experiments were
carried out and it resulted in the discovery of another group of plants, where
the first stable product was organic acid. This acid was identified as
Oxaloacetic Acid (OAA).
·
Thus, assimilation of CO2 during photosynthesis
is carried out in two main ways:
o
The C3 pathway
o
The C4 pathway
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The
Calvin Cycle
·
In Calvin Cycle, Carbon atoms from CO2 are
fixed and are used to form three – Carbon Sugar. This process is dependent on
ATP and NADPH formed from light reactions. The light reaction is carried out in
thylakoid membrane while the Calvin Cycle takes place in stroma. The Calvin
cycle can be described in three stages:
o
Carboxylation
–
It is the fixation of CO2 in stable organic intermediate. It is an important
stage in Calvin Cycle where CO2 is utilized for carboxylation of RuBP in the
presence of enzyme RuBP carboxylase. It results in the formation of 2 molecules
of 3-PGA. RuBP carboxylase also helps in oxygenation activity and is therefore
also referred as RuBP carboxylase – oxygenase (RuBisCO).
o
Reduction
–
This stage includes series of reactions that result in the formation of
glucose. This step utilizes 2 molecules of ATP (for phosphorylation) and two
molecules of NADPH (for reduction per CO2 molecule). The fixation of 6
molecules of CO2 and 6 turns of cycle result in the removal of 1 molecule of
glucose from pathway.
o
Regeneration
– This
stage includes regeneration of CO2 acceptor molecule and requires 1 ATP for
phosphorylation to form RuBP.
· Following diagram represents the entire Calvin cycle, as discussed above in detail.
The cycle starts with carboxylation,
followed by reduction and then, finally regeneration. The last stage includes
regeneration of CO2 acceptor molecule and requires 1 ATP for phosphorylation to
form RuBP:
Photosynthesis in higher plants class 11 notes
Reactions
in Calvin Cycle
With reference to the above diagram, the reactions are
divided in three different stages:
·
Carbon
Fixation – A CO2 molecule combines with 5 C acceptor molecule and
RuBP. This step makes 6 Carbon compound that splits into 2 molecules of 3
Carbon compound and 3PGA. The reaction is catalyzed by RuBP carboxylase or
oxygenase.
·
Reduction – At
the second stage, ATP and NADPH are converted to 3 PGA molecules into molecules
of a three carbon sugar and G3P (glyceraldehyde–3–phosphate).
·
Regeneration
–
At the final stage, 3GP molecules go to make glucose while other may be
recycled to regenerate RuBP acceptor. The process of regeneration requires ATP
along with complex network of reactions.
o
For exiting cycle, three CO2 molecules enter
the cycle for exiting 3GP molecule. This provides three new atoms of fixed
carbon. Entering of 3CO2 molecules, results in regeneration of 3 molecules of
RuBP acceptor.
The C4
Pathway
·
Plants that carry out C4 pathways comprise of
specific enzyme that are located in two different cell types, i.e. Mesophyll
Cells and Bundle – Sheath Cells. This pathway is the method that is used by
plants to convert atmospheric carbon dioxide in chemical compound containing
four carbons.
·
This pathway is used by the plants in
subtropical areas such as Sugar Cane, Maize, Millet, Papyrus and Sorghum.
·
These plants are special and have several type
of leaf anatomy, i.e. they can tolerate higher temperature and also show
response to high light intensity.
·
This pathway is cyclic in nature. The primary
CO2 acceptor is 3-Carbon Molecule PEP (phosphoenol pyruvate) and is present in
mesophyll cells. PEPcase or PEP carboxylase is the enzyme that is responsible
for this fixation. It is important to note that the mesophyll cells do not have
RuBisCO enzyme and C4 acid OAA is formed within cells.
·
After this, 4 - carbon compounds like aspartic
acid or malic acid are formed in mesophyll cells which are then transported to
bundle sheath cells, where C4 acids are broken down to release Carbon Dioxide
(CO2) and three carbon molecules.
·
These 3 Carbon molecules are transported back
to mesophyll cells where it gets converted in PEP, thereby completing the
cycle. The CO2 released enters in bundle sheath cells and thereby the Calvin
pathway.
·
These bundle sheath cells have surplus of an
enzyme called RuBisCO (Ribulose Biphosphate Carboxylase – Oxygenase) and is
deficient in PEPcase.
Following diagram explains the entire C4 pathway as
discussed above:
MHI 10 Solved Assignment 2021-22 In English Medium
Photorespiration
Definition
of Photorespiration
·
Photorespiration is a biochemical process in
plants in which, especially under conditions of water stress, oxygen inhibits
the Calvin cycle, the carbon fixation portion of photosynthesis.
·
Photorespiration results in light dependent
uptake of O2 and release of CO2 and is associated with metabolism and synthesis
of small molecule named glycolate. This process simultaneously takes place in
green plants along with photosynthesis.
·
Its end result decreases the net amount of CO2
and both photosynthesis and photorespiration works opposite to each other.
Factors
affecting Photorespiration
·
The rate of photorespiration increases at any
time when the level of carbon dioxide is low and oxygen is high. Such condition
occurs when stomata remain partially closed or completely closed and
photosynthesis is underway.
·
Majority of time, the stomata of plants are
open, resulting in lowering down the rate of photorespiration. But when plants
become water stressed, they close stomata to prevent loss of water via
transpiration. Thus, on the other hand, restricts the normal exchange of gases.
The level of CO2 gradually rises as water splits during light reaction.
·
In desert and dry tropical areas,
photorespiration is reduced due to water stress and this on the other hand,
results in lowering down the potential of plant growth.
·
Some plants have adapted to this problem by
modifying the way they carry out photosynthesis. One of the common adaptations
is called C2 Metabolism in which
plants develop different leaf anatomy called Kranz Anatomy.
Photosynthesis in higher plants class 11 notes
Factors
affecting Photosynthesis
There are several factors that affect the rate of
photosynthesis. These factors are both internal and external factors:
1. Temperature –
When carbon dioxide, light and other factors are not limiting, photosynthesis
rate increases with the rise in temperature. The most preferred range of
temperature is 6° C – 37° C. High temperature results in inactivation of
enzymes and thereby affects enzymatically controlled dark reactions.
2. Carbon Dioxide Concentration – It
is the major limiting factor and its concentration is very low in atmosphere,
i.e. 0.03 – 0.04%. Increase in concentration to 0.05% causes increase in
fixation rate of CO2. Added to this, the C3 and C4 plants differently respond
to the concentration of carbon dioxide. The fact that C3 plants respond to
higher CO2 concentration by showing increased rate of photosynthesis leading to
higher productivity has been used for some greenhouse crops like bell pepper
and tomatoes.” Such plants are allowed to grow in CO2 enriched environment that
leads to higher yields.
3. Light – The light varies as
per quality, duration and intensity and has significant impact on the rate of
photosynthesis. For instance, there is a linear relationship between incident
light and CO2 fixation at low light intensities. Added to this, increase in the
incident light beyond point causes breakdown of chlorophyll and decrease in
photosynthesis.
4. Oxygen – Oxygen inhibits
photosynthesis in C3 plants but C4 plants show little effect. This is so
because C4 plants carry out photorespiration and high oxygen stimulates it. The
rate of photosynthesis increases with the reduction of concentration of oxygen.
5. Water – It is an essential
raw material for the assimilation of carbon. Less than one percent of absorbed
water is utilized in photosynthesis. The decrease of water content in soil
decreases the rate of photosynthesis as well. This is so because it results in
dehydration of protoplasm and also results in stomatal closure. Added to this,
it impairs enzymatic efficiency, affects its colloidal state, inhibits
respiration, etc.
6. Mineral elements –
These are also essential for the growth of plants and it includes Cu, Cl, Mg,
Fe, P and these are closely related with the process of photosynthesis.
7. Air pollutants –
Metallic and gaseous pollutants reduce photosynthesis. The pollutants include
SO2, oxidants, ozone and hydrogen fluorides
8. Chemical compounds –
Although, chemical compounds are present in very less quantity but even the
small quantity depresses the rate of photosynthesis. On contrary, increase in
the presence of chemical compound results in dying of cells.
Thus, there are several factors that affect the rate of
photosynthesis. Other factors include content
of chlorophyll, protoplasmic factor, accumulation of carbohydrates, etc.
.