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Photosynthesis in higher plants




PHOTOSYNTHESIS:

It is the physicochemical process by which green plants and certain other organisms use light energy to synthesise organic compounds.
IMPORTANCE:
  • It is the primary source of food for all living organisms.
  • It is the only process responsible for release of oxygen into the atmosphere by Green plants.

EARLY EXPERIMENTS:

Joseph Priestley:
He hypothesized that plants restore to the air whatever breathing animals and burning candle remove. (Bell jar experiment)
Ingenhousz:
He showed that only the green part of the plant could release oxygen.
Julius Von Sachs:
He showed that green substance chlorophyll is located in special bodies chloroplast.
Englemann:
He split the light using prism into its component parts and illuminated a green alga Cladophora, placed in a suspension of aerobic bacteria. He observed that the bacteria accumulated mainly in the region of blue and red light of the spectrum and described the action spectrum of photosynthesis.
Cornelius van Niel:
He conducted experiments with purple and green sulphur bacteria. He demonstrated that photosynthesis is essentially a light dependent reaction in which hydrogen from hydrogen donor reduces CO2 to carbohydrate.
2H2A  +  CO2 ---------  2A  +  CH2O  +  H2O
Purple and green sulphur bacteria uses H2S as hydrogen donor so the product is Sulphur and no oxygen is formed.

Reaction of photosynthesis

$6CO_2 + 12H_2O \overset{light}{\rightarrow} C_6H_{12}O_6 + 6H_2O + 6O_2$
 

SITE OF PHOTOSYNTHESIS:

It takes place in the cell organelle called chloroplast which are found in the mesophyll cells of the leaves.

Granum:
  • Photochemical phase occurs in this region (Light reaction)
  • synthesis of ATP and NADPH.
Stroma:
  • Biosynthetic phase occur in this region (Dark reaction)
  • Reduction of CO2 to CH2O and formation of sugars.

Pigments involved in photosynthesis:

chlorophyll a - main pigment
chlorophyll b - Accessory pigment
Carotene - Accessory pigment
Xanthophyll - Accessory pigment
ABSORPTION SPECTRUM:
It is the graph plotted with the amount of light absorbed by chlorophylls as a function of wavelength of light.
ACTION SPECTRUM:
It is the curve that depicts the relative rate of photosynthesis at different wavelengths of light.

MECHANISM OF PHOTOSYNTHESIS:

Lets discuss the mechanism of photosynthesis

Light Reaction (Photochemical Phase)

Photochemical phase directly depends on light. The pigments absorb the light energy & convert it into ATP.
Electronic ransport (ATP Synthesis) & Photolysis are the two major events in light reaction. The photosynthetic pigments are organized into two discrete light harvesting complexes within Photosystem I & Photosystem II.

PHOTOSYSTEMS (PS):

Each photosystem has one specific chlorophyll 'a' and many other pigments bound by proteins.
The chlorophyll 'a' forms the recaction centre where the actual reactions takes place while the other pigments molecule form the Light harvesting system called antennae.
In photosystem-I the reaction centre or chlorophyll 'a' absorbs light at 700nm known as P700.
In photosystem II the reaction centre or chlorophyll 'a' absorbs light at 680 nm known as P680.

ELECTRON TRANSPORT (Z-SCHEME) :-

The whole scheme of e- transport starting from photosystemII, the red light of 680 nm is absorbed by reaction centre of photosystem II, due to which e- becomes excited & jumps & then picked up by e- acceptor (uphill transport); which passes further to downhill to cytochrome complexes & then photosystem I. Now, like the photosystem II, the electron in the reaction centre of photosystem I also get excited on receiving red light of 700 nm & get transferred uphill to another acceptor and finally downhill to NADP+.


This scheme is called Z-SCHEME because of the characteristic slope 'At formed when all the carriers are placed in a sequence, according to their redox potential value.
 

PHOTOLYSIS OF WATER (SPLITTING OF WATER):

  • It is the process by which water is split into protons, electron & oxygen.
2H2O -----------> 4H+ + O2 + 4e-
  • The water splitting complex is associated with photosystem II which is located on inner surface of thylakoid membrane.
  • The electrons are used to replace electrons lost by photosystem II.
  • The oxygen is liberated into the atmosphere while the protons accumulate in the lumen of thylakoid.
 

PHOTO- PHOSPHORYLATION:


It is the process through which ATP is synthesized from ADP & inorganic phosphate by the cell- organelle with the help of energy from solar – radiation.
In occurs in two ways:
  1. Non – cyclic photo phosphorylation
  2. Cyclic photo – phosphorylation

NON – CYCLIC PHOTO-PHOSPHORYLATION:

  • It is a type of photophosphorylation in which both the photosystem (photosystem I and photosystem II) co – operate in light driven synthesis of ATP.
  • Both ATP & NADPH are formed during this reaction.
  • Since, the electron released from photosystem II does not returns to it, hence it is known as Non – cyclic photophosphorylation.
 
CYCLIC PHOTOPHOSPHORYLATION:
  • When only photosystem-I is functional, the electrons are circulated within photosystem- I and phosphorylation occur within the cyclic flow.
  • It takes place in the stromal lamellae of chloroplast because the stromal lamellae does not possess enzymes NADP reduetase & photosystem II.
  • So, the excited electrons do not pass on to NADP+ instead it gets cycled back to the photosystem- I complex. Hence, there is formation of ATP only.

CYCLIC PHOTOPHOSPHORYLATION
NON – CYCLIC PHOTOPHOSPHORYLATION
  1. Photosystem I is functional only. (occur in stromal lamellae).
  2. It is not connected to photolysis of water so no O2 is evolved.
  3. It generates ATP only.
  4. It is activated by light of wavelength of 700 nm.
  1. Both photosystem I & II are functional. (occur in Granal Thylakoid).
  2. It is connected with photolysis so O2 is evolved.
  3. It produces both ATP as well as NADPH.
  4. It is activated by light of wavelength of  680 nm as well as 700 nm.

CHEMIOSMOTIC / CHEMIOSYNTHETIC HYPOTHESIS: [PETER MITCHAELL]

ATP synthesis is linked to the development of proton gradient across the Thylakoid membrane of chloroplast. It results due to following reasons:
Since, the splitting of water molecule takes place on the inner side of the thylakoid membrane, the protons (hydrogen ions) accumulate within the lumen of thylakoid.
Transportation of protons takes place across the membrane, when the electrons move through the photosystem.
The primary acceptor of electrons is located towards the outer side of the membrane which transfers electrons to the proton carrier & not to the electron carrier. So, this molecule removes proton from the stroma while transporting an electron & release it into the lumen of the inner side of thylakoid membrane.
The enzyme NADP reductase is located on the stroma side; along with the electron coming from photosystem I, protons are necessary to reduce NADP to NADPH2
The gradient is broken down due to movement of protons across the membrane to the stroma through transmembrane channel Fo portion of ATP synthetase. Therefore proton gradient is necessary to release the energy (ATP).

DARK REACTION/ BIO – SYNTHETIC PHASE:
  • This phase doesn’t require direct sunlight but it depends on the products of light reaction i. e. ATP & NADPH.
  • In this process CO2 is reduced to form carbohydrates.
  • It takes place in the stroma of chloroplast by a series of enzyme catalysed reactions.

CALVIN CYCLE:- (C3 Pathway)
The path of carbon in the dark reaction was traced by Melvin Calvin; through a technique called autoradiography using C14, hence it is termed as Calvin cycle.
 
Calvin cycle consists of three phases:
  1. Carboxylation
  2. Reduction
  3. Regeneration of RuBP
Carboxylation: In this reaction utilization of CO2 takes place for the carboxylation of RuBP [Ribulose 1, 5 – Biphosphate]. This reaction is catalysed by the enzyme RuBP carboxylase Oxygenase (RuBisCo), which results in the formation of two molecules of 3-Phosphoglycerate (PGA).
$CO_2 + RuBP + H_2O \overset{RuBisCo}{\rightarrow} 2 [ 3 PGA]$
Reduction: In this reduction of PGA takes place leading to formation of glucose. In this step ATP & NADPH are utilized.
Regeneration of RuBP: The Regeneration of RuBP requires ATP molecule for phosphorylation.
 
Hence, for every CO2 molecule that enters the calvin cycle [CO2 fixation] requires three molecules of ATP & two molecules of NADPH. Thus, in order to produce 1 molecule of glucose through the Calvin Pathway, 18 ATP’s and 12 NADPH are required

 

C4 Pathway: [Hatch & Slack Pathway]

This pathway of Carbon – fixation occurs in plants like Maize, sugarcane, sorghum (monocot plants).
This was first reported by Hatch & Slack.

 Leaf Anatomy
 Leaf Anatomy
  1. Only one type of chloroplast is present.

 

  1. Less efficient in photosynthesis than C4 leaves.
  2. Only Mesophyll cells carry out photosynthesis.
  1. Kranz anatomy i. e. two types of cells, each with its own type of chloroplast is present.
  2. More efficient in photosynthesis than C3 leaves.
  3. Both mesophyll cells & bundle sheath cells carry out photosynthesis.

The Hatch & Slack pathway is also a cyclic process which occurs in the following steps:
  • In C4 plants phosphoenol Pyruvate (PEP) is the primary CO2 acceptor, present in the mesophyll cells & the reaction is catalysed by the enzyme phosphoenolpyruvate carboxylase (PEP – CASE).
  • The first stable product is oxalo acetic acid. (OAA), which is a four carbon – compound.
  • OAA is converted into malic acid & transported to bundle sheath cells where it is decarboxylated into pyruvic  acid (3 carbon compound), by releasing CO2.
  • The 3 – carbon compound is again transported back to the mesophyll cells where regeneration of PEP takes place, thus completing the cycle.

C3 Cycle (Calvin Cycle)
C4  Cycle (Hatch and Slack Cycle)
  1. Photosynthesis occurs in mesophyll cells.
  2. Kranz anatomy is absent.
  3. RuBP is the primary CO2 acceptor.
  4. 3 – phosphoglyceric acid. (3 – PGA), a three carbon compound is the first stable product.
  5. Chloroplast are only of 1 type i.e. Granal.
  6. They are photosynthetically less efficient & productivity is low for ex: rice, bean, wheat, potato.
  1. Photo occurs in microphyll & bundle sheath cells.
  2. Kranz anatomy is present.
  3. PEP is the primary CO2 acceptor.
  4. OAA, a 4 – C – compound is the first stable product.
  5. Chloroplasts are dimorphic i. e. Granal in the mesophyll & a granal in the bundle sheath cells.
  6. They are photosynthetically more efficient & productivity is high. For ex: maize, Amaranth, sugarcane.


Photorespiration: (Occurs only in C3 pathway)

  • In C3 plants, RuBP carboxylase, which is the main enzyme of photosynthesis, also function as oxygenase at high temperature & high O2 concentration.
  • In C3 plants, some O2 will bind to RuBisCO decreasing in Co2 fixation, hence, RuBP instead of converting into 2 – molecules of PGA gets binds to O2 forming 1 molecule of Phosphoglycerate & 1 molecule of 2 – C compound Phospho Glycolate in the pathway called photorespiration.
  • It is a wasteful process as:
    • There is no sugar or ATP formed.
  1. CO2 is released with the utilization of ATP.

 

FACTORS AFFECTING PHOTOSYNTHESIS:

Photosynthesis is affected by both internal & external factors (Internal = Plant Factors, External = Environmental Factors).
 
Internal Factors:
The internal or Plant factor includes the number of stomata, size, age & orientation, number of leaves, mesophyll cells, chloroplasts, internal CO2 concentration & amount of chlorophyll.
 
External Factors:
The external or environmental factors include availability of sunlight, temperature concentration of CO2 & water.
 
BLACKMAN’S LAW OF LIMITING FACTORS:-
"When a physiological process is controlled by a number of factors, the rate of reaction depends on the slowest factor."
This means that at a given time, only that at a given time, only the factor which is the least, (limiting), among all the factors will determine the rate of reaction.
 
Light:-
  • Light quality and light intensity influence photosynthesis.
  • Light of wavelength 400 nm to 700 nm is effective for photosynthesis & this light is known as photosynthetically Active radiation (PAR).
  • Intensity of Light  Rate of photosynthesis.
  • At higher light intensity the rate of photosynthesis dosen’’t increase because:
  1. Other factors needed for photosynthesis may be limiting.
  2. Destruction Or photo oxidation of chlorophyll.
 
TEMPERATURE:-
  • Enzyme controlled dark  reaction (bio – synthetic phase) are affected by change in temperature.
  • The C4 plants have higher temperature optimum 35°C – 40o C, while C3 plants have lower temperature optimum 20°C – 25o C.

CO2 CONCENTRATION:-
  • In C3 plants the rate of photosynthesis increases with increase in CO2 concentration and Saturation occurs beyond 450 micro/litre .
  • In C4 plants, the saturation of CO2 reached at a concentration of about 360 micro/litre.
 
Water:-
Water influence photosynthesis in two ways:-
  1. If available water decreases & plants shows water stress, the stomata close, hence, there will be a decrease in supply of CO2 for photosynthesis.
  2. The leaf becomes wilted & the surface area for photosynthesis decreases.
‚ÄčQUESTIONS:-
1.Photosynthetic pigments are located on which part of chloroplast.
Ans: Lipid part of thylakoid Membrane.

2.Cynobacteria & some other photosynthetic bacteria do not have chloroplast then how do they conduct photosynthesis?
Ans: Because, they had thylakoid suspended freely in the cytoplasm & also have bacterio – chlorophyll to conduct photosynthesis.

3.Suppose, there were plants that have a high concentration of chlorophyll b but lack chlorophyll a, would it carry out photosynthesis?
Then why do plants have chlorophyll b and accessory pigments?

Ans: No, it can’t carry out photosynthesis. As reaction centre is formed by chlorophyll a. the other/ accessory pigments absorb different wavelengths of light & pass on the energy to reaction centre for more efficient photosynthesis.

4.Why is the colour of leaf kept in dark frequently Yellow or pale green, which pigment do you think is more stable?
Ans: This is due to the disintegration of pigments i.e. the chlorophyll disappears after breakdown. Carotene pigment is more stable.

5.RuBisCo is an enzyme that acts both as carboxylase & oxygenase. Why do you think RuBisCo carry out more carboxylation in C4 plants?
Ans: RuBisCo is present in the bundle sheath cells of C4 plants. The mesophyll cells of C4 plants have a mechanism to concentrate the CO2 in the bundle sheath cells. Since, the concentration of CO2 in the bundle sheath is higher RuBisCo functions as Carboxylase.