PHOTOSYNTHESIS IN HIGHER PLANTS

 

PHOTOSYNTHESIS IN HIGHER PLANTS

PHOTOSYNTHESIS AND ELECTRON TRANSPORT

PHOTOSYNTHESIS

Photosynthesis can be defined as an enzyme regulated anabolic process of manufacturing organic compounds inside the chlorophyll containing cells using carbon dioxide and water with the help of sunlight as a source of energy.

The organisms which perform photosynthesis are called photoautotrophs. They include green plants, red algae, brown algae, green algae, several types of protists, cyanobacteria and some bacteria as well Phototrophic bacteria employ hydrogen donors other than water.

Key Features of Photosynthesis

Following is the empirical equation which represents the total process of photosynthesis for oxygen releasing organisms:

CO₂ + H₂O -----^LIGHT------->  [CH₂O] + 0₂ [CH₂O is carbohydrate]

Cornelius van Niel discovered that photosynthesis is a light dependent process in which oxidation of oxygen (H₂O) or hydrogen donor and reduction of carbon dioxide into carbohydrate takes place, this can be represented by the following equation:

2H₂A + CO₂  -----^LIGHT-------> 2A + CH₂O + H₂о

The process of photosynthesis can be represented by the following equation:

6CO₂ + 12 H2O -----^LIGHT------->  C₆H₁₂O₆ + 6H₂O + 60₂ [C₆H₁₂O₆ is glucose]

Where Does Photosynthesis Place?

Takes Main site of photosynthesis is green parts of plants, Le. chloroplast. Higher plants have discoid and lens shaped chloroplasts.

Structure of Chloroplast

A double membrane surrounds the chloroplast Within the stroma is a chlorophyll system with a double-membrane sac. There are thylakoids in the stroma. To make grana, they are piled one on top of the other. Thylakoids are individual sacs within each granum Chlorophylls, carotenes, and xanthophylls are all pigments found in thylakoid membranes.

These pigments are fat soluble and are found in the lipid component of the membrane, as well as absorbing light in the visible region of spectrum.

Important

The mesophyll cells in the leaves contain a large number of chloroplasts, which are responsible for carbon fixation. Membranous System consists of:

(1) Grana

(2) Stroma lamellae

(3) Matrix stroma

Functions of membranous system are:

(1) Trapping solar energy.

(2) Synthesis of assimilatory powers, i.e. ATP and NADPH

Solar energy is trapped by chlorophyll pigment and stored as chemical energy in the form of ATP and reducing power in the form of NADPH in the photochemical phase of photosynthesis known as light reaction. Water splitting produces oxygen in the light reaction. An electron is raised to a higher energy level when a photon is absorbed by a chlorophyll molecule. A photon must have a particular amount of energy termed quantum energy in order to boost an electron. When a molecule absorbs a photon, it enters an energy-rich excited state. When the light source is turned off, the high-energy electrons quickly return to their usual low-energy orbitals, reverting the excited molecule to its original stable form, known as the ground state.

The biochemical phase of photosynthesis known as dark reaction does not depend on light directly, but is dependent on products of light reaction (ATP and NADPH). A sequence of enzyme-catalyzed events takes place in the stroma lead to carbon fixation. Molecules of ATP and NADPH are generated in the thylakoids during light reactions used in the stroma where carbohydrates are synthesised. The stroma is the place where carbohydrates are synthesised by using ATP and NADPH produced inside the thylakoids during light reaction Calvin, Benson and their colleagues discovered the process of carbon fixation in the dark reaction, that leads to the formation of sugar and starch via intermediary molecules. Calvin was awarded the Nobel Prize for this discovery in 1961.

How Many Pigments are Involved in Photosynthesis?

Chlorophyll is a green pigment found in plants. It has a significant role to perform, like, it provides green colour to plants. The chloroplast of the mesophyll cells of leaves contains it. They can absorb solar energy and use it to convert carbon dioxide to carbohydrates (food). Chlorophyll not only aids in the collection of sunlight by plants, but it also aids in the release of oxygen by plants. Water splitting complex or oxygen evolving complex which is located on inner side of thylakoid membrane is responsible for splitting of water and releasing of oxygen. Chlorophyll a and chlorophyll b are the two kinds of chlorophyll Accessory pigments include chlorophyll b. xanthophyll, and carotenoids absorb light and transfer the energy to chlorophyll a because it is primary pigment. These accessory pigments work as shield pigments because they protect chlorophyll a from photooxidation.

Paper chromatography: It is a technique used in plants to identify different pigments in the leaf.

To separate the different pigments by paper chromatography following steps are followed:

(1) Concentrate the extracted chlorophyll solution by evaporation.

(2) Apply a drop of it at one end, 2 cm away from edge of a strip of chromatography paper and allow it to dry thoroughly.

(3) Take a mixture of petroleum ether and acetone in the ratio of 9: 1 at temperature of 40°C to 60°C.

(4) Hang the strip in the jar with its loaded end dipping in the solvent.

(5) Close the jar tightly and keep it for an hour.

(6) The pigments separate into distinct green and yellow bands of chlorophyll and carotenoid respectively.

Type of Pigment	Name of Pigment	Colour	Function
Chief Pigment	Chlorophyll a	Bright or blue green	Absorption of light.
Thylakoid pigments / Accessory pigments	Chlorophyll b Xanthophyll Carotenoid	Yellow green Yellow Yellow to yellow orange	Absorption of light and pass energy to chlorophyll a

 

Absorption and Action Spectrum

All photosynthetic pigments have the ability to absorb light of various wavelengths. A graph can be used to show how much light is absorbed by different pigments at different wavelengths. Absorption spectrum is a graph that shows the absorption of light at different wavelengths by different pigments.

The absorption spectrum of chlorophyll a and b clearly reveals that more light is absorbed in the visible spectrum's blue, violet, and red wavelengths. The action spectrum is a graph that depicts the rate of photosynthesis at various wavelengths of light. The absorption spectra of chlorophyll a and b show a close association with the relative rate of photosynthesis at different wavelengths.

The wavelengths of light absorbed (red and blue) by chlorophyll pigments are remarkably comparable to the wavelengths that cause photosynthesis. and according to a comparison of the absorption spectrum for chlorophyll pigments and the action spectrum for chlorophyll pigments, the absorption and action spectra are very similar. To look at it another way, the absorption and action spectra of chlorophyll a and b are identical.

An overlapping of action spectrum of photosynthesis over absorption spectrum of chlorophyll a, states that most of the photosynthesis occurs in the red and blue regions of the spectrum and some photosynthesis occurs at the other wavelengths of the visible spectrum.

WHAT IS LIGHT REACTION?

We're all aware that photosynthesis requires the presence of sunlight Did you know, though, that chloroplast absorbs only the blue and red light wavelengths from the sun? Yes, you are accurate. Let's have a look at how the Light Reaction works.

In the chloroplast of the mesophyll cells of the leaves, a light-dependent process takes place. The chloroplasts are double-membraned cell organelles made up of thylakoids, which are stacked disc like structures. The pigment chlorophyll, which is essential for the process, is found on the membrane of these thylakoids, where the light reaction takes place.

The light reaction's main objective is to produce organic energy molecules like ATP and NADPH, which are required for the dark reaction.

The red and blue segments of white light are absorbed by chlorophyll, and photosynthesis is high at these wavelengths. The photosynthetic pigments are arranged in Light Harvesting Complex. Light Harvesting Complex (LHC) is a pigment bounded protein molecule. It is of two types: PS I and PS II.

When light strikes a plant, the chlorophyll pigment absorbs it and the electrons within it. are activated. This activity takes place within a photosystem, which is a complicated protein system. PS I and PS Il are two photosystems that are intimately connected. The excited chlorophyll pigments give up their electrons, and water is split to release four H ions, four electrons, and oxygen to compensate for the loss of electrons. The electrons that escape the PS Il are transferred to an electron transfer chain or ETC. Finally, the electrons are used to form NADPH. While electrons are dealt with the accumulation of H* ions within the thylakoid lumen is equally important. The hydrogen ions that accumulate inside the lumen generate a positive gradient, and when the enzyme ATP synthase is present, these H° ions interact with ADP in the surrounding area to make ATP. The waste product oxygen is released into the atmosphere by the plant, and some of it is utilised in photorespiration if the plant requires it.

Light reaction/ Photochemical phase consists of.

(1) Absorption of light

(2) Photolysis of water

(3) Release of oxygen

(4) Formation of ATP and NADPH

(5) Formation of high energy chemical intermediates.

Difference between PS I and PS II

PHOTOSYSTEM I	PHOTOSYSTEM II
The system is located in the non-appressed part of grana thylakoids as well as stroma thylakoids.	Photosystem II is present in the appressed part of granal thylakoids.
Chlorophyll to carotenoid ratio is high.	Chlorophyll to carotenoid ratio is Low.
Chlorophyll a content is more than twice that of chlorophyll b.	Chlorophyll a and chlorophyll b are approximately equal.
Its photocentre is P700.	Its photocentre is Р680.
It receives electrons from photosystem II.	Electrons are received from photolytic reaction.
Photosystem I can perform cyclic photophosphorylation independently.	It performs non-cyclic photophosphorylation in conjuction with photosystem I.
It is not connected with photolysis of water.	Photosystem II is connected with photolytic oxidation of water.
Usually, it hands over its electron to NADP+.	Usually, it hands over its electron to PS I.

 

THE ELECTRON TRANSPORT

The electron transport chain is a series of electron carriers (groups of proteins) over which electrons pass in a downhill pathway releasing energy at every step. This energy is used to create a proton gradient which helps in the production of ATP (Adenosine Triphosphate). This ATP is used for cellular function in metabolic activities. A proton gradient is formed throughout the process when protons are pushed from the mitochondrial matrix into the cell's intermembrane space, which also aids In ATP synthesis.

Because it relies on a higher concentration of protons to generate "proton motive force," the usage of a proton gradient is sometimes referred to as the chemiosmotic mechanism that drives ATP production. The number of protons pumped across the inner mitochondrial membrane is exactly proportional to the amount of ATP produced. The electron transport chain is made up of a succession of redox processes in which electrons are transferred from a source molecule to an acceptor molecule via protein complexes.

The proton gradient is created as a result of these reactions, allowing mechanical work to be transformed into chemical energy and produces ATP. In eukaryotes, the complexes are embedded in the cristae, the inner mitochondrial membrane.

Transfer of electrons takes place from high energy to low energy and low potential to high potential.

Splitting of Water

Splitting of water provides new electrons to photosystem II, water molecules are split into three main components, namely, protons, electrons and oxygen, respectively. Protons from splitting water are used as components of reaction that makes NADPH. Electrons formed in water splitting replace electrons that are lost in PS IL Oxygen formed is liberated into the atmosphere.

Important

Location of splitting of water. Thylakoid membrane's inner side Provides electrons to PS II

                            2H₂O -------->    4H⁺ + O₂ + 4e^-

Cyclic Photophosphorylation and Non-cyclic Photophosphorylation

When photosystem I is illuminated, electrons move out of and back into the reaction centre of the photosystem. The photophosphorylation of ADP to create ATP occurs in tandem with the cyclic electron flow named Cyclic photophosphorylation. Because only photosystem I is involved in this process, photolysis of water and the subsequent evolution of oxygen does not occur.

Non-cyclic photophosphorylation involves both PS I and PS Il photosystems. The electron transport chain begins with the release of electrons from PS II in this situation.

High-energy electrons emitted from PS Il do not return to PS II in this chain, but instead reach PS I through an electron transport chain, where they are donated to convert NADP to NADPH. In the dark process, the reduced NADP (NADPH) is used for CO₂ reduction. The oxidation of water molecules is caused by electron-deficient PS II.

Protons, electrons, and oxygen atoms are all liberated as a result. PS II takes up electrons to revert to a reduced state, while NAD accepts protons and oxygen is liberated. As a result of this mechanism, high-energy electrons liberated from PS Il do not return to PS II, and ATP is formed. This process is called non-cyclic photophosphorylation.

Cyclic Photophosphorylation	Non-Cyclic Photophosphorylation
It is performed by photosystem I independently.	It is performed by collaboration of both photosystems I and II.
An external source of electrons is not required because the same electrons get recycled.	The process required an external electron donor.
It is not connected with photolysis of water. Therefore, no oxygen is evolved.	It is connected with photolysis of water and liberation of oxygen.
It synthesises only ATP.	Non-cyclic photophosphorylation is not only connected with ATP synthesis but also the production of NADPH.
It operates under low light intensity. anaerobic conditions or when CO2availability is poor.	Non-cyclic photophosphorylation takes place under optimum light, aerobic conditions and in the presence of CO2.
The system does not take part in photosynthesis except uncertain bacteria.	The system is connected with CO2 fixation in all plants.
It occurs mostly in stromal or Intergranal thylakoids.	It occurs in granal thylakoids.

 

Chemiosmotic Hypothesis

It was proposed by Mitchell (1961). Electron transport, both in respiration and photosynthesis produces a proton gradient. The gradient develops in the outer chamber or inter-membrane space of mitochondria and inside the thylakoid lumen in chloroplasts.

(1) Lumen of thylakoid becomes enriched with H° ion due to photolytic splitting of water.

(2) Primary acceptor of electron is located on the outer side of thylakoid membrane. It transfers its electrons to an H-carrier. The carrier removes a proton from matrix while transporting electrons to the inner side of the membrane. The proton is released into the lumen while the electron passes to the next carrier.

(3) NADP reductase is situated on the outer side of thylakoid membrane. It obtains electrons from PS I and protons from matrix to reduce NADP to NADP + H⁺ state.

The consequence of the three events is that the concentration of protons decreases in matrix or stroma regions while their concentration in the thylakoid lumen rises resulting in a decrease in pH.

A proton gradient develops across the thylakoid. The proton gradient is broken down due to the movement of protons through transmembrane channels, CF_o of ATPase (CF_o - CF₁ particle). The rest of the membrane is impermeable to H+ ions. CF_o provides facilitated diffusion to H or protons. As protons move to the other side of ATP, they bring about confirmation change in CF₁ or ATPase or coupling factor. The transient CF₁ particle of ATPase enzyme form ATP from ADP and inorganic phosphate.

Therefore. ATP synthesis through chemiosmosis requires a membrane, a proton pump, a proton gradient or high concentration of H⁺ ions in the lumen. Proton diffuses across CF_o channels and releases energy that activates ATPase enzyme to catalyse ATP. One molecule of ATP is formed when 2H passes through ATPase.

Important

ATP synthesis through chemiosmosis requires a Membrane, Proton pump, Proton gradient and ATP synthase.

Points	CFo	CF1
Location	Submerged in thylakoid membrane.	Outer surface of thylakoid membrane.
Function	Forms a transmembrane channel that allows protons to diffuse more easily across the membrane.	Causes the enzyme to produce a large number of energy-dense ATP molecules.

C₃ AND C₄ PLANTS

WHERE ARE THE ATP AND NADPH USED?

The product of light reaction ATP and NADPH are used to incorporate carbon from CO2 into sugar. It occurs in the stroma of chloroplasts. This reaction is known as Dark reaction. The reaction itself does not require light but the process usually occurs in the light and continues for a short time after the plant is in dark as long as NADPH and ATP are there. For understanding of CO₂ fixation, Melvin Calvin identified the intermediate compounds and gave the detailed pathway of carbon in photosynthesis called Calvin cycle, in which he use the radioactive C¹⁴ in algal photosynthesis and finally discovered that the 3-carbon compound, i.e. 3 - phosphoglyceric acid was the first stable product of photosynthesis.

He also worked on that whether all plants formed the same 3-carbon compound or whether any other product formed in CO₂ fixation and after conducting experiments on a wide range of plants he found that again the first stable product is an organic acid but not 3-carbon atom compounds. But it was 4-carbon compound acid. This was oxaloacetic acid (OAA).

Then he concluded that CO₂ assimilation during photosynthesis to be of two types:

(1) Those plants who formed their first stable product C₃ acid (PGA) in CO2 fixation, i.e. C₃ pathway.

(2) Those who formed the first stable product C acid (OAA) in CO2 fixation, Le C pathway.

The Primary Acceptor of CO₂

Since the PGA is a 3-carbon compound, it was thought that CO₂ was primarily accepted by a 2-carbon to form a 3-carbon PGA; they spent many years to identifying the 2-carbon compound in chloroplast. Then they further investigate and finally experimentally proved that CO₂is primarily accepted by RuBP (Ribulose bisphosphate, 5-carbon compound) which produces two molecules of PGA.

The Calvin Cycle

Calvin and his colleagues investigated that some PGA is again transferred back to give RuBP to accept fresh CO₂ molecules and he operated it in a cyclic manner. This pathway occurs in all photosynthetic plants, whether they have C₃ or Ca pathways. The Calvin cycle is divided into three distinct stages:

(1) Carboxylation: Carboxylation is the addition of carbon dioxide with RuBP to form a stable intermediate organic compound. This reaction is catalysed in the presence of enzyme RuBP carboxylase oxygenase (RuBisCO) which forms two molecules of 3-phosphoglyceric acid (PGA). This is the first stable product of photosynthesis.

(2) Reduction: In the reduction, there are a series of reactions to form glucose. First, the ATP is required to phosphorylate the PGA and give rise to phosphoglyceric acid. Then this ATP is converted to ADP and second, this phosphoglyceric acid is reduced by NADPH to produce phosphoglyceraldehyde. Six tums of the Calvin cycle are required to synthesise one molecule of glucose.

(3) Regeneration: In this step, regeneration of RuBP takes place by ATP phosphorylation to produce RuBP. It is a crucial step to continue the cycle.

So, for every CO₂ molecule entering the three molecules of ATP and two molecules of NADPH are required. The net reaction of C₃ dark fixation of carbon dioxide is:

6 RuBP + 6 CO₂ + 18 ATP + 12 NADPH → C₆H₁₂0₆ + 18 ADP + 18 Pi + 18 NADP⁺

Important

To move one molecule of glucose. 6 turns of the Calvin cycle are required.

Important

The Calvin cycle is also known as C₃ cycle as the 1st stable compound of Calvin cycle is 3-carbon compound named 3-phosphoglyceric acid.

THE C₄ PATHWAY

The plants that perform C₄ cycle are found in tropical dry and hot regions. In this cycle, the first stable product of CO₂ fixation is oxaloacetic acid (OAA), a 4-carbon compound. These plants use O3 pathway as the main biosynthetic pathway. These are different from C₃ plants because C₄ plants have a special leaf anatomy called Kranz anatomy, they tolerate high temperatures and respond to high intensities of light. In Kranz's anatomy of leaf, vascular bundles are surrounded by a layer of large size bundle sheath cells that contain large numbers of chloroplasts, no intercellular spaces, thick walls impervious to gasses and lack grana i.e. their chloroplast is agranal type.

This pathway is also called the Hatch and Slack pathway. The processes include in the pathway are:

First, the carbon dioxide combines with 3-carbon molecules called phosphenol pyruvate (PEP) in mesophyll cells in the presence of the enzyme PEP carboxylase. Then it is converted to 4-carbon compound called oxalo acetic, acid, then malic acid or aspartic acid which are transported to cells in a bundle sheath. In bundle sheath cells, the malic acid is decarboxylated to carbon dioxide and 3-carbon molecules. Then these 3-carbon molecules return to the mesophyll cell where it is converted to РЕР. The carbon dioxide released in bundle sheath cells enters the C₄ cycle where these cells with the help of RuBisCO fix CO₂ to sugars. The Calvin pathway is common for both C₃ and C₄ plants.

Important

The C₄ Plants contain dimorphic chloroplasts, that means, chloroplasts in mesophyll cells are granal whereas in bundle sheath cells they are agranal.

C3 Plants (Calvin Cycle)	C4 Plants (Hatch-slack Cycle)
Ribulose bisphosphate is the first acceptor of СO2.	Phosphoenolpyruvate is the first acceptor of СO2, while ribulose bisphosphate is the second acceptor.
Phosphoglyceric acid is the first acceptor of СO2.	Oxalo acetic acid is the first product.
СO2 compensation point is 25-100 ppm.	СO2 compensation point is 0-10 ppm.
Mesophyll cells perform complete photosynthesis.	Mesophyll cells perform only initial fixation.
In higher plants operating C3 cycle, the chloroplasts are all granal.	There are two types of chloroplast, grana in mesophyll cells and agranal in bundle sheath cells.
The rate of carbon assimilation is low.	The rate of carbon assimilation is quite rapid.
At low temperature, C3 plants are more efficient while at high temperature their photosynthetic activity is comparatively reduced.	C4 plants are less efficient than C3 plants at low temperature but they have a higher net assimilation at high temperature.
Fixation of one molecule of СO2 uses 3 ATP and 2NADPH.	Fixation of one molecule of СO2 requires 5 ATP and 2NADPH.
C3  plants usually perform photosynthesis only when stomata are open.	C4  plants perform photosynthesis even when stomata are closed.

 

PHOTORESPIRATION

The photorespiration is the light-dependent process of oxygenation of RuBP. During photorespiration, RuBisCO shows oxygenation activity. It shows important difference between C₃ plants and C₄ plants. In the first step of the Calvin cycle, the RuBP combines with carbon dioxide to form PGA in the presence of enzyme RuBisCO. RuBisCO is the active site for both carboxylation and oxygenation but has more affinity for carbon dioxide as compared to oxygen and it is related to concentration of O₂ and CO₂ which determines the binding state of the enzyme.

In C₃ plants, some O₂ will bind to RuBisCO it occurs when CO₂ concentration is low. RuBP oxygenase instead of fixing carbon dioxide oxidises RuBP to produce PGA and 2-carbon atom phosphoglycolate. This pathway is called photorespiration. There is neither synthesis of sugar nor ATP as well as no synthesis of NADPH whereas C₄ plants are exception, in this, the enzyme is located at bundle sheath cells where the concentration of CO₂ is high due to C₄ acid decarboxylate and releases CO₂. Thus the RuBisCO is purely carboxylase in nature in C₄ plants and they have a high tolerance to temperature. Thus they show high productivity.

The site for photorespiration is chloroplast, peroxisome and mitochondria. It was discovered by Dicker and Tio (1959) in Tobacco Plant.

PHOTOSYNTHESIS FACTORS AFFECTING

The rate of photosynthesis is very important for determining productivity. It is affected by several factors, both internal and external. The internal factors include the size, number, age and orientation of leaves, mesophyll cell and chloroplast and internal CO₂ concentration whereas the external factor includes the availability of sunlight, temperature, CO₂ concentration and water. So these several factors interact and affect photosynthesis, when several factors affect any biochemical process then the Law of limiting factors comes.

Blackman's law of limiting factor

It states that if a chemical process is affected by more than one factor, then its rate will be determined by the factor which is nearest to its minimal value, it is the factor which directly affects the process if its quantity is changed.

Light

The main source of light for green plants for photosynthesis is solar radiation. Light varies in quality of light, intensity and duration of exposure of light. There is a linear relationship between incident light and carbon dioxide fixation.

Under low intensity, the rate of photosynthesis is low whereas at high intensity, the rate of photosynthesis also increases. The light intensity at which the rate of photosynthesis and rate of respiration is equal is called light compensation point.

As the light intensity increases, the rate of photosynthesis increases.

The light intensity at which a plant can achieve maximum amount of photosynthesis is called light saturation point. Beyond the saturation point, the rate of photosynthesis begins to decline. But it Is important to note that light is rarely a limiting factor in nature except for plants in shade or dense forests. Because light saturation occurs at 10% of the total sunlight.

Carbon Dioxide Concentration

It is the major limiting factor for photosynthesis as their concentration is low in nature (0.03 to 0.04 per cent). If its concentration increases upto 0.05%. the rate of photosynthesis also increases. The C₃ and C₄ plants respond differently to CO₂ concentration.

When CO₂ concentration is reduced, there comes a point at which plant parts stop absorbing carbon dioxide from their environment It is called CO₂ compensation point or threshold value. At this value, CO₂ fixed during photosynthesis is equal to CO₂ evolved in respiration and photorespiration. The value is 25-100 ppm for CO₃ plants and 0-10 ppm for C₄ plants. The optimum CO₂ concentration for C₄ plants is 360 ppm and more than 450 ppm for C₃ plants. This is called saturation point. The tomato and bell pepper plants are allowed to grow in CO₂ rich atmosphere for high productivity.

Temperature

The dark reactions are temperature controlled as enzymes deactivate at high temperatures. The requirement of temperatures varies with the plants for optimum photosynthesis like C₄ plants respond to high temperatures whereas the C₃ plants to low temperature. The temperature for photosynthesis also depends upon the habitat of plants. The optimum temperature for  C₃ plants is 10-25 degree Celsius while for C₄ plants it is 30-45 degree Celsius. At this temperature range, both C₃ and C₄ plants show maximum photosynthesis.

Water

Water is the most important material for photosynthesis. If the water supply is affected then the rate of photosynthesis also decreases as the stomata remain close in water stress conditions and this also makes leaves wilt.