Photosynthesis In Higher Plants

Chlorophyll is the pigment that is present in green plants. It contains Magnesium as a central atom. Without magnesium, chlorophyll cannot capture the sun's energy that is required for photosynthesis. Magnesium in plants is located in the enzymes, in the heart of the chlorophyll molecule.

• Maximum absorption of light takes place by chlorophyll a.

• Photosynthesis in plants converts solar energy into chemical energy using electrons and protons from water.

• In the chloroplast, chlorophyll a is the pigment that absorbs the sunlight.

• Sunlight is converted to chemical energy in the form of ATP.

• Light energy is converted to chemical energy when a photo-chemically excited special chlorophyll molecule of the photosynthetic reaction center loses an electron, undergoing an oxidation reaction.

Photosynthetically Active Radiation (PAR) is the range of light wavelength that is best fit for photosynthesis to happen. Photosynthesis is a process that requires light energy and optimally occurs in the range of 400 to 700 nanometer (nm). This range is also known as visible light.

Because chlorophyll, the pigment involved in photosynthesis is green (for the most part, there are other forms that are not green) and because it is green it reflects green wavelengths of light and does not absorb them. Plants need to absorb light to perform photosynthesis.

Chemosynthetic bacteria, unlike plants, obtain their energy from the oxidation of inorganic molecules, rather than photosynthesis.

Chemosynthetic bacteria are chemoautotrophs since they are able to use the energy stored in inorganic molecules and convert them in organic compounds.

The Proton gradient drives ATP Synthesis.

The electrochemical proton gradient across the inner mitochondrial membrane is used to drive ATP synthesis in the process of oxidative phosphorylation.

The device that makes this possible is a large membrane-bound enzyme called ATP synthase.

Light reaction occurs inside thylakoids. It is dependent upon light. It involves photolysis of water and production of assimilatory power.

Electrons released during photolysis of water are picked up by P680 photocenter of photosystem II. From here electrons pass over a series of carrier which include PQ, cytochrome b-f complex and PC.

While passing over cytochrome complex, the electron losses sufficient energy for creation of proton gradient and ATP from ADP and inorganic phosphate. From PC electron is picked by the trap centre P700 of photosystem I which pushes out electron after absorbing light energy.

Electron passes over carriers FeS, ferredoxin and NADP-reductase which gives electron to NADP⁺ for combining with H⁺ to produce NADPH.

Biosynthetic phase catalyses assimilation of CO₂ to carbohydrates. The reactions are known as carbon reactions. They occur in the stroma of chloroplasts. The reactions do not require light. Instead assimilatory power (ATP and NADPH) produced during photochemical phase is used in fixation and reduction of CO₂. All the enzymes required for the process are present in the matrix or stroma of the chloroplast.

PEP is the primary CO₂ acceptor in C4 plants.

In C4 plants, initial fixation of CO₂ or carboxylation occurs in mesophyll cells. The chloroplasts of mesophyll cells possess enzyme PEP carboxylase for initial fixation of CO₂. The primary acceptor of CO₂ is phosphoenol pyruvate. PEP combines with CO₂ in the presence of PEP carboxylase to form oxaloacetate.

Light reactions occur inside the thylakoids, especially those of grana region. It involves two types of reactions: photolysis of water and production of assimilatory power. The phenomenon of breaking up of water into hydrogen and oxygen in the illuminated chloroplasts is called photolysis. Water splitting complex is associated with Photosystem II which itself is physically located on the inner surface of thylakoid membrane.

Light reactions are also known as photolysis reactions. They are responsible for converting solar energy into chemical energy in the form of ATP and NADPH.

The correct sequence of electron flow is: Photosystem II � Plasstoquinone � Cytochrome � Photosystem II � Ferredoxin

PEP carboxylase is not found in C3 plants. The three most important roles that PEP carboxylase plays in plants and bacterial metabolism are in the C4 cycle, the CAM cycle, and the citric acid cycle.

Carboxylation is the addition of CO₂ to an acceptor. Photosynthetic carboxylation requires ribulose-1,5-bisphosphate or RuBP as acceptor of CO₂ and RuBP-carboxylase oxygenase as enzyme in C3 plants. In C4 plants, initial fixation of CO₂ occurs in mesophyll cells. The primary acceptor of CO₂ is phosphoenol pyruvate which combines with carbon dioxide in presence of PEP carboxylase to form oxaloacetic acid.

The first stable product synthesized is OAA i.e. oxaloacetate.

In C4 plants, initial fixation of CO₂ or carboxylation occurs in mesophyll cells. The chloroplasts of mesophyll cells possess enzyme PEP carboxylase for initial fixation of CO₂. The primary acceptor of CO₂ is phosphoenol pyruvate. PEP combines with CO₂ in the presence of PEP carboxylase to form oxaloacetate.

(a) This structure is present in plant cell. (b) Chloroplast has the ability to self-replicate and hence it can be passed on to the progeny. (c) (1) shows dark reaction and (2) shows the site of replication of chloroplast.

(a) It shows the decomposition of water molecule. It takes place in PS II. It is located on the inner surface of thylakoid membrane. (b) Splitting of water continuously provides electrons to the electron transport chain; further steps of photosynthesis.

Chloroplasts are not present in cyanobacteria’s and other photosynthetic bacteria. But in their inner membrane they have folds where photosynthesis takes place. You have bluish pigment phycocyanin to take photo-synthesis with solar energy.

(a) outer side of thylakoid membrane (Grana-lamella) (b) ATP molecules

The monocots cannot carry out girdling experiments. The monocot stem is scattered over all of the width of the trunk by vascular bundles. So, we can't reach a particular band of girdling phloem’s.

(a) 3 molecules of ATP are required for phosphorylation and 2 molecules of NADPH are required for reduction of carbon dioxide. (b) This reaction occurs in the stroma of chloroplast.

Moonlight does not have enough energy to excite chlorophyll molecules. Hence, moonlight cannot support photosynthesis.

(a) In C4 plants the process is called the Hatch & Slack Pathway, the glucose synthesis process. M D Hatch and C R Slack discovered it in 1977.

(b) Steps leading to the formation of carbohydrate following the division of the water molecule. This happens in a cyclical manner and is called Calvin Cycle. This was first discovered by Melvin Calvin and his co-workers.

(c) PEP carboxylase is an enzyme. It is present in mesophyll cells of C4 plants. It fixes carbon to form oxaloacetate.

(d) The specialized sclerenchyma cells present around vascular bundles in the veins of C4 plants are called bundle sheath cells.

NADP reductase enzyme is located on the outer side of thylakoid membrane. It facilitates breakdown of proton gradient to release energy, i.e. NADPH.

Two parts of the enzyme are ATPase: the head of F0 and the head of F1. The F0 head is towards the thylakoid’s inner side while F1 is towards the thylakoid's outer side. In F1 part of the enzyme, the conformation changes.

ATP & NADPH

Carbon is fixed to a 3-carbon compound in case of C3 pathways, i.e. 3-PGA.In C4 pathways, on the other hand, carbon, i.e. oxaloacetic acid, is fixed in a four-carbon compound. Thus, number of carbon atoms in the end product is the basis for designating C3 and C4 pathways of photosynthesis.

Succulent seedlings are known to keep their stomata closed for transpiration during the day. This prevents carbon dioxide from entering throughout the day. These plants have designed a special approach for ensuring the daily supply of carbon dioxide. These plants fix carbon dioxide at night in the form of malic acid.

Accessory pigments are known to support chlorophyll in the trapping of solar radiation. The accessory pigments are chlorophyll b, xanthophylls and carotenoids. They help collect and transfer solar radiation to chlorophyll a. In light harvesting, accessory pigment thus has a supporting role.

Dark reactions include the biosynthesis phase in which carbohydrate synthesis occurs. This part of the photosynthesis is not directed on light and is therefore referred to as dark reaction. This does not mean, however, that dark reaction occurs in full darkness but continues even during the day. Dark reaction therefore requires indirectly light but is not dependent on light.

In certain ways, photosynthesis and breathing are related. We know that the breathing of carbohydrates involves the oxidation of energy. Since carbohydrates are prepared for breathing during photosynthesis, breathing can therefore not take place without photosynthesis. In addition, breathing also needs oxygen that is a photosynthesis by-product. Similarly, carbon dioxide is an important raw material for photosynthesis and much of the carbon dioxide comes as a by-product of respiration.

When a plant is kept in the dark and ventilated correctly, carbon dioxide is still present.

However, it's sun-free. Therefore, photosynthesis cannot be performed in this plant. The plant can be supplied with adequate amount of water to maintain its survival. However, due to lack of nutrients, the seed would eventually die.

Light is seldom a photosynthesis limiting factor, because light saturation is 10% of the full sunlight. Light is rarely a limiting factor, with the exception of plants in shade or in dense forest. Photosynthetic organisms are found at different depths on oceans, and they are sufficient to execute photosynthesis by using enough light. In addition, the photosynthetic pigments of these organisms show considerable variations. These pigments help these organisms even in low light conditions to perform photosynthesis.

Light, as only 10% of full sunlight is sufficient in light saturation, is seldom a limiting factor in photosynthesis. Less light may slow the rate of photosynthesis in plants that grow under thick canopy in the woods, but photosynthesis cannot stop at daily times. Therefore, photosynthesis can take place. These plants.

Both carbon dioxide and oxygen are related to Rubisco. However, Rubisco is competitive to bind one of them. This means that the enzyme would be used as carboxylase in the event of a higher CO₂ concentration. However, the enzyme would act as oxygenase in cases of higher concentration of O₂.

Photosynthesis is an enzyme process through which the process is mediated. We are aware of the optimum temperature range of enzymes. An enzyme could not function if the temperature exceeds this range. This causes a decrease in the rate of photosynthesis at higher temperatures.

The movement of ions across and down a semi-permeable membrane is called chemical movement. In ATP synthesis, ions move through the thylakoid membrane, which is a semi-permeable diaphragm, during the light photosynthesis reaction.

In addition, proton pumps pump ions with a proton gradient, which is finally synthesized in ATP. Because of these factors, ATP synthesis is called a chemiosmotic phenomenon during light reaction

By experimenting at the University of California, Melvin Calvin and his staff found the complete way for sugar synthesis. The following measures were used: carbon dioxide, a radioactive isotope of carbon, was labelled C14 and was supplied to the plants for this experiment.

The path C14 was then monitored and analysed, e.g. in dark and in light, under different conditions.

Once the experiment with the live plant was over, the plant was killed and the labelled compound was extracted from the dead plant for further analysis.

Based on the analysis in live and dead plants, Calvin and his co-workers finally discovered the pathway during light independent reactions.

In the course of the Calvin cycle, the reduction of one CO₂ molecule requires two ATP molecules and two NADPH molecules.We know glucose is a 6-carbon compound, so six carbon dioxide molecules are required to produce a single glucose molecule.Hence, six turns of Calvin Cycle are required to generate one mole of glucose.

Photophosphorylation is the synthesis of ATP from ADP and inorganic phosphate in the presence of light.

When only PS I is functional, the electron is circulated within the photosystem and the phosphorylation occurs due to cyclic flow of electrons.

While the membrane or lamellae of the grana have both PS I and PS II the stroma lamellae membranes lack PS II as well as NADP reductase enzyme. The excited electron does not pass on to NADP+ but is cycled back to the PS I complex through the electron transport chain. The cyclic flow hence, results only in the synthesis of ATP, but not of NADPH+ H⁺. Cyclic photophosphorylation also occurs when only light of wavelengths beyond 680 nm are available for excitation.

C4-plants, such as maize, sugar cane etc are found in Kranz anatomy.These plants can withstand high temperatures and high light intensity. These plants are also suitable for a limited nitrogen and carbon dioxide supply.Unlike C3 plants, these plants do not contain photo-suspiration. This helps to create optimal glucose levels. Consequently, in contrast to C3, C4 plants produce more biomass.

Cyanobacteria are unicellular prokaryotic organisms. Besides, some primitive cellular cell organelles, they have photosynthetic lamellae where photosynthetic pigments are present. There are chlorophyll-a c, phycocyanin and phycoerythrih. These coloured pigments impart typical blue green colour to the bacteria and enable them to manufacture food for themselves and aquatic animals, Green plants are multi cellular organisms capable of making food by using carbon dioxide, water and light energy in special cell organelles called chloroplast. , So, bacteria and green plants make food for living organisms on earth.

Chloroplast and mitochondria contain additional genomic DNA. This improves organelles ' reproduction. Thus, semi-autonomous organelles are called Chloroplast and mitochondria, which contain additional genomic DNA. Because of this, they can replicate themselves. These organelles are therefore called semi-autonomous.

Extra genomic DNA is present in the chloroplast and mitochondria. This enhances the replication of these organelles. These organelles are therefore called semi-autonomous.

(a) Monocot plants show bundle sheath cells and mesophyll cells. (b) Oxaloacetic acid (c) PEP carboxylase or PEP case.

(a) Photorespiration

(b) It is not essential for survival.

(c) Hydrogen peroxide

(d) It occurs in chloroplast, mitochondria and peroxisome.

a) Euphorbia is a CAM plant. It fixes CO₂ during night and uses it in daytime. It will be able to survive in hot tropical climate. Maize is a tropical plant. It undergoes C4 cycle and can easily survive in hot climate.

b) Maize being a C4 plant is more efficient in terms of photosynthetic activity C4 plants have low CO₂ compensation point (0-10 ppm) and rate of carbon assimilation is rapid. Rate of photorespiration is negligible. C4 plants can perform photosynthesis even when the stomata are closed.

c) Euphorbia has large sized succulent cells and stomata having reniform guard cells closed during the day. Maize plant leaves have kranz anatomy. Stomata possess dumbbell shaped guard cells. The stomata remain open during the day.

The photosynthesis may also occur in this part if the part of the plant is green.

Therefore, it's wrong to say that only in plant leaves do photosynthesis occur. Most of the other components of the plant can be photosynthesized. Examples include (a) Chlorophyll roots of Trap and Timisoara and photosynthesis.

(b) The stem is changed into a fleshy green structure in Opuntia and the leaves are altered in spines to reduce transpiration. (b)Photosynthesis occurs in the stem of these plants. Opuntia's modified stem is named phylloclade.

(c) Sepals are green and perform photosynthesis at most plants.

(a) Outer side of thylakoid membrane

(b) Inner side of thylakoid membrane

(c) Stroma of chloroplast

(d) Chloroplast

(e) Chloroplast

Pigments are substances capable of absorbing light at certain wavelengths. This means that various pigments can absorb light of various colours. So, if pigment A can become excited by colour X, colour Y will stimulate pigment B. Once the pigment gets excited after light is absorbed, it provides energy for the future photosynthesis so that light energy can be used.

The most abundant pigment in plants is chlorophyll a. This pigment demonstrates the optimal efficiency between blue and red-light wavelengths

Other pigments also show optimal efficiency between blue and red light, as shown in these charts. In this chart, it is clear that the photosynthesis is on the best level between the blue and rot wavelengths (displaying a rate of photosynthesis).This is mainly due to chlorophyll a. This is also due to the action of accessory pigments, however. This means that in the range of red and blue light the rate of photosynthesis is higher.

The first chart shows the photosynthesis rate; by oxygen release. The second diagram shows the absorption by chlorophyllin a of various light wavelengths; the rate of photosynthesis is superimposed. In the peaks and troughs of both the graphs appear to imitate each other. The diagrams peak at a wavelength of over 400 nm, which is equal to blue. The two diagrams then appear dry. Then it peaks again between 600 and 700 nm, which is the same as red. This therefore shows that the spectrum of action and absorption overlaps. The black line shows the photosynthesis action spectrum, while the blue line displays the spectrum of absorption. Four 30–470 nm, 660–670 are the peak wavelength.

Under the following conditions, C4 plants are superior to C3 plants: Even at low CO₂ levels, C4 plants can carry out photosynthesis. At approximately 360 L-1, C4 plants show saturation whereas C3 plants show only after 450 L-1 saturation. The availability of CO₂ is therefore a limited factor for C3 plants; not C4 plants.

C4 plants have a higher optimum temperature, but C3 plants have a lower optimum temperature. Therefore, even at high temperatures, C4 plants can perform photosynthesis; this is not the case with C3 plants.

Because of Kranz anatomy, C4 plant is not affected by high levels of oxygen in the atmosphere. Kranz anatomy ensures that photorespiration in C4 plants is not carried out. But with C3 plants, this is not the case.

(a) The rates of photosynthesis indicate the action spectrum. The measurement is oxygen release the oxygen releases on y-axis and the wavelength on x-axis can be compared to the action spectrum. The absorption by pigments of different wavelengths, such as chlorophyll a as shown on this graph, can be demonstrated.

(b) The absorption of various wavelengths for a particular pigment can be done by the processing, such as chlorophyll a, or chlorophyll b, or any other pigment.

Important light reaction events and final products are as follows. It is important to note that these events are not the order in which they happen but the order in which the scientists discover them.

Light absorption: The Light Harvesting Complex (LHC) performs light absorption.

Each photosystem has a light harvesting system of all pigments (except one chlorophyll-a molecule). The antennas are also known as these pigments. These pigments absorb light from different wavelengths to improve the efficiency of the system. As a reaction centre, the single chlorophyll a molecule works.

Water division: sunlight energy is used to divide the water molecule into hydrogen ion and

NADP⁺ to NADPH is used for the extra electron released after the split.

Oxygen release: after division of the water molecule oxygen discharges into the stomach.

High-energy intermediate formation (ATP and NADPH): two energy-rich combinations ATP and NADPH are formed at the end of an electricity reaction.

When only PS I is functional, the electron is circulated within the photosystem and the phosphorylation occurs due to cyclic flow of electrons.

The type of phosphorylation process possible here is: cyclic photophosphorylation. Photophosphorylation is the synthesis of ATP from ADP and inorganic phosphate in the presence of light.

Both carbon dioxide and oxygen have affinity with the enzyme of Rubisco. This is competitive, however. It is more closely related to CO₂ than to oxygenation. It is the relative CO₂ and O₂ concentration that affects Rubisco’s binding to a particular molecule. Since it acts as both carboxylase and oxygenase, RUBP carboxylase–Oxygenase is better referred to.

Rubisco binds CO₂ to 3PGA in C3 plants. In C3 plants, Rubisco is binding with O₂ in a process known photorespiration to generate phosphoglycerate and phosphoglycerate. Photo vacuum reduces carbon attachment in C3 plants

Rubisco catalyses the production of oxaloacetic acid in C4 plants due to the increase in carbon dioxide levels in the vascular bundle. In this case, photorespiration is avoided and carbon fixation is more efficient.

The leaf anatomy of the C4 plants is special. The vascular bundles in leaves contain a sheath of large cells. Kranz Anatomy is called. In particular, chloroplast impregnates several layers of cells around the bundle cell. These cell thick walls cannot be exchanged for gasses. The sheath is without intercellular space.

C4 plants over C4 plants are securely fastened with the bundle sheath.It helps to increase the level of carbon dioxide in the leaves.We know that it is competitive to bind Rubisco to CO₂ and oxygen.

We also know that a certain molecule is affected by the relative concentration of carbon dioxide and oxygen. The high carbon dioxide concentration in these plants guarantees that the whole Rubisco binds to carbon dioxide and that no oxygen is binding.

Therefore, in these plants there is no photo vacuuming. This helps to prevent resource wastage and carbon fixation in these plants is highly efficient. These plants eventually produce more biomass than the C3 plants.

Rubisco and PEP case are the two important enzymes of C3 and C4 pathways. Rubisco is the primary CO₂ receiver in C3 facilities while PEP case is the main CO₂ receiver for C4 facilities.

Rubisco’s role: The Rubisco RUBP carboxylase is also called oxygenase. It has carboxylase as well as oxygenase functions, as the name suggests. The enzyme has more carbon dioxide potential than oxygen affinity. Some of the enzyme, however, is linked to oxygen

C3 plants a photorespiration process. Photo vacuum is a wasteful process because it does not produce a product that is useful for plants.In addition, carbon fastening in C3 plants is also reduced.

PEP case Role: this is the primary accepter of carbon dioxide in C4 plants. In these plants, however, Rubisco is also available. A4-carbon oxaloacetic (OAA) compound PEP case binds to carbon dioxide eventually becomes carbon dioxide.

Carbon dioxide is lastly produced in the cycle of Calvin. Keep in mind that both C3 and C4 plants use the Calvin Cycle.

Rubisco catalyses the first step during the Calvin cycle to transform carbon dioxide into sugar. In all photosynthetic organisms worldwide, this enzyme is present. From cyanobacteria to the floors of large trees; literally, Rubisco is present all over the place.

All carbon in the biosphere can be said to be derived from Rubisco carbon fixation. This is therefore the world's largest enzyme.

C4 plants over C3 plants have a definite benefit from the bundled sheath. The sheath contributes to increased concentration of carbon dioxide in the leaves. We are aware of the competition between Rubisco and carbon dioxide and oxygen. We also know that the relative carbon dioxide and oxygen concentration affects the binding of this enzyme to a specific molecule. The high carbon dioxide concentration in the plants makes all Rubisco bind to carbon dioxide, and no oxygen binding exists.

In these plants, therefore, photorespiration does not occur.