Photosynthesis: Turning Light Energy into Chemical One

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Photosynthesis is a chemical reaction that takes place inside a plant, producing sugar type food for the plant to survive. Carbon dioxide, water and light are all needed for photosynthesis to take place. It happens in the leaves of a plant and the other green portions as well.

  • Photosynthesis is a compound word made up of ‘photo’ meaning ‘light’ and ‘synthesis’ meaning ‘to put together’.
  • The word photosynthesis is first used in 1898 by Charles Reid Barnes (1858-1910).
  • Light is essential for photosynthesis to take place.

The process of photosynthesis is also known as oxidation-reduction process as through this process, water is oxidized (and oxygen emerges as a byproduct) and on the other hand, carbon-di-oxide is turned into starch by reduction reaction.

6CO2 + 12H2O + Light energy → C6H12O6 + 6O2 + 6H2O.

Photosynthetic Organs

All plants  that use  photosynthesis to make sugars contain chlorophyll and some other photosynthetic pigments. Therefore if a plant does not contain chlorophyll and other pigments, it won’t be able to photosynthesize.

In higher plants, chloroplasts are highly seen in pallisade and spongy parenchyma cells of mesophyll tissue where the most photosynthesis occurs.

On the other hand, bacteria & cyanobacteria do not have chloroplast. They have chromatophore. This chromatophore is their only photosynthetic pigment. The whole-body part of these photosynthetic bacteria, cyanobacteria and unicellular algae takes part in photosynthesis as they contain photosynthetic pigment in whole body.


Requirements for Photosynthesis
(Pigments, Light, Water, CO2)

There are major 3 types of photosynthetic pigments, namely;

  1. Chlorophyll
  2. Carotenoids
  3. Phycobilins

Chlorophyll

  • Chlorophyll-a (C55H72O5N4Mg)
  • Chlorophyll-b (C55H70O6N4Mg)

Carotenoids

  • Carotenes (C40H56O)
  • Xanthophylls (C40H5602)

Phycobilins

  • Phycoerythrobilin (C34H46O8N4)
  • Phycocyanobilin (C34H44O8N4)

 

Wavelength of light absorbed by pigments of plant

Sunlight is the only source of photosynthesis . Only 40% of sunlight fall on earth. It has known that the only 83% of this light is absorbed by leaves  and only 0.5 to 3.5% are absorbed by chlorophyll .Plant pigment molecules absorb only light in the wavelength range of 760 nm to 390 nm; this range is referred to as photosynthetically-active radiation. Violet and blue has the shortest wavelengths and the most energy, whereas red has the longest wavelengths and carries the least amount of energy.

Using different wavelengths it has seen that in red (the wavelength between 610-760 nm) and blue (wavelength between 400 -510 nm) light most of the photosynthesis happens.

Supply of Water for Photosynthesis

Water is very much essential for photosynthesis. Aquatic plants can take water by  their whole body. But  higher plants absorb water from soil and supplies to the mesophyll tissue of leaves. Less than 0.01% of this absorbed water is used by green fresh leaf  .

Entrance of CO2 in Mesophyll Tissue

Another important thing is carbon-di-oxide for photosynthesis. In air almost 0.004% CO2 is present. The carbon-di-oxide enters the leaves of the plant through the stomata present on their surface. The carbon dioxide enters the leaves of the plant through the stomata present on their surface. Sometimes, CO2 is absorbed by lenticels and cuticles. When the plant is photosynthesising during the day, these features allow carbon dioxide to diffuse into the spongy mesophyll cells, and oxygen to diffuse out of it. To get to the spongy mesophyll cells inside the leaf, gases diffuse through small pores called stomata.


Photosynthetic Unit

In higher plants there are different kinds of chlorophyll, carotene and different layers of protein on the surface of thylakoid. These pigments together make a special decoration. These color pigments and proteins are called photosynthetic unit. Some of these units make a set 200-300 of this set make a chlorophyll -a. The unit of photosynthetic unit is known as “Quantosome”. Every thylakoyid is having much quantosome. These are also known as photosystem I and photosystem II.

 

 

Two Pigment System Theory of Photosynthesis of Photosystem

The two pigment system theory of photosynthesis was proposed by Emerson. He observed Emerson Effect in 1957. Beyond wavelengths of 680 nm i.e., far-red region, there is a decrease in photosynthetic yield compared to the red region of electromagnetic spectrum. This decrease is known as the Red drop. Red drop occurs because of the decrease in quantum yield. Quantum yield is the number of oxygen molecules released per light quanta absorbed. Red drop effect was proposed by Emerson and Lewis. They performed their experiment on chlorella plant.

Emerson Effect was observed by Robert Emerson in 1957. Emerson effect shows that photosynthesis proceeding very slowly when the light of 700 nm wavelength is used can be increased by illuminating with light of shorter wavelength i.e., 650 nm. So, Emerson effect is related to increase in photosynthesis when lights of two different wavelengths are provided together. Emerson effect is also known as Enhancement effect.

From red drop and enhancement experiments in the light reaction of photosynthesis, it was concluded that two photosystems are present which work at different wavelengths. It is evident of the presence of two photosystems i.e. Photosystems I and II.

Photosystem 1

The two main multi-subunit membrane protein complexes differ in their absorbing wavelength, where the photosystem I or PS 1 absorbs the longer wavelength of light which is 700 nm . It is located on the outer surface of the thylakoid membrane. P700 is the photo center.The pigments in the photosystem 1 absorb longer wavelengths of light which is 700 nm (P700).This system is involved in both cyclic as well as non-cyclic photophosphorylation. No photolysis occur.Photosystem I or PS 1 contains chlorophyll A-670, chlorophyll A-680, chlorophyll A-695, chlorophyll A-700, chlorophyll B, and carotenoids. The primary function of the photosystem I is in NADPH synthesis, where it receives the electrons from PS II.The PSI is made up of two subunits which are psaA and psaB.

Photosystem II

Photosystem II or PS II is the protein complex that absorbs light energy, involving P680, chlorophyll and accessory pigments and transfer electrons from water to plastoquinone and thus works in dissociation of water molecules and produces protons (H+) and O2.It is located on the inner surface of the thylakoid membrane.P680 is the photo center.The pigments in the photosystem2 absorb shorter wavelengths of light which is 680 nm (P680).This system is involved in both cyclic photophosphorylation.Photolysis occurs in this system.Photosystem II or PS 2 contains chlorophyll A-660, chlorophyll A-670, chlorophyll A-680, chlorophyll A-695, chlorophyll A-700, chlorophyll B, xanthophylls and phycobilins.The primary function of the photosystem II is in the hydrolysis of water and ATP synthesis.The PS II is made up of two subunits made up of D1 and D2.


Electron Transport System in Thylakoid

By electron-microscope some ovul granules are seen ,which are almost 10X20 nm. In every Quantosome there is either a p700 or an P680 which is ready to take sun ray for reaction.

there are also some very essential chemical tools for carring electron of higher energy. They converts NADP & ADP to NADPH2 & ATP. These  Electron carriers are known as thaylakoid electron transport system (TETS).

  1. Pheophytin (Ph)
  2. Plastoquinone (PQ)
  3. Cytochrome (Cyt.)
  4. Plastocyanin (PC)
  5. Ferredoxin (Fd)
  6. NADP reductase

Mechanism of Photosynthesis

Scientist Blackman(1905) first divides the photosynthesis into two different phases

  1. Light dependent phase
  2. Light neutral phase

 

A. Light Dependent Phase

The reaction of this phase only happens when there is light on thylakoid membrane. In this phase light energy  is  converted into chemical energy. The reflected photon from sunlight is absorbed by chlorophyll. When the chlorophyll is excited electron contained higher energy comes out and placed in the electron carrier. This gained energy is transferred by ATP and NADPH2 but the chlorophyll loses the electron and synthesis the water. This is called the Photooxidation of H2O.

4H2O+2ADP+2Pi+2NADP+Light+ chlorophyll→2ATP+2NADPH+H++2H2O+O2

The production of ATP by the light dependent reactions is called photophosphorylation, as it uses light as an energy source. This NADPH and ATP is called the power of integration.

Photophosphorylation has two kinds:

  1. Noncyclic photophosphorylation

2. Cyclic photophosphorylation

 

  1. Noncyclic photophosphorylation

In a process called non – cyclic photophosphorylation (the “standard” form of the light-dependent reactions), electrons are removed from the water and passed through PSII and PSI before ending up in NADPH.

Here are the basic steps

  1. Light absorption in PSII. …
  2. ATP synthesis. …
  3. Light absorption in PSI. …
  4. NADPH formation.

2. Cyclic photophosphorylation:

This transport chain produces a proton-motive force, pumping H+ ions across the membrane; this produces a concentration gradient that can be used to power ATP synthase during chemiosmosis. This pathway is known as cyclic photophosphorylation, and it produces neither O2 nor NADPH.

Differences Between Cyclic and Noncyclic Photophosphorylation

B. Light Independent Phase

No light is needed directly to disperse the carbon, this phase is known as light independent phase. This process can be happen both of the day and night. But in the day time it happens highly. This light independent phase is also called the carbon cycle. Light reactions harness energy from the sun to produce chemical bonds, ATP, and NADPH. These energy-carrying molecules are made in the stroma where carbon fixation takes place. .

There are three pathway of carbon, which are recognized.

  1. Calvin Cycle (C3 cycle)
  2. Hatch ANd Slack Cycle (C4 cycle)
  3. CAM process (Crassulacean Acid Metabolism)

 

  1. Calvin Cycle

The Calvin cycle (also known as the Benson- Calvin cycle ) is a set of chemical reactions that take place in chloroplasts during photosynthesis. The cycle is light-independent because it takes place after the energy has been captured from sunlight. This cycle is also known as C3 cycle. The Calvin cycle is named after Melvin Calvin, who won the 1961 Nobel Prize in Chemistry for finding it. Peanuts, cotton, sugar beets, tobacco, spinach, soybeans, and most trees are C3 plants. Most lawn grasses such as rye and fescue are C3 plants. C3 plants have the disadvantage that in hot dry conditions their photosynthetic efficiency suffers because of a process called photorespiration.

  • the first stage of the Calvin cycle, light-independent reactions are initiated; CO2is fixed from an inorganic to an organic molecule.
  • In the second stage, ATP and NADPH are used to reduce 3-PGA into G3P; then ATP and NADPH are converted to ADP and NADP+, respectively.
  • In the last stage of the Calvin Cycle, RuBP is regenerated, which enables the system to prepare for more CO2to be fixed.

 

Rubisco: (ribulose bisphosphate carboxylase) A plant enzyme which catalyzes the fixing of atmospheric carbon dioxide during photosynthesis by catalyzing the reaction between carbon dioxide and RuBP.

Ribulose bisphosphate: An organic substance that is involved in photosynthesis, reacts with carbon dioxide to form 3-PGA.

2. Hatch And Slack Cycle (C4 cycle)

The Calvin Cycle is the means by which plants assimilate carbon dioxide from the atmosphere, ultimately into glucose. Plants use two general strategies for doing so. The first is employed by plants called C3 plants (most plants) and it simply involves the pathway described above. Another class of plants, called C4 plants employ a novel strategy for concentrating the  CO2  prior to assimilation. C4 plants are generally found in hot, dry environments where conditions favor the wasteful photorespiration reactions of RUBISCO, as well as loss of water. In these plants, carbon dioxide is captured in special mesophyll cells first by phosphoenolpyruvate (PEP) to make oxaloacetate. The oxaloacetate is converted to malate and transported into bundle sheath cells where the carbon dioxide is released and it is captured by ribulose-1,5-bisphosphate, as in C3 plants and the Calvin Cycle proceeds from there. The advantage of this scheme is that it allows concentration of carbon dioxide while minimizing loss of water and photorespiration.

C4 plant is a plant that cycles carbon dioxide into four-carbon sugar compounds to enter into the Calvin cycle. These plants are very efficient in hot, dry climates and make a lot of energy. Many foods we eat are C4 plants, like corn, pineapple, and sugar cane.

  • Differences between C3 and c4 plants:

3. CAM Process (Crassulacean Acid Metabolism)

A carbon fixation pathway that evolved in some plants as an adaptation to arid conditions, in which the stomata in the leaves remain shut during the day to reduce evapotranspiration, but open at night to collect carbon dioxide (CO2).

Xerophytes, such as cacti and most succulents, also use phosphoenolpyruvate (PEP) carboxylase to capture carbon dioxide in a process called crassulacean acid metabolism (CAM). In contrast to C4 metabolism, which physically separates the CO2 fixation to PEP from the Calvin cycle, CAM temporally separates these two processes.

CAM plants. Some plants that are adapted to dry environments, such as cacti and pineapples, use the crassulacean acid metabolism (CAM) pathway to minimize photorespiration. This name comes from the family of plants, the Crassulaceae, in which scientists first discovered the pathway.

CAM plants have a different leaf anatomy from C3 plants, and fix the CO2 at night, when their stomata are open. CAM plants store the CO2 mostly in the form of malic acid via carboxylation of phosphoenolpyruvate to oxaloacetate, which is then reduced to malate. Decarboxylation of malate during the day releases CO2 inside the leaves, thus allowing carbon fixation to 3-phosphoglycerate by RuBisCO. Sixteen thousand species of plants use CAM

Cross section of agave, a CAM plant: Cross section of a CAM (crassulacean acid metabolism) plant, specifically of an agave leaf. Vascular bundles shown. Drawing based on microscopic images courtesy of Cambridge University Plant Sciences Department.


Factors of Photosynthesis

Factor # 1. Carbon Dioxide

CO2 concentration of the atmosphere is 0.036% or 360 ppm (360 µl. l-1). It is a limiting factor for C3 as the available CO2 concentration is lower than the optimum for photosynthesis. Increase in of photosynthesis in most C3 plants . A decline is observed beyond 0.1%.

When CO2 concentration is reduced, there comes a point at which illuminated plant parts stop absorbing carbon diox­ide from their environment. It is known as CO2 compensation point or thresh­old value. At this value CO2 fixed in photosynthesis is equal to CO2 evolved in respiration and photorespiration. The value is 25-100 ppm in C3 plants and 0-10 ppm in C4 plants. The reason for low compensation value for C4 plants is the greater efficiency of CO2-fixation through PEP-carboxylase.

Factor # 2. Light Intensity

It varies with latitude, altitude, season, topography, presence or absence of light interceptors like clouds, dust, fog, humidity, etc.

As the light intensity (µ mol. m-2. s–1) increases, the rate of photosynthe­sis (p mol. CO2. M-2.s-1) also increases. The light intensity at which a plant can achieve maximum amount of photosyn­thesis is called light saturation point. Its value is 800-1000 ft. candles (10% of full sunlight) in shade plants, 50-70% of full sunlight in C3 sun plants and up to 200% of full sunlight in C4 sun plants, (e.g., Sugarcane, Fig. 13.30).

Beyond saturation point (it is seldom realised in nature in C4 sun plants) the rate of photosynthesis begins to decline. The phenomenon is called solarisation.

 

Factor # 3. Quality of Light

The range of photo-synthetically active radiation (PAR) lies between 400 – 700 nm. Maximum photosynthesis occurs in blue-violet and red regions of the light spectrum where most of the absorption is carried out by chlorophylls. Red light favours carbohydrate accumulation while blue light stimulates protein synthesis.

Minimum photosynthesis occurs in the green wavelengths. Plants growing under the canopy of other trees receive very little red and blue-violet light because of its absorption by leaves of the canopy. They receive more of green light that is transmitted through the tree leaves. As a result the rate of photosynthesis of herbs, shrubs and other undergrowth’s in a forest is comparatively low. Ultra-violet rays are harmful.

Factor # 4. Duration of Light

Continuous photosynthesis can occur in continuous illumination without any harm to the plant though the rate of photosynthesis may slightly decline after six days.

Factor # 5. Temperature

It does not influence light reactions of photosynthesis but affects the enzyme-controlled dark reactions. The minimum temperature at which most plants start photosynthesis is 0°-5°C but it can be as low as -20°C for lichens and -35°C for some gymnosperms. The maximum temperature at which photosynthesis can occur is 55°C in some desert plants and 75°C for hot spring algae.

The optimum temperature is 10°-25°C for C3 plants and 30-45 °C for C4 plants. When temperature is increased from minimum to optimum, the rate of photosynthesis doubles for every 10°C rise in tempera­ture. Above the optimum temperature, the rate of photosynthesis shows an ini­tial increase for short duration but later declines. This decline with time is called time factor (Fig. 13.31).

At low temperature enzymes become inactive. C4 plants show little photo­synthesis even at not so low tempera­ture (2-10°C) because their enzyme pyru­vate phosphate dikinase is particularly sensitive to it. C3 plants show different responses to lower temperatures de­pending upon their adaptability. At high temperature C3 plants are more affected because of increased affinity of Rubisco to oxygen.

Factor # 6. Oxygen

Small quantity of oxygen is essential for photosynthesis except in some anaerobic bacteria. C3 plants show optimum photosynthesis at low oxygen concentration. For example, Bjorkman (1968) found in beans the rate of photosynthesis at 2.5% oxygen was twice as compared to normal atmospheric concentration.

The possible reasons are

(i) Oxygen takes part in oxidation of photosynthetic pigments, intermediates and enzymes in the presence of strong light (photo-oxidation).

(ii) Oxygen is a strong quencher of excited state of chlorophyll.

(iii) Oxygen competes with CO2 for reducing power. However, this effect is not known in C4 plants.

(iv) It converts RuBP-carboxylase to RuBP-oxygenase. At a very high oxygen content the rate of photosynthesis begins to decline in all plants. The phenomenon is called Warburg effect.

 

Factor # 7. Water (Soil Water)

Water supplies H+ and electrons for carbon dioxide fixation. However, less than 1% of the total water absorbed is utilized in photosynthesis. The rest is lost in transpiration.

Even a slight increase in transpiration reduces the leaf hydration that cuts down photosynthesis by causing stomatal closure and hence decreased CO2 absorption, loss of leaf turgidity, reduced absorption of solar radiations and decreased enzymatic activity.

There is reduction in leaf water potential. It reduces leaf growth and hence photosynthetic area. Thus photo- synthesis is more sensitive to dehydration than any other metabolic process.

Factor # 8. Air Pollutants

Dust and smoke particles present in the atmosphere reduce photo­synthesis by reducing light penetration and forming a layer over the plants. Sulphur dioxide, nitrogen oxides, hydrogen fluorides and other air pollutants also decrease photosynthesis.

Factor # 9. Minerals (Nutrient Supply)

Nitrogen is component of chlorophyll, enzymes, elec­trons transport chains and various structures of photosynthetic cells. Magnesium is a component of chlorophyll.

Fe, Cu and Mn are required for synthesis of chlorophyll. Mn and Cl are linked to photolysis of water. P as phosphate is essential for ATP synthesis. Enzyme activators of photosynthesis include potassium and sulphur. Lower availability of any of these minerals reduces rate of photosynthesis.

Internal or Plant Factors (Leaf or Genetic Factors)

Factor # 10. Age

As a leaf develops, the rate of photosynthesis rises with the age till it becomes maximum at full maturity. Afterwards the rate of photosynthesis begins to decline (Fig. 13.32) as ageing or senescence brings about deacti­vation of enzymes and degeneration of chlorophyll.

Factor # 11. Chlorophyll

Photosynthesis does not occur in the absence of chlorophyll. There­fore, variegated leaves produce less organic food than the completely green leaves.

How­ever, there is no proportionality between the rate of photosynthesis and amount of chloro­phyll. For example, sun plants contain less chlorophyll as compared to shade plants but the rate of photosynthesis in bright light is much higher in sun plants than in shade plants.

Factor # 12. Hormones

Cytokines and gibberellins increase the rate of photosynthesis but abscisic acid reduces the same.

Factor # 13. Leaf Anatomy

Leaf anatomy influences the rate of diffusion of CO2 into the mesophyll cells, availability of light, rate of translocation of end products, etc.

The important anatomical structures influencing them are size, structure, position and frequency of sto­mata, thickness of epidermis and cuticle, distribution and number of vascular strands, size and distribution of intercellular spaces, etc. Kranz anatomy is specific for C4 plants. Leaves with Kranz anatomy are more efficient in photosynthesis because of the division of labour between mesophyll and bundle sheath cells.

Factor # 14. Accumulation of End Products

Slow rate of translocation causes accumulation of photosynthetic end products during afternoon. This reduces the rate of photosynthesis.

Photosynthesis and why it’s important

Photosynthesis is plants taking in water, carbon dioxide, and light to make sugar and oxygen. This is important because all living things need oxygen to survive. All producers make oxygen and sugar for the secondary consumers and then the carnivores eat animals that eat the plants. If there is not enough producers to make the sugar and oxygen that all living things need then all living things will die out. Without producers CO2 couldn’t be taken in by the plants to be turned back into oxygen that all animals need for survival. Also, without producers, herbivores wouldn’t have anything to eat and they would all die leaving carnivores without food to eat and all life forms wouldn’t be on Earth. Photosynthesis happens in the Chloroplast of the plant’s organelle, if the Chloroplast wasn’t in the plant Photosynthesis couldn’t happen. If there was no sunlight for the plant to absorb then Photosynthesis couldn’t happen. If living things didn’t breathe out CO2 then the plants would die and living things would have no way of getting oxygen. These are the reasons why Photosynthesis is important to plants and all loving things on the planet.

Reasons Why Photosynthesis Is Important:

  • It is the number one source of oxygen in the atmosphere.
  • It contributes to the carbon cycle between the earth, the oceans, plants and animals.
  • It contributes to the symbiotic relationship between plants, humans and animals.
  • It directly or indirectly affects most life on Earth.
  • It serves as the primary energy process for most trees and plants.