Photosynthesis in higher plants

Introduction

Most of the plants are autotrophs as they are able to produce their necessary food on their own. Plants use Photosynthesis for their food production. Photosynthesis is a scientific process in which plants use mineral, water, and carbon dioxide to produce energy( food)  while releasing a byproduct- oxygen which is necessary for all of us to survive. For years, many experiments have been conducted to investigate the possible mechanism behind the process. Some of the questions like why sunlight is necessary for photosynthesis and why carbon is utilized instead of oxygen has been explained by various scientists. Let`s discuss some of the most popular theories and the important concepts of photosynthesis in higher plants.

 

 

Factors affecting Photosynthesis

Several factors affect the process of photosynthesis. Some of them are found within the plant while others are from the external source. The most common factors on which photosynthesis rely are temperature, carbon dioxide levels, light, oxygen, minerals, pollutants and some chemical compounds surrounding the plants. Let`s understand how each of these factors are linked to the photosynthesis. 

 

 

Temperature

Both the high and low temperatures affect the rate of photosynthesis, provided, the carbon dioxide and other factors are neutral. We can conclude this phenomenon by using a simple experiment by placing 5 similar plants in 5 different temperatures. The plants are exposed to the optimum temperature of  6° C – 37°   grow well as this is the most preferred range of temperature required for photosynthesis i. On the other hand, raise in the temperature results in the inactivation of enzymes and thereby affects enzymatically controlled dark reactions which result in slower rates of photosynthesis.

 

 

The concentration of Carbon Dioxide

Carbon Dioxide is the major limiting factor influencing the rate of photosynthesis. Our atmosphere has carbon dioxide concentration as low as 0.03 – 0.04%; which is very lass than all other gases. An increase in concentration to 0.05% causes a rise in the level of CO2 fixation which can eventually increase the rate of photosynthesis. Added to this, the C3 and C4 plants respond differently to the concentration of carbon dioxide. “The C3 plants respond to higher CO2 concentration in a positive manner while the latter response negatively. This principle has been used for some greenhouse-crops like bell pepper and tomatoes.” Such plants are allowed to grow in a CO2 enriched environment as we can expect better yields in a quick time.

 

 

Light 

The intensity of light varies between different zones as the climates found to be different at each zone. The amount of exposure of light interferes with the levels of co2 fixation by the plants. As the light differs in terms of the quality, duration, and intensity on each geographical area, it has a significant impact on the rate of photosynthesis,.We can observe a linear relationship between the incident light and CO2 fixation at low light intensities. Added to this, an increase in the incident light beyond a certain point causes the breakdown of chlorophyll which can limit photosynthesis.

 

 

Oxygen 

Oxygen inhibits photosynthesis in C3 plants but C4 plants display slightly positive effect. This is because C4 plants carry out photorespiration in the presence of higher oxygen levels while the others not. Therefore, the rate of photosynthesis increases as the concentration of oxygen is decreased.

 

 

Water 

It is an essential raw material necessary for carbon assimilation. For a healthy photosynthesis, water levels in the soil must be rich. However, only  1% of absorbed water is being utilized for the photosynthesis and the rest of the water is being drained away through transpiration. The decrease in water content in soil results in the poor photosynthesis as the stomata are frequently closed during the dehydrated season which results in the decreased levels of photosynthesis. 

 

 

Mineral elements 

Minerals are essential for the growth of plants. Some of the minerals related to photosynthesis are  Cu, Cl, Mg, Fe, P.

 

 

Air pollutants 

Metallic and gaseous pollutants reduce photosynthesis, some of them are SO2, oxidants, ozone and hydrogen fluorides.

 

 

Chemical compounds

The chemical compounds in the soil are scarce but some of them are really essential. However,  excess of a chemical composition may result in the death of plant cells

 

 

Early experiments on Photosynthesis

1. Jan Baptista van Helmont`s experiment

Jan Baptista van Helmont, a Belgian chemist, physiologist, and physician have attempted to explain the science behind photosynthesis during the 1600s. He performed a series of experiments on a willow tree. He kept the tree under a neutral environment and he watered it for over 5-years period. Helmont has concluded that the tree has grown as a result of the nutrients it had received from the water but not due to the soil. The argument that plants do not use soil for their growth was rejected, instead,  people accepted that water contributes to the growth of plants.

 

2. Joseph Priestley`s experiment

 

Later on, Joseph Priestley - another scientist has worked to discover the mechanism behind photosynthesis. Born in the year 1733, he became a chemist, minister, natural philosopher, educator, and political theorist. He had placed a well-lit-candle along with a plant concealed inside a glass jar that had no chance to receive air from outside. The flame was off quickly. In another jar, he kept a candle without lighting it along with a mouse. After a brief duration mouse went fainted. But when the mint plant was placed in another jar with the candle burning, the mouse stayed alive. This proved that both candles and mice require air. After 44 years- In 1774, the results of Priestley’s experiments were published in “Experiments and Observations of Different Kinds of Air, Volume I.” 

 

3. Jan Ingenhousz`s experiemnts

Jan Ingenhousz’s -a Dutch chemist, biologist and physiologist has performed a set of experiments in the late 1770s. He had placed submerged plants under sunlight and then moved them to the shade. He observed that plants produced small bubbles while they were under the influence of sunlight. Again he transferred them under the shade where bubbles disappeared! .Based on this, he concluded that plants use light to produce oxygen.

 

4. Jean Senebier`s experiment

In 1796, Jean Senebier-  a Swiss botanist has demonstrated that plants absorb carbon dioxide and release oxygen in the presence of the sunlight.

 

5. Nicolas-Théodore de Saussure

In the early 1800s Nicolas-Théodore de Saussure demonstrated that, although plants need carbon dioxide, the growth of the plants needs the contribution of water more than anything.

 

6. Julius Robert Mayer

In the 1840s Julius Robert Mayer, a German physician and physicist stated that energy can be neither created nor destroyed.  This is known as the first law of thermodynamics. He proposed that plants convert light energy into chemical energy. From 1862-64 Julius Sachs investigated the way starch is produced under the influence of light with the help of chlorophyll.

 

6. Cornelis Van Niel

In the 1930s Cornelis Van Niel proposed an equation for photosynthesis. This is popularly known as, general equation of photosynthesis.It is written as CO2 + 2H2A + Light energy → [CH2O] + 2A + H2O. Most scientifically accepted equation for photosynthesis is -

https://ncerthelp.com/ncertimages/class%2011/biology/CH%2013/Cass11bioCH13001.jpg

 

 

Chemical nature of Photosynthesis

Photosynthesis takes place in the chloroplast- the green part of a plant leaf. Chloroplasts are the specialized cells that contain green pigments called chlorophylls. Chlorophylls assist in trapping the sunlight necessary for photosynthesis thereby allowing the redox to happen. Redox id the combination of reductions and oxidation( redox) that converts water, carbon dioxide and plant minerals into energy and oxygen.  In an oxidation-reduction reaction, there is a loss of electrons by water while there is a gain of electrons takes place with the help of carbon dioxide. The carbon dioxide enters into the Mesophyll Cells through small pores known as stomata. At the same time, water for the process is transported via the root system with the help of the plant`s vascular tissues. The membrane system (the stroma lamellae, grana) present in the chloroplast traps the light energy to synthesize  ATP and NADPH. 

 

 

 

 

Role of mesophyll in photosynthesis

Mesophyll Cells contain an outer cell wall, cell membrane, cytoplasm, chloroplast, vacuole, and nucleus. Cell Wall is an outermost layer assist in providing mechanical and structural support to maintain the shape of the cell. It also assists in protecting the cells against pathogens while keeping a check on the direction of the plant`s growth. The cytoplasm also provide space to a variety of cell organelles and enzymes involving in photosynthesis. Cell Membrane acts as a selectively permeable layer that entertains only the necessary materials while repelling the unnecessary materials. Chloroplasts contain chlorophyll- a green coloured substance that absorbs the light energy while the vacuole help in holding the moisture to keep the plant turgid. The nucleus contains DNA that can control various activities of the cell during the process.

 

 

 

 

Pigments in Photosynthesis

Pigments can absorb light at a specific wavelength. 4 distinct types of pigments in leaves are Chlorophyll-A (bright or blue-green in chromatogram), Chlorophyll-B (yellow-green), Carotenoids (yellow to yellow-orange) and Xanthophyll (yellow coloured) pigments. Photosynthesis takes place usually in red and blue regions of the spectrum however some photosynthesis can occur through other wavelengths. Chlorophyll A is the major pigment help in trapping the light-energy whereas the other pigments act as accessory pigments.

 

 

Photosynthetic Reactions

There are 2 types of Photosynthetic Reactions; Light-Dependent Reaction and Light Independent Reaction (Dark reaction). Light Dependent Reaction is the one in which the energy emitted by sunlight is absorbed by chlorophyll to produce ATP and NADPH (electron carrier molecule). On the other hand, light Independent Reaction or dark reaction is also known as the Calvin Cycle during which the energized electrons found in the leaves help in making simple carbohydrates.

 

 

Photosystem

Photosystems are the structural and functional units of protein complexes that aid in the photosynthesis. The protein complexes in the photosystem join together to conduct the primary photochemistry of photosynthesis, I, e absorption of light, the transfer of energy and electrons from sunlight into the mesophyll. Photosystems are found in the thylakoid membranes of plants, algae, and cyanobacteria. There are 2 kinds of photosystems,  PS-I and PS-II.  In PS II, the reaction centre chlorophyll-a absorbs 680 nm of wavelengths of light resulting in the excitation and jumping of electrons out of their orbits. The excited electrons are captured by the electron acceptors that transfer them into 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 centre are also excited when they receive red light of wavelength 700 nm. 

 


Cyclic and Non – Cyclic Photo – Phosphorylation

 

 

The photophosphorylation process results in the movement of the electrons in a cyclic manner. The cyclic electron movement help in synthesizing ATP molecules Hence it is called cyclic photophosphorylation. In this process, plant cells convert ADP to ATP for immediate energy for the cells. This process usually takes place in the thylakoid membrane and uses Photosystem I and the chlorophyll P700. During the cyclic photophosphorylation,  the electrons are transferred back to P700 instead of moving into the NADP from the electron acceptor. Such downward movement of electrons from an acceptor to P700 aids in the formation of ATP molecules.  The photophosphorylation process which results in the movement of the electrons in a non-cyclic manner for synthesizing ATP molecules using the energy from excited electrons provided by photosystem II is called non-cyclic photophosphorylation. Here the complete movement of the electrons is in a unidirectional or in a non- cyclic manner. During non-cyclic photophosphorylation, the electrons released by P700 are carried by primary acceptor and are finally passed on to NADP. Here, the electrons combine with the protons – H+ which is produced by splitting up of the water molecule and reduces NADP to NADPH2.

 

 

 

 

Chemiosmotic Hypothesis

Peter Mitchell has proposed the Chemiosmotic hypothesis for the first time. According to his hypothesis, in order to drive the synthesis of ATP, a proton-motive force was responsible. The protons are pumped alongside the inner mitochondrial membrane when the electrons move through their transfer chain. Such a movement results in a proton gradient with a low pH in the inter-membranous space and higher pH in the mitochondrial matrix. The pH gradient acts as a battery that stores energy to produce ATP.

 


Biosynthetic Phase of photosynthesis

It is a process that occurs in the stroma with the help of a series of enzyme-mediated reactions.
This is the phase in which the carbon dioxide is reduced to carbohydrates. The other term used for the biosynthetic phase is carbon fixation. Carbon fixation utilizes ATP and NADPH produced in the light phase to convert carbon dioxide into carbohydrates. 

 

 

The Calvin Cycle

There are a series of cycles that drive photosynthetic reactions. Some of them are biosynthetic phase, dark reactions, Calvin cycle and light-independent reactions (photosynthetic carbon reduction). They all occur within the stroma, the fluid-filled area of a chloroplast outside the thylakoid membranes. The 3 phases of the light-independent reactions are collectively called the Calvin cycle. The 3 phases are carbon fixation, reduction reactions, and ribulose 1,5-bisphosphate (RuBP) regeneration. Calvin cycle although it happens during day time –in the presence of sunlight, it is known by the name dark reaction. In the Calvin Cycle, Carbon atoms found in the CO2 are fixed and are used to form 3 Carbon Sugar. The ATP and NADPH that are formed from light reactions help the Calvin cycle.

 

 

 

3-phases of Calvin cycle (Reactions in Calvin Cycle)

The reactions in the Calvin cycle are divided into 3 different stages:

  • Carbon Fixation – 1  CO2 molecule interacts with 5 C acceptor molecule in the presence of  RuBP to generate 6 Carbon compound,  that splits into 2 molecules of 3 Carbon compound and 3PGA. Carboxylase or oxygenase. Is the enzyme help in this process?

  • The reduction in the 2nd in which ATP and NADPH are converted to 3 PGA molecules - three-carbon sugar and G3P (glyceraldehyde–3–phosphate).

  • Regeneration is the last phase where 3GP molecules assist in making glucose while others may be recycled to regenerate RuBP acceptor.

 

 

The C4 Pathway

C4 pathways in plants help to convert the atmospheric carbon dioxide into a chemical compound containing 4 carbons. The 2 main cell types participate in the C4 pathway are  Mesophyll Cells and Bundle – Sheath Cells.  This is the pathway used by the plants such as Sugar Cane, Maize, Millet, Papyrus and Sorghum and they are found in the subtropical regions. In a C4 pathway, the primary CO2 acceptor is 3-Carbon Molecule PEP (phosphoenolpyruvate) found in mesophyll cells. PEP case or PEP carboxylase is the enzyme 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. The 3 Carbon molecules are transported back to mesophyll cells where it is converted into PEP, thereby completing the cycle. The CO2 released enters in bundle sheath cells and Calvin pathway. These bundle sheath cells have a surplus of the enzyme called RuBisCO (Ribulose Biphosphate Carboxylase – Oxygenase) and are deficient in PEPcase. 

 

 

Photorespiration

Photorespiration is a process that simultaneously takes place in green plants along with photosynthesis. It is a biochemical process that occurs during adverse situations like water stress. Photorespiration results in light-dependent uptake of O2 and release of CO2 to form a glycolate. In the end, the process helps to decrease the net amount of utilization of CO2.

 



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