Category: Biology

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Types of Respiration

Respiration is classified into two types as aerobic and anaerobic respiration (Figure 14.4)

Aerobic Respiration

Respiration occurring in the presence of oxygen is called aerobic respiration. During aerobic respiration, food materials like carbohydrates, fats and proteins are completely oxidised into CO₂, H₂O and energy is released. Aerobic respiration is a very complex process and is completed in four major steps:

1. Glycolysis
2. Pyruvate oxidation (Link reaction)
3. Krebs cycle (TCA cycle)
4. Electron Transport Chain (Terminal oxidation).

Anaerobic Respiration

In the absence of molecular oxygen glucose is incompletely degraded into either ethyl alcohol or lactic acid (Table 14.1). It includes two steps:

1. Glycolysis
2. Fermentation

Aerobic Respiration	Anaeorbic Respiration
1. It occurs in all living cells of higher organisms	1. It occurs yeast and some bacteria
2. It requires oxygen for breaking the respiratory substrate	2. Oxygen is not required for breaking the respiratory substrate
3. The end products are CO2 and H2O	3. The end products are alcohol, and CO2 (Or) lactic acid
4. Oxidation of one molecule of glucose produces 36 ATP molecules	4. Only 2 ATP molecules are produced
5. It consists of four stages-glycolysis, link reaction, TCA cycle and electron transport chain	5. It consists of two stages-glycolysis and fermentation
6. It occurs in cytoplasm and mitochondria	6. It occurs only in cytoplasm

An oxidation-reduction (redox) reaction is a type of chemical reaction that involves a transfer of electrons between two species. An oxidation-reduction reaction is any chemical reaction in which the oxidation number of a molecule, atom, or ion changes by gaining or losing an electron.

Because any loss of electrons by one substance must be accompanied by a gain in electrons by something else, oxidation and reduction always occur together. As such, electron-transfer reactions are also called oxidation-reduction reactions, or simply redox reactions.

Remember that although redox reactions are common and plentiful, not all chemical reactions are redox reactions. All redox reactions involve complete or partial transfer of electrons from one atom to another. Both reactions above are examples of oxidation-reduction reactions.

Oxidation-reduction reaction, also called redox reaction, any chemical reaction in which the oxidation number of a participating chemical species changes.

Simple Redox Reactions

• Write the oxidation and reduction half-reactions for the species that is reduced or oxidized.
• Multiply the half-reactions by the appropriate number so that they have equal numbers of electrons.
• Add the two equations to cancel out the electrons. The equation should be balanced.

What is the only pattern for redox reactions? Describe how redox reactions always occur. Redox reactions always occur together; if one reactant in a chemical reaction is oxidized, then another reactant must be reduced.

Double-replacement reactions such as the one below are not redox reactions because ions are simply recombined without any transfer of electrons. Note that the oxidation numbers for each element remain unchanged in the reaction.

The batteries which are used for generating DC current use redox reaction to produce electrical energy. Batteries also called as electrochemical cells used in our day-to-day life are also based on redox reactions. For example, storage cells which are used in vehicles to supply all the electrical needs of the vehicles. These reactions cannot be reversed once they occur, because the product escapes the surface immediately.

Oxidation reactions may be exothermic or endothermic. Eg. All combustion reactions are exothermic and ionisation reaction giving positive ions are endothermic.

The key difference between redox and nonredox reactions is that in redox reactions, the oxidation state of some chemical elements changes from one state to another state whereas, in nonredox reactions, the oxidation states of chemical elements do not change.

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Redox Reactions

NAD⁺ + 2e^– + 2H⁺ → NADH + H⁺
FAD + 2e^– + 2H⁺ → FADH₂

When NAD⁺ (Nicotinamide Adenine Dinucleotide-oxidised form) and FAD (Flavin Adenine Dinucleotide) pick up electrons and one or two hydrogen ions (protons), they get reduced to NADH + H⁺ and FADH₂ respectively.

When they drop electrons and hydrogen off they go back to their original form. The reaction in which NAD⁺ and FAD gain (reduction) or lose (oxidation) electrons are calledredox reaction (Oxidation reduction reaction). These reactions are important in cellular respiration.

An oxidation-reduction (redox) reaction is a type of chemical reaction that involves a transfer of electrons between two species. An oxidation-reduction reaction is any chemical reaction in which the oxidation number of a molecule, atom, or ion changes by gaining or losing an electron.

The reaction in which one substance gets oxidised and other gets reduced is known as redox reaction. Example: ZnO + C → Zn + CO. Here, C is oxidised to CO because oxygen is being added and ZnO is reduced to Zn because oxygen is being removed. Therefore, it is a redox reaction.

Keep this in mind as we look at the five main types of redox reactions: combination, decomposition, displacement, combustion, and disproportion. Combination. Combination reactions “combine” elements to form a chemical compound.

• Decomposition
• Displacement
• Combustion
• Disproportionation

Because any loss of electrons by one substance must be accompanied by a gain in electrons by something else, oxidation and reduction always occur together. As such, electron-transfer reactions are also called oxidation-reduction reactions, or simply redox reactions.

Remember that although redox reactions are common and plentiful, not all chemical reactions are redox reactions. All redox reactions involve complete or partial transfer of electrons from one atom to another. Both reactions above are examples of oxidation-reduction reactions.

Oxidation-reduction reaction, also called redox reaction, any chemical reaction in which the oxidation number of a participating chemical species changes.

Write the oxidation and reduction half-reactions for the species that is reduced or oxidized. Multiply the half-reactions by the appropriate number so that they have equal numbers of electrons. Add the two equations to cancel out the electrons. The equation should be balanced.

Double-replacement reactions such as the one below are not redox reactions because ions are simply recombined without any transfer of electrons. Note that the oxidation numbers for each element remain unchanged in the reaction.

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Structure of ATP

Respiration is responsible for generation of ATP. The discovery of ATP was made by Karl Lohman (1929). ATP is a nucleotide consisting of a base-adenine, a pentose sugar-ribose and three phosphate groups. Out of three phosphate groups the last two are attached by high energy rich bonds (Figure 14.3). On hydrolysis, it releases energy (7.3 K cal or 30.6 KJ/ATP) and it is found in all living cells and hence it is called universal energy currency of the cell.

ATP is an instant source of energy within the cell. The energy contained in ATP is used in synthesis carbohydrates, proteins and lipids. The energy transformation concept was established by Lipman (1941).

ATP is a nucleotide that consists of three main structures: the nitrogenous base, adenine; the sugar, ribose; and a chain of three phosphate groups bound to ribose. The phosphate tail of ATP is the actual power source which the cell taps.

The structure of ATP is a nucleoside triphosphate, consisting of a nitrogenous base (adenine), a ribose sugar, and three serially bonded phosphate groups. ATP is commonly referred to as the “energy currency” of the cell, as it provides readily releasable energy in the bond between the second and third phosphate groups.

It allows the cell to store energy briefly and transport it within the cell to support endergonic chemical reactions. The structure of ATP is that of an RNA nucleotide with three phosphates attached. As ATP is used for energy, a phosphate group or two are detached, and either ADP or AMP is produced.

ATP and ADP are two types of nucleotides mainly involved in the transfer of energy between biochemical reactions in the cell. Both ATP and ADP are composed of a ribose sugar, adenosine, and phosphate groups. ATP molecule is composed of three phosphate molecules while ADP is composed of two phosphate molecules.

ATP is composed of ribose, a five-carbon sugar, three phosphate groups, and adenine, a nitrogen-containing compound (also known as a nitrogenous base).

From the perspective of biochemistry, ATP is classified as a nucleoside triphosphate, which indicates that it consists of three components: a nitrogenous base (adenine), the sugar ribose, and the triphosphate.

Give three examples of how ATP is used in organisms. ATP is used to build large molecules such as proteins, to temporarily store energy in the form of fat, and to allow for all types of cellular transport.

Its Structure. The ATP molecule is composed of three components. These phosphates are the key to the activity of ATP. ATP consists of a base, in this case adenine (red), a ribose (magenta) and a phosphate chain (blue).

All living things, plants and animals, require a continual supply of energy in order to function. The energy is used for all the processes which keep the organism alive. This special carrier of energy is the molecule adenosine triphosphate, or ATP.

The process of phosphorylating ADP to form ATP and removing a phosphate from ATP to form ADP in order to store and release energy respectively is known as the ATP cycle. The energy within an ATP molecule is stored in the phosphate bonds of the ATP. When a cell needs energy, a phosphate is removed from ATP.

Any metabolic process that requires oxygen to occur is referred to as aerobic. Humans, most other multicellular organisms, and some microorganisms require oxygen for the efficient capture of the chemical energy from food and its transformation into the cellular energy form known as ATP.

Energy is stored in the bonds joining the phosphate groups (yellow). The covalent bond holding the third phosphate group carries about 7, 300 calories of energy.

Adenosine triphosphate (ATP), energy-carrying molecule found in the cells of all living things. ATP captures chemical energy obtained from the breakdown of food molecules and releases it to fuel other cellular processes.

For example, both breathing and maintaining your heartbeat require ATP. In addition, ATP helps to synthesize fats, nerve impulses, as well as move certain molecules into or out of cells. Some organisms, such as bioluminescent jellyfish and fireflies, even use ATP to produce light.

All organisms need energy. Life depends on the transfer of energy. ATP is an important source of energy for biological processes. In photosynthesis energy is transferred to ATP in the light-dependent stage and the ATP is utilised during synthesis in the light-independent stage.

ATP is required for various biological processes in animals including; Active Transport, Secretion, Endocytosis, Synthesis and Replication of DNA and Movement.

The Adenosine triphosphate (ATP) molecule is the nucleotide known in biochemistry as the “molecular currency” of intracellular energy transfer; that is, ATP is able to store and transport chemical energy within cells. ATP also plays an important role in the synthesis of nucleic acids.

Beginning with energy sources obtained from their environment in the form of sunlight and organic food molecules, eukaryotic cells make energy-rich molecules like ATP and NADH via energy pathways including photosynthesis, glycolysis, the citric acid cycle, and oxidative phosphorylation.

The energy released by ATP is released when a phosphate group is removed from the molecule. ATP has three different phosphate groups, but the bond holding the third phosphate group is unstable and is very easily broken. Where does ADP come from? When phosphate is removed, energy is released and ATP becomes ADP.

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Gaseous Exchange

The term respiration was coined by Pepys (1966). Respiration is a biological process in which oxidation of various food substances like carbohydrates, proteins and fats take place and as a result of this, energy is produced where O₂ is taken in and CO₂ is liberated. The organic substances which are oxidised during respiration are called respiratory substrates.

Among these, glucose is the commonest respiratory substrate. Breaking of C-C bonds of complex organic compounds through oxidation within the cells leads to energy release. The energy released during respiration is stored in the form of ATP (Adenosine Tri Phosphate) as well as liberated heat. Respiration occurs in all the living cells of organisms. The overall process of respiration corresponds to a reversal of photosynthesis. C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (686 K cal or 2868 KJ)
(1K cal = 4.184 KJ)

Depending upon the nature of respiratory substrate, Blackman divided respiration into,

1. Floating Respiration
2. Protoplasmic Respiration

When carbohydrate or fat or organic acid serves as respiratory substrate and it is called floating respiration. It is a common mode of respiration and does not produce any toxic product. Whereas respiration utilizing protein as a respiratory substrate, it is called protoplasmic respiration. Protoplasmic respiration is rare and it depletes structural and functional proteins of protoplasm and liberates toxic ammonia.

Compensation Point

At dawn and dusk the intensity of light is low. The point at which CO₂ released in respiration is exactly compensated by CO₂ fixed in photosynthesis that means no net gaseous exchange takes place, it is called compensation point. At this moment, the amount of oxygen released from photosynthesis is equal to the amount of oxygen utilized in respiration.

The two common factors associated with compensation point are CO₂ and light (Figure 14.2). Based on this there are two types of compensation point. They are CO₂ compensation point and light compensation point. C₃ plants have compensation points ranging from 40-60 ppm (parts per million) CO₂ while those of C₄ plants ranges from 1-5 ppm CO₂.

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Photosynthesis in Bacteria

Though we study about bacterial photosynthesis as the last part, bacterial photosynthesis formed first and foremost in evolution. Bacteria does not have specialized structures like chloroplast. It has a simple type of photosynthetic apparatus called chlorosomes and chromatophores (Table 13.6). Van Neil (1930) discovered a bacterium that releases sulphur instead of oxygen during photosynthesis.

Photosynthesis in Plants	Photosynthesis in Bacteria
1. Cyclic and non-cyclic phosphorylation takes place	1. Only cyclic phosphorylation takes place
2. Photosystem I and II involved	2. Photosystem I only involved
3. Electron donor is water	3. Electron donor is H<sub>2</sub>S
4. Oxygen is evolved	4. Oxygen is not evolved
5. Reaction centres are P700 and P680	5. Reaction centre is P<sub>870</sub>
6. Reducing agent is NADPH + H+	6. Reducing agent is NADH + H+
7. PAR is 400 to 700 nm	7. PAR is above 700 nm
8. Chlorophyll, carotenoid and xanthophyll	8. Bacterio chlorophyll and bacterio viridin
9. Photosynthetic Apparatus – Chloroplast	9. It is chlorosomes and chromatophores

Here, electron donor is hydrogen sulphide (H₂S) and only one photosystem is involved (PS I) and the reaction centre is P₈₇₀. Pigments present in bacteria are bacteriochlorophyll a, b, c, d, e and g and carotenoids. Photosynthetic bacteria are classified into three groups:

1.Green Sulphur Bacteria.
Example: Chlorobacterium and Chlorobium.

2. Purple Sulphur Bacteria.
Example: Thospirillum and Chromatium.

3. Purple Non-Sulphur Bacteria.
Example: Rhodopseudomonas and Rhodospirillum.

Cyanobacteria contain chlorophyll while other forms of bacteria contain bacteriochlorophyll. Cyanobacteria perform photosynthesis using water as an electron donor in a similar manner to plants. This results in the production of oxygen and is known as oxygenic photosynthesis.

There are several groups of bacteria that undergo anoxygenic photosynthesis: green sulfur bacteria, green and red filamentous anoxygenic phototrophs (FAPs), phototrophic purple bacteria, phototrophic acidobacteria, and phototrophic heliobacteria.

There are several groups of bacteria that undergo anoxygenic photosynthesis: green sulfur bacteria, green and red filamentous anoxygenic phototrophs (FAPs), phototrophic purple bacteria, phototrophic acidobacteria, and phototrophic heliobacteria.

The main purpose of photosynthesis is to convert radiant energy from the sun into chemical energy that can be used for food. Cellular respiration is the process that occurs in the mitochondria of organisms (animals and plants) to break down sugar in the presence of oxygen to release energy in the form of ATP.

Oxygenic photosynthetic bacteria perform photosynthesis in a similar manner to plants. They contain light-harvesting pigments, absorb carbon dioxide, and release oxygen. Cyanobacteria or Cyanophyta are the only forms of oxygenic photosynthetic bacteria known to date.

An example of photosynthesis is how plants convert sugar and energy from water, air and sunlight into energy to grow. The water from the leaves evaporates through the stomata, and filling its place, entering the stomata from the air, is carbon dioxide. Plants need carbon dioxide to make food.

Algae are sometimes considered plants and sometimes considered “protists” (a grab-bag category of generally distantly related organisms that are grouped on the basis of not being animals, plants, fungi, bacteria, or archaeans).

In all phototrophic eukaryotes, photosynthesis takes place inside a chloroplast, an organelle that arose in eukaryotes by endosymbiosis of a photosynthetic bacterium (see Unique Characteristics of Eukaryotic Cells). These chloroplasts are enclosed by a double membrane with inner and outer layers.

The most influential bacteria for life on Earth are found in the soil, sediments and seas. Well known functions of these are to provide nutrients like nitrogen and phosphorus to plants as well as producing growth hormones. By decomposing dead organic matter, they contribute to soil structure and the cycles of nature.

Bacteria are classified into five groups according to their basic shapes: spherical (cocci), rod (bacilli), spiral (spirilla), comma (vibrios) or corkscrew (spirochaetes). They can exist as single cells, in pairs, chains or clusters.

Photosynthesis converts solar energy into chemical energy. Photosynthesis produces carbohydrates. Plants need sunlight, carbon dioxide, water, nutrients, and chlorophyll to complete photosynthesis. Plants use chlorophyll, water, and carbon dioxide to make sugar.

Photosynthesis is the biochemical process in which energy from sunlight is converted by plants, algae, and some bacteria into sugars, which are used by the organism as food. However, there is a least one exception: a little bacterium deep under the Pacific Ocean which manages photosynthesis without sunlight.

Cyanobacteria are oxygenic photosynthetic bacteria. They harvest the sun’s energy, absorb carbon dioxide, and emit oxygen. Like plants and algae, cyanobacteria contain chlorophyll and convert carbon dioxide to sugar through carbon fixation.

According to the diagram of photosynthesis, the process begins with three most important non-living elements: water, soil, and carbon dioxide. Plants begin making their ‘food’, which basically includes large quantities of sugars and carbohydrate, when sunlight falls on their leaves.

The reactants for photosynthesis are light energy, water, carbon dioxide and chlorophyll, while the products are glucose (sugar), oxygen and water.

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Factors Affecting Photosynthesis

In 1860, Sachs gave three cardinal points theory explaining minimum, optimum and maximum factors that control photosynthesis. In 1905, Blackman put forth the importance of smallest factor. Blackman’s law of limiting factor is actually a modified Law proposed by Liebig’s Law of minimum. According to Blackman, “When a process is conditioned as to its rapidity by a number of separate factors, the rate of the process is limited by the pace of the lowest factor”.

To conclude in an easy way “at any given point of time the lowest factor among essentials will limit the rate of photosynthesis”. For example, when even sufficient light intensity is available, photosynthesis may be low due to low CO₂ in the atmosphere.

Here, CO₂ acts as a limiting factor. If CO₂ is increased in the atmosphere the rate of photosynthesis also increases. Further increase in photosynthesis is possible only if the available light intensity is also increased proportionately (Figure 13.21).

Factors affecting photosynthesis are further grouped into External or Environmental factors and Internal factors.

I. External Factors:
Light, carbon dioxide, temperature, water, mineral and pollutants.

II. Internal Factors:
Pigments, protoplasmic factor, accumulation of carbohydrates, anatomy of leaf and hormones.

External Factors

1. Light

Energy for photosynthesis comes only from light. Photooxidation of water and excitation of pigment molecules are directly controlled by light. Stomatal movement leading to diffusion of CO₂ is indirectly controlled by light.

a. Intensity of Light:

Intensity of light plays a direct role in the rate of photosynthesis. Under low intensity the photosynthetic rate is low and at higher intensity photosynthetic rate is higher. It also depends on the nature of plants. Heliophytes (Bean Plant) require higher intensity than Sciophytes (Oxalis).

b. Quantity of Light:

In plants which are exposed to light for longer duration (Long day Plants) photosynthetic rate is higher.

c. Quality of light:

Different wavelengths of light affect the rate of photosynthesis because pigment system does not absorb all the rays equally. Photosynthetic rate is maximum in blue and red light. Photosynthetically Active Radiation (PAR) is between 400 to 700 nm. Red light induces highest rate of photosynthesis and green light induces lowest rate of photosynthesis.

2. Carbon Dioxide

CO₂ is found only 0.3% in the atmosphere but plays a vital role. Increase in concentration of CO₂ increases the rate of photosynthesis (CO₂ concentration in the atmosphere is 330 ppm). If concentration is increased beyond 500ppm, rate of photosynthesis will be affcted showing the inhibitory effect.

3. Oxygen

The rate of photosynthesis decreases when there is an increase of oxygen concentration. This Inhibitory effect of oxygen was first discovered by Warburg (1920) using green algae Chlorella.

4. Temperature

The optimum temperature for photosynthesis varies from plant to plant. Temperature is not uniform in all places. In general, the optimum temperature for photosynthesis is 25°C to 35°C. This is not applicable for all plants.

The ideal temperature for plants like Opuntia is 55°C, Lichens 25°C and Algae growing in hot spring photosynthesis is 75°C. Whether high temperature or low temperature it will close the stomata as well as inactivate the enzymes responsible for photosynthesis (Figure 13.22).

5. Water

Photolysis of water provides electrons and protons for the reduction of NADP, directly. Indirect roles are stomatal movement and hydration of protoplasm. During water stress, supply of NADPH + H⁺ is affected.

6. Minerals

Deficiency of certain minerals affect photosynthesis e.g. mineral involved in the synthesis of chlorophyll (Mg, Fe and N), Phosphorylation reactions (P), Photolysis of water (Mn and Cl), formation of plastocyanin (Cu).

7. Air pollutants

Pollutants like SO₂, NO₂, O₃ (Ozone) and Smog affects rate of photosynthesis.

Internal Factors

1. Photosynthetic Pigments
It is an essential factor and even a small quantity is enough to carry out photosynthesis.

2. Protoplasmic Factor
Hydrated protoplasm is essential for photosynthesis. It also includes enzymes responsible for Photosynthesis.

3. Accumulation of Carbohydrates
Photosynthetic end products like carbohydrates are accumulated in cells and if translocation of carbohydrates is slow then this will affect the rate of photosynthesis.

4. Anatomy of Leaf
Thickness of cuticle and epidermis, distribution of stomata, presence or absence of Kranz anatomy and relative proportion of photosynthetic cells affect photosynthesis.

5. Hormones
Hormones like gibberellins and cytokinin increase the rate of photosynthesis.

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Photorespiration or Cycle or Photosynthetic Carbon Oxidation (PCO) Cycle

Respiration is a continuous process for all living organisms including plants. Decker (1959) observed that rate of respiration is more in light than in dark. Photorespiration is the excess respiration taking place in photosynthetic cells due to absence of CO₂ and increases of O₂ (Table 13.5). This condition changes the carboxylase role of RUBISCO into oxygenase.

Photorespiration	Dark respiration
1. It takes place in photosynthetic green cells	1. It takes place in all living cells
2. It takes place only in the presence of light	2. It takes place all the time
3. It involves chloroplast, peroxisome and mitochondria	3. It involves only mitochondria
4. It does not involve Glycolysis, Kreb’s Cycle, and ETS	4. It involves glycolysis, Kreb’s Cycle and ETS
5. Substrate is glycolic acid	5. Substrate is carbohydrates, protein or fats
6. It is not essential for survival	6. Essential for survival
7. No phosphorylation and yield of ATP	7. Phosphorylation produces ATP energy
8. NADH2 is oxidised to NAD+	8. NAD+ is reduced to NADH2
9. Hydrogen peroxide is produced	9. Hydrogen peroxide is not produced
10. End products are CO2 and PGA	10. End products are CO2 and water

C₂ Cycle takes place in chloroplast, peroxisome and mitochondria. RUBP is converted into PGA and a 2C-compound phosphoglycolate by Rubisco enzyme in chloroplast. Since the first product is a 2C-compound, this cycle is known as C₂ Cycle. Phosphoglycolate by loss of phosphate becomes glycolate. Glycolate formed in chloroplast enters into peroxisome to form glyoxylate and hydrogen peroxide.

Glyoxylate is converted into glycine and transferred into mitochondria. In mitochondria, two molecules of glycine combine to form serine. Serine enters into peroxisome to form hydroxy pyruvate. Hydroxy pyruvate with help of NADH + H⁺ becomes glyceric acid.

Glyceric acid is cycled back to chloroplast util ising ATP and becomes Phosphoglyceric acid (PGA) and enters into the Calvin cycle (PCR cycle). Photorespiration does not yield any free energy in the form of ATP. Under certain conditions 50% of the photosynthetic potential is lost because of Photorespiration (Figure 13.20).

Significance of Photorespiration

1. Glycine and Serine synthesised during this process are precursors of many biomolecules like chlorophyll, proteins, nucleotides.
2. It consumes excess NADH + H⁺ generated.
3. Glycolate protects cells from Photo oxidation.

Carbon Dioxide Compensation Point

When the rate of photosynthesis equals the rate of respiration, there is no exchange of oxygen and carbon dioxide and this is called as carbon dioxide compensation point. This will happen at particular light intensity when exchange of gases becomes zero. When light is not a limiting factor and atmospheric CO₂ concentration is between 50 to 100 ppm the net exchange is zero.

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Crassulacean Acid Metabolism or CAM Cycle

It is one of the carbon pathways identified in succulent plants growing in semi-arid or xerophytic condition. This was first observed in crassulaceae family plants like Bryophyllum, Sedum, Kalanchoe and is the reason behind the name of this cycle. It is also noticed in plants from other families Examples: Agave, Opuntia, Pineapple and Orchids.

The stomata are closed during day and are open during night (Scotoactive). This reverse stomatal rhythm helps to conserve water loss through transpiration and will stop the fixation of CO₂ during the day time. At night time CAM plants fix CO₂ with the help of Phospho Enol Pyruvic acid (PEP) and produce oxalo acetic acid (OAA). Subsequently OAA is converted into malic acid like C₄ cycle and gets accumulated in vacuole increasing the acidity.

During the day time stomata are closed and malic acid is decarboxylated into pyruvic acid resulting in the decrease of acidity. CO₂ thus formed enters into Calvin Cycle and produces carbohydrates (Figure 13.19).

Significance of CAM Cycle

1. It is advantageous for succulent plants to obtain CO₂ from malic acid when stomata are closed.
2. During day time stomata are closed and CO₂ is not taken but continue their photosynthesis.
3. Stomata are closed during the day time and help the plants to avoid transpiration and water loss.

Crassulacean acid metabolism (CAM) is a photosynthetic adaptation to periodic water supply, occurring in plants in arid regions (e.g., cacti) or in tropical epiphytes (e.g., orchids and bromeliads). CAM plants close their stomata during the day and take up CO₂ at night, when the air temperature is lower.

Crassulacean acid metabolism, also known as CAM photosynthesis, is a carbon fixation pathway that evolved in some plants as an adaptation to arid conditions that allows a plant to photosynthesize during the day, but only exchange gases at night.

In CAM plants, carbon dioxide is only gathered at night, when the stomata open. Click for more detail. During the day, the malic acid is converted back to carbon dioxide. This type of photosynthesis is known as Crassulacean Acid Metabolism because of the storage of carbon dioxide at night as an acid.

Crassulacean acid metabolism (CAM) is an important elaboration of photosynthetic carbon fixation that allows chloroplast-containing cells to fix CO₂ initially at night using phosphoenolpyruvate carboxylase (PEPC) in the cytosol.

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. Image of a succulent.

Biochemical studies indicate that photorespiration consumes ATP and NADPH, the high-energy molecules made by the light reactions. Thus, photorespiration is a wasteful process because it prevents plants from using their ATP and NADPH to synthesize carbohydrates.

The CAM plants close their stomata at the time of day and open it in night. When the stomata remains closed then this will prevent the loss of water by the process of transpiration. This process also prevents the carbon dioxide gas being entering into the plant leaves.

Crassulacean acid metabolism (CAM) is a photosynthetic adaptation to periodic water supply, occurring in plants in arid regions (e.g., cacti) or in tropical epiphytes (e.g., orchids and bromeliads). CAM plants close their stomata during the day and take up CO₂ at night, when the air temperature is lower.

Unlike plants in wetter environments, CAM plants absorb and store carbon dioxide through open pores in their leaves at night, when water is less likely to evaporate. During the day, the pores, also called stomata, stay closed while the plant uses sunlight to convert carbon dioxide into energy, minimizing water loss.

Without enough light, a plant cannot photosynthesise very quickly – even if there is plenty of water and carbon dioxide and a suitable temperature. Increasing the light intensity increases the rate of photosynthesis, until some other factor – a limiting factor – becomes in short supply.

But there are few plants like Peepal which gives out Oxygen at night by CAM photosynthesis which are the plants the Crassulaceae family. During daylight hours, plants take in carbon dioxide and release oxygen through photosynthesis, and at night only about half that carbon is then released through respiration.

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Hatch & Slack Pathway or Cycle or Dicarboxylic Acid Pathway or Dicarboxylation Pathway

Till 1965, Calvin cycle is the only pathway for CO₂ fixation. But in 1965, Kortschak, Hart and Burr made observations in sugarcane and found C₄ or dicarboxylic acid pathway. Malate and aspartate are the major labelled products. This observation was confirmed by Hatch & Slack in 1967.

This alternate pathway for the fixation of CO₂ was found in several tropical and sub-tropical grasses and some dicots. C₄ cycle is discovered in more than 1000 species. Among them 300 species belong to dicots and rest of them are monocots.

C₄ plants represent about 5% of Earth’s plant biomass and 1% of its known plant species. Despite this scarcity, they account for about 30% of terrestrial carbon fixation. Increasing the proportion of C₄ plants on earth could assist biosequestration of CO₂ and represent an important climate change avoidance strategy.

C₄ pathway is completed in two phases, first phase takes place in stroma of mesophyll cells, where the CO₂
acceptor molecule is 3-Carbon compound, phosphoenol pyruvate (PEP) to form 4-carbon Oxalo acetic acid (OAA). The first product is a 4-carbon and so it is named as C₄ cycle.

oxalo acetic acid is a dicarboxylic acid and hence this cycle is also known as dicarboxylic acid pathway (Figure 13.18). Carbon dioxide fixation takes place in two places one in mesophyll and another in bundle sheath cell (di carboxylation pathway).

It is the adaptation of tropical and sub tropical plants growing in warm and dry conditions. Fixation of CO₂ with minimal loss is due to absence of photorespiration. C₄ plants require 5 ATP and 2 NADPH + H⁺ to fix one molecule of CO₂.

Stage: I Mesophyll Cells

Oxaloacetic acid (OAA) is converted into malic acid or aspartic acid and is transported to the bundle sheath cells through plasmodesmata.

Stage: II Bundle Sheath Cells

Malic acid undergoes decarboxylation and produces a 3 carbon compound Pyruvic acid and CO₂. The released CO₂ combines with RUBP and follows the calvin cycle and finally sugar is released to the phloem. Pyruvic acid is transported to the mesophyll cells.

Significance of C₄ Cycle

1. Plants having C₄ cycle are mainly of tropical and sub-tropical regions and are able to survive in environment with low CO₂ concentration.
2. C₄ plants are partially adapted to drought conditions.
3. Oxygen has no inhibitory effect on C₄ cycle since PEP carboxylase is insensitive to O₂.
4. Due to absence of photorespiration, CO₂ Compensation Point for C₄ is lower than that of C₃ plants (C₄ Cycle) are given in table 13.4

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Dark Reaction or Cycle or Biosynthetic Phase or Photosynthetic Carbon Reduction (PCR) Cycle

Biosynthetic phase of photosynthesis utilises assimilatory powers (ATP and NADPH + H⁺) produced during light reaction are used to fix and reduce carbon dioxide into carbohydrates. This reaction does not require light. Therefore, it is named Dark reaction. Ribulose 1, 5 bisphosphate (RUBP) act as acceptor molecule of carbon dioxide and fix the CO₂ by RUBISCO enzyme.

The first product of the pathway is a 3 – carbon compound (Phospho Glyceric Acid) and so it is also called as C₃ Cycle. It takes place in the stroma of the chloroplast.

M. Melvin Calvin, A.A. Benson and their co-workers in the year 1957 found this path way of carbon fixation. Melvin Calvin was awarded Nobel Prize for this in 1961 and this pathway named after the discoverers as Calvin-Benson Cycle. Dark reaction is temperature dependent and so it is also called thermo-chemical reaction.

Dark reaction consists of three phases: (Figure 13.16).

1. Carboxylation (fixation)
2. Reduction (Glycolytic Reversal)
3. Regeneration

Phase 1 – Carboxylation (Fixation)

The acceptor molecule Ribulose 1, 5 Bisphosphate (RUBP) a 5 carbon compound with the help of RUBP carboxylase oxygenase (RUBISCO) enzyme accepts one molecule of carbon dioxide to form an unstable 6 carbon compound. This 6C compound is broken down into two molecules of 3-carbon compound phospho glyceric acid (PGA) (Figure 13.17).

Phase 2 – Glycolytic Reversal / Reduction

Phospho glyceric acid is phosphorylated by ATP and produces 1, 3 bis phospho glyceric acid by PGA kinase. 1, 3 bis phospho glyceric acid is reduced to glyceraldehyde 3 Phosphate (G-3-P) by using the reducing power NADPH + H⁺. Glyceraldehyde 3 phosphate is converted into its isomeric form di hydroxy acetone phosphate (DHAP).

Phase 3 – Regeneration

Regeneration of RUBP involves the formation of several intermediate compounds of 6-carbon, 5-carbon,4-carbon and 7- carbon skeleton. Fixation of one carbon dioxide requires 3 ATPs and 2 NADPH + H⁺, and for the fixation of 6 CO₂ requires 18 ATPs and 12 NADPH + H⁺ during C₃ cycle. One 6 carbon compound is the net gain to form hexose sugar.

Overall Equation for Dark Reaction:
6CO₂ + 18ATP + 12NADPH + H⁺ → C₆H₁₂O₆ + 6H₂O + 18Pi + 12NADP⁺

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Photophosphorylation

Phosphorylation taking place during respiration is called as oxidative phosphorylation and ATP produced by the breakdown of substrate is known as substrate level phosphorylation. In this topic, we are going to learn about phosphorylation taking place in chloroplast with the help of light. During the movement of electrons through carrier molecules ATP and NADPH + H⁺ are produced.

Phosphorylation is the process of synthesis of ATP by the addition of inorganic phosphate to ADP. The addition of phosphate here takes place with the help of light generated electron and so it is called as photophosphorylation. It takes place in both cyclic and non-cyclic electron transport.

Cyclic Photophosphorylation

Cyclic photophosphorylation refers to the electrons ejected from the pigment system I (Photosystem I) and again cycled back to the PS I. When the photons activate P700 reaction centre photosystem II is activated. Electrons are raised to the high energy level.

The primary electron acceptor is Ferredoxin Reducing Substance (FRS) which transfers electrons to Ferredoxin (Fd), Plastoquinone (PQ), cytochrome b6-f complex, Plastocyanin (PC) and finally back to chlorophyll P700 (PS I).

During this movement of electrons Adenosine Di Phosphate (ADP) is phosphorylated, by the addition of inorganic phosphate and generates Adenosine Tri Phosphate (ATP). Cyclic electron transport produces only ATP and there is no NADPH + H⁺ formation.

At each step of electron transport, electron loses potential energy and is used by the transport chain to pump H⁺ ions across the thylakoid membrane. The proton gradient triggers ATP formation in ATP synthase enzyme situated on the thylakoid membrane.

Photosystem I need light of longer wave length (> P700 nm). It operates under low light intensity, less CO₂ and under anaerobic conditions which makes it considered as earlier in evolution (Figure 13.13).

Non-Cyclic Photophosphorylation

When photons are activated reaction centre of pigment system II(P680), electrons moved to the high energy level. Electrons from high energy state passes through series of electron carriers like pheophytin, plastoquinone, cytochrome complex, plastocyanin and finally accepted by PS I (P700). During this movement of electrons from PS II to PS I ATP is generated (Figure 13.16).

PS I (P700) is activated by light, electrons are moved to high energy state and accepted by electron acceptor molecule ferredoxin reducing Substance (FRS). During the downhill movement through ferredoxin, electrons are transferred to NADP⁺ and reduced into NADPH + H⁺ (H⁺ formed from splitting of water by light).

Electrons released from the photosystem II are not cycled back. It is used for the reduction of NADP⁺ into NADPH + H⁺. During the electron transport it generates ATP and hence this type of photophosphorylation is called non-cyclic photophosphorylation. The electron flow looks like the appearance of letter ‘Z’ and so known as Z scheme.

When there is availability of NADP⁺ for reduction and when there is splitting of water molecules both PS I and PS II are activated (Table 13.3). Non-cyclic electron transport PS I and PS II both are involved co operatively to transport electrons from water to NADP⁺ (Figure 13.14).

Bio Energetics of Light Reaction

• To release one electron from pigment system it requires two quanta of light.
• One quantum is used for transport of electron from water to PS I.
• Second quantum is used for transport of electron from PS I to NADP⁺
• Two electrons are required to generate one NADPH + H⁺
• During Non-Cyclic electron transport two NADPH + H⁺ are produced and it requires 4 electrons.
• Transportation of 4 electrons requires 8 quanta of light

Chemiosmotic Theory

Chemiosmosis theory was proposed by P. Mitchell (1966). According to this theory electrons are transported along the membrane through PS I and PS II and connected by Cytochrome b6-f complex. The flow of electrical current is due to difference in electrochemical potential of protons across the membrane.

Splitting of water molecule takes place inside the membrane. Protons or H⁺ ions accumulate within the lumen of the thylakoid (H⁺ increase 1000 to 2000 times). As a result, proton concentration is increased inside the thylakoid lumen. These protons move across the membrane because the primary acceptor of electron is located outside the membrane.

Protons in stroma less in number and creates a proton gradient. This gradient is broken down due to the movement of proton across the membrane to the stroma through CFO of the ATP synthase enzyme. The proton motive force created inside the lumen of thylakoid or chemical gradient of H⁺ ion across the membrane stimulates ATP generation (Figure 13.15).

The evolution of one oxygen molecule (4 electrons required) requires 8 quanta of light. C₃ plants utilise 3 ATPs and 2 NADPH + H⁺ to evolve one Oxygen molecule. To evolve 6 molecules of Oxygen 18 ATPs and 12 NADPH + H⁺ are utilised. C₄ plants utilise 5 ATPs and 2 NADPH + H⁺ to evolve one oxygen molecule. To evolve 6 molecules of Oxygen 30 ATPs and 12 NADPH + H⁺ are utilised.