Plant Physiology - Respiration .. By UK sir

Cellular Respiration

-    The cells of living Organisms get energy by "burning process." They "burn" glucose in the process called cellular respiration.

-    Cellular respiration is a process that all living things use to convert glucose into energy.

-    It is of Two types

1.       Aerobic Respiration (Completely oxidized with the help of Oxygen)

2.       Anaerobic Respiration (Incomplete break down without the use of Oxygen)

Compensation Point: Rate of Photosynthesis is equivalent to rate of Respiration.

Respiratory Quotient: Ratio of volume of Carbon dioxide produced to the volume of oxygen consumed in Respiration process in particular time period.

-          If RQ = 1, then Respiration is Aerobic and respiratory substrate is Carbohydrate. (6CO₂/ 6O₂ )

-          If RQ < 1, then Respiration is Aerobic and respiratory substrate is Fat/ Protein. (0.7/0.9)

-          If RQ >1, then Respiration is Aerobic and respiratory substrate is Organic Acids. (Ex- in Oxalic acid break RQ= 4.0 )

-          If RQ = 0, then Respiration is in Succulent plants, which do not produce CO2 during night (Special case  )

1. Aerobic Respiration

Reaction:

C₆H₁₂O₆+ 6O₂ ------------------à 6CO₂+ 6H₂O+ Energy (~686 Kcal)

                                     

-    During the process of glycolysis, glucose is oxidized to carbon dioxide and water.

-    Energy released during the reaction is captured by the energy-carrying molecule ATP.

-    Cellular respiration is a collection of three unique metabolic pathways: Glycolysis, The Citric acid cycle (TCA cycle) and The Electron transport chain (oxidative phosphorylation). 

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Glycolysis is an anaerobic process, while the other two pathways are aerobic.

i) Glycolysis (Common pathway)

-       Glycolysis (which is also known as the glycolytic pathway or the Embden -Meyer hof -Parnas pathway.

-       This pathway is anaerobic and takes place in the cytoplasm of the cell. 

-       This pathway breaks down 1 glucose molecule and produces 2 pyruvate molecules. 

-       It is a sequence of 10 chemical reactions taking place in most cells .

-       There is two halves of glycolysis, with five steps in each half.

-       The first half is known as the “energy requiring” steps. This half splits glucose, and uses up 2 ATP.

-       In the second half, the “energy releasing: steps, 4 molecules of ATP and 2 NADH₂ are released. 

-       Through the process reduction, electrons are transferred to 2 NAD+ (nicotinamide adenine diphosphate) to produce two molecules of NADH₂.

-       Pyruvate molecules produced during glycolysis then enter the mitochondria.

-       Where they each converted into a compound known as Acetyl coenzyme A.

-       Which then enters the TCA cycle

-       Glycolysis has a net gain of  2 ATP molecules and 2 NADH₂. (Total 8 ATP)

 

 

Pyruvate Oxidation

-          In eukaryotes, pyruvate oxidation takes place in the mitochondria.

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          Pyruvate oxidation can only happen if oxygen is available.

-          In this process, the pyruvate created by glycolysis is oxidized

-          This process include Pyruvate dehydrogenase enzyme and also releases NADH₂ and CO₂.

ii) Tri carboxylic acid cycle

-          The TCA cycle (which is also known as the Krebs cycle or citric acid cycle) plays a central role in the breakdown or catabolism of organic fuel molecules.

-          The Krebs cycle is the second pathway in cellular respiration, and it also takes place in the mitochondria.(in the matrix)

-         


This pathway is a closed loop: the final step produces the compound needed for the first step. (for which called as cycle)

-          The cycle is made up of eight steps catalyzed by eight different enzymes that produce energy at several different stages.

-          Most of the energy obtained from the TCA cycle, however, is captured by the compounds NAD+ ( Nicotinamide adenine dinucleotide) and Flavin adenine dinucleotide (FAD) and converted later to ATP.

-          The citric acid cycle provides a net gain of 2 CO₂, 2 GTP or ATP, and 6 NADH₂ and 2 FADH₂.

-          These molecules go on to fuel the third stage of cellular respiration

-          Whereas carbon dioxide, which is also produced by the TCA cycle, is released as a waste product.

iii) Oxidative phosphorylation

-          The third stage, electron transport, takes place on the inner membrane.

-          In the oxidative phosphorylation stage, each pair of hydrogen atoms removed from NADH₂ and FADH₂ provides a pair of electrons

-          That electron moves through the action of a series of iron-containing hemoproteins, the cytochromes and some carrier proteins.

-          Eventually they reduce one atom of oxygen to form water.

-          However transfer of one pair of electrons from NADH₂ to oxygen results in the formation of three molecules of ATP.

Electron transport chain

-          This is the series of steps by which electrons flow to oxygen permits a gradual lowering of the energy of the electrons. This part of the oxidative phosphorylation stage is sometimes called the electron transport chain.

Chemiosmotic Hypothesis:

-          The pumping of hydrogen ions across the inner membrane creates a greater concentration of these ions in the inter membrane space than in the matrix – producing an electrochemical gradient.

-          This gradient causes the ions to flow back across the membrane into the matrix, where their concentration is lower.

-          The flow of these ions occurs through a protein complex, known as the ATP synthase complex.

-          The ATP synthase acts as a channel protein, helping the hydrogen ions across the membrane.

-          It also acts as an enzyme, forming ATP from ADP and inorganic phosphate.

-          It is the flow of hydrogen ions through ATP synthase that gives the energy for ATP synthesis.

 

Total Number of ATP ?

Glycolysis	2 ATP	2 NADH2 = 6 ATP	2 +6 = 8 ATP
Decarboxylation of Pyruvate	-----	2 NADH2 = 6 ATP	6 ATP
Kreb’s Cycle	2 ATP/GTP	6 NADH2 = 18 ATP 2 FADH2 = 4 ATP	2 + 22 = 24 ATP
			Total =  38 ATP

2 ATP utilized to pump 2 NADH₂ synthesized during Glycolysis.

So The net gain is considered as 36 ATP in Eukaryotes.

 

2) Anaerobic Respiration:

-          Anaerobic respiration is the type of respiration through which cells can break down sugars to generate energy in the absence of oxygen.

-          In both anaerobic and aerobic cellular respiration, electrons extracted from a fuel molecule are passed through an electron transport chain, to produce ATP.

-          Some organisms use sulfate as the final electron acceptor at the end of the transport chain, while others use nitrate, sulfur, or one of a variety of other molecules.

-          Some bacteria and archaea, that live in low-oxygen environments dependent on anaerobic respiration to break down fuels.

-          For example, some archaea called methanogens can use carbon dioxide as a terminal electron acceptor, producing methane as a by-product.

-          Methanogens are found in soil and in the digestive systems of ruminants, a group of animals including cows and sheep.

-          Similarly, sulfate-reducing bacteria and Archaea use sulfate as a terminal electron acceptor, producing hydrogen sulfide (H₂​S) as a byproduct.

Fermentation

-          Fermentation is another anaerobic (non-oxygen-requiring) pathway for breaking down glucose, performed by many types of organisms and cells.

-          In fermentation, the only energy extraction pathway is glycolysis, with one or two extra reactions for the end product.

-          Glycolysis is common pathway. But, in fermentation, the pyruvate made in glycolysis does not continue through oxidation and the citric acid cycle.

-          Here the electron transport chain does not run. 

A) Lactic acid fermentation

-          In lactic acid fermentation, NADH transf
ers its electrons directly to pyruvate, generating lactate as a byproduct.

-          The bacteria that make yogurt carry out lactic acid fermentation.

-          Occurs in Lactobacillus, in some fungi and Muscles

-          Muscle cells also carry out lactic acid fermentation, when they have too little oxygen for aerobic respiration to continue.

Lactic acid produced in muscle cells is transported through the bloodstream to the liver, where it’s converted back to pyruvate and processed normally in the remaining reactions of cellular respiration.

B) Alcohol fermentation

-          Another fermentation process is alcohol fermentation, in which NADH donates its electrons to a derivative of pyruvate, producing ethanol.

-          It is common in Fungus (Like Rhizopous, yeast) and Bacteria.

-          Going from pyruvate to ethanol is a two-step process.

-          In the first step, a carboxyl group is removed from pyruvate and released in as carbon dioxide, producing a two-carbon molecule called acetaldehyde.

-          In the second step, NADH passes its electrons to acetaldehyde, regenerating NAD+ and forming ethanol.

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          Alcohol fermentation by yeast produces the ethanol found in alcoholic drinks like beer and wine. 

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