NEET Biology Notes Respiration in Plants
Gaseous Exchange in Plants
Each part of the plant takes part in gaseous exchange. Roots, stems and leaves all respire but at lower rates than the animals.
During photosynthesis, plants require large amount of gases exchange. For 1 this, purpose each leaf is well adapted. Leaves have plenty of stomata to carry out gas exchange. When in cells photosynthesis occur, availability of 02 is abundant, since, 02 is released from the cell.
In thick woody stems and roots, the living cells are organised in thin layers inside and beneath the bark. They also have openings called lenticels.
Respiration
It is a catabolic process of oxidation-reduction reaction, in which the complex organic food materials are broken down to form simpler end products with the
stepwise release of energy and carbon dioxide.
The breaking of C—C bond of complex compounds through oxidation with in the cells, leading to release of considerable amount of energy is called respiration.
The compounds that are oxidised during this process are known as respiratory substrates.
Carbohydrates are mainly used to release energy, but proteins, fats and even
organic acids can be used as respiratory substances in some plants, under certain conditions. When fats and carbohydrate are the respiratory substrates in respiration, it is called floating respiration. During oxidation within a cell, the energy is released in a series of slow stepwise reactions controlled by enzymes and trapped as chemical energy in the form of ATP.
ATP acts as the energy currency in the cell. This energy is utilised in various energy-requiring processes of the organisms and the carbon skeleton produced during respiration is used as precursors for biosynthesis of other molecules in the cell.
Cellular Respiration
It is the process, in which the chemical energy stored in a glucose molecule is released by oxidation.
The respiration, which occurs in. the presence of oxygen (02) is called aerobic respiration and the respiration, which the occurs in absence of oxygen is called anaerobic respiration.
During aerobic respiration, AT4!5 are formed in mitochondria that is why mitochondrion is known as Power house of the cell. ATP is known as universal energy carrier or the energy currency of cells. Hydrolysis of ATP releases 30.6 kj energy for every mole of ATP. ATPase enzyme catalyses the condensation reaction of ATP.
Cells make ATP through autotrophic metabolism by photophosphorylation and through heterotrophic metabolism by substrate level phosphorylation and oxidative phosphorylation.
Anaerobic Respiration
It is an enzyme mediated energy liberating stepwise catabolic process. The incomplete breakdown of organic substrates occurs without using oxygen as an oxidant. H20 is not produced as end product in this process.
Efficiency of anaerobic respiration is very low as out of 686 kcal of energy from a glucose molecule, only 14.6 kcal (2ATP = 2 x 7.3 kcal) energy is produced. The process of anaerobic respiration takes place completely in cytoplasm only.
In microorganisms, the term ‘anaerobic respiration’ is replaced by fermentation (Cruickshanic; 1897), which is known after the name of its major product, e.g., alcoholic fermentation and lactic acid fermentation. Glycolysis is common to both aerobic and anaerobic respiration.
Pyruvic acid formed in glycolysis is then transformed anaerobically into different products depending upon the enzyme present in the microorganism. Anaerobic respiration can be shown as Decarboxylation
Here, both NADH + H+ molecules are used in reduction of acetaldehyde hence, net ATP production is only 2.
Fermentation
It is similar to anaerobic respiration. The difference is that substrate breakdown in fermentation is extracellular, whereas it is intracellular in anaerobic respiration. The two most common types of fermentation are alcoholic fermentation and lactic acid fermentation.
- Alcoholic Fermentation
It is common in yeast (Saccharomyces). Yeast cells release enzymes in surrounding medium and the breakdown of the substrate takes place outside the cell. Here, also end product is ethyl alcohol, C02 and energy. Ethyl alcohol is end product of alcoholic fermentation.
- Lactic Acid Fermentation
In lactic acid fermentation, lactic acid is end product and carried out by lactic acid bacteria. These bacteria can ferment lactose sugar formed in milk. Enzyme involved in this fermentation is Lactic Dehydrogenase (LDH), which is produced and released by the bacterium. Ethyl alcohol is end product of alcoholic fermentation and lactic acid in lactic acid fermentation.
Glycolysis
The scheme of glycolysis was given by Gustav Embden, Otto Meyerhof and J Parnas and is also called as EMP pathway. This process is common in both aerobic and anaerobic organisms.
Steps of Glycolysis
- It occurs in cytoplasm of the cell. In glycolysis, glucose undergoes partial oxidation to form two molecules of pyruvic acid.
- Glucose and fructose are phosphorylated to give rise to glucose-6-phosphate by the activity of hexokinase. The sequential steps of glycolysis are given ahead Glucose (6C)
Pyruvic acid is the key product of glycolysis. This pyruvic acid can undergo in three major pathways depending on different cells. These are lactic acid fermentation, alcoholic fermentation and aerobic respiration as discussed earlier in this chapter. Fermentation occurs in many prokaryotes and unicellular eukaryotes.
For complete oxidation of glucose to C02 and H20, organisms follow Krebs’ cycle, as well as electron transport chain. It is also called as aerobic respiration because needs oxygen.
Aerobic Respiration
The crucial events of aerobic respiration are
- The complete oxidation of pyruvate by the stepwise removal of all the hydrogen atoms, leaving three molecules of C02.
- The passing on of the electrons removed from hydrogen atoms and transferred to molecular 02 with simultaneous synthesis of ATP.
- The first process occurs in matrix of mitochondria, while the second process occurs on the inner membrane of mitochondria. Pyruvate, the product of glycolysis undergoes oxidative decarboxylation by a complex set of reactions catalysed by pyruvic dehydrogenase.
Oxidative Decarboxylation of Pyruvic Acid
- The acetyl Co-A then enters a tricarboxylic acid cycle or Krebs’ cycle.
- In the presence of sufficient Oz, each three carbon- pyruvate molecules (CH3COCOOH) enters in the mitochondrial matrix where its oxidation is completed by aerobic means.
- This reaction is also called as the transition reaction or link reaction between glycolysis and Krebs’ cycle.
Krebs’ Cycle or Tricarboxylic Acid (TCA) Cycle The TCA cycle starts with the condensation of acetyl group with Oxaloacetic Acid (OAA) and water to yield citric acid.
Steps of Krebs’ Cycle
The complete steps of TCA cycle are given in the following figure
The Summary For This Phase Of Respiration Is Given as
In Krebs’ cycle, glucose has been broken down to release CO 2 and eight molecules of NADH + H+ two of FADH2 and just two molecules of ATP
Electron Transport Chain (ETS)
Electron Transport Chain (ETC) or Respiratory Chain (RC) is present in the inner membrane of mitochondria. When the electron pass from one carrier to another in electron transport chain, they are coupled to ATP synthase for the production of ATP from ADP and inorganic phosphate (Pi). A diagrammatic representation of electron flow via various electron carrier complexes is shown in figure.
The enzymes of inner membrane appear to exist as components of these five complexes. The first four members among these complexes constitute the electron transport system, while the 5th complex is connected with oxidative phosphorylation, i.e. conservation and transfer of energy with ATP synthesis.
Oxidative Phosphorylation
The following steps in the respiratory process are to release and utilise the energy stored in NADH + H+ and FADH2. This is possible when they are oxidised through the electron transport system and the electrons are passed onto 02 resulting in the formation of H20. The metabolic pathway through, which the electron passes from one carrier to another is called Electron Transport System (ETS) and it is present in the inner mitochondrial membrane. Unlike photophosphorylation, where it is the light energy that is utilised for the production of proton gradient required for phosphorylation, in respiration it is the energy of oxidation-reduction utilised for the same process. It is for this reason, the process is called oxidative phosphorylation.
Steps of Electron Transport System
Step I 2NADH + 2H+ from glycolysis yields 4 ATP via route 2 of ETC (glycerol phosphate shuttle) or 6 ATP via route 1 (malate-aspartate shuttle).
Step II 2NADH + 2H+ from pyruvate oxidation yields 6ATP molecules in route 1 of ETC.
Step III 6NADH + 6H+molecules from TCA (Krebs’ cycle) yield 18 ATP molecules in route 1 of ETC.
Step IV 2FADH2 molecules from TCA cycle yield 4 ATP molecules in route 2 of ETC. Hence, ETS alone produces 32 or 34 ATP.
The presence of oxygen is vital, since it derives the whole process by removing hydrogen from the system. Oxygen acts as the final hydrogen acceptor.
Amphibolic Pathway
All carbohydrates are first converted to glucose before they are used for respiration. Fats would need to be broken down into glycerol and fatty acids first. Proteins would be degraded by proteases and the individual amino acids enters the pathway at some stage within the Krebs’ cycle or even as pyruvate or acetyl Co-A.
The respiratory pathway comes in way both during breakdown and synthesis of fatty acids. Similarly, during breakdown and synthesis of protein too, respiratory intermediates form the link.
Breaking down processes in the living organism is catabolism and synthesis is anabolism.
Because the respiratory pathway is involved in both anabolism and catabolism, it would hence be considered the respiratory pathway as an amphibolic pathway rather than as a catabolic one.
Energy Relations
2ATP from glycolysis 4- 2GTP from TCA cycle and 32/34 ATP from ETS/ETC =38/36 ATP molecules (34 or 36 ATP + 2 GTP) are produced from one glucose molecule. Generally, the distinction between ATP and GTP is not made, while giving the total yield of glucose respiration. A cytoplasmic enzyme, nucleoside diphosphate kinase readily converts the GTP formed in TCA cycle to ATP.
In prokaryotic cells, oxidation of glucose molecule always yields 2 ATP molecules as NADH + H+ is not to enter mitochondria, which are absent here. Overall result of aerobic respiration during complete oxidation of one molecule of glucose is
- Release of 6C02
- Utilisation of 602
- Formation of 12H20
The last step of aerobic respiration is oxidation of reduced coenzymes, i.e. NADH2 and FADH2 by molecular oxygen through FAD, Co-Q (ubiquinone).
Respiratory Quotient (RQ)
The ratio of the volume of C02 evolved to the volume of 02 consumed in respiration is called Respiratory Quotient (RQ) or respiratory ratio.
The complete combustion of glucose, produces C02 and H20 and end products, yields energy most of which is given out as heat.
The glucose molecule is oxidised in several small steps, i.e. glycolysis, Krebs’ cycle and electron transport chain.
Some conditions of RQ are as follow :
(i) When carbohydrates are used as substrates and are completely oxidised, the RQ will be 1.
(ii) When fats are used in respiration, the RQ is less than 1. Calculations for a fatty acid, tripalmitin, if used as a substrate.
(iii) When proteins are respiratory substrates the ratio would be about 0.9.
(iv) It is important that in living organisms respiratory substrates are often more than one; pure proteins or fats are never used as respiratory substrates.
Factor Affecting Respiration
Various factors affecting respiration can be summarised as:
- Amount of oxygen Rate of respiration is directly proportional to amount of oxygen available.
- . Intensity of light Rate of respiration is directly proportional to intensity of light.
- Temperature The rate of respiration is directly proportional to temperature, i.e. the rise in temperature leads to increased metabolism (cellular respiration) while with lowering in temperature, its rate decreases.
- Dehydration Rate of respiration is inversely . proportional to dehydration, i.e. scarcity of moisture or water in the surroundings/body. It decreases with increase in dehydration in body.
- . Minerals Rate of respiration is directly proportional to availability of minerals as some of them play vital role in the process, i.e. acts as catalysts on reaction centres.
- Tissue injury The rate of respiration is directly proportional to tissues injury.