CBSE Class 11 Biology Revision Notes Chapter 14

CBSE Class 11 Biology Revision Notes Chapter 14 – Respiration in Plants

This chapter discusses respiration in plants, types of respiration, glycolysis, fermentation and many more important topics. Students can refer to the revision notes for this chapter prepared by Extramarks to better comprehend the topics of this chapter. With the help of these Chapter 14 Biology Class 11 notes, students can get a thorough explanation of all topics covered in the chapter and also get their conceptual doubts cleared.

Access Class 11 Biology Chapter 14- Respiration in Plants

Some of the important topics covered in the chapter and in the revision notes by Extramarks are listed below.

14.1 Do Plants Breathe?

Plants are quite similar to the other living organisms on earth. As a result, plants breathe. But, the process of breathing in the case of plants is quite different from that of animals. Unlike animals, plants do not have organs that will help them in breathing. But they have organelles which are known as stomata and lenticels. These tiny openings or spores help plants breathe. 

Types of Respiration:

Respiration in plants can be categorized into two different types based on the presence of oxygen during cellular respiration. These are known as aerobic and anaerobic Respiration.

Aerobic Respiration:

Aerobic Respiration forms water and carbon dioxide using raw materials, including oxygen. This usually occurs in the cell’s mitochondria with an energy release of almost 2870 KJ. 

There are two stages of aerobic respiration. One is glycolysis, and the other one is the citric acid cycle. This was first discovered in the striated muscles of insects by Kollicker. The word “mitochondria” was given by C.Benda. 

The mitochondria are known as the powerhouse of the cell. It is the site for cellular respiration. Hoyeboom proposed this for the first time. Mitochondria consist of DNA, RNA, and proteins, along with ribosomes. They are known as semi-autonomous organelles. Usually, these appear on the inner parts of the eukaryotic cells. These cells have a symbiotic relationship, so they are known as endosymbionts. 

14.2 Glycolysis:

Glycolysis is the first step in the case of both aerobic and anaerobic forms of respiration. When the glucose undergoes oxidation, it forms pyruvic acid. It needs a series of reactions catalyzed by the presence of an enzyme. 

It is also known as the EMP pathway based on the biologist who discovered it. The names of these scientists are Gustav Embden, Otto Meyerhof and J.Paranas. 

Glycolysis forms two different molecules of pyruvic acid after the process of oxidation. The two major phases are :

1. Preparatory phase and cleavage
2. Oxidative and payoff phase

 Let us look closely at the steps involved in both of these phases. 

1. Phosphorylation of glucose needs energy in the form of ATP and hexokinase enzymes, leading to the formation of glucose 6 phosphatases. 
2. The conversion of glucose 6 phosphate into fructose 6 phosphate, which is the isomeric form of the other one, is done with the help of an enzyme known as phosphoglucose isomerase. 
3. The remaining steps involved in glucose and fructose metabolism are all the same. 
4. The one molecule of ATP helps the fructose 6 Phosphate convert into fructose 1,6 bisphosphate. 
5. The Fructose 1,6 bisphosphate is then broken down into parts, forming two molecules of 3 carbon compounds. These are glyceraldehyde phosphate and dihydroxyacetone phosphate. With the help of aldolase enzymes, it is possible to catalyze as both compounds can be converted. 
6. Glyceraldehyde 3 phosphate is formed by the isomerization of Dihydroxyacetone phosphate, which leads to the production of two molecules of glyceraldehyde 3 phosphates. 
7. Each molecule converts to triose phosphate with the help of oxidation, known as 1,3 bisphosphoglyceric acid. It will produce two electrons and two protons, of which one proton and two electrons are used to reduce. 
8. Every molecule of 1,3 bisphosphoglyceric acid is converted to 3 phosphogylceric acids, and the ATP molecule is released. In this step of ATP generation, the substrates group and other metabolites are added to ADP. This shall result in the formation of ATP. This is known as the substrate level synthesis of substrate-level phosphorylation. The ATP synthesis that takes place in the chloroplast is quite different from the one which takes place in mitochondria. 
9. Every 3 phosphoglyceric acids are converted to form 2 phosphoglycerates.
10. Now every 2 phosphoglycerides are converted into 2 phosphoenolpyruvate, leading to water molecules and ATP production. 
11. Every phosphoenol pyruvate converts into pyruvic acid because of the enzymes known as pyruvate kinase. In this step, the phosphate group will convert directly to ATP from ADP with the release of ATP molecule, which is also a substrate level ATP synthesis. 

The ATP net gain will be 8 instead of 2. So, during glycolysis, two pyruvic acid molecules and eight molecules of ATP are produced. 

The main product formed during glycolysis is pyruvic acid (pyruvate). The metabolic fate of pyruvic acid will take place in three ways that depend on oxygen’s presence. It is the basic need of an organism. 

14.3 Fermentation:

Fermentation is the process which occurs in the absence of oxygen and that involves incomplete oxidation of food materials. It leads to the formation of CO2. 

The production of CO2 and ethanol as end products is known as anaerobic respiration, fermentation, or zymosis. This process takes place with the help of yeast. Gay Lussac was the first one to discover the fermentation process, while Shank was the one to name it “fermentation”. 

Fermentation takes place in different organisms, including prokaryotes, unicellular eukaryotes, and seeds germinate in anaerobic conditions. Fermentation is usually of two types: 

1. Alcoholic fermentation
2. Lactic acid fermentation

Alcoholic Fermentation:

Alcoholic fermentation forms carbon dioxide and ethanol. There are two steps. The first one is glycolysis, which forms pyruvic acid from glucose molecules. It then gets converted and forms carbon dioxide and ethanol. The two steps are: 

1. Conversion of Pyruvic acid into carbon dioxide and acetaldehyde with the help of an enzyme known as pyruvic and decarboxylase. 
2. An enzyme called alcohol dehydrogenase, along with coenzymes, leads to the acetaldehyde process that forms ethanol and carbon dioxide. In this process, 8 ATP molecules are released. 

In animals, anaerobic respiration occurs in skeletal muscles that use energy in anaerobic respiration.

Lactic Acid Fermentation:

The process results in the formation of lactic acid and is used in the milk industry. It is used in the muscle cells of vertebrates as well. 

It is the curd formation due to the presence of a bacteria known as Lactobacillus, which results in the formation of NAD+. No carbon dioxide is produced in this case. This reaction occurs due to the enzyme called dehydrogenase enzyme. The total net gain of ATP is two. 

14.4 Aerobic respiration:

During glycolysis, the end product produced is pyruvic acid, which is a 3-carbon compound. It usually occurs in the cytoplasm of the cell. 

The steps involved in this process are:

1. Pyruvic acid is oxidized completely by removing hydrogen atoms and that too in a step-by-step manner. 
2. After the production of ATP, the electrons will move towards oxygen molecules, and usually, this process takes place in the inner mitochondrial membrane of the cell. 
3. The molecules of carbon dioxide are removed, which occurs in the mitochondrial matrix of the cell. 

When the pyruvic acid enters the mitochondria, it takes part in the citric acid cycle, and the oxidation produces carbon dioxide. This process is known as oxidative decarboxylation. 

In this process, the pyruvic acid first undergoes decarboxylation and is then oxidized with pyruvate dehydrogenase. The remaining pyruvic acid molecules will combine with coenzyme A and will form acetyl coenzyme in the presence of magnesium. Coenzyme  A is a sulfur-containing compound, and acetyl CoA acts as the connecting link between glycolysis and the citric acid cycle. 

The aerobic oxidation of pyruvic acid is named the link reaction, and its result is in the form of NADH. The reduction of NAD+ obtains this. The process of glycolysis produces two molecules of pyruvic acid from one glucose molecule after oxidation during aerobic oxidation. Thus two molecules of NADH are formed. Hence the net energy gain is 6 molecules of ATP. 

14.4.1 Tricarboxylic Acid Cycle:

The cycle involves the formation of carbon dioxide and water after the complete oxidation process of pyruvic acid. It usually occurs in a stepwise series reaction and needs oxygen. This usually takes place in the mitochondria of the cell. 

Hans Krebs made the formula that includes the cycle’s steps, hence it is named as Krebs cycle. The first compound of the cycle is tricarboxylic acid and citric acid. Thus the cycle is known as the citric acid cycle. It has three acids, so it is known as tricarboxylic acid. 

The respiratory substrate in the TCA cycle is acetyl coenzyme A while the 4 carbon compound is known as oxaloacetate acid. It is the acceptor molecule. This cycle has 4 dehydrogenation reactions and two decarboxylations reactions. 

The reduction of coenzymes will lead to the formation of carbon dioxide. 

1. One molecule of acetyl coenzyme A and 4 carbon oxaloacetic acids are combined to form citric acid. This reaction takes place with the help of an enzyme known as citrus synthase. The reaction needs a water molecule which results in the formation of CoA. 
2. The isomerization process occurs to produce citric acid with the help of water molecules. 
3. Dehydrogenation is the process of conversion of isocitric acid to oxaloacetic acid. During this process, NAD+ is reduced to form NAD+H+. 
4. The formation of alpha-ketoglutaric acid occurs, which is a five-carbon compound. It occurs by the process of decarboxylation of oxalosuccinic acid. 
5. Then a four-carbon compound known as succinyl CoA is formed by decarboxylation.
6. GTP will be formed along with succinic acid because of the loss of CoA from succinyl. The GTP then formed will be transferred to one of the phosphates of ADP, forming ATP.
7. The dehydrogenation process will convert succinic acid to fumaric acid, a four-carbon compound. FAD will get converted to FADH.
8. The next step is when malic acid is formed from fumaric acid after adding water molecules to it. 
9. Finally, malic acid will convert to oxaloacetic acid, and NAD+ will be reduced. 
10. The oxaloacetic acid formed will combine with acetyl CoA and cause a new cycle to begin.
11. The citric acid cycle will cause the oxidation of acetyl Co-A, which results in the replenishment of oxaloacetic acid and causes the regeneration of NAD+ and FAD+ from NADH. 
12.  During glycolysis, two molecules of pyruvic acid and only one molecule of glucose are required, along with the formation of two molecules of acetyl CoA. 

FADH2 is going to produce 2 molecules of ATP. This process is known as oxidative phosphorylation. So, the overall energy gain is 12 ATP in the overall citric acid cycle. When one molecule of glucose undergoes aerobic respiration, 38 ATP molecules are produced. 

Aerobic Respiration ATP Production

In the case of several eukaryotic cells, there is a need for 2 molecules of ATP to help transfer NADH into mitochondria produced during the glycolysis process. It will then again undergo oxidation. So, the total gain of energy will become 36 molecules of ATP. 

This will release 45% of the energy stored in the 38 molecules of ATP for oxidizing one molecule of glucose. In contrast, the remaining energy is lost as heat during aerobic respiration. 

14.4.2 Electron Transport System and Oxidative Phosphorylation:

In dehydrogenation, electrons and hydrogen ions are removed from the substrates, including glyceraldehyde 3 phosphates, pyruvic acid, alpha-ketoglutaric acid, succinic acid, malic acid, and isocitric acid during aerobic respiration. This results in the removal of energy. The removed hydrogen ions and electrons will be combined with coenzymes like NAD+ and FAD. This will form NADH and FADH2.

The energy released from electrons gets stored in the bonds formed between NADH and H and FAD and H. 

When NADH and FADH₂ go through oxidation, the electrons of the hydrogen atoms are transferred with the help of various kinds of electron carriers to the oxygen, which is arranged in a particular order known as the electron transport chain. It is also named as mitochondrial respiratory chain or electron transport system.

The electron transport system is placed in the inner mitochondrial membrane. Every member of ETS individually will be known as electron carriers: Flavin, protein, quinines, and cytochromes. Flavin is FMN, FeS is iron-sulfur protein, and quinones are present in the membrane. There are mobile electron carriers, while ubiquinone is a common kind of quinine and is a phenolic compound. 

Lastly, the cytochromes act as both enzymes as well as electron carriers. The cytochromes present in ETS are Cyt c1, Cyt c, Cyt a and Cyt a3. These all contain iron which works as an activator, except Cyt a3, which contains copper and iron. 

Download CBSE Class 11 Biology Chapter 14 Notes 

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