Respiration In Plants

The ultimate electron acceptor of respiration in aerobic organisms is oxygen because in, the electron transport chain, oxygens act as the final electron acceptor and it is a highly oxidizing agent, and therefore it is a dominant electron acceptor. The other options are incorrect as they are not as highly oxidizing like oxygen.

Phosphorylation of glucose during glycolysis is catalyzed by hexokinase. The six carbon glucose molecule is phosphorylated with the presence of ATP to form a glucose-6- phosphate enzyme. Hexokinase enzyme is used to catalyze this reaction. The other options are not involved in phosphorylation of glucose during glycolysis.

Pyruvic acid is the key product of glycolysis, which can have many metabolic fates. Under the aerobic condition, it forms Acetyl CoA + CO₂ which is further converted to CO₂ + H₂O after citric acid cycle. The other options are incorrect as they are the products for anaerobic fermentation, i.e., Lactic acid, Ethanol + CO₂ and oxidized product of Acetyl CoA in the citric acid cycle, i.e., CO₂ + H₂O.

Electron Transport System (ETS) is located in the inner mitochondrial membrane. In the cristae of mitochondria, ETS is carried out. It comprises of many carrier molecules known as cytochromes and also, other enzymes. The other options are incorrect as they are not the location of ETS.

The rate of respiration depends upon the overall metabolic activity of a particular tissue in a plant. Among the given options germinating seed has the highest metabolic activity, and therefore, it has the highest respiration rate.

• The aerobic process of respiration takes place only in the presence of oxygen, but the role of oxygen is limited to the terminal stage of the process.

• During terminal oxidation, oxygen acts as final hydrogen acceptor and combines with H⁺and forms the water.

• Pyruvate is formed by glycolytic catabolism of carbohydrates in the cytoplasm.

• In fermentation, there is incomplete oxidation of glucose under anaerobic conditions.

• During the conversion of succinyl-CoA to succinic acid, a molecule of GTP is synthesised.

Mitochondria are called powerhouses of the cell because they are responsible for the generation of most of the ATP. Thus, they are responsible for producing energy in the cell and known as the powerhouse of the cell.

After the complete oxidation of a glucose molecule, 38 ATP are formed along with H₂O, CO₂. Oxidative phosphorylation takes place in the inner mitochondrial membrane of the cell. The other options are incorrect as these products are not the end products of oxidative phosphorylation and these products take part in the overall respiration process.

A. Molecular oxygen – ii. hydrogen acceptor

The ultimate electron acceptor of respiration inaerobic organisms is oxygen because in the electron transport chain, oxygens act as the final electron acceptor and it is a highly oxidizing agent and therefore it is a powerful electron acceptor.

B. Electron acceptor – iii. Cytochrome C

The cytochrome C is a protein enzyme which is located between the inner and outer mitochondrial membrane. It is responsible for the transfer of electron and it is the major electron acceptor.

C. Pyruvate dehydrogenase – iv. acetyl CoA

Pyruvate dehydrogenase complex (PDC) is a complex of enzymes that converts the pyruvate into the acetyl-CoA by a process known as pyruvate decarboxylation.

D. Decarboxylation – i. α - Ketoglutaric acid

α - Ketoglutaric acid is a key intermediate in the citric acid cycle i.e., tricarboxylic cycle (TCA cycle). The decarboxylation oxalosuccinate forms the alpha - ketoglutaric acid in a decarboxylation reaction of the TCA cycle.

The energy which gets released during the oxidation of compounds in respiration is stored as Adenosine Triphosphate i.e., ATP. The ATP is stored in the form of chemical bonds.
ADP + IP + energy → ATP (ADP = Adenosine Diphosphate and IP = Inorganic phosphorous)
When energy is required then, this bond energy is broken and utilised
ATP → ADP + IP + energy.

Energy cannot be used directly, and the body needs to store and adequately spend the energy. The energy currency is the energy which is stored and released in living organisms. Adenosine Triphosphate, i.e., ATP acts as energy currency in plants and animals.

Respiratory Quotient (RQ) indicate which type of substrate, i.e., carbohydrate, fat or protein is getting oxidized by measuring the volume of carbon dioxide, i.e., CO₂ released or evolved by the volume of oxygen O₂ consumed. It is a unitless quantity. The ratio helps in determining the type of substrate.

A stands for volume of carbon dioxide, i.e., CO₂ released or evolved.

B stands for the volume of oxygen O₂ consumed.

RQ less than 1 = fat or protein.

RQ equal to 1 = carbohydrate like glucose.

RQ higher than 1 = substrate can be organic acid like malic acid.

The F0 – F1 particles are present in the inner mitochondrial matrix. The F1 particle is a peripheral membrane protein complex, and it has the site for the synthesis of ATP from ADP and inorganic phosphate.

Anaerobic respiration occurs in man when there is intensive exercise or stress on a muscle which leads to inadequate oxygen in the cells, the pyruvic acid gets reduced to lactic acid, and this is known as homolactic fermentation.

Anaerobic respiration occurs in yeasts when there occurs incomplete oxidation of glucose, and there isn’t any presence of oxygen, i.e., anaerobic conditions. The pyruvic acid is reduced into C₂H₅OH, i.e., Ethanol and release CO₂. This is known as alcoholic fermentation.

The caloric value of 1 gm fat = 9.45 kcal/gm

The caloric value of 1 gm protein= 5.65 kcal/gm

The caloric value of 1 gm glucose which is a carbohydrate is = 4.1 kcal/gm

And caloric value of 0.5 g of protein + 0.5g glucose = 5.65/2 + 4.1 / 2 = 9.75 / 2 = 4.875 kcal/gm

Therefore, arranging them in ascending order based on their oxidation energy:

1 gm of glucose < 0.5 g of protein + 0.5g glucose < 1 gm of protein < 1 gm of fat.

The product of glycolysis (under hypoxia) in skeletal muscle Is lactic acid while anaerobic fermentation in yeast are ethanol and carbon dioxide i.e., C₂H₅OH and CO₂.

If a person is feeling dizzy, glucose or fruit juice is given immediately but not a cheese sandwich because:

i) Glucose or fruit juice is a simpler food product which is easily absorbed and assimilated by the body and it is easily oxidized to form energy.

ii) Cheese sandwich would take time for absorption and energy conversion.

Aerobic respiration is more efficient because:

i) Aerobic respiration produces more energy as compared to anaerobic respiration. It is a high energy yielding process.

ii) Anaerobic respiration produces less energy as compared to anaerobic respiration.

Efficiency is attributed to the number of ATP produced from one molecule of glucose. In Aerobic respiration produces 38 ATP molecules from a single glucose molecule while Anaerobic respiration produces only 2 ATP molecules from a single glucose molecule. Therefore, aerobic respiration is more efficient than anaerobic respiration.

• Under the Aerobic conditions, the metabolic product of pyruvic acid is Carbon Dioxide and Water, i.e., CO2 and H2O.

• Under the Anaerobic conditions (Hypoxia) in the skeletal muscles, the metabolic product of pyruvic acid is lactic acid.

• Under the Anaerobic conditions in the anaerobic organism like yeast, the metabolic product of pyruvic acid is ethanol and carbon dioxide, i.e., C2H5OH and CO2.

Anaerobic respiration occurs in organisms that live in aerobic condition like human beings when there is intensive exercise or stress on a muscle it leads to inadequate oxygen in the cells, the pyruvic acid gets reduced to lactic acid, and this is known as homolactic fermentation. In angiosperms, the germination of seeds leads to anaerobic respiration for releasing the energy for the seedling to emerge from the soil.

Oxygen plays an essential for aerobic respiration at the end of ETC, i.e., respiratory process because it removes the hydrogen from the electron transport system. Oxygen acts as the final hydrogen acceptor in the electron transport chain. If oxygen is not present, then the electrons could not pass through the co-enzymes. Oxygen is responsible for the formation of the proton pump in the ETS process as it gives energy by the oxidation-reduction process. If oxygen is absent, then there will be no formation of proton pump and ATP will not be produced by oxidative phosphorylation.

1) Step-wise oxidative breakdown:

During cellular respiration, all the energy which is contained in the respiratory substrate is not released all at once. Instead, it is released in a step-wise manner.

The energy is released by a series of slow steps which are controlled and moderated enzymatically. It is synthesized in the chemical energy in the form of ATP. Therefore, the energy release cannot occur directly, and it follows a series of steps of catabolic breakdown only when energy is required.

2) Organic substances (used as substrates):

Organic substances like carbohydrates used as substrates and oxidized during the respiratory process for releasing energy. Organic substances like protein, fats and organic acids are used as organic respiratory substances in some organisms and plants.

Although respiration is an energy producing process but ATP is also being used in some steps of the process. For example, the phosphorylation reactions in the glycolysis process in which glucose is converted to glucose – 6 – phosphate which consumes one ATP.

Although the overall ATP produced by the complete oxidation of the substrate is more than the amount of energy spent on some metabolic processes.

The ATP is used up in step E.

The amphibolic pathway is defined as a biochemical pathway which involves both anabolic and catabolic process.

• The respiration biochemical process has been conventionally considered to be a catabolic pathway as it comprises of the breakdown of substrates. But the breakdown of substrates serves as a part of the synthesis of other substrates.

• The fatty acids when respired are broken down into Acetyl CoA. Similarly, glycerol also enters a biochemical pathway and gets broken down into PGAL (3-phosphoglyceraldehyde).

But when there is a need for synthesizing fat in an organism. The Acetyl CoA is taken from the respiratory pathway. Respiration can cause both the breakdown as well as the synthesis of fatty acids, i.e., it is involved in both catabolism and anabolism. Therefore, it is to be an amphibolic pathway.

The energy currency is the energy which is stored and released in living organisms. Adenosine Triphosphate, i.e., ATP acts as energy currency in plants and animals.

Some other energy carriers present in a cell are:

• GTP (Guanine Triphosphate)

• ADP (Adenosine Diphosphate)

The substrate level phosphorylation occurs in the pay off phase of the glycolysis. These ATP are produced during the glycolysis of substrate level phosphorylation as these are formed without the electron transport system (ETS) and chemiosmosis. The ATP is directly formed from ADP and Inorganic phosphatase.

In the TCA cycle where there is substrate level phosphorylation the step in which succinyl Co-A is converted to the succinic acid and there is one GTP molecule is synthesised through the substrate level phosphorylation.

The GTP which is formed is used for making ATP.

a. The name of the process is cellular respiration.

b. It is a catabolic process because it involves the glucose molecule breakdown.

c. Raw materials involved in the cellular respiration process are Glucose molecule and oxygen.

During cellular respiration all the energy which is contained in the respiratory substrate is not released all at once, instead it is released in a step-wise manner. The energy is released by a series of slow steps which are controlled and moderated enzymatically. It is synthesized in the chemical energy in the form of ATP. Therefore, the energy release cannot occur directly. The advantage of stepwise release of energy is that each step helps in regulation of the energy. And energy is only released when formed or when required. Energy wastage as heat is avoided and energy is stored. Thus, releasing energy in multiple steps ensures efficient use of energy by the body.

The first cells on the earth managed to survive in an atmosphere that lacked O₂ because they were anaerobes ,i.e. they did not require oxygen for respiration and instead survived by the breakdown of other inorganic compounds like H₂S etc. They survived without the presence of oxygen.

Red muscle fibres in animals can work for more extended periods continuously as it has myoglobin, which is an oxygen transporting pigment. It gets bound to the oxygen molecule and stored as oxymyoglobin. During the muscle contraction, which occurs during extended periods of continuous work, the stored oxymyoglobin releases oxygen.

o anaerobic respiration.

Efficiency is attributed to the number of ATP produced from one molecule of glucose. In Aerobic respiration produces 38 ATP molecules from a single glucose molecule while Anaerobic respiration produces only 2 ATP molecules from a single glucose molecule. Therefore, aerobic respiration is more efficient than anaerobic respiration as in anaerobic respiration is because of the partial breakdown of the glucose, whereas aerobic respiration involves the complete breakdown of glucose.

a. Enzymes used in photosynthesis are:

• RuBP

• PEP carboxylase

• ATPase

b. Enzymes used in respiration are:

• Hexokinase

• ATPase

• Pyruvate dehydrogenase

• Cytochrome oxidase

c. Enzyme used in both in photosynthesis and respiration are:

• ATPase

A tree trunk exchanges gases with the environment although it lacks the stomata because it has lenticels through which oxygen and other gas exchange occurs. They are cells consisting of large intercellular pores.

Two energy yielding reactions of glycolysis:

• Conversion of 1,3 – bisphosphoglyceric acid (1,3 – BPGA) to 3 – phosphoglyceric acid (3 – PGA).

• Conversion of 3- phosphoenolpyruvate (3 - PEP) to pyruvic acid.

The site of pyruvate synthesis is Cytoplasm in the cell.

Pyruvic acid + CoA + NAD⁺ Acetyl CoA + CO₂ + NADH + H⁺

The important series of events of aerobic respiration that occur in the matrix of the mitochondrion and in the inner membrane of the mitochondrion are:

i) Citric Acid Cycle/ Tricarboxylic acid cycle: It occurs in the mitochondrial matrix of the cell.

ii) Electron transport system/ Oxidative phosphorylation: It occurs in the inner mitochondrial membrane of the cell.

The respiratory pathway is considered to be a catabolic pathway because it involves only the breakdown of substrate for example in glycolysis the glucose is broken down to form pyruvate. But in the Tricarboxylic cycle (TCA cycle) the pyruvate is further broken down to Acetyl CoA, and this Acetyl CoA can also be synthesised to fatty acid whenever it is required for fatty acid metabolism or synthesis. Thus, During the TCA cycle, it is observed that both breakdown and synthesis of substrates occur. Therefore, the TCA cycle exhibits Amphibolic nature as it shows both catabolism and anabolism.

a = Glyceraldehyde – 3 – phosphate

b = PEP (Phosphoenol pyruvic acid)

c = C₂H₅OH

d = Lactic acid

The process is known as fermentation. After the glycolysis process, which doesn’t require oxygen. We obtain pyruvic acid. Now if the cells have oxygen present, i.e., aerobic conditions are present in the cell, then this pyruvic acid goes on to the citric acid cycle or tricarboxylic cycle in the mitochondria. But when there are anaerobic conditions the pyruvic acid gets converted to either lactic acid and it is known as homolactic fermentation. For examples, when our muscles do not receive oxygen, then homolactic fermentation occurs.

Also in some micro-organisms which are naturally anaerobic like yeast. They form C₂H₅OH and release CO₂. This is known as alcoholic fermentation.

Two Applications of fermentation:

i) Fermentation is widely used for the production of alcoholic beverage products like Whisky, Beer, etc.

ii) Fermentation is widely used for the production of acids like citrus acid, lactic acid, alpha- amylase, etc.

A= ATP = It is the end product which is formed in ATP synthesis.

B = F1 particle : The F1 particle is a peripheral membrane protein complex and it has the site for the synthesis of ATP from ADP and inorganic phosphate.

C = Pi : It is the inorganic phosphatase which is present along with ADP for the formation of ATP.

D = 2H⁺ :For every ATP which is produced during the ATP synthesis process, the 2H⁺ ions passes through F0 from the inner mitochondrial space to the matrix down the electrochemical proton gradient.

E = Inner mitochondrial membrane = It is the membrane from which 2H⁺ ions pass through to the matrix.

The role of oxygen is critical with respect to Electron Transport System (ETS) because:

i) Oxygen plays a crucial role at the end of ETC because it removes the hydrogen from the electron transport system.

ii) Oxygen acts as the final hydrogen acceptor in the electron transport chain.

iii) If oxygen is not present, then the electrons could not pass through the co-enzymes.

iv) Oxygen is responsible for the formation of the proton pump in the ETS process as it gives energy by the oxidation-reduction process.

v) If oxygen is absent, then there will be no formation of proton pump and ATP will not be produced by oxidative phosphorylation.

The assumptions which are taken for the making of the respiratory balance sheet are:

i) The metabolic pathway for respiration is sequential as:

Glycolysis � Kreb’s Cycle � Electron transport system.

Each of the above occurs one after the other in sequential order.

ii) The NADH which is synthesized during the glycolysis process is taken from the cytoplasm to the mitochondria of the cell, where oxidation occurs along with electron transport.

iii) The intermediate which is formed by the various biochemical respiration pathways is not used as raw material for any other respiration pathway.

iv) Glucose is the sole respiratory substrate, and no other substrate can enter the glycolytic pathway at any particular step of the process.

These assumptions are not always valid for an organism because:

i) All the biochemical pathways do not follow a sequential order, and instead, they occur simultaneously.

ii) The substrates are constantly entering in the intermediate steps of the biochemical respiratory pathway.

iii) The ATP molecules are used whenever it is required and formed by the body.

iv) The enzyme functions rate is controlled by various functions, and it is not controlled by only a single mechanism from a single site.

Difference between fermentation and aerobic respiration:

Glycolysis is the process which involves the splitting of glucose(a 6C compound) into two molecules of pyruvic acid (a 3C compound). It occurs in the cytoplasm of the cell. The glycolysis process can be divided into two phases-

• Hexose phase and

• Triose phase.

The end products of glycolysis are two pyruvate (pyruvic acid) molecules, a total of four ATP molecules, and two molecules of NADH.

The reactions involved in the process of glycolysis are as follows:

1. The six carbon glucose molecule is phosphorylated with the presence of ATP to form a glucose-6- phosphate enzyme.

Hexokinase enzyme is used to catalyze this reaction.

2. Then Glucose-6- Phosphate is isomerized by using phosphoglucoisomerase to form a fructose-6- phosphate.

3. The Fructose-6- phosphate obtained from the previous reaction is further phosphorylated using ATP to form fructose 1,6-bisphosphate.

Phosphofructokinase is used to catalyze this reaction, and the ATP is dephosphorylated to form ADP.

4. The Fructose 1,6-bisphosphate gets cleaved by utilizing the enzyme aldolase. It gets cleaved into two molecules of 3 Carbon compounds called dihydroxy acetone phosphate (DHAP) and glyceraldehyde-3- Phosphate. These are isomeric triose compounds.

5. Dihydroxy acetone phosphate (DHAP) and the glyceraldehyde-3- phosphate are interconvertible with the use of triose.

6. Glyceraldehyde-3- phosphate is further phosphorylated and then oxidized in the presence of glyceraldehyde-3- phosphate dehydrogenase to form 1,3- bisphosphoglyceric acid.

7. Then 1,3-bisphosphoglyceric acid is dephosphorylated by phosphoglyceric kinase to form a molecule of 3-phosphoglyceric acid (3-PGA).

8. 3-phosphoglyceric acid (3-PGA) is then further converted by using phosphoglyceric mutase into a molecule of 2-phosphoglyceric acid.

9. 2-phosphoglyceric acid (2-PGA) is further dehydrated by using enolase into a molecule of 2- phosphoenol pyruvic acid.

10. At last 2-phosphoenol pyruvic acid is dephosphorylated to form pyruvic acid and the ADP

is also phosphorylated to form ATP. This reaction is catalyzed by using pyruvic kinase enzyme.

A simple flowchart of steps involvedin glycolysis is shown below :

Glycolysis occurs in both aerobic and anaerobic organisms. After glycolysis, if the cells have oxygen, then it goes on to a process known as a citric acid cycle or tricarboxylic cycle. And if there is no oxygen, then the fermentation process occurs.