Cellular Respiration (Short Questions and Answers)

Cellular Respiration

Short Questions and Answers

One mark questions with answers

1. Cellular respiration in the absence of molecular oxygen is

(a) photorespiration

(b) glycolysis

(c) EMP pathway

(d) HMS pathway

Answer: (b) glycolysis

2. The citric acid cycle initiates with

(a) succinic acid

(b) pyruvic acid

(c) acetyl coenzyme A

(d) fumaric acid

Answer: (c) acetyl coenzyme A

3. Acetyl CoA is formed by reaction of CoA with

(a) pyruvic acid

(b) acetic acid

(c) citric acid

(d) fumaric acid

Answer: (a) pyruvic acid

4. If less oxygen is absorbed and more CO2 liberated then the respiratory substrate is

(a) protein

(b) fats

(c) organic acid

(d) carbohydrates

Answer: (c) organic acid

5. In cellular respiration, the first molecule of carbon dioxide is liberated during

(a) glycolysis

(b) krebs cycle

(c) a stage between glycolysis and krebs cycle

(d) terminal oxidation

Answer: (c) a stage between glycolysis and krebs cycle

 6. The enzyme which converts glucose to glucose 6 phosphate is

(a) phosphorylase

(b) glucose 6 phosphatase

(c) hexokinase

(d) glucose synthetase

Answer: (c) hexokinase

 7. The main source of immediate energy for all biological process is

(a) sunlight

(b) ATP molecules

(c) NADP molecule

(d) both b and c

Answer: (b) ATP molecules

8. The conversion of pyruvate to acetyl coenzyme is known as

(a) krebs cycle

(b) oxidative decarboxylation

(c) fermentation

(d) all the above

Answer: (b) oxidative decarboxylation

9. The synthesis of ATP via electron flow through ETS with oxygen as the terminal electron acceptor is known as

(a) oxidative phosphorylation

(b) phosphorylation

(c) glycolysis

(d) all the above

Answer: (a) oxidative phosphorylation

10. Each molecule of FADH produces

(a) 2 ATP molecules

(b) 3 ATP molecules

(c) 1 ATP molecule

(d) no ATP molecule

Answer: (a) 2 ATP molecules

11. Anaerobic respiration is also called

(a) fermentation

(b) restoration

(c) fragmentation

(d) multiplication

Answer: (a) fermentation

12. The link between glycolysis and citric acid cycle is

(a) NAD

(b) FAD

(c) Acetyl CoA

(d) None

Answer: (c) Acetyl CoA

Two marks questions with answers

1. What is respiratory substrate? Give examples.

Answer: Respiratory substrates are the high energy compounds that are oxidized during respiration to release energy. Respiratory substrates include carbohydrates, fats and proteins. Of these carbohydrates such as glucose, fructose, sucrose or starch, insulin, hemicellulose etc., are the main substrates.

2. Differentiate between aerobic and anaerobic respiration.

Answer: Aerobic respiration occurs in all living cells whereas anaerobic respiration occurs only in some bacteria, fungi, germinating seeds, fleshy fruits etc. Aerobic respiration requires oxygen and anaerobic respiration does not require oxygen. The end products of anaerobic respiration are carbon dioxide and water. The end products of anaerobic respiration are carbon dioxide and alcohol.

3. Differentiate between respiration and combustion

Answer: Respiration is a vital process whereas combustion is a chemical process. In respiration there is very small rise in temperature whereas in combustion there is large increase in temperature. The energy released during respiration is again stored in chemical compounds, example ATP. Most of the energy released in combustion is lost in the form of heat.

4. What is glycolysis?

Answer: The sequence of reactions which converts glucose into pyruvic acid with the production of ATP is termed as glycolysis. It is a series of reactions in which 6 carbon glucose molecules converts into two 3 carbon molecules of pyruvic acid.  It produces 4 molecules of ATP.  Of these, two molecules of ATP are used up during degradation of glucose into pyruvic acid, hence, the net gain is 2 ATP molecules. All reactions of glycolysis occur in the cytoplasm.  Two molecules of NADH2 are also produced during glycolysis.

5. Differentiate between Krebs cycle and glycolysis.

Answer: Krebs cycle is a cyclic pathway whereas glycolysis is a linear pathway. Krebs cycle occurs inside mitochondria whereas glycolysis occurs inside cytoplasm. Krebs cycle occurs only in aerobic respiration whereas glycolysis is common to both aerobic and anaerobic modes of respiration. In Krebs cycle there is no consumption of ATP whereas glycolysis requires two molecules of ATP for the initial phosphorylation of substrate molecule.

6. What is oxidative phosphorylation?

Answer: Oxidative phosphorylation is the synthesis of energy-rich ATP molecules with the help of energy liberated during oxidation of reduced coenzymes (NADH2 and FADH2), produced in respiration.  The enzyme required for the synthesis of ATP is termed as ATP synthase. The enzyme ATP synthase is considered to be the 5th complex of electron transport chain. ATP synthase is located in F1 or headpiece of F0– F1 or elementary particles.

Three marks questions with answers

1. What do you understand by uncouplers of oxidative phosphorylation?

Answer: Uncouplers of oxidative phosphorylation in mitochondria inhibit the coupling between the electron transport and phosphorylation reactions and thus inhibit ATP synthesis without affecting the respiratory chain and ATP synthase.  Therefore, blocking oxidative phosphorylation effectively decreases ATP concentrations in the cell.

Uncoupling agents are compounds which dissociate the synthesis of ATP from the transport of electrons through the cytochrome system.  The electron transport continues to function, leading to oxygen consumption but phosphorylation of ADP is inhibited.

2. What is fermentation?

Answer: Fermentation is defined as the anaerobic breakdown of carbohydrates and other organic compounds into alcohols, organic acids, gases etc., with the help of microorganisms or their enzymes. It is a kind of anaerobic respiration carried out primarily by fungi and bacteria. In the process of fermentation pyruvate is anaerobically broken down to yield various products depending upon the organism and the types of tissue.  The two common products are ethyl alcohol and lactic acid.

3. What do you understand by Respiratory quotient?

Respiratory quotient is the ratio of the volume of carbon dioxide produced to the volume of oxygen consumed in respiration over a period of time.

RQ=Volume of CO2 release/volume of oxygen absorbed

The respiratory quotient besides other factors, depends on the type of respiratory substrate (e.g., glucose) and the completeness of oxidation.  It can be measured with the help of an apparatus called, Ganong’s respirometer.  The value of respiratory quotient can be 1, zero, more than 1 or less than 1.

4. Define respiration. Mention the two types of respiration.

Answer: Respiration can be defined as the process by which living cells break down complex high-energy food molecules into simple low energy molecules, releasing the energy trapped within the chemical bonds. Depending on the use of molecular oxygen in oxidation, respiration can be classified into two types:  aerobic and anaerobic respiration. Aerobic respiration: Most of the organism use molecular oxygen for oxidation of their foods.  It is termed as aerobic respiration or oxyrespiration.

Anaerobic respiration: When oxygen is not available for the oxidation of carbohydrates, then carbon dioxide and ethyl alcohol or lactic acid are formed.  This is termed as anaerobic respiration or an oxyrespiration.  It results in incomplete oxidation of the substrate, hence, the energy released is much less than in aerobic respiration.

5. What is ethyl alcohol fermentation?

Answer: It is quite common in fungi (e.g., Rhizopus, yeast) and bacteria.  Yeast can respire both aerobically and anaerobically.  Anaerobic respiration occurs in sugary solution if the fungus is not in contact with the atmosphere.  It causes fermentation.  In the presence of enzyme pyruvate decarboxylase and TPP (thiamine pyrophosphate), pyruvate is broken down to form acetaldehyde releasing carbon dioxide.

Pyruvate→Acetaldehyde+carbon dioxide

Acetaldehyde is reduced to ethyl alcohol or ethanol in the presence of enzyme alcohol dehydrogenase.  Hydrogen is obtained from NADH2 produced during oxidation of glyceraldehyde 3 phosphate to 1, 3 diphosphoglycerate in glycolysis.


6. What is lactic acid fermentation?

Answer: The fermentation occurs in lactic acid bacteria (e.g., Lactobacillus), some fungi and muscles.  Lactic acid produced in muscles is sent to liver to regenerate glucose.  In lactic acid fermentation pyruvate produced in glycolysis is directly reduced by NADH2 to form lactic acid.  No carbon dioxide is produced.  The enzyme is lactic dehydrogenase, which requires FMN (Flavin mononucleotide) and Zn2+.

Pyruvic acid+NADH2→lactic acid+NAD+