Ch21 Respiration

Question 1

Glycolysis (cytoplasm)

Answer 1

1. Break down of glucose to triose phosphate
   Glucose is activated by phosphorylation. Two ATP molecules are used. The activated glucose is broken down into two triose phosphate molecules.
2. Oxidation of triose phosphate to pyruvate
   Each of the two triose phosphate molecules loses hydrogen to form pyruvate. The hydrogen is accepted by NAD. NAD is reduced to NADH. Two ATP molecules are formed.

Question 2

Bridging between glycolysis and Krebs cycle (mitochondrial matrix)

Answer 2

A carbon atom is removed from pyruvate and released as carbon dioxide. The remaining 2-C compound loses hydrogen which is accepted by NAD to form NADH. The 2-C compound combines with coenzyme A to form acetyl coenzyme A.

Question 3

Krebs cycle (mitochondrial matrix)

Answer 3

1. Combination of acetyl-CoA with 4-C compound
   Acetyl-CoA combines with a 4-C compound in the mitochondrial matrix. The 2-C acetyl group is donated to the 4-C compound to form a 6-C compound. Coenzyme A is released.
2. Regeneration of 4-C compound
   The 6-C compound loses two carbon atoms which are released as carbon dioxide. The 6-C compound loses hydrogen atoms. The hydrogen atoms are accepted by carrier molecules. Three molecules of NAD are reduced to NADH. One molecule of FAD is reduced to FADH. The energy released from the conversion is used to make one ATP molecule.

Question 4

Oxidative phosphorylation (inner membrane)

Answer 4

1. NADH loses hydrogen and is oxidized to NAD. The hydrogen atoms split into hydrogen ions and electrons. By losing hydrogen, NAD is regenerated. It can accept hydrogen in glycolysis and the Krebs cycle again.
2. The electrons take part in a series of redox reactions along an electron transport chain. The chain consists of electron carriers embedded in the inner membrane of mitochondria. The energy released in the reactions is used to make ATP.
3. The hydrogen ions and electrons are transferred to oxygen, the final electron acceptor, to form water.
   (Also for FADH to FAD)

Question 5

Comparison between aerobic respiration and photosynthesis.

Answer 5

• site of occurrence
• type of metabolism
• energy transformation
• activation process
• cyclic process
• electron transportation and formation of ATP

Question 6

Investigation of oxygen consumption in grasshopper

Answer 6

• use potassium hydroxide solution to absorb carbon dioxide.
• assumption: amount of gas removed = amount of oxygen absorbed by grasshopper
• The water level in the delivery tube rises. The carbon dioxide originally present in the flask and that produced during respiration are absorbed by potassium hydroxide solution. As oxygen is consumed by the grasshopper, the air pressure in the conical flask drops and becomes lower than atmospheric pressure. The atmospheric pressure pushes the coloured water in the beaker downwards and the water level in the delivery tube decreases.

Question 7

Investigation of carbon dioxide production in germinating seeds

Answer 7

• during germination, the rate of respiration of the seeds increases rapidly to break down the food stored in the seeds, producing large amount of carbon dioxide in a short period of time
• boiling the seeds: kills the seeds, no more enzymatic reaction -> no respiration
• sterilize the surface of the seeds: prevent microorganisms present on the seeds from affecting the results
• soak the germinating seeds: provide favourable conditions
• The hydrogencarbonate indicator in the tube with boiled seeds remains red while that in the tube with germinating seeds turns yellow after a few hours. Germinating seeds give out carbon dioxide.

Question 8

Investigation of carbon dioxide production in a living mouse

Answer 8

• Potassium hydroxide in flask A (before mouse) remains clear. It is used to absorb carbon dioxide in the air, preventing carbon dioxide from entering the bell jar. The lime water in flask B (before mouse) remains clear. It is used to confirm that the air entering the bell jar contains no carbon dioxide.
• The lime water in flask C (after mouse) turns milky after 30 minutes. Carbon dioxide is present in the air leaving the bell jar, which is given out by the living mouse.

Question 9

Investigation of heat production in germinating seeds

Answer 9

• when germinating seeds carry out respiration to break down the stored food in the seeds, heat is released
• vacuum flask: traps any heat produced by the seeds
• The temperature of the flask with germinating seeds increases while that of the flask with boiled seeds does not. Germinating seeds release heat.
• Warm air rises as it is less dense than air. Inverting the vacuum flasks traps the warm air and minimizes heat loss

Question 10

Investigation of heat production in a living mouse

Answer 10

• differential air thermometer
• opening the clips before the experiment: balance the air pressure, hence the liquid level
• The coloured liquid level in arm B (with mouse) falls and that in arm A rises. The mouse releases heat which warms up the air in tube B. The air expands and pushes the liquid level in arm B downwards
• Cotton wool: used as insulator to prevent heat loss to the surroundings
• Modifications:
  1. Use a smaller container -> less air have to be heated up by the mouse
  2. Use a test tube of different shape -> increase the surface area

Question 11

Investigation of the rate of respiration in a living mouse by measuring the rate of carbon dioxide production

Answer 11

• In the control set-up (without mouse), there is a decrease in the mass of flask C (after bell jar) due to the evaporation of water from the potassium hydroxide solution

Question 12

Investigation of the rate of respiration in germinating seeds by measuring the rate of oxygen consumption

Answer 12

• syringe: adjust the levels of coloured liquid in both arms to the same height, readings shows volume of oxygen consumed
• The liquid level in arm B (with germinating seeds) rises while that in arm A (without seeds) falls. Respiring seeds take in oxygen and give out carbon dioxide. The carbon dioxide is absorbed by soda lime. The reduction in tube B causes the liquid level in arm B to rise.
• Water bath: prevent the experimental results from being affected by changes in temperature in the surroundings

Question 13

Effect on temperature on respiration

Answer 13

1. At low temperatures (below 40C), the rate of respiration increases because the enzymes involved in the reactions become more active at higher temperatures
2. When the temperature reaches above 40C, the rate of respiration decreases, The enzymes involved in the reactions of aerobic respiration are denatured

Question 14

Alcoholic fermentation

Answer 14

Pyruvate formed from glycolysis is reduced to alcohol. Carbon dioxide is released and NAD is regenerated to pick up hydrogen in glycolysis.

Question 15

Lactic acid fermentation

Answer 15

Pyruvate accepts hydrogen from NADH and is reduced to lactic acid. NAD is regenerated and can be used in glycolysis again.

Question 16

Significance of lactic acid fermentation in skeletal muscles

Answer 16

During strenuous exercise, skeletal muscles need a lot of energy from breaking down glucose. Through the blood supply of oxygen to the muscles increases, there is still an insufficient supply of oxygen for complete oxidation of glucose in muscles. In addition to aerobic respiration, muscles also carry out lactic acid fermentation to convert pyruvate to lactic acid. This provides additional energy in a very short time, muscles can contract more powerfully and at a higher rate.

Question 17

Removal of lactic acid.

Answer 17

• oxygen debt: the additional amount of oxygen required to remove all the lactic acid
• keep breathing fast and deeply for a period of time to take in extra oxygen
• with more oxygen, more NAD is regenerated in the electron transport chain, then the lactic acid can be oxidized by NAD to pyruvate, which can be converted to acetyl-CoA for entering the Krebs cycle

Question 18

Similarities of aerobic and anaerobic fermentation

Answer 18

• release energy from the oxidative breakdown of organic substances
• transfer energy to the energy carrier ATP, and some energy is lost as heat
• consist a number of reactions controlled by enzymes

Question 19

Differences between aerobic and anaerobic respiration

Answer 19

Aerobic:
- occurs in cytoplasm and mitochondria
- oxygen is required
- organic substances are completely broken down into carbon dioxide and water
- a larger amount of energy is released (38 ATP per glucose)

Anaerobic:
- occurs only in cytoplasm
- oxygen is not required
- organic substances are partially broken down to form lactic acid or ethanol and carbon dioxide
- a much smaller amount of energy is released (2 ATP per molecule)