Cellular Respiration, practice questions

B, ATP is the cell's immediate energy currency; it is broken down to release energy where needed and rebuilt using energy from respiration, while glucose is the stored fuel.

C, Aerobic respiration uses oxygen to break glucose fully into carbon dioxide and water, releasing energy; option D is photosynthesis written in reverse.

C, Glycolysis occurs in the cytoplasm of every cell and does not need oxygen; only the later aerobic stages take place in the mitochondria.

D, The energy-rich aerobic stages take place in the mitochondria, so cells with a high energy demand, such as muscle cells, contain a large number of mitochondria.

B, Yeast ferments glucose into ethanol and carbon dioxide; lactic acid is the product in human muscle, and water forms only in aerobic respiration.

B, When the oxygen supply falls short of demand, muscle cells respire anaerobically, producing lactic acid and a small amount of energy.

B, Carbon dioxide turns limewater cloudy (milky); clear limewater would show that no carbon dioxide was released.

B, Boiled seeds are dead and do not respire, so they act as a control, showing that any change in the living set-up is due to respiration.

B, The oxygen debt is the extra oxygen taken in after exercise to break down the lactic acid that built up, which is why breathing stays heavy once exercise stops.

C, Complete breakdown of glucose into carbon dioxide and water releases almost all of its chemical energy, while fermentation leaves much of that energy locked in ethanol.

• Manipulated variable: the contents of the tube (living seeds, boiled seeds, or glass beads).
• Responding variable: whether, and how quickly, the limewater turns cloudy.
• Tube 1 (living seeds): limewater turns cloudy because the respiring seeds release carbon dioxide.
• Tube 2 (boiled seeds): limewater stays clear because the dead seeds do not respire.
• Tube 3 (glass beads): limewater stays clear because beads are non-living and do not respire, acting as a further control.
• Controlled variable: for example the temperature, the volume of limewater, or the mass of material in each tube (any one).

• The process is anaerobic respiration (fermentation).
• Word equation: glucose → ethanol + carbon dioxide + energy.
• Observation 1: bubbles of gas pass through the limewater, which turns cloudy because carbon dioxide is produced.
• Observation 2: the smell of ethanol develops, or the temperature of the solution rises slightly as energy is released.
• The gas is carbon dioxide, identified by the cloudy limewater.
• A warm temperature is used so the enzymes in the yeast work faster, increasing the rate of fermentation.

• Aerobic respiration releases a large amount of energy and produces carbon dioxide and water.
• Anaerobic respiration releases only a small amount of energy and produces lactic acid.
• Oxygen debt is the extra oxygen the body needs after exercise to break down the lactic acid that accumulated.
• During the race the oxygen supply could not meet the muscles' demand, so lactic acid built up.
• After the race, breathing stays deep and fast to take in the extra oxygen required.
• This extra oxygen is used to break down the accumulated lactic acid, repaying the oxygen debt.

Respiration transfers energy stored in the chemical bonds of glucose into ATP, a small molecule that acts as the cell's immediate, usable form of energy. ATP is broken down wherever energy is needed in the cell, releasing that energy instantly, then rebuilt using more energy from respiration, so the cell is never short of an energy supply.

Aerobic respiration completely breaks down glucose using oxygen, producing carbon dioxide, water and a large amount of energy: glucose + oxygen gives carbon dioxide + water + energy. It takes place mainly in the mitochondria and is far more efficient than any anaerobic pathway, which is why muscle and liver cells contain especially large numbers of mitochondria.

When the oxygen supply cannot meet a muscle's demand during vigorous exercise, human cells respire anaerobically: glucose gives lactic acid + a small amount of energy. Lactic acid accumulates in the muscle, lowers its pH, and contributes to the fatigue and cramp felt during and shortly after hard exercise.

Yeast cells respire anaerobically by fermentation, breaking glucose down into ethanol and carbon dioxide while releasing a small amount of energy: glucose gives ethanol + carbon dioxide + energy. This reaction is exploited industrially, the carbon dioxide makes bread dough rise during baking, and the ethanol is the basis of beer and wine production.

Aerobic respiration releases far more energy from each glucose molecule than either anaerobic pathway, because it breaks glucose down completely into carbon dioxide and water rather than only partially into lactic acid or ethanol, both of which still contain a large amount of unreleased chemical energy.

During vigorous exercise, lactic acid builds up faster than it can be removed, creating an oxygen debt. After exercise stops, breathing and heart rate stay raised so that extra oxygen can be taken in to break down the accumulated lactic acid, which is why a person keeps panting after a hard sprint has finished.

Every cell begins respiration the same way: glycolysis breaks glucose down in the cytoplasm, releasing a small amount of energy without needing oxygen. If oxygen is available, the products of glycolysis then pass into the mitochondria, where the aerobic stages take place and release most of the total energy obtained from the glucose molecule.

The ATP produced by respiration powers almost every activity of a living organism, including muscle contraction for movement, active transport of substances across cell membranes, the biosynthesis of large molecules such as proteins during growth, transmission of nerve impulses, and the generation of heat that keeps body temperature stable in warm-blooded animals.

Respiration is commonly investigated by detecting one of its products or measuring one of its reactants: limewater turns milky when carbon dioxide from respiring seeds or organisms is bubbled through it, a thermometer or insulated flask can detect the heat released by germinating seeds, and a respirometer measures the volume of oxygen consumed over time.

Fermentation by yeast is used directly in the food industry: in baking, the carbon dioxide produced makes bread dough rise and gives bread its texture, while in brewing, the ethanol produced from sugar is the alcohol present in beer and wine. Both processes rely on exactly the same anaerobic pathway taught in this chapter.