Honors Biology: Unit 6 - Cellular Respiration Check & Challenges

Pg. 134:

1. How are carbon skeletons involved in biosynthesis and decomposition reactions?

Carbon skeletons are involved in biosynthesis because they are products of cell respiration, which are used to provide energy to perform the cellular work. Additionally, when carbon skeletons undergo a decomposition reaction, energy is created, which may be able to fuel biosynthesis.

2. What is the advantage of the stepwise nature of the reactions of cell respiration?

The advantage of the stepwise nature of the reactions of cell respiration is that less energy is wasted; the body can use glucose much more efficiently. By oxidizing glucose in small steps, just enough energy is produced to generate one molecule of ATP, where the energy is then stored. Releasing glucose all at once or only having one reaction of cell respiration would waste a lot of energy and cause burnout.

3. Why is glucose not a direct source of cellular energy?

Glucose is not a direct source of cellular energy because the body utilizes it much less efficiently than ATP. If glucose was a direct source of that energy, then the body would overheat/cook itself by burning it all at once. Instead, glucose is converted to ATP first, a safer and more efficient source of energy.

4. Compare the process of cell respiration to the operation of a car’s engine.

The process of cell respiration and the operation of a car’s engine are kind of similar. When you want to drive your car, you first start the engine and allow the gasoline to burn slowly, taking it step-by-step like how glucose is oxidized, releasing energy to drive the creation of ATP. You don’t drop a match in the gas tank; that would blow you and the car up. That is much like how if all the energy in glucose was released at once, which would heat the organism until it cooked itself, which is not a smart idea.

5. Breathing is also sometimes called respiration. What could be the connection between your need for cell respiration and your need to breathe?

The connection between the need for cell respiration and the need to breathe is that you need O2 (oxygen) via your airway because it is one of the reactants for cellular respiration; you actually have to breathe, otherwise cell respiration would not function properly. They are not the same thing, but they have some similarities. Also, when you exhale, you breathe out the carbon dioxide produced as an end product for cellular respiration.

6. Compare the three cellular hydrogen carriers and describe their functions.

Three cellular hydrogen carriers are NADH, NADPH, and FADH2. NADH and NADPH are identical, except for the fact that there is an extra phosphate group in NADPH; like NADPH, NADH is oxidized and reduced in a cycle; it is the reduced form of NAD+, which is the oxidized form. NADH also synthesizes ATP, using the electron transport system. FADH2, on the other hand, is produced when FAD combines with 2 H in one step of the Krebs cycle. It acts as a high energy electron carrier that carries electrons to the electron transport chain.

 Pg. 144:

1. What are the products of each stage of respiration?

The products of each stage of respiration are as follows: in glycolysis, ATP, pyruvate, NADH, and carbon skeletons are produced (2 molecules each); in the Krebs cycle, 2 CO2 molecules, 3 NADH molecules, 1 FADH2 molecule, and 1 ATP molecule are produced.

2. How does the presence or absence of oxygen determine the fate of pyruvate in cells?

The presence of oxygen determines the fate of pyruvate in cells; with enough oxygen, the Krebs cycle can work, but with insufficient oxygen, animal and some bacteria cells reverse the oxidation produced by the pyruvate, and the cell will be forced to convert NADH and pyruvate into NAD+ and lactate (3-carbon acid); NAD- cycles back to glycolysis, which provides small amounts of ATP until more oxygen becomes available. This forms an anaerobic pathway called lactic-acid fermentation, which occurs in the cytoplasm.

3. Compare the functions of the two mitochondrial membranes to the functions of the inner and outer chloroplast membranes.

The two mitochondrial membranes (also called inner and outer) have similar functions to the inner and outer membranes of a chloroplast. The mitochondrial membranes consist of proteins and the double layer of lipid molecules (typical), but the inner membrane contains so many enzymes that it’s more protein-based than lipid-based. The inner membrane also has many folds/cristae to extend inside to the mitochondrion; enzymes on the cristae are involved in the electron transport system, ATP formation enzymes, and some enzymes of the Krebs cycle (though most of those enzymes are located in the matrix). The outer membrane regulates the movement of molecules flowing in and out of the mitochondria. The similarities with the chloroplast membranes are that the outer membrane controls what leaves and enters and that the inner membrane synthesizes ATP, whether it’s through the electron transport chain or the photosystems (I & II).

4. How many molecules of carbon dioxide result from the breakdown of one molecule of glucose in aerobic respiration?

6 molecules of carbon dioxide result from the breakdown of one molecule of glucose in aerobic respiration.

5. How is oxygen involved in cell respiration?

Oxygen is involved in cell respiration (the last step, as it’s the final electron acceptor that helps electrons move down the electron transport chain) by the fact that it is needed to oxidize glucose. Without it, cells must ferment glucose, which forms only 2 ATP molecules per glucose molecule; with oxygen present, organisms can obtain much more energy from their food, and can release the energy more efficiently as well. Stage 3 of respiration, which is the electron transport system, needs oxygen to be used to produce much of the ATP in ATP synthesis.

6. How many times more ATP does aerobic respiration produce than does fermentation?

Aerobic respiration produces 36 ATP molecules, while fermentation produces only 2 ATP molecules. Conclusively, aerobic respiration produces 18 times (18x) the number of ATP molecules compared to the less efficient fermentation.

Pg. 148:

1. Why is the Krebs cycle so important in metabolism?

The Krebs cycle is so important in metabolism because it is involved in not only the breakdown of carbohydrates, but also fats and proteins. This cycle breaks down acetate from fatty acids and sugars. The Krebs cycle also produces the building blocks for biosynthesis. It also plays an important part in aerobic respiration. Lastly, the Krebs cycle helps to fuel the production of ATP.

2. Why is oxygen necessary for the respiration of fat?

Oxygen is necessary for the respiration of fat because the process skips glycolysis. Thus, cells are not able to ferment fatty acids the same way that they can with carbohydrates. So, without oxygen, most of the energy in fat cannot be transferred to ATP; it can’t be respirized.

3. Of what adaptive value is the alternative pathway of respiration in plants?

The alternative pathway of respiration in plants has an important adaptive value, which is heat. More specifically, the energy of the electron flow in this pathway enables more heat energy and less ATP to be produced, compared to normal respiration. Some examples of how more heat energy can benefit a plant include a strong odor that attracts insects, allowing for pollination, and the melting of snow over a certain plant, which helps it grow through the hole and still get pollinated, thus surviving.

4. How do plants and animals benefit from the heat energy released in respiration?

Plants and animals both benefit from the heat energy released in respiration. Plants use the heat energy to get pollinated through various means, including attracting certain insects with certain odors and melting through snow to survive. Animals use the heat energy to assist with hibernation, which is active at the end; animals must quickly raise their body temperature to normal levels in order to fully awaken. Animals use the thermal energy to stay alive, as do plants.

5. Discuss the control of glucose oxidation and ATP production in cells.

The control of glucose oxidation and ATP production in cells is important and must be regulated. Supply and demand determine whether glucose is broken down in respiration or converted into starches and fats. When animals need to use their energy rapidly, cells absorb glucose from the blood to produce ATP. Conversely, when energy demand is low, cells use excess glucose to synthesize glycogen, and then fat.

6. Describe how fats and proteins are brought into the Krebs cycle.

Fats and proteins can both be brought into the Krebs cycle. Fats are brought in by first breaking down into glycerol and fatty acid. They are then converted into CoA, or acetyl CoA, which then enters the Krebs cycle. Proteins are brought into the Krebs cycle by having digestive enzymes break proteins down into amino acids first, other enzymes remove the amino groups and ammonia is converted into safer nitrogen compounds, which may be recycled or excreted. The carbon skeletons remaining from some amino acids can undergo reactions that form 4- or 5- carbon acids, oxaloacetate or ketoglutarate, which can finally enter the Krebs cycle.