Honors Biology: Cellular Respiration Learning Targets

Related State of MI learning standards:

• HS-LS1-5 Use a model to illustrate how photosynthesis transforms light energy into stored chemical energy.

• HS-LS1-6 Construct and revise an explanation based on evidence for how carbon, hydrogen, and oxygen from sugar molecules may combine with other elements to form amino acids and/or other large carbon-based molecules. 

• HS-LS1-7 Use a model to illustrate that cellular respiration is a chemical process whereby the bonds of food molecules and oxygen molecules are broken and the bonds in new compounds are formed resulting in a net transfer of energy. 

• HS-LS2-3 Construct and revise an explanation based on evidence for the cycling of matter and flow of energy in aerobic and anaerobic conditions. 

• HS-LS2-5 Develop a model to illustrate the role of photosynthesis and cellular respiration in the cycling of carbon among the biosphere, atmosphere, hydrosphere, and geosphere.

1. Relate cell structures and functions to the process of cellular respiration.

• For glycolysis, describe the location in the cell where this occurs and any cell structures that contribute to its functioning. What role do these cell structures play?

Glycolysis occurs in the cytosol/cytoplasm, or the fluid of the cell. The mitochondria is not needed for the functioning of glycolysis. The cytoplasm serves as a medium for chemical reactions, such as glycolysis; it is a jelly-like substance between the nucleus and the cell membrane.

• For the link reaction and Krebs Cycle, describe the location in the cell where these occur and any cell structures that contribute to their functioning. What role do these cell structures play?

The link reaction and Krebs cycle occur in the matrix of the mitochondria. The link reaction takes the products of glycolysis, the first stage of cellular respiration, and converts them into the reactants of the Krebs cycle (pyruvate is converted into acetyl-CoA by an enzyme called CoA). The role of the mitochondria is to serve as the powerhouse of the cell, or generate chemical energy needed to fuel the cell’s chemical reactions. Essentially, the energy produced by the mitochondria can help the link reaction and Krebs cycle take place.

• For the electron transport chain, describe the location in the cell where this occurs and any cell structures that contribute to its functioning. What role do these cell structures play?

The electron transport chain occurs on the cristae, or the inner membrane of the mitochondria. The inner membrane of a cell is folded to form the cristae; the folds allow a much greater amount of enzymes of the electron transport chain and ATP Synthase to be packed into the mitochondria. Thus, because the electron transport chain occurs there, it maximizes the efficiency of synthesizing ATP, which is its function. Additionally, the cytochromes, a series of enzymes and other proteins that are embedded in the inner membranes of mitochondria, help to transport electrons through the system. The ATP synthase of the cristae (mitochondria) also contributes to the functioning of the electron transport chain by helping to produce ATP.

2. Illustrate and describe the energy conversions that occur during cellular respiration.

• Write the equation for cellular respiration. Identify the reactants and products overall, as well as for each of the three major processes involved.

The equation for cellular respiration is C6H12O6 + 6O2 → 6CO2 + 6H2O + ATP. The reactants are glucose and oxygen. Glucose is used in glycolysis, while the oxygen is used in the electron transport chain. The products are carbon dioxide, water, and ATP. Carbon dioxide is produced in the Krebs cycle, water is produced in the electron transport chain, and the ATP can technically be produced in all three major processes, glycolysis, the Krebs cycle, and the electron transport chain.

• Trace the path of a molecule of glucose through the entire process of cellular respiration. What happens to it in each of the different stages? This question will be more helpful for studying if you can be specific and detailed in your answer.

First, the molecule of glucose enters the first stage of cellular respiration, glycolysis, which breaks it down into two pyruvate molecules, producing ATP and NADH in the cytosol/cytoplasm. Two phosphates attach to glucose to form a new 6-carbon compound. This provides energy to rearrange the molecule. 2 ATP also turns into 2 ADP. The compound then splits into 2 PGAL, which is oxidized, 2 NAD+ is reduced into 2 NADH, and each PGAL gets a phosphate; 2 3-carbon molecules are formed. Phosphates are then removed, producing 2 pyruvates. The phosphates attached to ADP form ATP (4 ADP to 4 ATP). The products of glycolysis are 2 pyruvates, 2 NADH, and 2 ATP. Next, the 2 pyruvic acids enter the mitochondrial matrix, in which the Krebs cycle breaks it down and forms CO2, ATP, NADH, and FADH2. Pyruvate diffuses into the mitochondria, where enzymes release a molecule of carbon dioxide from each pyruvate molecule, leaving a molecule of acetate; pyruvate is oxidized and NAD+ is reduced. Acetyl CoA enters the Krebs cycle, citrate is produced, which is rearranged and citrate is oxidized, NAD+ is reduced into NADH and releases CO2, which occurs once more. Then, rearrangement, oxidation, NAD and FAD are reduced, and oxaloacetate is regenerated. The products of the Krebs cycle include 6 CO2, 8 NADH, 2 FADH2, 2 ATP. Since Krebs begins with 2 molecules of pyruvate, it must make 2 turns per molecule of glucose. The next and final stage is the electron transport chain and chemiosmosis which converts energy in NADH & FADH2 to ATP. Hydrogen and electrons are added to oxygen to form H2O. NADH and FADH2 carry hydrogen atoms to the electron transport system, which separates the hydrogen atoms into electrons and protons. Cytochromes embedded in the inner membranes of the mitochondria transfer electrons step by step throughout the system; the last cytochrome is an enzyme that combines electrons and protons with oxygen, forming water. At each transfer, the electrons release free energy that enables enzymes in the inner membrane to actively transport the protons from the matrix into the intermembrane space. As those protons become highly concentrated, they tend to diffuse back into the matrix of the mitochondrion, passing through the ATP-synthase enzyme complex, where ATP is synthesized. The electron transport chain, in summary, produces ATP, a process known as oxidative phosphorylation. That’s what happened to the original glucose molecule.

3. Compare and contrast anaerobic and aerobic processes. 

• What types of organisms use only aerobic respiration? Anaerobic respiration? Both? Give examples of each.

The types of organisms that use only aerobic respiration are animals, plants, protists, and fungi. The types of organisms that use only anaerobic respiration include some species of bacteria and archaea. The types of organisms that use both aerobic and anaerobic respiration are many bacteria and archaea, which make them facultative aerobes, as they can survive for long periods with or without oxygen, switching between fermentation and aerobic respiration depending on oxygen’s availability.

• Describe the differences in the glycolysis pathway for aerobic respiration vs. anaerobic respiration.

The glycolysis pathway for aerobic respiration is different from the glycolysis pathway for anaerobic respiration. For aerobic respiration, glycolysis stops at producing the 2 pyruvates, which is then used in the Krebs cycle via the link reaction; some products of the Krebs cycle then undergo the electron transport chain to produce ATP. Meanwhile, for anaerobic respiration/fermentation, glycolysis attempts to make ATP, though only 2 ATP compared to the 38 ATP molecules per glucose molecule (2 from glycolysis, 2 from the Krebs cycle, and roughly 34 from the electron transport chain). Lactic acid is a product of anaerobic respiration; muscles make acid so cells can’t contract, causing pain. It goes into the blood to the liver, then can be converted back to pyruvate, then glucose in the liver. Or ethyl alcohol and carbon dioxide fermentation may occur, which occurs until ethyl alcohol reaches concentration that inhibits fermentation.

• Explain why an organism that uses aerobic respiration could grow larger and more complex over time, compared with an organism that only utilizes anaerobic respiration.

An organism that uses aerobic respiration could grow larger and more complex over time compared to an organism that only utilizes anaerobic respiration. This is because aerobic respiration produces more ATP than anaerobic respiration does. Since ATP drives many of the processes in living cells, like those contributing to growth, more ATP means more growth.

4. Explain how carbohydrates, proteins, and lipids are utilized as energy sources in the human body.

• Describe the differences in how carbohydrates, proteins, and lipids are metabolized. Be sure to include where each happens and what part of the cellular respiration process each type of macromolecule participates in.

Carbohydrates like glucose are metabolized by going through the all processes of cellular respiration, including glycolysis, which occurs in the cytoplasm, the Krebs cycle, which occurs in the mitochondrial matrix, and the electron transport chain, which occurs in the inner membrane of the mitochondria (cristae). Fats/lipids are metabolized by not going through glycolysis, but enzymes in the mitochondria break down the fatty acids into acetate, coenzyme A transfers acetate to the Krebs cycle, but they can’t be fermented or go through anaerobic respiration, and most of the energy in fat cannot be transferred to ATP. In proteins, digestive enzymes break down the proteins to amino acids, enzymes remove the amino groups and convert the ammonia to urea, and the carbon skeletons remaining from some amino acids can undergo reactions that form 4- or 5-carbon acids (oxaloacetate or ketoglutarate), which can enter the Krebs cycle. The reactions of cell respiration, especially of the Krebs cycle, contribute to decomposition and the biosynthesis of carbohydrates, lipids, and proteins.

5. Describe how organisms acquire energy indirectly from sunlight.

• Compare and contrast the process of cellular respiration with the process of photosynthesis.

Cellular respiration is a decomposition pathway that provides the energy cells need to function in the form of ATP. Photosynthesis is the synthesis process by which plants use sunlight, water, and carbon dioxide to create oxygen and sugar (food), that can be later released by cellular respiration to fuel the organism’s activities. So, it stores energy, while cell respiration releases energy. Photosynthesis has two stages, the light reactions and the Calvin cycle, while cellular respiration has three stages, glycolysis, the Krebs cycle, and the electron transport chain - chemiosmosis. Plants perform both photosynthesis and cell respiration, while animals can only perform the latter. Also, photosynthesis takes place in the chloroplasts of plant cells, while cellular respiration can take place in the cytoplasm, mitochondrial matrix, and cristae. Lastly, photosynthesis produces the oxygen that cell respiration needs to make ATP, among water and carbon dioxide that photosynthesis actually needs.

• Draw a picture of a tree with leaves, the sun, some rain, and an animal (an herbivore). Use this to diagram how photosynthesis and cellular respiration are interconnected.