Yeast are facultatively anaerobic which means that they perform fermentation only under anaerobic conditions. In the presence of O₂, the yeast will perform aerobic metabolism. The roommate has left the bucket open to the air, so the yeast will have access to a continuous supply of O₂.  With O₂, the yeast will replenish their NAD⁺ through electron transport in the mitochondria. Alcohol and CO₂ (which produces carbonation) are produced by the fermentation pathway which occurs significantly only in the absence of O₂.
 
 

View the question.

Yeast are facultatively anaerobic which means that they perform fermentation only under anaerobic conditions. Certain bacteria are obligately anaerobic and can exist only in O₂-free circumstances. In contrast, many organisms can only survive in aerobic conditions. Humans need O₂ to survive, but their cells can utilize fermentation when O₂ levels are low. In the presence of O₂, the yeast will perform aerobic metabolism. The roommate has left the bucket open to the air, so the yeast will have access to a continuous supply of O₂. With O₂, the yeast will replenish their NAD⁺ through electron transport in the mitochondria. In the mitochondria, even more NAD⁺ are converted to NADH by the TCA cycle. However, all these NADH, including those from glycolysis, will be converted back to NAD⁺ when they donate their electrons to the electron transport chain. The electron transport chain pumps H+ across the mitochondrial inner membrane to create a H+ gradient. This proton gradient is then used to make ATP energy. Because yeast only produce alcohol and CO₂ (which produces carbonation) through the fermentation pathway, the presence of O₂ will prevent any build up of either alcohol or CO².

• Irrelevant information is left out. 

• The key points are clearly stated in the first two sentences. 

• All of the highlighted information is extraneous. The question is about yeast only and fermentation only. 

 
• Much of the pertinent information is buried between two large sections of irrelevant information. 

Yeast are facultatively anaerobic which means that they perform fermentation only under anaerobic conditions. In the presence of O₂, the yeast will perform aerobic metabolism. The roommate has left the bucket open to the air, so the yeast will have access to a continuous supply of O₂.  With O₂, the yeast will replenish their NAD⁺ through electron transport in the mitochondria. Alcohol and CO₂ (which produces carbonation) are produced by the fermentation pathway which occurs significantly only in the absence of O₂.

View the question.

Yeast can choose between using oxygen and not using oxygen. They will ferment stuff (1) only when there is not enough oxygen around. In the presence of O₂, the yeast want to use the oxygen (2), so they do. The roommate has left the bucket open to the air, so the yeast will have access to a continuous supply of O₂.  With O₂, the yeast will do what they need to do without making alcohol and carbonation. Alcohol and carbonation are produced by the pathway that does not require O₂, and this obviously only occurs significantly in the absence of O₂.

• The appropriate technical terms (highlighted) are used when pertinent.  

• A lack of scientific terms makes the answer less clear. 

 
• Use of slang. 

 
• Teleology and anthropomorphism. 

Yeast are facultatively anaerobic which means that they perform fermentation only under anaerobic conditions. In the presence of O₂, the yeast will perform aerobic metabolism. The roommate has left the bucket open to the air, so the yeast will have access to a continuous supply of O₂. With O₂, the yeast will replenish their NAD⁺ through electron transport in the mitochondria. Alcohol and CO₂ (which produces carbonation) are produced by the fermentation pathway which occurs significantly only in the absence of O₂.

View the question.

Yeast are facultatively anaerobic which means that they perform fermentation only under anaerobic conditions. In the presence of O₂, the yeast will perform aerobic metabolism.  With O₂, the yeast will replenish their NAD⁺ through electron transport in the mitochondria. Alcohol and CO₂ (which produces carbonation) are produced by the fermentation pathway which occurs significantly only in the absence of O₂.

Cause and effect relationships are made obvious by the sentence. 

Without the sentence about the bucket, the answer does not attempt to solve the problem outlined by the question. Without this sentence, you would have explained facultative anaerobic metabolism, but would not have demonstrated that you could use this knowledge in a practical setting to answer the question.

Yeast are facultatively anaerobic which means that they perform fermentation only under anaerobic conditions. In the presence of O₂, the yeast will perform aerobic metabolism. The roommate has left the bucket open to the air, so the yeast will have access to a continuous supply of O₂.  With O₂, the yeast will replenish their NAD⁺ through electron transport in the mitochondria. Alcohol and CO₂ (which produces carbonation) are produced by the fermentation pathway which occurs significantly only in the absence of O₂.
 
 

View the question.

Yeast are facultatively anaerobic which means that they perform fermentation only under anaerobic conditions. Certain bacteria are obligately anaerobic and can exist only in O₂-free circumstances. In contrast, many organisms can only survive in aerobic conditions. Humans need O₂ to survive, but their cells can utilize fermentation when O₂ levels are low.

It addresses the question.

It addresses a different question. 

The answer above could be considered a good answer to a question which asks you to compare different organisms that use different types of metabolism. However, it is a poor answer to the question that was asked.

Yeast are facultatively anaerobic which means that they perform fermentation only under anaerobic conditions. In the presence of O₂, the yeast will perform aerobic metabolism. The roommate has left the bucket open to the air, so the yeast will have access to a continuous supply of O₂. With O₂, the yeast will replenish their NAD⁺ through electron transport in the mitochondria. Yeast perform the following reaction only in anaerobic conditions.

View the question.

• It uses a diagram to convey information. 

 
• There is adequate text to address the question. 

• There is only a diagram without any explanation of how it relates to the question. 

 
• The instructor is left wondering if you know how this relates to the question.