Question

much of the carbon dioxide produced by algae is not excreted as a metabolic waste because it
1 can be used for photosynthesis
2 cannot pass through cell membranes
3 is needed for aerobic respiration
4 is used for the hydrolysis of proteins

Explanation:

Algae are photosynthetic organisms. Photosynthesis uses carbon dioxide ($\ce{CO_2}$) as a raw material to produce glucose and oxygen. So the carbon dioxide produced (e.g., from respiration) can be reused in photosynthesis instead of being excreted. Option 2 is wrong as $\ce{CO_2}$ can pass through cell membranes (it's a small gas molecule). Option 3: Aerobic respiration produces $\ce{CO_2}$, it doesn't need $\ce{CO_2}$ as a reactant (aerobic respiration uses glucose and oxygen). Option 4: Hydrolysis of proteins involves water and enzymes, not $\ce{CO_2}$. So the correct reason is it can be used for photosynthesis.

Answer:

1. can be used for photosynthesis