This is any functional structure within the confines of a cell; literally a. Small organ; it usually has a membrane-based structure

The correct answer is cell organelles.

Explanation:

A unique membrane-bound composition, which is found inside the eukaryotic cell's cytoplasm, that is, in plants and animal cells is termed as cell organelle. Some of the examples of cell organelles found within the cells of eukaryotes are endoplasmic reticulum, Golgi complex, mitochondria, chloroplast, etc.  

All these organelles play an essential function for the proper functioning of the cell, like chloroplasts found in plants play an essential role in the process of photosynthesis, and mitochondria both in plants and animals are regarded as the powerhouse of the cell.  

both anaerobic and aerobic respiration use glucose (or other carbohydrates), proteins and lipids to produce atp and nadh. cell uses those energy molecules, obtained from food and   breaks them down during the chemical reactions of respiration. in anaerobic respiration, nadh is regenerated to nad+ by enzymes lactate dehydrogenase (lactate is produced, e.g. in muscle cells) or by alcohol dehydrogenase where ethanol is produced (alcohol fermentation), while in aerobic respiration, electrons from nadh are deposited in the mitochondria to regenerate nad+ (electron transport chain. anaerobic respiration produces 2 atp per molecule of glucose compared to aerobic respiration where about 38 atp molecules are produced.