Cells are the basic unit of life. Thus, cells are responsible for maintaining a stable internal environment within living organisms. How do humans continue to maintain a stable internal environment if billions of cells die within their bodies each day?
A.
New cells are eaten in the form of food.
B.
The cells must be able to reproduce.
C.
Exposure to the Sun stimulates cell growth.
D.
Humans need fewer cells as they get older.

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.