Final answer:
Organisms deep under water such as tubeworms rely on glycolysis due to the absence of oxygen needed for the Krebs cycle or ETC. They have adapted to their anoxic environment by using chemosynthesis, a metabolic process that enables survival without sunlight and oxygen by harvesting energy from chemical reactions.
Step-by-step explanation:
An organism deep under water that only performs glycolysis and not the Krebs cycle or the electron transport chain (ETC) likely does so because it resides in an environment where oxygen is scarce or absent. Such organisms have adapted to use anaerobic metabolism to survive. Early life on Earth existed in an oxygen-poor atmosphere, which suggests that organisms evolved from a common ancestor with a metabolism suited to those conditions. In environments such as the deep ocean, where there is no sunlight for photosynthesis, organisms like tubeworms rely on chemosynthetic bacteria, which harvest energy from chemical compounds through chemosynthesis rather than sunlight.
Glycolysis is a highly conserved metabolic pathway found in nearly all living organisms and can occur without oxygen, making it critical for anaerobic respiration. The process generates ATP, which is essential for cellular functions, even in the absence of the Krebs cycle and ETC. This pathway is especially important for organisms, like certain extremophiles and red blood cells, which either cannot utilize oxygen or lack mitochondria.