Question

nadh and fadh₂ carry the electrons theyve gained to the etc. those electrons will eventually react with oxygen, the final electron acceptor in the etc, to form water
but what happens if oxygen runs out? this happens regularly in a wide array of animals - especially in muscle tissue. when humans exercise strenuously, for example when sprinting at top speed or holding a challenging yoga pose for a long time, the demand for atp is extremely high. the etc runs at full throttle, and oxygen - rich blood cant be delivered to the muscle tissue fast enough, no matter how well - trained the individual is. so oxygen runs out
to understand what happens in response, its critical to take a closer look at glycolysis: the first major stage in cellular respiration. remember that glycolysis is a series of ten reactions that yields both atp and nadh.
in this diagram, the blue circles represent carbon atoms in the reactants and products of the 10 reactions and the yellow circles phosphate groups; the reactions are numbered in the sequence they occur.
a couple notes:

• reactions 2, 5, 8, and 9 are essentially molecular re - arrangement reactions.
• reaction 5 changes the top product of reaction 4 to the bottom product of reaction 4, so reactions 6 - 10 happen twice for every molecule of glucose that enters glycolysis.

analyze the diagram of the ten reactions in glycolysis. which statements accurately describe the process of glycolysis? select all that apply.
□ glycolysis starts with an \investment phase\ that adds phosphate groups to 6 - carbon sugars and uses 2 atp.
□ in reaction 6, an isolated phosphate group, not one that is part of atp, is added to a 3 - carbon molecule.
□ glycolysis yields 1 nadh for every molecule of glucose that enters.
□ glycolysis has a net yield of 2 atp, because 4 atp are produced but 2 atp are invested.
□ reaction 4 splits a 6 - carbon substrate into a total of four 3 - carbon molecules.

Explanation:

• First statement: Glycolysis indeed has an investment phase where 2 ATP are used to add phosphate groups to glucose (a 6 - carbon sugar).
• Second statement: In reaction 6 of glycolysis, an inorganic phosphate (not from ATP) is added to a 3 - carbon molecule.
• Third statement: For each glucose molecule, glycolysis yields 2 NADH (because reactions 6 - 10 happen twice as per the note in the problem).
• Fourth statement: 4 ATP are produced in glycolysis (reactions 7 and 10, each happening twice) and 2 are invested, so the net yield is \(4 - 2=2\) ATP.
• Fifth statement: Reaction 4 splits a 6 - carbon substrate into two 3 - carbon molecules (not four).

Answer:

• Glycolysis starts with an "investment phase" that adds phosphate groups to 6 - carbon sugars and uses 2 ATP.
• In reaction 6, an isolated phosphate group, not one that is part of ATP, is added to a 3 - carbon molecule.
• Glycolysis has a net yield of 2 ATP, because 4 ATP are produced but 2 ATP are invested.