The oxidative reactions of cellular respiration were the focus of intense study in the 1930s.These reactions are represented in a linear pathway, as they were thought to occur.Each product is designated as a lettered compound (A through H) in Figure 13-16.

\begin{array}{ccccccccccc}A&\longrightarrow&B&\longrightarrow&C&\longrightarrow&D&\longrightarrow&E&\longrightarrow&F&\longrightarrow&G&\longrightarrow&H\\&\text { citrate }&&\text { isocitrate }&&\alpha \text {-ketoglutarate }&&\text { succinyl CoA }&&\text { succinate }&&\text { fumarate }&&\text { malate }\\&\text { oxaloacetate }\\\end{array}

Figure 13-16
A.What was the first observation that Krebs made when he added malonic acid to the minced muscle samples, and what was his conclusion about how and where it was acting in the reactions he was studying?
B.What happens when the malonate block is introduced and subsequently compound A is added in excess? What is the result if compound G is added after the block, instead of A? How did Krebs attempt to reconcile these two results?
C.What additional observation led Krebs to hypothesize that what was previously thought to be a linear sequence of reactions is actually a cyclic sequence of reactions? How did this idea further explain the earliest observations that the addition of any single compound in the pathway greatly increases oxygen uptake by the muscle tissue?

The Oxygen-Dependent Reactions Required for Cellular Respiration Were Originally Thought

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