(a) addition of malate to an extract of muscle cells stimulates oxygen consumption by the extract. explain why malate stimulates oxygen consumption.
(b) malonate is an inhibitor of succinate dehydrogenase. if malonate is added to a suspension of bacteria that are catabolizing glucose then oxygen consumption is inhibited. explain why malonate inhibits oxygen consumption.

Oxygen consumption is a measure of the activity of the first two stages of cellular respiration: glycolysis and the Kreb's cycle.  The addition of malate stimulates the citric acid cycle and thus stimulate respiration.

Explanation:

The added malate serves a catalytic role, because it is regenerated in the later part of the citric acid cycle. With higher concentrations of oxaloacetate or malate, higher flux of acetyl CoA will be utilized into the citric acid cycle increasing the oxygen consumption much greater levels.

The  inhibition  of  succinate oxidation  by  malonate  is a well  known  phe-  

nomenon.  Since  the  oxidation  of  succinate  to  fumarate  is  an  integral  

part  of  the  Krebs  cycle  of  oxidations,  it  has been generally  assumed that  

the  inhibitory  effect  of  malonate  upon  the  oxidation  of  any  member  of  

the  cycle  is the  result  of  the  inhibition  of  the  succinate to  fumarate  step.   malonate  inhibits  oxidations  in  the  cycle  by  at  least  two  mechanisms: in  addition  to  the  inhibition  resulting  from  a block  of  succinate oxidation,  malonate  inhibits  oxidation  by  another  mechanism that  is believed  to  involve  combination  with  magnesium  ions.