Question

most cells cannot harness heat to perform work because...
heat does not involve a transfer of energy.
cells do not have much heat; they are relatively cool.
temperature is usually uniform throughout a cell.
heat can never be used to do work.
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Explanation:

Heat is a form of energy transfer. Cells are relatively cool compared to some environments but that's not the main reason. Heat can be used to do work in some systems (like heat engines which rely on temperature gradients). In a cell, temperature is usually uniform. For work to be done using heat (based on thermodynamics, like in a heat - engine - like process), there needs to be a temperature gradient (a hot and a cold region). Since cells don't have significant internal temperature gradients, they can't harness heat to perform work in a thermodynamic work - extraction sense.

Answer:

temperature is usually uniform throughout a cell.