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Thermodynamic efficiency can be expressed as the ratio of Work done(W) to Energy invested (Q).

Thermodynamic efficiency= W/Q

How can one measure work done by a biological system?

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Shouldn't this be W/Q? If Q/W then "thermodynamic efficiency" is very high when you do little work but put in a lot of energy. Doesn't seem very efficient. – Conner Aug 16 '12 at 20:38
@Conner Thanks for noticing. Corrected it. – Chirag Aug 16 '12 at 21:13
up vote 3 down vote accepted

Measuring the work done by a biological system seems pretty impossible. Imagine how many different ways one cell of your body uses energy (ATP). You can't really measure all the work done by every cell on a macro scale. Metabolic efficiency has been defined as... "health". That seems just a little ambiguous. That's why we use things like averages to determine if energy use is normal or not, such as in metabolic age.

In short, work is a more tangible term in discrete physics examples, but there is so much complicated energy use in biological systems that total systemic work can't be easily defined.

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