Yeast respiration and artificial electron carriers

What you’ll learn

• Interpret the reduction of a supplied redox indicator.
• Compare endpoint times while accounting for oxygen and control treatments.

Understand the idea

Core practical 15 uses an artificial hydrogen carrier, or redox indicator, to investigate respiration in yeast. The indicator's colour depends on its oxidation state. In a suitable assay, metabolically transferred reducing equivalents change the dye's colour. The dye is a proxy for respiratory activity rather than a direct measurement of ATP production, oxygen consumption or cell number. Use the supplied information about its oxidised and reduced colours.

A fixed small colour-change endpoint can give a relative rate proportional to reciprocal time when dye amount, yeast quantity and the relevant conditions are matched. Temperature, pH and substrate availability can affect metabolism. Oxygen may reoxidise a reduced indicator, so different mixing or air exposure can change the visible endpoint without reflecting only a difference in the underlying respiratory reactions.

A non-living or otherwise inactive control can help show that a response depends on active biological material, while a dye-and-substrate blank checks non-biological change. A control must be interpreted specifically: an inactive preparation differs in many ways from living cells and does not identify one particular enzyme. Classroom yeast investigations follow the approved supervised method; these questions use supplied observations and do not specify growth conditions.

Key formulae

$$\mathrm{relative\ rate}\propto\frac1{t_{\mathrm{endpoint}}}$$

$$\frac{r_B}{r_A}=\frac{t_A}{t_B}\quad\text{for equal endpoint progress}$$

Worked example

In a supplied redox-indicator record, matched active-yeast samples reach the same small endpoint in 90 s with substrate A and 45 s with substrate B. An inactive-yeast control does not reach the endpoint during the observation period. Compare relative rates and state what the control supports.

1. For matched endpoints, the relative rates are proportional to $1/90$ and $1/45$.
2. The B-to-A ratio is $90/45=2$.
3. The inactive control supports a contribution from active biological processes, but does not prove that one named enzyme alone caused the response.

Answer: B has twice the relative endpoint rate; the control supports dependence on active yeast in this assay.