Temperature and Respiration in Yeast

Why temperature changes the rate at all

At low temperatures, molecules have relatively little kinetic energy, so the enzyme-controlled reactions of respiration proceed slowly and the overall rate is low. As the temperature rises, molecules move faster and successful enzyme-substrate interactions become more frequent, so the rate of respiration climbs towards an optimum. Push the temperature past that optimum, though, and the enzymes involved begin to lose their functional shape; their activity falls, and so does the rate of respiration. At a sufficiently high temperature, respiration can become extremely slow or stop almost entirely.

The pattern worth holding in your head as a single chain is: low temperature gives slow respiration, rising temperature gives faster respiration up to an optimum, and temperature in excess of that optimum gives falling respiration. A fair conclusion is built from the data you actually collect, never from the assumption that a higher temperature is automatically a faster one — once the temperature is too high for the enzymes involved, the rate goes down, not up.

Investigating the relationship

A suitable investigation uses carbon dioxide production as an indication of how fast yeast is respiring. Yeast is supplied with a respiratory substrate such as glucose, and identical yeast mixtures are then held at different temperatures — a water bath is a convenient way to set and maintain each one. The carbon dioxide produced over a fixed period can be collected and measured, for example with a gas syringe.

In this investigation, temperature is the independent variable, and the rate of respiration — estimated from carbon dioxide produced per unit time — is the dependent variable. Everything else needs to be held constant so temperature is the only thing genuinely responsible for any difference you observe: the amount and concentration of yeast, the amount and concentration of glucose, the pH, and the total measurement time all need to match across every temperature you test.

The rate itself is calculated as the volume of carbon dioxide produced divided by time. Repeating each measurement and calculating a mean makes the result more reliable, and it also gives you a way to spot an anomalous result rather than treating every single data point as equally trustworthy.

Reading the pattern correctly

If a question gives you a rate graph, faster respiration plots higher — that part is intuitive. But if the apparatus instead measures the time taken to reach a fixed endpoint, the relationship flips: faster respiration means a shorter time, not a larger number. Before you interpret any graph or table in this topic, check which of the two you’re actually looking at.

Don’t confuse

There isn’t a single universal “optimum temperature” for yeast respiration worth memorising as a number — different experimental setups genuinely show different approximate optima, and the current syllabus requires the effect of temperature, not a fixed figure. Keep temperature as the only variable that changes between your test conditions; changing yeast concentration or glucose concentration alongside it breaks the fair test. And a single reading at each temperature is weaker evidence than a repeated, averaged one — that’s a reliability point examiners look for specifically, not just a nice-to-have.