Required Practical 9 Investigating the Rate of Respiration

Question 1

Why are yeast used in respiration investigations?

Answer 1

Yeast are single-celled and respire both aerobically and anaerobically. There are no ethical considerations when using yeast.

Question 2

What is dehydrogenation?

Answer 2

Dehydrogenation is the removal of a hydrogen. It happens at various stages in aerobic respiration as well as in anaerobic respiration. The hydrogen atoms reduce hydrogen carriers FAD and NAD during glycolysis.

Question 3

Why are DCPIP or methylene blue used as indicators in respiration investigations?

Answer 3

These substrates are reduced instrad of NAD and FAD and change colour when reduced. In doing so we can see when respiration is taking place.

Question 4

What colour change do DCPIP or methylene blue undergo when reduced (when respiration is taking place?)

Answer 4

They change from blue to colourless

Question 5

How can DCPIP or methylene blue be used to measure the rate of respiration.

Answer 5

The rate of decolourisation can be used as a measure of rate of respiration.

Question 6

How can temperature be controlled?

Answer 6

Water bath

Question 7

When investigating the substrate concentration (glucose concentration), which other factors need to be controlled to make the investigation valid?

Answer 7

1. Volume of dye added: if there is more dye molecules present then the time taken for the colour change to occur will be longer.
2. Volume of yeast suspension: when more yeast cells are present the rate of respiration will be inflated.
3. Type of substrate: yeast cells will respire different substrates at different rates.
4. Temperature: an increase or decrease in temperature can affect the rate of respiration due to energy demands and kinetic energy changes. The temperature of the dye being added also needs to be considered.
5. pH: a buffer solution can be used to control the pH level to ensure that no enzymes are denatured

Question 8

Why does the yeast solution need to be buffered?

Answer 8

Enzymes have an optimum pH. Buffering keeps the solution within an optimum range.

Question 9

What happens to the rate of respiration as the temperature increases?

Answer 9

As temperature increases the rate of respiration increases until the temperature is high enough to denature enzymes then the rate drops quickly.

Question 10

Why does the rate of respiration increase as temperature increases (to an optimum).

Answer 10

Respiration is controlled by enzymes and enzyme activiy increases as temperature increases.

Question 11

What are the limitations to this investigation?

Answer 11

1. This experiment is not measuring the rate of dehydrogenase activity directly (through measuring the rate of substrate use or product made) but is instead predicting what the rate would be measuring the rate of electron / hydrogen release from the reactions of respiration.
2. Distinguishing the end of the reaction and the colour change is subjective and therefore one person should be used to attempt to control this

Question 12

Why can a yellow colour be seen in the test tubes?

Answer 12

That is due to the colour of yeast.