Required Practical 9: Single-Celled Organism Respiration

Use the centre’s approved single-celled organism culture, such as a suitable yeast preparation, and vary one factor such as temperature or substrate concentration. A method may measure carbon dioxide production, oxygen uptake in an appropriate respirometer, or a validated indicator response.

Choose the measurement to match the biological question. Yeast can produce carbon dioxide through more than one metabolic condition, so carbon dioxide alone does not prove a particular oxygen-use pathway. Standardise the quantity and physiological state of the culture, substrate supply, pH and total volume as relevant.

Allow treatment components and apparatus to equilibrate where appropriate before starting measurements. Use the same sampling intervals and suitable independent replicates, and include a background or inactive-organism control when it helps distinguish metabolism from apparatus changes.

A sealed gas method needs leak checks and an interpretation of pressure or temperature effects. In a respirometer intended to measure oxygen uptake, carbon dioxide handling must be accounted for in the stated method. Warming trapped gas can change its volume without a corresponding change in respiration, so a control apparatus can be essential.

Calculate change per unit time over a defined interval and normalise to culture mass, cell number or another justified measure when comparing different amounts of organism. A fixed population-size label may still hide differences in viability or metabolic state.

A temperature rise can increase enzyme-controlled activity over a suitable range but eventually reduce it through stress or loss of function. Substrate limitation, oxygen availability and product accumulation can alter later readings. Use initial or otherwise justified comparable intervals rather than assuming the entire progress curve is linear.

Try each question before opening the worked response. Use precise biological terms, connect each cause to its effect, and check units and assumptions when interpreting data.