Respiration
Respiration is a series of enzyme-controlled reactions that occur in the cells of all living organisms. Its primary purpose is to release energy from glucose (or other organic molecules) so that it can be used for life processes.
Building on the concept of metabolism, this released energy is stored temporarily in a molecule called ATP (Adenosine Triphosphate). ATP acts as the immediate energy currency of the cell. When ATP is broken down to ADP (Adenosine Diphosphate), energy is released to power cellular activities.
Respiration can occur in two forms:
- Aerobic Respiration: Requires oxygen. Releases a large amount of energy.
- Anaerobic Respiration: Occurs without oxygen. Releases a small amount of energy.
The energy released from respiration is essential for the following processes:
| Process | Explanation |
|---|---|
| Muscle Contraction | Energy is needed to shorten and lengthen muscle fibers, allowing movement. |
| Protein Synthesis | Energy is required to join amino acids together to form proteins. |
| Cell Division | Energy is needed for the replication of DNA and the splitting of cells (mitosis). |
| Active Transport | Energy is used to move substances against a concentration gradient (e.g., root hair cells absorbing mineral ions). |
| Growth | Energy is used to build new cells and tissues from smaller molecules. |
| Passage of Nerve Impulses | Energy is needed to transmit electrical signals along neurons in the nervous system. |
| Maintenance of Body Temperature | In mammals and birds, energy is released as heat to maintain a constant internal temperature (homeostasis), especially in cold environments. |
Note: Plants also use energy from respiration for these processes, particularly protein synthesis and active transport in roots.
A chemical reaction in which glucose is broken down to release energy for use by the organism.
Yeast
Yeast is a single-celled fungus. It is a heterotroph, meaning it obtains its food from external sources. Crucially, yeast is facultatively anaerobic, meaning it can respire both aerobically (with oxygen) and anaerobically (without oxygen).
Anaerobic Respiration in Yeast (Fermentation)
When oxygen is absent, yeast breaks down glucose into ethanol and carbon dioxide. This process is called fermentation.
Equation:
\text{Glucose} \rightarrow \text{Ethanol} + \text{Carbon Dioxide} + \text{Energy (small amount)}
You are investigating the effect of temperature on the rate of anaerobic respiration in yeast. You measure the volume of carbon dioxide (CO_2) produced over time.
Data:
- At 10 minutes, 0 \text{ cm}^3 of CO_2 was produced.
- At 15 minutes, 8.0 \text{ cm}^3 of CO_2 was produced.
Calculation of Rate:
The rate is defined as the change in volume per unit time.
\text{Rate} = \frac{\text{Change in Volume}}{\text{Time Taken}}
\text{Rate} = \frac{8.0 \text{ cm}^3 - 0 \text{ cm}^3}{15 \text{ min} - 10 \text{ min}}
\text{Rate} = \frac{8.0 \text{ cm}^3}{5 \text{ min}} = 1.6 \text{ cm}^3/\text{min}
Key Point: Always include units in your final answer. The standard unit for gas volume rate is \text{cm}^3/\text{min} or \text{mm}^3/\text{s}.
Many students write that respiration produces or makes energy.
The Correct Understanding:
Energy cannot be created or destroyed, only transferred. Respiration releases energy stored in the chemical bonds of glucose. The energy is then transferred to ATP for use by the cell.
Why this matters: Examiners mark down 'produces energy' because it violates the law of conservation of energy. Use the phrase 'releases energy' or 'transfers energy from glucose to ATP'.
Use this explanation when asked to describe or explain the results of an investigation into how temperature affects the rate of respiration in yeast.
The Explanation:
The rate of respiration increases as temperature increases up to an optimum temperature (typically around 40^\circ\text{C} for yeast). This is because higher temperatures increase the kinetic energy of the molecules, leading to more frequent collisions between enzymes and substrates.
However, above the optimum temperature, the rate decreases rapidly. This is because the high heat causes the enzymes involved in respiration to denature. Denaturation changes the shape of the enzyme's active site, so the substrate can no longer bind, and the reaction stops.
Examiner Acceptance:
Cambridge accepts phrases like:
- 'Rate increases then decreases'
- 'Optimum temperature is reached at...'
- 'Enzymes denature above optimum temperature'
Reasoning: This directly addresses the biological mechanism (enzyme kinetics and denaturation) rather than just stating a trend. Simply saying 'it gets faster then slower' is insufficient without mentioning enzymes.
Experimental Design
To investigate the effect of temperature on respiration, you must control specific variables to ensure a fair test.
| Variable Type | Specific Variable | Reason for Control/Choice |
|---|---|---|
| Independent Variable | Temperature | This is the factor you deliberately change (e.g., 20^\circ\text{C}, 30^\circ\text{C}, 40^\circ\text{C}). |
| Dependent Variable | Rate of Respiration | Measured by the volume of CO_2 produced per minute, or the time taken to produce a fixed volume of gas. |
| Control Variables | Yeast Mass/Concentration | Must be constant so that the amount of enzyme is the same in each trial. |
| Glucose Concentration | Must be constant so that substrate availability does not limit the rate. | |
| pH | Enzymes are sensitive to pH; buffers may be used to keep it constant. | |
| Volume of Solution | Ensures consistent surface area for gas collection. |
Method Overview:
- Prepare yeast suspension and glucose solution.
- Place the mixture in a water bath at a specific temperature (e.g., 30^\circ\text{C}) for a few minutes to equilibrate.
- Add an oil layer on top of the solution if measuring anaerobic respiration to prevent oxygen from entering (keeping it anaerobic) while allowing CO_2 to escape into a gas syringe or collection tube.
- Measure the volume of CO_2 produced over a set time interval.
Data Handling and Graphing
When plotting results for this investigation:
- Axes: The independent variable (Temperature) goes on the x-axis. The dependent variable (Rate of Respiration or Volume of Gas) goes on the y-axis. Both axes must be labelled with units.
- Scale: Use a linear scale that allows the data to occupy at least half the grid in both directions.
- Plotting: Plot points accurately (\pm half a small square).
- Line of Best Fit: Draw a smooth curve of best fit. Do not join the dots with straight lines unless specifically asked for a bar chart or if the data is discrete categories (which temperature is not, it is continuous).
Interpreting the Graph:
The graph typically shows a bell-shaped curve:
- Rising phase: Rate increases with temperature.
- Peak: The optimum temperature.
- Falling phase: Rate drops sharply as enzymes denature.
- Muscle contraction; 2. Protein synthesis; 3. Active transport. (Other acceptable answers: cell division, growth, maintenance of body temperature, passage of nerve impulses).