Key concepts in biology Revision Notes

Temperature and Enzyme Activity

Enzymes have an optimum temperature, typically around 37°C for human enzymes. Below this, activity increases with temperature as kinetic energy rises. Above the optimum, the enzyme denatures irreversibly, losing its active site shape and function.

pH and Enzyme Activity

Each enzyme has an optimum pH. Changes in pH outside this narrow range alter the charges on the amino acids within the enzyme, disrupting its tertiary structure and active site, leading to denaturation and loss of activity.

Substrate Concentration and Enzyme Activity

Increasing substrate concentration generally increases reaction rate until all enzyme active sites are occupied (saturation). At this point, the enzyme is working at maximum capacity, and further increases in substrate concentration have no effect.

Denaturation

Denaturation is the irreversible change in an enzyme's three-dimensional shape, particularly its active site, caused by extreme temperatures or pH. A denatured enzyme loses its catalytic function.

Variables and Controls

The independent variable is pH. Use buffer solutions of different pH values (e.g., pH 2, 4, 6, 7, 8, 10). The dependent variable is the rate of enzyme activity, measured as the time taken for a specific change to occur (e.g., starch disappearance) or the amount of product formed (e.g., oxygen produced). Important control variables include temperature (use a water bath to maintain a constant temperature, typically 25°C or 37°C), enzyme concentration (use the same volume and concentration of enzyme solution for each pH), and substrate concentration (use the same volume and concentration of substrate solution). Consistent mixing and reaction volume are also crucial. Accurate measurement of volumes using pipettes and observing changes consistently are vital for reliability.

Method: Amylase and Starch

1. Prepare a series of buffered solutions at different pH values. 2. Set up a water bath at a constant temperature (e.g., 30°C). 3. Add equal volumes of amylase solution and starch solution to separate boiling tubes, then place them in the water bath to equilibrate to temperature. 4. Add a drop of iodine solution to each well of a spotting tile. 5. Mix the amylase and starch solutions in one boiling tube, starting a stopwatch immediately. 6. Every 30 seconds, use a clean pipette to take a small sample and add it to a well on the spotting tile containing iodine. 7. Record the time it takes for the iodine solution to remain brown/orange (indicating no starch is present). 8. Repeat steps 3-7 for each pH buffer. 9. Calculate the rate of reaction as 1/time (e.g., 1/seconds) for each pH. Safety: Wear eye protection. Amylase is a protein, treat as potential irritant. Iodine can stain, avoid contact with skin and clothing. Dispose of biological waste appropriately. Wash hands thoroughly.

Data Analysis and Interpretation

Plot a graph with pH on the x-axis and the rate of reaction (1/time) on the y-axis. The graph should show an optimum pH where the enzyme activity is highest, indicated by the shortest reaction time (and thus the highest 1/time value). At pH values significantly above or below the optimum, the enzyme will denature, causing the active site to change shape and reducing its ability to bind to the substrate, leading to a slower reaction rate or no reaction at all. The graph should typically show a bell-shaped curve. Evaluating results involves looking for anomalies and considering the precision of timing and pH control. The percentage error in measuring time can be calculated as (uncertainty in time / measured time) x 100%. For example, if a stopwatch has an uncertainty of +/- 0.5 s, and a reaction takes 60 s, the percentage error is (0.5/60) * 100 = 0.83%. Reducing uncertainty in pH measurement might involve using a pH meter instead of universal indicator paper. Repeat trials at each pH are crucial for improving reliability and identifying anomalous results, allowing for calculation of mean rates.

Calculating Reaction Rate

The rate of an enzyme-catalysed reaction is calculated as the change in the amount of substrate or product over a given time period. The formula is generally: Rate = Change in quantity / Time taken. Quantity can be mass (e.g., grams, milligrams), volume (e.g., cm³, mL), or moles of product formed or substrate consumed. Time can be in seconds, minutes, or hours. Ensure consistent units for both quantity and time.

Factors Affecting Enzyme Rate

Several factors influence the rate of enzyme activity. Increased temperature generally increases the rate up to an optimum, after which denaturation occurs, reducing the rate. pH also has an optimum, outside which the enzyme denatures. Increasing substrate concentration increases the rate until all active sites are saturated. Increasing enzyme concentration directly increases the rate, assuming sufficient substrate is available. Understanding these influences is key to interpreting rate calculations.

Interpreting Rate Graphs

When enzyme activity is plotted against time, the initial rate of reaction is often the steepest part of the curve. This is usually the maximum rate as substrate concentration is highest. As the reaction proceeds, substrate is consumed, and the rate typically slows down. Calculating the rate at different points on the curve (e.g., by finding the gradient) can provide insight into the reaction's kinetics.

Designing and Conducting the Potato Osmosis Practical

Use a cork borer and ruler to cut potato cylinders of consistent dimensions, recording all measurements precisely. Blot each cylinder dry before and after soaking to avoid surface liquid errors. Submerge cylinders in labeled beakers containing a range of sucrose concentrations (e.g., 0.0 M–0.8 M). Keep the volume of solution, potato type, temperature, and soaking duration constant across samples. After set incubation (e.g., 24 hours), blot dry again and record the final mass on a calibrated balance. Calculate the percentage change in mass for each sample.

Safety, Evaluation, and Interpretation

Handle sharps with care, cutting away from yourself on a safe surface under supervision. Wear goggles if splashing is possible. Dispose of potato pieces as biological waste. To interpret, graph the data with sucrose concentration (x) and percentage change in mass (y). The point where the graph crosses the x-axis marks the isotonic concentration. Consistent results across repeats indicate good reliability; evaluate equipment uncertainty and consider improvements (such as finer balances) to reduce percentage error, supporting more accurate conclusions.