Key concepts in biology Revision Notes

Metric Prefixes for Small Scales

Biologists often deal with very small dimensions when studying cells and their organelles. To manage these small numbers effectively, the metric system uses prefixes that indicate specific powers of 10. Understanding these prefixes is essential for interpreting and converting measurements accurately. The common prefixes used for small scales are milli (m), micro (µ), nano (n), and pico (p).

Conversion Factors for Common Prefixes

It is important to remember the conversion factors for each prefix. A milli-unit is 10^-3 (one thousandth) of the base unit. A micro-unit is 10^-6 (one millionth) of the base unit. A nano-unit is 10^-9 (one billionth) of the base unit. A pico-unit is 10^-12 (one trillionth) of the base unit. To convert from a larger unit to a smaller unit, you multiply by the appropriate positive power of 10. To convert from a smaller unit to a larger unit, you divide by the appropriate positive power of 10 (or multiply by the negative power).

Practical Applications in Cell Biology

These units are widely used in cell biology. For instance, typical animal cells are about 10-100 µm in diameter, while bacteria are usually 1-10 µm. Viruses are even smaller, often measured in nanometres (e.g., 20-400 nm). Molecular structures like DNA strands or atoms require picometre scales. Accurate conversions are vital when comparing sizes, calculating concentrations, or working with microscopy data.

Key SI Prefixes for Tiny Measurements

Milli (10⁻³), micro (10⁻⁶), nano (10⁻⁹), and pico (10⁻¹²) represent fractional values of a metre. For example, 1 µm = 0.000001 m (micro), and 1 nm = 0.000000001 m (nano). Recognising these allows rapid conversion between units in calculations involving cells.

Standard Form and Unit Conversion

Express values as a × 10ᵇ, with 1 ≤ |a| < 10, for clarity and simplicity. To convert between SI prefixes, use appropriate factors of ten. For example, to go from micrometres to nanometres, multiply by 1000 (since 1 µm = 10⁻⁶ m and 1 nm = 10⁻⁹ m).

Enzyme Structure and Active Site

Enzymes are proteins with a complex 3D shape. A crucial part of this shape is the active site, a pocket or groove where a specific substrate molecule binds. The active site's shape is complementary to the substrate, much like a lock fits a specific key.

Mechanism of Action: Lock and Key, Induced Fit

Substrates bind to the enzyme's active site, forming an enzyme-substrate complex. The 'lock and key' model suggests a perfect fit. The 'induced fit' model refines this, stating that the enzyme's active site can slightly change its shape to better accommodate the substrate upon binding, optimising the reaction. Products are then released, and the enzyme is ready for another reaction.

Enzyme Specificity

Due to the unique and complementary shape of its active site, each enzyme is typically specific to only one type of substrate or a very small group of structurally similar substrates. This specificity ensures that the correct biochemical reactions occur within the cell.

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.