Key concepts in biology revision notes

Variables and Controls

The independent variable is the food sample being tested. The dependent variable is the colour change observed with the reagent. Control variables include: volume of food sample, volume of reagent, concentration of reagent, temperature and heating time (for Benedict's test). A negative control (e.g., distilled water) and a positive control (e.g., glucose solution for Benedict's) should be used to confirm reagents are working correctly and to establish baseline colours.

Reproducible Method

For each food test: 1. Prepare a small sample of the food, crushing if solid, and dissolving in water if possible. 2. Place a small amount into a clean test tube. 3. Add the specific reagent for the target molecule (see below). 4. Observe and record any colour changes. For Benedict's test, heat gently in a water bath to 80°C for 5 minutes. For emulsion test, add ethanol, shake, then add water. Repeat for different food samples and controls.

Risk Assessment and Safety

Wear eye protection to prevent chemical splashes. Benedict's solution, iodine solution, sodium hydroxide, and copper(II) sulfate are irritants; avoid skin contact and wash hands thoroughly. Keep Bunsen burners (if used for heating water baths) away from flammable materials. Handle hot test tubes with test tube holders. Dispose of chemical waste according to school guidelines.

Result Interpretation and Evaluation

A positive test indicates the presence of the molecule. For example, blue-black with iodine means starch is present. Evaluate results against controls; a negative control should show no positive result. Identify sources of error such as contamination (using unwashed test tubes) or incorrect heating. Improve reliability by repeating tests multiple times per sample. A graph of results is not suitable as the data is qualitative (colour changes). Uncertainty in qualitative data is about subjective interpretation of colour, which can be reduced by comparing to a colour chart and having multiple observers. Percentage error is not applicable for qualitative data.

Improving Methods and Reliability

Method improvements focus on accuracy and precision, e.g., using measuring cylinders for consistent volumes. Reliability improvements are about reducing random errors and ensuring consistent results, e.g., repeating tests multiple times and calculating averages (though not for qualitative colour data), or by taking observations from multiple independent assessors to reduce subjective bias in colour interpretation. For Benedict's test, using a thermostatically controlled water bath ensures consistent heating.

Basic Calorimetry Principle

Calorimetry measures energy released from burning food. Heat from combustion is transferred to a known mass of water, causing its temperature to increase. This temperature change allows calculation of the energy released, providing an estimate of the food's energy content.

Key Formula for Energy Calculation

The energy absorbed by water is calculated using E = mcΔT. E represents energy in Joules (J), m is the mass of water in grams (g), c is the specific heat capacity of water (approximately 4.2 J/g°C), and ΔT is the change in water temperature in degrees Celsius (°C).

Factors Affecting Accuracy

Accuracy can be reduced by incomplete combustion of the food, which means not all potential energy is released. Significant heat loss to the surroundings also leads to underestimation. Improvements include insulating the apparatus and shielding from drafts to minimise heat loss.

Experimental Setup

A common setup involves suspending a food sample beneath a test tube or beaker containing a measured volume of water. The food is ignited, and the flame heats the water. A thermometer in the water records the temperature change from start to finish.

Light Microscope Limitations

Light microscopes use visible light and glass lenses. Their resolution is limited by the wavelength of light, typically to about 0.2 micrometres. This means they cannot resolve structures smaller than this, such as many organelles. Magnification is also lower compared to electron microscopes.

Electron Microscope Advantages

Electron microscopes use a beam of electrons and electromagnetic lenses. Electrons have a much shorter wavelength than visible light, resulting in significantly higher resolution (down to approximately 0.2 nanometres) and much greater magnification. This allows for detailed visualisation of sub-cellular structures and organelles, such as ribosomes and the internal structures of mitochondria, which are not visible with light microscopes. This enhanced clarity and detail have profoundly increased our knowledge of cell biology.

Impact on Understanding

The improved resolution and magnification provided by electron microscopy have enabled scientists to identify and characterise numerous organelles previously invisible or poorly understood. By seeing these structures in detail, researchers could better deduce their functions. For example, the detailed structure of mitochondria revealed by electron micrographs helped confirm their role in cellular respiration, and the discovery of ribosomes clarified the sites of protein synthesis. This led to a more comprehensive and accurate model of cell structure and function.

Understanding Osmosis and Mass Change

In osmosis, water moves across a partially permeable membrane. If a cell or tissue is placed in a solution with a higher water concentration (more dilute), water enters the cell, causing it to gain mass. Conversely, if placed in a solution with a lower water concentration (more concentrated), water leaves the cell, leading to a loss of mass.

Formula for Percentage Change in Mass

To calculate the percentage change in mass, use the formula: Percentage change in mass = ((Final mass - Initial mass) / Initial mass) × 100. A positive result indicates a gain in mass, while a negative result indicates a loss of mass.

Interpreting Results

A percentage gain suggests the tissue is in a hypotonic (more dilute) solution relative to its internal concentration, meaning water moved in. A percentage loss suggests the tissue is in a hypertonic (more concentrated) solution, meaning water moved out. A zero percentage change would imply the solution is isotonic (equal concentration).