Key concepts in biology Revision Notes

Eukaryotic Cells: Animal and Plant

Eukaryotic cells, such as those in animals and plants, are complex, featuring a true nucleus and various membrane-bound organelles. Animal cells include the nucleus (controls cell, contains DNA), cell membrane (controls entry/exit), mitochondria (site of aerobic respiration, energy release), and ribosomes (protein synthesis). Plant cells have these too, plus a cell wall (structural support), chloroplasts (site of photosynthesis), and a large central vacuole (maintains turgor, stores substances). The specialised functions of these organelles are critical for the survival and specific roles of animal and plant cells within their respective organisms.

Prokaryotic Cells: Bacteria

Prokaryotic cells, like bacteria, are simpler and lack a membrane-bound nucleus; their genetic material (chromosomal DNA) is free in the cytoplasm. They also contain small, circular pieces of DNA called plasmids, which can carry genes for antibiotic resistance. Like all cells, bacteria have a cell membrane to regulate internal conditions and ribosomes for protein synthesis. Additionally, many bacteria possess flagella, which are whip-like structures enabling movement, allowing the cell to navigate towards resources or away from threats. The absence of complex organelles reflects their generally smaller size and often rapid reproduction rates.

Structure-Function Relationships

The relationship between a cell's subcellular structures and their functions is fundamental to biology. For example, the extensive inner membrane folds (cristae) in mitochondria increase the surface area for aerobic respiration, maximising energy production. Chloroplasts contain stacks of thylakoids (grana) for efficient light absorption during photosynthesis. The rigid cell wall of plants provides structural support against osmotic pressure, preventing the cell from bursting. Ribosomes, being vital for protein synthesis, are found in all cell types. Flagella provide motility for some bacteria, allowing them to move in their environment. These specialisations ensure cells can perform their roles effectively.

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.

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.

Sperm Cell Adaptations

Sperm cells are male gametes, highly adapted for fertilisation. They possess an acrosome, a cap-like structure at the head containing enzymes crucial for breaking down the outer layers of the egg cell. The head also contains a haploid nucleus, carrying half of the organism's genetic information. The mid-piece is packed with mitochondria, which produce the large amount of ATP (energy) needed to power the tail. The tail (flagellum) is long and whip-like, enabling the sperm cell to swim towards the egg.

Egg Cell Adaptations

Egg cells (ova) are female gametes, adapted to be fertilised and support early embryonic development. They are typically much larger than sperm cells and are rich in nutrients stored within their cytoplasm, providing an energy source for the developing embryo before implantation. Like sperm, they contain a haploid nucleus, contributing the other half of the genetic material. A critical adaptation occurs after fertilisation: the cell membrane undergoes changes to prevent additional sperm from entering, a mechanism known as the cortical reaction, which ensures only one sperm fertilises the egg.

Ciliated Epithelial Cell Adaptations

Ciliated epithelial cells are specialised for moving substances across surfaces. They are characterised by the presence of numerous cilia, which are small, hair-like projections on their apical surface. These cilia beat rhythmically in coordinated waves. In the trachea and bronchi, they sweep mucus, dust, and pathogens upwards and out of the respiratory tract. In the oviducts (fallopian tubes), their cilia help to move the egg cell from the ovary towards the uterus after ovulation.