Key concepts in biology revision notes

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.

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.

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.

Diffusion

Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration. This is a passive process, meaning it does not require metabolic energy from the cell. It occurs down a concentration gradient until equilibrium is reached. Substances like oxygen and carbon dioxide move into and out of cells via diffusion. The rate of diffusion is affected by factors such as temperature, surface area, concentration gradient, and the distance particles need to travel.

Osmosis

Osmosis is the net movement of water molecules across a selectively permeable membrane from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution). It is a special case of diffusion involving only water. This passive process is crucial for maintaining turgor in plant cells and preventing animal cells from bursting or shrivelling. Water potential refers to the likelihood of water molecules to move out of or into a solution.

Active Transport

Active transport is the movement of particles across a cell membrane against their concentration gradient, from a region of lower concentration to a region of higher concentration. This process requires energy, typically in the form of ATP, which is produced by respiration. Specific carrier proteins in the cell membrane are involved in binding to the substance and moving it across. Examples include the absorption of mineral ions by plant roots and glucose absorption in the small intestine.