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.

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.

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.

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).