Cells as the basis of life

Every living thing is made of one or more cells, and every cell comes from a pre-existing cell. This idea, the cell theory, is the foundation for the whole Year 11 course. Cells fall into two broad groups. Prokaryotic cells (bacteria and archaea) are small, usually 1–10 µm, and lack a membrane-bound nucleus. Their DNA is a single circular chromosome in a region called the nucleoid, often with extra small loops called plasmids.

They have 70S ribosomes, a cell wall (peptidoglycan in bacteria), and sometimes a capsule, flagella and pili. Eukaryotic cells (animals, plants, fungi, protists) are larger, usually 10–100 µm, and are compartmentalised by internal membranes into organelles.

• Nucleus – double membrane (nuclear envelope) with pores; holds linear chromosomes; nucleolus makes ribosomal subunits.
• Ribosomes – 80S, free or on rough ER; site of protein synthesis.
• Rough ER – folds and transports proteins; smooth ER – lipid synthesis and detoxification.
• Golgi apparatus – modifies, sorts and packages proteins into vesicles for secretion.
• Mitochondria – double membrane, inner membrane folded into cristae; site of aerobic respiration.
• Chloroplasts (plants only) – double membrane, thylakoids stacked into grana; site of photosynthesis.
• Lysosomes (mainly animal) – contain digestive enzymes; vacuole – large permanent central vacuole in plants maintains turgor.
• Cell wall (plants: cellulose) – support and resistance to bursting.

Plant cells have a cellulose wall, chloroplasts and a large central vacuole; animal cells lack these but have centrioles and often small vacuoles and lysosomes. Both have a cell membrane, cytoplasm, nucleus, mitochondria, ER, Golgi and ribosomes.

Microscopy and cell size. A light microscope uses visible light and lenses; total magnification = eyepiece × objective. Resolution is limited to about 0.2 µm, so organelles smaller than this (ribosomes, ER detail) cannot be seen. A wet mount is made by placing a thin specimen in a drop of water or stain (e.g. iodine for onion cells, methylene blue for cheek cells) on a slide and lowering a coverslip at an angle to avoid air bubbles.

Cell size can be estimated by counting how many cells fit across the field of view or by using a mini-grid (a transparent grid of known square size placed over the specimen). The electron microscope (1930s onward) uses electron beams, giving resolution near 1 nm and revealing membranes, ribosomes and the internal structure of mitochondria. This led to the fluid mosaic model and the endosymbiotic theory. Later fluorescence and confocal microscopy let scientists track labelled molecules in living cells.