Explain cell theory and compare prokaryotic and eukaryotic cells.

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Cell theory

The cell is the foundation idea of biology because every organism, from a single bacterium to a blue whale, is built from cells. The modern cell theory has three core statements:

1. All living things are made of one or more cells.
2. The cell is the smallest unit that shows all the characteristics of life.
3. All cells come from pre-existing cells by division.

This theory developed over time as microscopes improved. Robert Hooke first described cork cells, and later workers such as Schleiden, Schwann, and Virchow added the ideas that all organisms are cellular and that cells reproduce only from other cells. The third statement directly rejects spontaneous generation, the old belief that living things could form from non-living matter.

Features shared by all cells

Despite huge variety, every cell has a few features in common, which reflects their shared evolutionary origin:

• A plasma membrane that separates the inside of the cell from its surroundings and controls what enters and leaves.
• Cytoplasm (or cytosol), the watery internal environment where chemical reactions occur.
• Genetic material in the form of DNA, which carries the instructions for building proteins.
• Ribosomes, the structures that build proteins.

Because all cells share these features, biologists treat them as strong evidence that all life descends from a common ancestor.

Prokaryotic cells

Prokaryotic cells are the cell type found in bacteria and archaea. The word prokaryote means before the nucleus, and the defining feature is that they have no membrane-bound nucleus. Key features include:

• A single circular chromosome free in the cytoplasm, in a region called the nucleoid.
• No membrane-bound organelles such as mitochondria or chloroplasts.
• Generally very small, around 1 to 10 micrometres across.
• A cell wall outside the membrane, and sometimes extra DNA in small rings called plasmids.

Prokaryotes usually live as single cells. They still carry out respiration and, in some cases, photosynthesis, but these processes happen at the plasma membrane rather than inside organelles.

Eukaryotic cells

Eukaryotic cells are found in animals, plants, fungi, and protists. The word eukaryote means true nucleus. Their defining feature is a membrane-bound nucleus that contains the DNA, packaged with proteins into linear chromosomes. They also have:

• Membrane-bound organelles such as mitochondria, the endoplasmic reticulum, and the Golgi apparatus, which separate different chemical jobs into compartments.
• A larger size, typically 10 to 100 micrometres.
• Often a complex internal skeleton (the cytoskeleton) that gives shape and moves materials.

Plant cells add a cellulose cell wall, chloroplasts for photosynthesis, and a large central vacuole. Animal cells lack these but may have features such as centrioles.

Why cell size matters

Cells stay small because they rely on diffusion across the membrane to exchange materials. A small cell has a large surface area relative to its volume, so materials move in and out quickly enough to meet the cell's needs. This is why most cells, even in large organisms, are microscopic, and why large organisms are built from many small cells rather than a few giant ones.