In Darwin's time living cells were regarded as simple bags of chemicals that could have arisen spontaneously from organic compounds. However, it is now clear that cells contain intricate biochemical machinery. The steps by which this machinery may have originated are unknown and difficult to imagine. Thus it is no longer justifiable to simply take it for granted that living cells have evolved from chemicals by physical processes. Some important structures of typical plant and animal cells are depicted in this illustration.

The ribosomes manufacture protein molecules by following blueprints encoded in messenger RNA. Although they appear here as mere dots, the ribosomes have a complex structure.

The endoplasmic reticulum consists of a complex of membranes that form internal compartments used in the synthesis and transport of various compounds produced by the cell.

The nucleus contains the hereditary material, DNA, which carries instructions for the operation and perpetuation of the cellular machinery. Complex molecular processes are involved in replicating the DNA.

The nucleolus is a factory for the partial manufacture of ribosomes.

The microtubules form a complex latticework that gives form to the cell and enables it to systematically move and change shape.

Some cells possess cilia, whiplike structures that execute a swimming stroke through the action of an internal arrangement of sliding rods.

Lysosomes contain enzymes that break down unwanted material within the cell.

The chloroplasts found in plant cells are complex chemical factories that carry out photosynthesis the storage of solar energy in the form of sugar molecules.

The cellular membrane is equipped with many complex protein molecules that regulate the passage of molecules into and out of the cell and act as sensors informing the cell of external conditions.

The mitochondria are chemical factories that generate energy for the cell through the controlled breakdown of food molecules.