Chemotaxis is the process of how cells follow chemical gradients. For example, if you get a small scratch on your skin, the healing is helped by a type of white blood cell called neutrophils that removes bacteria and other foreign materials from your wound. The neutrophils’ ability to leave the bloodstream and navigate to the tissue in the injury area seems uncanny, but is all because of the process of chemotaxis. This process is not only used by humans—or any other eukaryotic organisms—but is also used by prokaryotic bacterial cells that use the gradients to locate food. The tiny creatures use a kind of sampling mechanism to determine the direction of the chemical gradient, then move in that same direction. Eukaryotic cells do not move to find a chemical gradient, but rely on their size to measure the spatial differences across the cell body.

Yes, cells can survive in complete darkness. A theory on the origin of life argues that living systems arose in small compartments of total darkness located within iron sulfide rocks—originally formed by hot springs on the ocean floor. Today, plant root cells live in total darkness and carry out all normal plant cell activities, with the exception of photosynthesis. And, if you think about it, cells located in the very middle of the human body also live in a dark environment!