In an episode of the original Star Trek series titled "The Immunity Syndrome," the Enterprise encounters a gigantic single-celled organism resembling an amoeba. The organism is perhaps thousands of kilometers across, much larger than the Enterprise. In fact, the Enterprise penetrates the organism's cell membrane and goes into its cytoplasm to destroy it. Could such an organism be possible?
A single cell depends upon the process of diffusion to get materials across its membrane and move materials within it. Diffusion is the movement of a substance from an area of high concentration to an area of low concentration. You can smell an onion cut in the kitchen in another room because odor molecules from the onion move from an area of high concentration (the onion) to an area of low concentration (the kitchen and other rooms). For cells, diffusion works most efficiently over short distances (1 to 100 microns, 1 micron is 1 millionth of a meter).
In addition to short distances, cells need a large surface area for diffusion to be efficient. Most cells are spherical or cuboidal. Let's look at an example of a spherical cell with a radius (r). The volume of a sphere is given by the formula V = 4/3 r3, while the surface area is given by the formula A = 4 r2. As the cell grows and r increases, the volume gets much larger than the surface area (volume is a cubic function of r, while, surface area is a square function of r). As the cell grows, diffusion can no longer bring materials into the center of the cell because the distance becomes too great. So, a practical limit on the size of a single cell is about 100 microns in diameter or less. Gigantic single cells like that depicted in the Star Trek's "The Immunity Syndrome" could not survive. Large organisms, including humans, are made of many small cells and use circulatory systems to deliver oxygen and nutrients to cells and to remove carbon dioxide and wastes from them.
On a related note, several 1950s films such as "Them" depict large insects like ants and spiders that are as large or larger than a man. Even "Harry Potter and the Chamber of Secrets" has a huge spider and many spiders the size of dogs. Insects have no active ventilation system (such as lungs) to bring in air. Instead, they rely on a system of branching tubes called trachioles to bring air close enough to each cell in their body and rely on diffusion of air through these tubes. The larger the insect, the greater the distance the air must travel and the less efficient diffusion becomes. So, this is why you do not see gigantic spiders and ants roaming the planet.
Another reason for not seeing large insects is that their long thin legs would not support large bodies in normal Earth gravity. So, the alien from the "Alien" movie series would probably not be able to walk around in normal gravity. The largest land animal is the African elephant and it has four large, broad legs to support its body weight. The appearance of large aliens is still a popular attraction in many sci-fi films, but you do not see large single-celled aliens much anymore.
Science continues to evolve and new discoveries are made all of the time, so someday we may look back on some of these mistakes with the amused nostalgia that viewers of films like "Robinson Crusoe on Mars" feel today. But as long as the science is plausible, the film doesn't push our willingness to suspend disbelief and (some would say, most importantly) the story is engaging, we can always enjoy science fiction.
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