Scale in Biology
For centuries, curious observers have been grappling with the question “What is life?” Early naturalists documented observations about the living world long before anyone knew about cells. Their descriptions captured considerable detail, highlighting structures that exposed patterns that in turn hinted at relationships within and among organisms. Microscopy expanded the field of exploration, revealing hidden layers of life and paving the way for newer vistas of scientific inquiry. Here, the use of the term “scale” refers to relative size but also establishes new boundaries for comparison and exploration. To provide perspective on this site’s exploration of life from a macro to a microscopic scale, consider that the palm of an average human hand is approximately 10 centimeters across (or about 4 inches) and keep that in mind on the journey that follows, further and further into the microscopic world.
Written by Christina Wilson Bowers; Amherst College; Massachusetts, USA
Initial Idea by Katrina Velle; University of Massachusetts Dartmouth; Massachusetts, USA
Background images by (from the left) Igor Siwanowicz, HHMI’s Janelia Research Campus; National Institute of Allergy and Infectious Diseases, U.S. National Institutes of Health (NIH); and David Bushnell, Ken Westover, and Roger Kornberg, Stanford University, California, USA
Checking out a Chinese water dragon
Life is easy to recognize when it stares back at you with a magnetic gaze like that of this Chinese water dragon, which measures about one meter long (about 3 feet), including its tail. This reptile would be roughly 10 human-palm–widths long, according to the estimate noted above.
Even keeping in mind that human visual perception varies from individual to individual, it is reasonable to expect that you would notice a Chinese water dragon sunning itself in the distance well before you met one face to face. This image allows you to examine a water dragon at close range. Notice the patterns on its skin, including the dark stripe along the side of its face tracing a path to its ear. What underlying structures would allow the skin to grow this way?
Every so often, this reptile sheds its skin, allowing for the growth and replacement of worn-out cells. If you could peer into the cells beneath this creature’s surface, and then into the proteins within its cells, you might begin to understand how these invisible parts work together to form a living lizard.
Image by Igor Siwanowicz, HHMI’s Janelia Research Campus
Assessing spun glass
This spun glass caterpillar is able to eat in peace thanks to its glitzy protective shield. You would have to look closely to spot this creature in the wild, wandering around and consuming the leaves of a swamp oak, because it’s only about 1 centimeter long (a little less than half an inch), or roughly one-tenth the width of an average human palm. The fine hairs and translucent pointed extensions conceal the face of this little eating machine, warning would-be predators not to touch. And if you look closely at the extensions on its body, you can just make out green veins filled with chlorophyll pigment that was extracted from its last leafy meal. This caterpillar is small enough that the only real sign of it you might notice would be the trail of chomped leaf edges it leaves behind as it journeys along.
Image by Igor Siwanowicz, HHMI’s Janelia Research Campus
Rating rotifers
Some animals are small enough that they’re nearly invisible to the naked eye. Imagine a whole organism – with tissues and organ systems, including muscles and a digestive tract – that’s small enough to hide within the period at the end of this sentence. Meet the rotifer, named for the crown of cilia (tiny hairs stained red in this image) that sends currents containing food into its open mouth. A single rotifer, which is composed of about 1,000 cells, can range from half a millimeter to 1 millimeter (the smallest division on a metric ruler). That means it would take about 100 rotifers, laid out tail to crown, to stretch the width of an average human palm.
Rotifers are born with a complete set of cells. Unlike a human embryo, which grows and develops through cell division and specialization, rotifers become larger through cell growth alone.
Antonie van Leeuwenhoek, a 17th-century Dutch microbiologist, saw rotifers tumbling around when he viewed a drop of pond water with his hand lenses – famously calling them “animalcules.” He was the first person to describe the wide variety of rotifer body forms, which he did with painstaking attention to detail. Some rotifers spend their lives swimming free, while others grasp surfaces with a sticky “foot,” stretching it out, snapping it back, and lurching forward as they hunt for food. Imagine what van Leeuwenhoek must have thought upon his first sight of these complete creatures – existing beyond the realm of sight, eating, exploring, and very much alive.
Image by Igor Siwanowicz, HHMI’s Janelia Research Campus
Quantifying crossed hairs
Human hairs are visible to the naked eye, but they are very thin, and their dimensions are best appreciated when viewed with a light microscope. The width of the hairs shown here is 75 micrometers. About 1,300 hairs laid side by side would span the palm of an average human hand.
Hair is made of a protein called keratin, the same fibrous protein found in skin and fingernails. Keratin proteins are assembled into strands of hair in our hair follicles. Given what hair is made of and how it is built, would you consider hair to be alive in its own right?
Image by Michael Peres; Rochester Institute of Technology; New York, USA
Measuring a macrophage
If the cell is the basic unit of life, look no further than the mighty macrophage for an example of a relatively large unit. These cells, which measure about 20 micrometers across, prowl around your tissues, removing threats like cancerous cells and invading pathogens. The term macrophage is derived from a phrase meaning “big eater” in Greek. Macrophages move independently around your body, following chemical trails that signal infection or malfunction. Their flexible membranes engulf threats, sending them down the pathway toward destruction within the compartments of the cell. Without macrophages, our bodies would be vulnerable to infection and certain types of cancer.
It would take about 5,000 macrophage cells lined up end to end to span the width of an average human palm.
Image by National Institute of Allergy and Infectious Diseases, NIH
Counting out bacteria
According to some estimates, your body contains roughly equal numbers of human cells and bacterial cells. The microbes that occupy our bodies are distinct from our own cells, most obviously in size and structure. However, the communities of microbes that live on and inside us play a vital role in how our bodies function.
Bacteria perform all the functions of life within an efficient but tiny package that’s about 1 to 5 micrometers in length. Unlike a typical so-called eukaryotic cell, bacteria consist of a small volume of cytoplasm encased within an “envelope” of membranes – and sometimes a cell wall. Bacteria are always scanning for signals that could lead them toward food sources or away from hazards. Their high surface-to-volume ratio, a large-on-the-outside/small-on-the-inside arrangement, optimizes their sensing and processing of environmental signals – a necessity for these tiny cells, which have no room inside to stockpile resources.
Bacteria reproduce by a process called binary fission (splitting in two). Sometimes, two daughter cells will stay attached to one another for a while at the end of the process. If you look closely at this image, you can see signs of recent cell division.
If you were able to see a bacterium with your naked eye, a line of bacteria stretched across the width of an average human palm would number 100,000 cells!
Image by National Institute of Allergy and Infectious Diseases, NIH
Evaluating HIV
A single human immunodeficiency virus (HIV) – a number of which are shown here in yellow, nestled against a blue-green human H9 T cell – measures 100 nanometers across. Viruses are abundant, always existing in proximity to our own cells, hitching a ride and using our resources. Because of viruses’ simple structure and reliance on other cells’ molecular machinery to reproduce, there is ongoing debate about whether or not they should be considered “living.”
Virus particles (also called virions) hijack the molecular mechanisms of the host cells they infect to reproduce and spread. Even though they can lie dormant for a time, HIV virions in the end kill the cells they’ve infected. For example, HIV infects T cells, which are a crucial part of the human immune system; HIV virions are able to neutralize these crucial guardian cells, despite being 1,000 times smaller than their target.
You would need to line up about 1,000,000 (a million) HIV virions end to end to span the width of an average human palm.
Image by National Institute of Allergy and Infectious Diseases, NIH
Appraising RNA polymerase
Enzymes are molecular machines that do the work of living cells. These protein-based molecules come in a diverse array of shapes and sizes that support their varied functions. They can be found floating in a cell’s cytoplasm; suspended within the cell membrane; tucked away inside organelles in eukaryotic cells; or even free floating in the circulatory, digestive, and/or lymphatic systems of animals.
The enzyme shown here is about 15 nanometers across and is one of the most important enzymes of the living world. This RNA polymerase is nature’s decoder. A polymerase is an enzyme that builds a polymer, or chain of repeating subunits. RNA polymerase can be seen here “reading” a segment of DNA (the double-stranded blue, purple, yellow, and orange molecule in front of the enzyme). The RNA molecules created by RNA polymerase are strings of RNA nucleotides (the single-stranded pink and purple molecule) that are delivered to ribosomes for interpretation and protein synthesis.
It would take a line of about about 60,000,000 (60 million) RNA polymerase enzymes placed side by side to stretch across the width of an average human palm.
Image by David Bushnell, Ken Westover, and Roger Kornberg; Stanford University
Determining the diameter of DNA
This image shows a section of a DNA chain that measures 2.5 nanometers wide. If you can imagine these DNA segments lined up side by side and stretching across the width of an average human palm, the chain would repeat a dizzying 360,000,000 (360 million) times.
DNA can form chains that are much longer than the width of the DNA molecule. In fact, a bacterial cell like Escherichia coli has around 4,200 genes packaged into sections of DNA that if stretched out would measure 1.5 millimeters long – a little longer than a rotifer and 1,500 times longer than the diameter of the cell that contains them!
A typical human cell like a macrophage has about 2 meters of DNA (coding roughly 20,000 genes) packaged in its nucleus – about twice the length of a Chinese water dragon and roughly 20 times longer than the width of an average human palm.
Image by David Bushnell, Ken Westover, and Roger Kornberg; Stanford University
Finding unity amidst diversity
The narrower our view, the more similar life’s systems appear. Every cell in your body contains the same genetic code. The same physical DNA sequences are packaged in the cells of your skin as in those of your muscles and even the neurons of your brain. Under the guidance of RNA polymerase and an expansive suite of collaborating factors, our cells express our genes in different ways in response to specific cues. Even within a single human body, there is a diversity of form and function that is made possible by the blueprints provided in the genetic code.
DNA (as well as RNA) codifies and unites all life on Earth. Every cell in every organism on the planet, both extant and extinct, shares the key feature of having a genetic code. Throughout the history of life, the nucleotide sequences of A, G, C, and T (adenine, guanine, cytosine, and thymine) have been copied faithfully in preparation for cell division, connecting and communicating across generations. DNA tells a story at all scales of life, providing the blueprints for the size, structure, and function not only of the organisms highlighted here but of every living entity.
Image by National Institute of Allergy and Infectious Diseases, NIH
For suggestions on how to incorporate this journey into your teaching, see our “Implementation Suggestions.”