Setting aside skin pigmentation (because it’s trivial), we are left with three major colors. Blood is red, urine is light yellow, and feces are brown. Those three are tightly related, and why I bring up the topic. Blood as you know hauls oxygen around. Heme has an iron atom in the middle, surrounded by five-member rings containing nitrogen atoms (pyrrole). Those rings allow the iron to shuttle electrons in and out quickly and without much energy. Otherwise, the iron would just rust, not reversibly and way too slowly. Unfit for purpose. So, blood containing heme is important for large animals because most of their cells are remote from air. Blood cells are disassembled and replaced rapidly because they have pretty rough ‘lives’. After being pushed through ‘tight-fit’ capillaries millions of times, they get ragged. This recycling is efficient but not completely so. Pyrrole rings that don’t get made into new heme are disposed of in two ways. Urobilin exits the pee way and there’s your yellow. Stercobilin exits the poop way and there’s your brown. Whole thing is in service to transporting oxygen to inwardly remote cells. Ever had a big bruise? I know you have. And that you’ve observed that ‘stranded’ blood losing its pyrrole electron helpers and iron changing in color from red through others to green as iron atoms slowly lose their oxygen. Muscles (our linear motors) use heme in a closely related way and they also send it to recyclers. Maybe we’ll look at those details later. For now I’ll only mention that cooked meat gravy is brown and hope that information above does not make you appreciate it less. If yellow, would you appreciate it more? Capillaries are small (and many) because they need to be. Evolution cannot beat physical limitations of oxygen diffusion. Many for sure here know this story, but having perhaps hooked readers with the excretory rainbow… You have two capillary networks. Oxygen-source is in lungs. Oxygen-sink is everywhere else. These two loops are pumped by one heart. It has 4 chambers (two on each loop) because inflation and ejection need to be valved separately. Sure, rotary pumps might work instead, but wheels are rare in organismal evolution. They’d also put two more squishes on red blood cells for each full loop. Add to trauma for our suffering little heme-helpers. Probably increase your protein requirements, and add to color of your pyrrolic outputs.
Pill bugs? My understanding is some plankton sized critters have rotary motion of their flagellum. Not a rotation but more like a whip motion that resembles a cork screw: Bob Wilson
When the topic of flagellin arises, we're talking Pandora's Box, as the flagellin & its various components are the subject of molecular machines; Molecular Machines Museum & so what are we to surmise? how is THAT possible ... Motors .... stators .... U-joints .... propellers .... which of these specialized individual components came first .... .
Sometimes you want to chat about something and not have to explain the basics. So I use noise canceling ear buds at bars not so much for muting the volume as much as the content. Bob Wilson
Dovetail'ing & Segway'ing back to OP's topic (i think) (mis?)understanding Bob's 'flagellin' reference as an example that maybe it's not so .... that "wheels are rare" as tochatihu states .... because observationally - at the molecular level - inexplicably - there they are. No hem carrying blood cells necessary. The lowly flagellin necessarily raises a question from anyone with an ounce of inquiry; "how can this be? " ... such complexity at such a basic level. .
There ar ion channels in the cell membrane for moving things in and out, and one looping within the membrane could have been the first nano wheel.
Flagellar rotary motors are an excellent example. Because they are rare only in the sense that most biological motion is linear. Also because a sneaky teacher will say something that is not quite true, hoping that smart listeners will correct things. And that such corrections are memorable. Talking about pee and poo and sex are not the only tricks in the book...
One motor 'runs backwards' to directly convert mechanical force to voltage. So we hear Prestin - Wikipedia
