Biological complexity: homeostasis from feedback loops (Introduction)

by David Turell , Friday, March 02, 2018, 15:47 (2458 days ago) @ David Turell

I have mentioned feedback loops over and over as a explanation of automaticity in life. They are a requirement for automatic homeostasis:

https://aeon.co/essays/the-feedback-loop-is-a-better-symbol-of-life-than-the-helix?utm_...

"The union of genetics with molecular biology has undoubtedly created a powerful new science. By unpicking the molecular interactions inside cells, we’ve been able to develop drugs to mimic or interfere with specific processes. The discovery of enzymes that synthesise and edit DNA laid the foundations for genetic engineering, which is usually discussed in terms of its commercial applications, but whose most common use has always been in pure research. In this way, advances in knowledge create the tools to advance knowledge, in a virtuous circle. Nevertheless, we appear to have come to a threshold. The more we know about the molecular processes, the less sense the gene-centric perspective makes.

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"Schooling and flocking behaviours are, of course, restricted to large groups of individuals from a single species. Might self-organising properties operate even at the level of multi-species ecosystems? There’s reason to think that they do. Vegetation on arid soils will, for example, arrange itself into groups with plain soil between them: the plants in the groups all benefit from their mutual ability to help rare rainfall penetrate the ground. And in simple experimental microbial communities (not, in fact, different species, but yeasts genetically engineered to have different metabolisms that can co-operate to use environmental nutrients), the different types of individual self-organise into mixed clusters that bring co‑operators together. Many ecologists believe that large-scale ecosystems of co‑operating organisms, for example trees and fungi, show similar self‑organising behaviour.

"All of these examples, and many more like them, turn out to have something in common when analysed at the mechanistic level: in each case, what has been achieved so far by the system is used to control its current behaviour. This type of control is called feedback, and is represented by a loop feeding information from the output of a process back to its input.

"In the case of the cytoskeleton, the stability of a filament depended on whether it was carrying a mechanical force, which in turn depended on whether it was in the right place to connect to cell junctions. The achievement of a filament (to be in a useful place or not) is therefore fed back to decide what it will do next (survive, or be disassembled). In the case of the blood capillaries, the extent to which present growth has been adequate to bring enough oxygen into the tissues is fed back, via VEGF, to control whether the capillaries continue to grow or remain as they are. And the same principle seemed to explain the schooling of fish: any error in an individual fish’s relative positions and direction compared to its neighbours is used to modify its swimming, to make the error smaller. Seen from the abstract perspective of feedback loops, adaptive self-organisation looks more or less the same across all scales of life, from the architecture of subcellular assemblies to the arrangements of co-operating species in ecosystems.

"In this sense, the loop is a near-universal symbol of living processes.

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"The DNA helix is important, of course. But the most important thing it does is make proteins that can operate in regulatory loops. These loops can also operate at the molecular level: genes make proteins, but these proteins determine which genes are ‘off’ and which are ‘on’ (as HIF1A does), making a control loop at even the molecular level. Unlike the helix, loops also operate at scales far above the molecular, covering a range of sizes from bacterial colonies to the vast ecosystems of the rainforest – perhaps to the ecosystem of the entire Earth. Beyond Earth, life without DNA is just about thinkable (one can imagine alternative strategies for storing information). Life without feedback loops, though? I have never met any biologist who can imagine that."

Comment: You may think my brain is rigid, but in medical school I was raised on feedback loops. That is how life works, and it creates reactions and responses that are automatic even as environmental challenges change. 'A' responds to a stimulus by triggering 'B', which nudges 'C', which tells 'D' which informs 'E' which comes back to 'A' to change its output. All the letters are complex protein molecules, and some of them are giant enzymes. Each step is a modulation of response. DNA simply codes the manufacture of the proteins. But the entire structure is designed to work harmoniously by God. I've skipped the author's long examples. The whole essay is worth reading.