Reacting to Changes: Computers and Cells

Computers

In computer systems, you sometimes want the system to do X when Y happens. For example, whenever you get an email, you want the computer to make a sound. But, sometimes, the computer has no way of knowing that the condition is triggering right now. Instead, it has to constantly check if the condition has already triggered. In this example, it would constantly check if you have gotten a new email.

There is an interesting trade-off with the frequency of checking. The more often the computer checks if you have an email, the faster it can notify you. If it checks every hour, then in the worst case, it could be an hour late in telling you about it. If it checks every minute, then it could only be a minute late in notifying you. But the more often it checks, the more energy it wastes because most of the time you won’t have any emails anyways.

This is a common problem and common trade-off in tech. But, it turns out, it’s a trade-off our cells have to worry about too!

Living Cells

Living cells are all about reacting to stimuli. But how do they do that?! There is no brain inside cells that says do X when Y happens. Instead, cells survive by setting up a system such that the stimulus chemically causes a reaction that helps respond to the stimulus.

A prime example is how mammals process glucose. Right after you eat, there is a lot of glucose in your blood and the cells in your body need to process it. We achieve this in two steps.

First, the beta cells in your pancreas activate. The chemical reaction in the beta cells requires a very high level of glucose. When they don’t have that, their reactions aren’t running. When they have it, their chemical reactions proceed and release insulin. The rest of the cells in our body are set up so that their chemical reactions become much more efficient at processing glucose when insulin is present. So, we deal with a high glucose level because beta cells react to the glucose levels and the rest of the cells react to the output of beta cells.

But sometimes, the cell needs to react to the lack of something - how does it do that?

The main example is oxygen¹. When a cell is in an oxygenated environment, it properly respires and consumes energy. When the cell is in a low oxygen environment, it has to shift its processes and prepare for a harsher environment. But how do you cause the absence of something to trigger/participate in the chemical reaction? You poll, just like the computers!

The cell does this by constantly generating a protein called HIF1a. HIF1a is easily destroyed in the presence of oxygen. So, if HIF1a survives, you know you are in a low-oxygen environment. HIF1a also happens to be a protein that is able to go into the cell nucleus and activate different genes that help with dealing with low oxygen environments! For example, it can activate metabolic processes that are less efficient but that can generate energy without oxygen. It can also activate genes that reduce the overall activity of the cell, reducing energy demands.

The HIF1a acts like a polling mechanism. The cell produces it constantly and when the protein survives, the cell uses that as a signal to trigger the low-oxygen responses.

The cell has to waste energy creating a lot of HIF1a only for them to get destroyed almost all the time. But if we created it less frequently, we would react more slowly to low oxygen levels because we would be “polling” less often. So, just like computer systems, the cells have to manage the trade-off between energy use and response times. Just the difference is that the cells started doing it a few hundred million years before computers.