I wanted to introduce a curious piece of biochemistry in an easy-to-understand way. The story has interesting implications for how evolution works and perhaps even the origin of life.
The system has some unique features that speak about how biology works.
Simple calcium signaling
The cells in your body send signals to each other usually through biochemicals such as hormones. Some of those chemicals enter the cell and do anything in it but usually, those chemicals interact with the molecules on the surface of a cell causing those molecules to release something. in the cell. Often, it involves calcium.
This diagram shows a way that can happen. The blue area is outside a cell; the beige is inside a cell. The gray barbells are the cell membrane, which separates the inside from the outside. The green thing is a receptor; in this case, a calcium channel usually closes until certain hormones arrive or attach and cause it to open, causing calcium to enter the cell.
That calcium causes a series of biochemical events that lead the cell to do whatever it needs to do. In this case, calcium acts as a messenger; it carries a signal from the membrane to the inside of the cell.
Second messenger calcium signaling
Often, the first action of the receptor is to produce some other molecule which is caused by the appearance of calcium in a cell which then affects the cell somehow. This image shows a stimulated EGFR receptor, which is caused by a long series of substances created that eventually cause calcium to be released from the Endoplasmic Reticulum (ER) store.
That calcium continues to carry the signal to other parts of the cell. Biologists refer to calcium as the second messenger even though, in some cases, it is more like the 5th messenger.
Here is the strange part
Many signaling systems use calcium as the first or second messenger. This diagram shows some of these paths but many people find it not really possible to show most of them on a chart. Each of the objects sitting in the cell membrane are receptors for different types of signals.
Each of these signals is translated into higher calcium levels. It is very strange to have so many messages relayed by the same messenger. Here are just a few of the messages it carries:
Muscles use calcium to control circulation.
Neurons use calcium to control the release of neurotransmitters.
Many endocrine organs use calcium to control hormone release.
Calcium is involved in the activation of T cells and B cells in the immune system.
Calcium release is an important step in the process by which eggs become embryos and play important roles in many other stages of embryonic development.
Calcium is involved in cell proliferation (reproduction of new cells).
Calcium regulates programmed cell death, apoptosis.
Note that the last two items in this list are contradictory. Exactly how does a substance indicate both cell growth and cell death? This is an active research area and I cannot answer that question properly.
The other strange part
Calcium is harmful. It can change proteins so that they do not work and react with phosphate so that it is not used. It can be an important cause of brain damage. Your cells have complex machinery to keep it at very, very, very low level.
This system has been around for a very, very, long time.
All plants and animals use calcium as a signal. Although many single cell organisms use calcium as a signal for many different things.
Calcium is one of the most widely used secondary messengers.
This chart lists the more common ones.
They all resemble calcium in each of them is used for many messages. Some, like CO, are toxic.
Conclusion about evolution Evolution never uses a simple solution if a complexity works as well.
Calcium signaling is a good example of this. Sometimes it takes a few biochemical steps before the calcium is finally released from its store. And sometimes it takes many other steps before the increased amount of calcium is converted to the appropriate biochemical action. Evolution wants to reuse organs and biochemicals. Calcium is a good example of this. Over the course of evolutionary history, animals and plants have evolved in many ways to use calcium to signal new things, sometimes evolving conflicting messages from the same messenger.
