Four Seats

Inside a red cell

It was red, and it was packed. The spark had brought them into a red blood cell, and the cell was so full of one kind of molecule that there was hardly room for anything else. Hundreds of millions of them, bumping, rolling, pressed close.

"Haemoglobin," said Danya, who had read the word on a poster at Brightfield School. "It's the stuff that carries the oxygen."

Each one was made of four chains, curled up and packed together into one rounded lump. Two chains were one kind, two were another kind, fitted together like four pieces of a puzzle. Tucked into each chain was a flat ring with an iron atom at its centre.

"Four rings," said Beto, counting. "Four seats."

An oxygen molecule bumped past, rattled about, bumped into one of the iron atoms and stuck there. One seat full.

Danya's sum

Danya cares about things being fair, and he counts things. He counted now.

"Right," he said. "Four seats. Each seat has the same chance of getting an oxygen. So it's just four separate chances, one for each seat. Every seat on its own."

It was a good, tidy idea, and the Fold did not care about it at all.

Because the first oxygen had stuck, and the whole molecule shifted. Not much. Its four chains moved a tiny bit against each other, the way a folding chair settles when you put weight on one corner. And in the new shape, the other three seats held oxygen more easily than before.

The next oxygen that bumped into an iron stuck almost at once. Then another. Then the last.

"That's not four separate chances," said Beto. "The first one changes the others."

Danya stared. "So the more seats are full," he said slowly, "the easier the rest fill up."

"And the other way?"

They waited, and before long they saw it. Somewhere the cell passed through a place with very little oxygen around, and an oxygen came loose from its seat and drifted off. The molecule shifted back a little toward its first shape, and the next oxygen came loose more easily, and the next.

"It's all or mostly nothing," said Beto.

"Not all or nothing," said Danya. "But close. That's why it works. In your lungs there's loads of oxygen, so the seats nearly all fill up. In a hot leg that's been running, there's not much, so loads of them let go at once."

He thought about his legs, burning on the grass. "And running makes your muscles a bit acidic, too," he added, because he had read about that as well. "That makes it let go even more."

"Your blood is clever," said Beto.

"It isn't clever," said Danya. "It doesn't plan anything. It's a shape that changes when oxygen sticks to it. The clever bit is that the shape does that."

How can anyone be sure?

Of course he asked. He always asks.

Some of it was measured. People had grown crystals of haemoglobin with oxygen and without it, and fired X-rays through both, and the two shapes were slightly different, just as they had seen. Other people had measured how much oxygen blood holds at each amount of oxygen around it, and drawn the curve, and the curve was S-shaped: flat at the bottom, steep in the middle, flat at the top. Four separate chances would have made a smooth bend instead.

Some of it was predicted. AlphaFold had predicted each of the two kinds of chain, one at a time, and it was very sure about almost all of each one. Only the very first piece of each chain was pencil.

"But it did one chain at a time," said Danya. "No rings. No oxygen. So it can't show the shape changing."

"So the seats came from the crystals," said Beto.

"The seats came from the crystals," said Danya. "And the S came from the curve. And we came from the spark."

The spark flickered.