Caffeine keeps you awake

Section 4 of 4

A cup of coffee keeps many people awake. Caffeine does it by sitting in a receptor and blocking a messenger.

Your cells run on energy carried by a molecule called ATP, and as they use it they leave adenosine behind. While you are awake, adenosine builds up in your brain. It fits adenosine receptors on nerve cells and switches them on, and switched-on adenosine receptors slow those cells down, so you feel sleepy. While you sleep, the adenosine level falls again.

Adenosine is adenine, one of the four bases in DNA, joined to a ribose sugar ring. Caffeine has two joined rings of the same sizes, with nitrogen atoms in the same four places.

Adenosine, C₁₀H₁₃N₅O₄Adenine, one of the bases in DNA, joined to a ribose sugar ring. Adenine is a six-membered ring and a five-membered ring sharing one edge, with four nitrogen atoms in its rings and an -NH₂ group.OOHOHOHNNNNNH2
Adenosine, C₁₀H₁₃N₅O₄: adenine, the two joined rings with an -⁠NH₂ group, joined to a ribose sugar ring.
Caffeine, C₈H₁₀N₄O₂A six-membered ring and a five-membered ring sharing one edge, like the rings of adenine, with four nitrogen atoms in the rings, two C=O groups and three methyl groups on nitrogen atoms.OONNNN
Caffeine, C₈H₁₀N₄O₂: the same pair of rings, with two C=O groups and three methyl groups on its nitrogens.

Sitting in the lock

Caffeine's rings fit the part of the receptor's pocket where adenosine's adenine sits. It has no sugar ring, and the receptor stays switched off. While caffeine sits in the pocket, adenosine cannot get in, so its message to slow down is blocked and you feel more awake.

Adenosine switches its receptor on, and caffeine blocks it Two panels. Left: adenosine, drawn as a blue pair of joined rings, a six-membered ring and a five-membered ring, sits in the pocket of a receptor protein, shaded teal. Its amber sugar ring, joined to the five-membered ring, rests on the protein surface beside the pocket. The receptor is switched on. Right: caffeine, drawn as the same blue pair of joined rings with no sugar ring, sits in the same pocket. The receptor stays switched off: it is blocked. adenosine receptor switched on caffeine receptor blocked
Adenosine's rings (blue) and sugar ring (amber) switch its receptor on. Caffeine fits the same pocket with its rings alone, and the receptor stays off.

Your liver breaks caffeine down: half of it is gone after about 3 to 7 hours, and the time varies a lot from person to person.

Where caffeine comes from

Coffee beans, tea leaves, cacao beans and kola nuts all hold caffeine. It tastes bitter and helps protect the plants from animals that eat them. Chocolate also holds theobromine, caffeine with one methyl group fewer, which blocks adenosine receptors more weakly. Dogs break theobromine down very slowly, so chocolate is poisonous to dogs.

Quick check

Why does a cup of coffee make you feel less sleepy?

That's the end of lesson 28

In this lesson you found out:

  • What hormones and neurotransmitters are, and how they fit their receptors
  • How your body builds dopamine and serotonin from amino acids
  • Why your body makes just one hand of adrenaline
  • How caffeine keeps you awake
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