Margaret Atwood - the Amoeba's Tale

Here is the gist of Margaret Atwood's tale

At midday an amoeba is sealed into a test-tube full of food. The amoeba feeds and divides in two every two minutes. By midnight all the food is gone, the test-tube is full of amoebas and they start to die from starvation. At what time would the amoeba realise that there was a problem? (That is if they were capable of rational thought.) Well, at two