How do organisms generate energy when oxygen is not available?

Fermentation releases energy from food molecules by producing ATP.

Explanation:

In the absence of oxygen, fermentation releases energy from food molecules by producing ATP.

Hope this helps...

Extra Information:

And assuming you are using the Biology Foundations Reading and Study Guide Workbook, more information regarding this topic can be found on page 123 (Chapter 10, Cellular Respiration).

they deliver oxygen to the blood