How do radioactive isotopes differ from isotopes?

In organic compounds, groups of atoms directly involved in chemical reactions are called ________. 

A base ________. 

The functional compartments of a cell are called ________________. 

People with lactose intolerance do not have enough ________. 

DNA nucleotides include ________. 

Plants use which molecule as an energy-storing carbohydrate? 

The branched structure of human lungs significantly increases the surface area for gas exchange which increases the efficiency of gas exchange with the lungs.  Which theme of biology does this example illustrate? 

Which substance is NOT recycled but moves through an ecosystems? 

Sugar dissolves when stirred into coffee. The coffee is the ________, the sugar is the ________, and the sweetened coffee is the ________.