Across
1
Center of an atom or cell
5
Form of thermal energy
6
Gas essential for breathing
7
Biological catalyst
12
Unit of energy
Down
2
Preserved remains of ancient life
3
Unit of heredity
4
Blocking of light by a celestial body
8
Tissue that contracts for movement
9
Curved glass that refracts light
10
Substance with pH below 7
11
Second lightest element