Which of the following is a difference between chlorophyll a and chlorophyll b? Chlorophyll a contains a Mg in its ring structure, whereas chlorophyll b contains iron. Chlorophyll a is a pigment, and chlorophyll b is the enzyme that transfers excited electrons from chlorophyll a to electron carriers of the thylakoid membrane. Chlorophyll a and b absorb light energy at slightly different wavelengths. Chlorophyll a absorbs yellow light, and chlorophyll b absorbs green.