Light-dependent Reactions

1. Pigments absorb light energy and give up electrons.
2. Two water molecules are split to produce one molecule of oxygen. The nucleotides ATP and NADPH form.
3. Electrons are replaced in the pigment molecules that first gave them up.

Electrons expelled from a chlorophyll molecule go through one or two electron transport systems, resulting in formation of ATP and NADPH.

Your textbook describes cyclic and noncyclic pathways of electron flow in great detail. This is what you need to know about these pathways:

The cyclic pathway is an ancient way to make ATP from ADP. It evolved in early bacteria.

The noncyclic pathway evolved later. Both pathways are still in use in all photosynthetic organisms. The noncyclic pathway operates at a higher energy level, allowing the formation of NADPH from NADP+. Water molecules are split into hydrogen and oxygen. This pathway allowed for the release of large quantities of oxygen into the atmosphere.

Thus, oxygen is a by-product of photosynthesis. Since about 2 billion years ago, oxygen has been accumulating in the atmosphere making aerobic respiration in animals possible.

These animations (Audio - Important) describe the noncyclic pathway:
light dependent reaction 1.
light-dependent reaction 2
light dependent reaction 3.
light dependent reaction 4

REVIEW: When light excites chlorophyll, the chlorophyll molecule
Answer absorbs the energy and moves an electron to a higher energy state.