Date ______
The Light Reactions of Photosynthesis
Name _________________
Suppose that you could observe what happens inside a chloroplast as sunlight strikes a leaf. Within the thylakoid
membrane, chlorophyll and other molecules are arranged in clusters called photosystems. Each photosystem contains a
few hundred pigment molecules, including chlorophyll a, chlorophyll b, and carotenoids. This cluster of pigment
molecules acts like a light-gathering panel, somewhat like a miniature version of a solar collector.
When light strikes the chloroplast, pigment molecules absorb the energy. This energy jumps from molecule to molecule
until it arrives at the reaction center.
Each time a pigment molecule absorbs light energy, one of the pigment's electrons gains energy—the electron is raised
from a low-energy "ground state" to a high-energy "excited state." This excited state is very unstable. Almost
immediately, the excited electron falls back to the ground state and transfers the energy to a neighboring molecule. The
energy transfer excites an electron in the receiving molecule. When this electron drops back to the ground state, it
excites an electron in the next pigment molecule, and so on. In this way, the energy "jumps" from molecule to molecule
until it arrives at what is called the reaction center of the photosystem.
The reaction center consists of a chlorophyll a molecule located next to another molecule called a primary electron
acceptor. The primary electron acceptor is a molecule that traps the excited electron from the chlorophyll a molecule.
Other teams of molecules built into the thylakoid membrane can now use that trapped energy to make ATP and NADPH.
Two photosystems are involved in the light reactions, as shown in Figure 8-10. The first photosystem traps light energy
and transfers the light-excited electrons to an electron transport chain. This photosystem can be thought of as the
"water-splitting photosystem" because the electrons are replaced by splitting a molecule of water. This process releases
oxygen as a waste product, and also releases hydrogen ions.
The electron transport chain connecting the two photosystems releases energy, which the chloroplast uses to make ATP.
This mechanism of ATP production is very similar to ATP production in cellular respiration. In both cases, an electron
transport chain pumps hydrogen ions across a membrane—the inner mitochondrial membrane in respiration and the
thylakoid membrane in photosynthesis. The main difference is that in respiration food provides the electrons for the
electron transport chain, while in photosynthesis light-excited electrons from chlorophyll travel down the chain.
The second photosystem can be thought of as the "NADPH-producing photosystem." This photosystem produces NADPH
by transferring excited electrons and hydrogen ions to NADP+. Figure 8-11 shows a mechanical analogy for the light
reactions. Note how the light energy "bumps up" the electrons to their excited state in each photosystem.
Date ______
The Light Reactions of Photosynthesis
Name _________________
In this "construction analogy" for the light reactions, the input of light energy is represented by the large yellow mallets.
The light energy boosts the electrons up to their excited states atop the platform in each photosystem. The energy
released as the electrons move down the electron transport chain between the photosystems is used to pump hydrogen
ions across a membrane and produce ATP.
The light reactions convert light energy to the chemical energy of ATP and NADPH. But recall that photosynthesis also
produces sugar. So far no sugar has been produced. That is the job of the Calvin cycle, which uses the ATP and NADPH
produced by the light reactions.
Date ______
The Light Reactions of Photosynthesis
Name _________________
1. In the diagram color code the following structures:
o A molecule of chlorophyll
o Photosystem I
o Photosystem II
o Phospholipid bilayer
o ATP synthase
o Electron transport chain
2. Complete the Venn Diagram comparing and contrasting Photosystem I and Photosystem II
Photosystem II
Photosystem I
Date ______
The Light Reactions of Photosynthesis
Name _________________
3. The main function of the electron transport chain is to pump H+ ions into the inside of the thylakoid which
requires energy.
a. This is an example of what type of transport.
b. Explain why this process requires energy?
c. Where do the proteins get the energy to pump the H+ ions.
4. During the light reaction, a molecule of water is split apart. Explain what happens to each of the following
a. Electrons:
b. Hydrogen ions:
c. Oxygen Atom:
5. At photosystem I, NADP+ is converted into NADPH.
a. Explain the difference between NADP+ and NADPH.
b. Explain how NADP+ is converted into NADPH.
6. Once the electrons are inside of the thylakoid, they move to the stroma without the use of energy. The only way
that H+ ions can move into the stroma is to pass through a protein called ATP synthase.
a. Explain why it does not take any energy for the H+ ions to move to the stroma.
b. This is an example of what type of transport.
c. Explain why H+ ions cannot pass through the phospholipid bilayer.
7. ATP Synthase is an enzyme and like all enzymes it speeds up a chemical reaction.
a. What is the reaction that is catalyzed by ATP Synthase?
b. If cell was unable to maintain homeostasis and the temperature became too hot how could this prevent ATP
synthase from performing its function?
c. Explain how an increasing the concentration of H+ ions in the stroma could prevent ATP synthase from
performing its function.