Photosynthesis: Part 5: Light Reactions | HHMI BioInteractive Video

Summary

The light reactions of photosynthesis occur in the thylakoid membranes, where light is absorbed by protein-pigment complexes called photosystems. These photosystems convert light energy into chemical energy by exciting electrons and transferring them through an electron transport chain. The process involves water splitting, oxygen release, and the creation of a proton gradient. The energy from light is stored in NADPH and ATP, which can later be used in the Calvin cycle. This continuous cycle, powered by light, facilitates energy conversion and electron transfer to sustain the plant's metabolic processes.

Takeaways

• 😀 Light is absorbed by protein-pigment complexes known as photosystems, which are essential for the light reactions of photosynthesis.
• 😀 There are two photosystems: Photosystem I and Photosystem II, both of which help convert light energy into chemical energy.
• 😀 Photosystems work by exciting electrons, which are transferred through an electron transport chain on the thylakoid membrane.
• 😀 The process of electron transport involves molecules passing electrons from one to the next, generating energy to fuel the synthesis of ATP and NADPH.
• 😀 Chlorophyll in Photosystem II absorbs light, exciting electrons that are passed to electron carriers, starting the electron transport chain.
• 😀 Water molecules split in Photosystem II, releasing electrons, protons, and oxygen as byproducts. The oxygen is released into the atmosphere.
• 😀 Electrons from Photosystem II move to the cytochrome complex, which uses energy from the electrons to pump protons into the lumen of the thylakoid.
• 😀 The excited electrons are passed to Photosystem I, where they are re-excited by light, and then passed to a third electron carrier.
• 😀 The final electron acceptor in the light reactions is NADP+, which combines with electrons to form NADPH, storing energy for later use.
• 😀 A proton gradient is created across the thylakoid membrane, and protons diffuse through ATP synthase, which uses this energy to produce ATP.

Q & A

What are the thylakoid membranes responsible for in photosynthesis?

The thylakoid membranes contain specialized molecules that work together to perform the light reactions of photosynthesis, where light energy is converted into chemical energy.

What are photosystems, and how do they function?

Photosystems are protein-pigment complexes in the thylakoid membrane that absorb light energy. There are two photosystems, photosystem I and photosystem II, and they use light energy to excite electrons and move them through an electron transport chain.

How does light energy get transformed into chemical energy in photosynthesis?

Light energy is absorbed by the photosystems, exciting electrons that are transferred through an electron transport chain on the thylakoid membrane. This process converts light energy into chemical energy, which is stored in molecules like NADPH and ATP.

What happens to water molecules during the light reactions?

Water molecules are split during the light reactions, releasing electrons that replace those lost from photosystem II. This process also produces oxygen as a byproduct, which is released into the air, and hydrogen ions (protons) that accumulate in the thylakoid lumen.

What is the role of the cytochrome complex in the electron transport chain?

The cytochrome complex uses the energy from the excited electrons to pump additional protons into the thylakoid lumen, helping to establish a proton gradient across the thylakoid membrane.

What happens to the electrons after they pass through photosystem II?

After electrons pass through photosystem II, they lose most of their energy and are passed to photosystem I, where they are re-excited by photons of light.

How does NADPH form in the light reactions?

In photosystem I, excited electrons are passed to an enzyme that interacts with NADP+, the final electron acceptor. This results in the formation of NADPH, which stores energy from light.

What is the proton gradient, and how is it used in ATP synthesis?

The proton gradient is created by the accumulation of protons (hydrogen ions) in the thylakoid lumen. This gradient is used by ATP synthase to generate ATP by combining ADP with inorganic phosphate as protons diffuse back into the stroma.

What is the role of ATP synthase in the light reactions?

ATP synthase uses the energy stored in the proton gradient across the thylakoid membrane to combine ADP with inorganic phosphate to form ATP, which stores chemical energy for the Calvin cycle.

How do ATP and NADPH contribute to the Calvin cycle?

ATP and NADPH produced in the light reactions provide the energy and reducing power required for the Calvin cycle, which uses these molecules to fix carbon dioxide and produce sugars.