Light Reactions and Energy Flow During Photosynthesis 🌞

The bipyridillium herbicides bind to electrons that would normally interact with Fd, therefore inhibiting its function. Analyze the figure depicting the events of the light reactions, and identify which of the following would occur in the presence of a bipyridillium herbicide.

NADPH production will be inhibited

Cytochrome Complex

group of reversibly oxidizable and reducible proteins that forms part of the electron transport chain between photosystem II and photosystem I

What is the source of energy for this process?

photons of light from the sun

The following conditions were found in a mutant cell:ATP levels are unaffected.The strength of the proton gradient is unaffected. NADP+ levels increase, NADPH becomes depleted. Use this information to predict which component of the light reactions and chemiosmosis is not functioning normally in this mutant cell.

photosystem I

P700

reaction center of photosystem I

P680

reaction center of photosystem II

NADP+ reductase

enzyme that transfers a proton and two electrons from ferredoxin to NADP+, forming NADPH (2 ELECTRONS are required).

The triazine herbicides bind to molecules within the primary electron acceptor of photosystem II and block electron flow. Analyze the figure depicting the events of the light reactions, and identify which of the following would occur in the presence of a triazine herbicide. What will occur?

-The proton gradient across the thylakoid membrane will be dissipated -Water consumption will decrease -ATP production will be inhibited -NADPH production will be inhibited

Place the steps of the process in the context of the locations in the chloroplast.

1) PS II and PS I along with the cytochrome complex span the thylakoid membrane extending to both sides. When electrons move between these complexes via electron carriers these carriers are positioned either within the thylakoid membrane (Pq), along the thylakoid space side of the membrane (Pc) or on the stromal side (Fd). 2) NADP+ Reductase is located in the stroma such that the NADPH produced will be present in the stroma. 3) ATP synthase is positioned in the thylakoid membrane such that H+ ions move down their concentration gradient through the enzyme complex from the thylakoid space to the stroma. The potential energy of the H+ ion gradient drives the production of ATP, which is released into the stroma.

Trace the path of electrons through PS II and PS I

1. H2O---> 2. P680---> 3. primary electron acceptor in PS II----> 4. Pq----> 5. cytochrome complex---> 6. Pc---> 7. P700---> 8. primary electron acceptor in PS I----> 9. Fd----> 10. NADP+ reductase----> 11. NADPH

Trace the path of energy through the light reactions, PS II?

1. Sun---> 2. PS I light-harvesting complex pigments---> 3. P700---> 4. primary electron acceptor in PS I----> 5. Fd----> 6. NADP+ reductase----> 7. NADP+ + H+ (becomes NADPH)

Trace the path of energy through the light reactions, PS I?

1. Sun---> 2. PS II light-harvesting complex pigments---> 3. P680---> 4. primary electron acceptor in PS II---> 5. Pq---> 6. cytochrome complex---> 7. thylakoid H+ ion concentration gradient---> 8. ATP Synthase---> 9. ADP +Pi (becomes ATP)

The following conditions were found in a mutant cell: ADP accumulates, ATP becomes depleted.The strength of the proton gradient increases. NADPH levels are unaffected. Use this information to predict which component of the light reactions and chemiosmosis is not functioning normally in this mutant cell.

ATP Synthase

NADPH

An electron carrier involved in photosynthesis. Light drives electrons from chlorophyll to NADP+, forming NADPH, which provides the high-energy electrons for the reduction of carbon dioxide to sugar in the Calvin cycle.

When is a low-energy electron transferred in the light reactions of photosynthesis?

Between the cytochrome complex and Pc

Think about the flow of energy through the light reactions. When is energy transferred from one electron to another, rather than transferring with a high-energy electron?

Between the pigment molecules of the light-harvesting complex and the special pair of chlorophyll a pigment molecules

How does the energy move through PS II and PS I?

In each of these photosystems, energy from the sun excites the electron in a light-harvesting complex pigment. This energy is passed from the electron of one pigment to the next until it reaches P680/P700. After this point the electron carrying the energy is passed onto the primary electron acceptor.

Where does the energy that passes through photosystem I end up?

NADPH

PQ (Pronator quadratus)

Plastoquinone (PQ) is an isoprenoid quinone molecule involved in the electron transport chain in the light-dependent reactions of photosynthesis

Explain how this process produces ATP?

The energy from the high-energy electron is used by the cytochrome complex to pump H+ ions against their gradient into the thylakoid space. The result is a high concentration of H+ ions in the thylakoid space. This gradient is used to drive ATP synthase. As H+ ions move through ATP synthase back to the stroma the potential energy of the concentration gradient is used to drive the ATP synthase that produces ATP from ADP and Pi.

How does the energy transferred to the cytochrome complex lead to ATP production?

The energy is used to create a high concentration of H+ in the thylakoid space that drives ATP synthase.

Explain how this process produces NADPH?

The high-energy electron passed through photosystem I is transferred to the electron carrier Fd and passed on to the NADP+ reductase enzyme which combines the electron with NADP+ and H+to create NADPH.

A plant is exposed to a high intensity of light but is not effectively carrying out photosynthesis. Predict why this might be the case using the diagram as a guide.

The light has a high proportion of energy at the 700 nm but not the 680 nm wavelength

What is the source of the electrons used to replenish the special pair of chlorophyll a pigment molecules in Photosystem II?

Water (H20)

Explain the role of water in this process?

Water is an electron donor. When P680 in PS II passes its electron to the primary electron acceptor P680 is now missing an electron. These electrons are replaced with electrons split from a water molecule. Additionally the H+ ions produced from splitting water contribute to the H+ ion concentration gradient within the thylakoid. The oxygen produced is released by the plant.

Fd

an iron containing protein which acts as an electron carrier

NADP+

carrier molecule that transfers high-energy electrons from chlorophyll to other molecules