Photosynthesis 03: The Light-Harvesting Reactions Part 2

Main goal of the light-harvesting reactions

To capture sunlight and convert it into chemical energy stored in ATP and NADPH.

Location of the light-harvesting reactions

In the thylakoid membranes of chloroplasts.

Two photosystems involved in non-cyclic photophosphorylation

Photosystem II ($\text{P680}$) and Photosystem I ($\text{P700}$).

Photosystem that initiates non-cyclic photophosphorylation

Photosystem II ($\text{P680}$).

Photosystem II (PSII)

A protein-pigment complex in the thylakoid membrane that absorbs light energy to initiate the light-dependent reactions.

$$\text{P680}$$

The reaction-center chlorophyll $a$ complex of Photosystem II.

Effect of photon absorption on Photosystem II

Light energy excites an electron in $\text{P680}$ from a ground energy state to a high-energy excited state.

Pheophytin

The primary electron acceptor in Photosystem II that receives an excited electron from $\text{P680}$.

Redox state of $\text{P680}$ after transferring an electron to pheophytin

It becomes oxidized ($\text{P680}^+$) due to the loss of an electron.

Redox state of pheophytin upon accepting an electron

It becomes reduced due to the gain of an electron.

Mechanism by which Photosystem II replaces lost electrons

Through photolysis, which splits water molecules to release replacement electrons.

Photolysis

The light-driven splitting of water into electrons, protons ($\text{H}^+$), and oxygen ($\text{O}_2$).

Balanced chemical equation for photolysis

$$2\text{H}<em>2\text{O} \rightarrow 4\text{H}^+ + 4e^- + \text{O}</em>2$$

Enzyme complex responsible for splitting water in PSII

The oxygen-evolving complex (OEC).

Why oxidized PSII acts as an extremely strong electron acceptor

Losing an electron makes $\text{P680}$ highly electronegative and oxidized ($\text{P680}^+$), allowing it to pull electrons away from water.

Thermodynamic classification of photolysis (endergonic vs. exergonic)

Endergonic, because it requires continuous energy input supplied by sunlight.

Destination of electrons generated by photolysis

They reduce and replenish the oxidized $\text{P680}$ reaction center in Photosystem II.

Fate of the $\text{H}^+$ ions produced during photolysis

They accumulate in the thylakoid lumen, contributing directly to the proton gradient.

Fate of the oxygen produced during photolysis

It is released out of the plant into the atmosphere as a metabolic byproduct.

Source molecule of the oxygen released during photosynthesis

Water ($\text{H}<em>2\text{O}$), not carbon dioxide ($\text{CO}</em>2$).

Path of electrons after leaving Photosystem II

They pass through an electron transport chain embedded in the thylakoid membrane toward Photosystem I.

Purpose of the electron transport chain between PSII and PSI

To transfer electrons energetically downhill and harness released energy to pump $\text{H}^+$ ions across the membrane.

Plastoquinone (PQ)

A lipid-soluble mobile electron carrier that transfers electrons and protons through the thylakoid membrane.

Events occurring when plastoquinone is reduced

It accepts two electrons from PSII and picks up two protons ($\text{H}^+$) from the stroma.

Events occurring when plastoquinone reaches the lumen side of the thylakoid membrane

It becomes oxidized by transferring electrons onward to the cytochrome complex and releases $\text{H}^+$ ions into the thylakoid lumen.

Role of the cytochrome complex

Transfers electrons along the ETC while translocating $\text{H}^+$ ions from the stroma into the thylakoid lumen.

Purpose of pumping $\text{H}^+$ ions into the thylakoid lumen

To generate an electrochemical proton gradient that stores potential energy for ATP synthesis.

Photosystem I (PSI)

A protein-pigment complex in the thylakoid membrane that absorbs light energy to re-energize electrons delivered from PSII.

$$\text{P700}$$

The reaction-center chlorophyll $a$ complex of Photosystem I.

Effect of light absorption on Photosystem I

Light energy excites electrons in $\text{P700}$ to a higher energy level, allowing their transfer to ferredoxin.

Destination of excited electrons leaving Photosystem I

They enter a secondary electron transport chain and are transferred to reduce $\text{NADP}^+$ into NADPH.

Ferredoxin

An iron-sulfur mobile protein that receives electrons from PSI and transfers them to $\text{NADP}^+$ reductase.

Outcome when $\text{NADP}^+$ is reduced

It gains two electrons and a proton ($\text{H}^+$) to form NADPH.

Role of NADPH in photosynthesis

Acts as a carrier of high-energy electrons to the Calvin cycle to reduce carbon dioxide into sugars.

Difference between $\text{NADP}^+$ and NADPH

$\text{NADP}^+$ is the oxidized electron carrier, whereas NADPH is the reduced form carrying high-energy electrons.

Difference between $\text{NAD}^+$ and NADH

$\text{NAD}^+$ is the oxidized electron carrier, whereas NADH is the reduced form carrying high-energy electrons.

Metabolic distinction between NADPH and NADH

NADPH is primarily used in anabolic photosynthetic pathways to synthesize sugars, whereas NADH is primarily used in catabolic cellular respiration to produce ATP.

Proton gradient

A concentration and electrical charge difference of $\text{H}^+$ across a membrane that stores potential energy.

Chloroplast compartment with higher $\text{H}^+$ concentration during light reactions

The thylakoid lumen.

Chloroplast compartment with lower $\text{H}^+$ concentration during light reactions

The stroma.

Proton-motive force

The electrochemical potential energy stored across a biological membrane by a proton concentration gradient.

Why $\text{H}^+$ ions cannot freely diffuse through the thylakoid membrane

Because they are charged ions and cannot pass through the hydrophobic core of the lipid bilayer without a transport protein.

ATP synthase

A membrane-spanning enzyme complex that utilizes the passive flow of $\text{H}^+$ ions down their gradient to synthesize ATP.

Direction of $\text{H}^+$ flow through ATP synthase

From the thylakoid lumen into the stroma.

Mechanism of ATP synthesis via ATP synthase

The movement of $\text{H}^+$ ions down their gradient powers the catalytic rotation of the enzyme, combining ADP and inorganic phosphate ($\text{P}_i$).

Chemical equation for ATP synthesis

$$\text{ADP} + \text{P}_i \rightarrow \text{ATP}$$

Photophosphorylation

The light-driven production of ATP from ADP and inorganic phosphate using energy derived from electron transport.

Reason the process is called non-cyclic photophosphorylation

Electrons flow unidirectionally from water to $\text{NADP}^+$ rather than returning to the original photosystem.

Final products of non-cyclic photophosphorylation

ATP, NADPH, and $\text{O}_2$.

Destination of ATP and NADPH following the light reactions

They diffuse into the stroma to fuel the light-independent reactions (Calvin cycle).

Primary contribution of ATP to the Calvin cycle

Chemical energy required to phosphorylate intermediates and drive the endergonic synthesis of sugars.

Primary contribution of NADPH to the Calvin cycle

High-energy electrons and reducing power required to reduce carbon intermediates into sugars.

Initial reaction when $\text{CO}_2$ enters the Calvin cycle

It is fixed to RuBP by RuBisCO to form 3-phosphoglycerate (3-PGA).

Molecule produced when 3-PGA is reduced by ATP and NADPH

Glyceraldehyde-3-phosphate (G3P), a three-carbon carbohydrate.

Fates of G3P produced in the Calvin cycle

Some is exported to synthesize glucose and other organic molecules, while the majority is recycled to regenerate RuBP.

Substances recycled back to the light reactions from the Calvin cycle

$\text{ADP}$, inorganic phosphate ($\text{P}_i$), and $\text{NADP}^+$.

Complete electron pathway in non-cyclic photophosphorylation

$$\text{H}_2\text{O} \rightarrow \text{PSII (P680)} \rightarrow \text{Pheophytin} \rightarrow \text{Plastoquinone} \rightarrow \text{Cytochrome complex} \rightarrow \text{Plastocyanin} \rightarrow \text{PSI (P700)} \rightarrow \text{Ferredoxin} \rightarrow \text{NADP}^+ \rightarrow \text{NADPH}$$

Sequence of events in non-cyclic photophosphorylation

1. PSII absorbs light and excites electrons. 2. Pheophytin accepts electrons. 3. Water splits to replace electrons. 4. Electrons move down the ETC to pump $\text{H}^+$. 5. PSI absorbs light and re-excites electrons. 6. Electrons reduce $\text{NADP}^+$ to NADPH. 7. $\text{H}^+$ flows through ATP synthase to produce ATP.

Main function of the $\text{H}^+$ gradient in non-cyclic photophosphorylation

To provide proton-motive force that drives the catalytic synthesis of ATP via ATP synthase.

Difference in how ATP and NADPH are synthesized during the light reactions

ATP is produced chemiosmotically via proton flow through ATP synthase, whereas NADPH is synthesized chemically when $\text{NADP}^+$ accepts electrons at the end of the electron transport chain.