Photosynthesis and Phototrophy: Key Concepts and Processes in Chloroplasts

Chapter 19

Phototrophy

The process of converting light (electromagnetic radiation) into chemical energy.

Phototrophs

Organisms that capture energy from light ("light-feeders").

Bacteriorhodopsin

A light-absorbing protein that pumps protons across the membrane to create a proton gradient.

Role of ATP synthase in phototrophy

Uses the proton gradient generated by light-driven proton pumps to make ATP.

Photosynthesis

Process that uses light energy and electrons from a donor (usually water) to convert CO2 into carbohydrates.

Oxygenic photosynthesis

Uses water as the electron donor and produces O2 as a byproduct.

Chemotrophs

Organisms that obtain energy from chemicals instead of light.

Autotrophs

Organisms that obtain carbon from CO2 rather than organic molecules.

Light reactions

Reactions that capture light energy to produce ATP and NADPH.

Dark reactions

Reactions that reduce CO2 into sugars (Calvin cycle).

Relationship between respiration and photosynthesis

Photosynthesis is essentially the reverse of respiration; one builds sugars, the other breaks them down.

Role of high-energy electrons in photosynthesis

Light boosts electrons to high energy so they can generate NADPH and a proton gradient.

Products of light reactions

ATP and NADPH.

Photosystems

Transmembrane protein complexes that absorb light and generate excited electrons.

Photosystem I

Produces NADPH but becomes electron deficient.

Photosystem II

Oxidizes water to restore electrons to PSI; generates O2 and contributes to proton gradient.

Flow of electrons between PSII and PSI

PSII → plastoquinone → cytochrome bf → plastocyanin → PSI.

Chloroplast

Organelle where photosynthesis occurs in eukaryotes.

Stroma

Fluid within inner membrane; site of dark reactions.

Thylakoid membrane

Membrane system that houses photosystems, ETC, and ATP synthase; site of light reactions.

Granum

Stacks of thylakoids.

Stroma lamellae

Connections between grana; help organize photosynthetic machinery.

Chlorophylls

Primary photoreceptor molecules; have conjugated bonds that allow absorption of visible light.

Structure of chlorophyll a

Similar to heme but contains magnesium instead of iron.

Absorption spectrum of chlorophyll a

Strong absorption in the visible-light region, corresponding to wavelengths where sunlight is maximal.

Photoexcitation

Absorption of light boosts chlorophyll electron from ground to excited state.

Photoinduced charge separation

Key event in photosynthesis where excited electron is transferred to an acceptor molecule, creating D+ and A−.

Reaction center

The specialized site where photoinduced charge separation occurs (contains a "special pair" of chlorophylls).

Cyclic electron flow (bacterial)

Allows production of ATP without production of reducing power.

Two-photosystem model

Plants and cyanobacteria use PSII followed by PSI to generate both a proton gradient and NADPH.

Photosystem II function

Transfers electrons from water to plastoquinone; generates O2 and releases protons into the lumen.

Plastoquinone (Q)

Lipid-soluble mobile electron carrier that cycles between Q and QH2.

Water-oxidizing complex (WOC)

Cluster containing manganese ions that oxidizes water to O2 and provides electrons to PSII.

P680+

Strong oxidant in PSII that extracts electrons from water.

Proton gradient from PSII

Protons released into lumen + protons taken up for Q reduction → forms gradient.

Cytochrome bf complex

Connects PSII and PSI; performs Q cycle and pumps protons.

Plastocyanin (Pc)

Copper-containing mobile protein that transfers electrons to PSI.

Q cycle in plants

Process in cytochrome bf that increases proton pumping and electron transfer efficiency.

Photosystem I function

Uses light energy to produce reduced ferredoxin (Fd), which helps generate NADPH.

P700

Special chlorophyll pair in PSI absorbing maximally at 700 nm.

Ferredoxin

A soluble iron-sulfur protein that carries electrons from PSI.

Ferredoxin-NADP+ reductase

Enzyme that reduces NADP+ to NADPH using electrons from ferredoxin.

Z scheme

Pathway of electron flow from water → PSII → cytochrome bf → PSI → NADP+.

Proton gradient in chloroplasts

Mostly due to pH difference across thylakoid membrane; drives ATP synthesis.

Chloroplast ATP synthase (CF1-CF0)

Enzyme complex that synthesizes ATP in the stroma using the proton gradient.

Direction of proton flow in chloroplasts

Protons flow from lumen → stroma through ATP synthase.

Regulation of chloroplast ATP synthase

Dependent on reduction of a disulfide bond in γ subunit via thioredoxin.

Epsilon (ε) subunit function

Has conformations that either inhibit ATP hydrolysis or promote ATP synthesis.

Cyclic photophosphorylation

Process where electrons cycle through PSI and cytochrome bf to make ATP without making NADPH.

Photon requirement

8 photons → 1 O2, 2 NADPH, 3 ATP.

Accessory pigments

Chlorophyll b and carotenoids; broaden absorption spectrum and funnel energy to reaction centers.

Resonance energy transfer

Excitation energy transferred from one pigment to another without moving electrons.

Light-harvesting complexes

Arrays of accessory pigments surrounding reaction centers to maximize light capture.

Carotenoids

Accessory pigments that protect against reactive oxygen and help in NPQ.

Nonphotochemical quenching (NPQ)

Mechanism that dissipates excess light energy as heat to prevent damage.

Role of PsbS in NPQ

Helps sense lumen pH and activate NPQ.

Stacked vs. unstacked thylakoids

Stacked = PSII rich; unstacked = PSI + ATP synthase rich.

Electron carriers between photosystems

Plastoquinone (Q) and plastocyanin (Pc).

Evolution of photosynthesis

Thought to originate in bacteria; oxygenic photosynthesis ~2 billion years old.

Artificial photosynthesis

Uses light-driven systems to split water and produce fuels like hydrogen.

Phototrophy

The process of converting light (electromagnetic radiation) into chemical energy.

Phototrophs

Organisms that capture energy from light ("light-feeders").

Bacteriorhodopsin

A light-absorbing protein that pumps protons across the membrane to create a proton gradient.

Role of ATP synthase in phototrophy

Uses the proton gradient generated by light-driven proton pumps to make ATP.

Photosynthesis

Process that uses light energy and electrons from a donor (usually water) to convert CO2 into carbohydrates.

Oxygenic photosynthesis

Uses water as the electron donor and produces O2 as a byproduct.

Chemotrophs

Organisms that obtain energy from chemicals instead of light.

Autotrophs

Organisms that obtain carbon from CO2 rather than organic molecules.

Light reactions

Reactions that capture light energy to produce ATP and NADPH.

Dark reactions

Reactions that reduce CO2 into sugars (Calvin cycle).

Relationship between respiration and photosynthesis

Photosynthesis is essentially the reverse of respiration; one builds sugars, the other breaks them down.

Role of high-energy electrons in photosynthesis

Light boosts electrons to high energy so they can generate NADPH and a proton gradient.

Products of light reactions

ATP and NADPH.

Photosystems

Transmembrane protein complexes that absorb light and generate excited electrons.

Photosystem I

Produces NADPH but becomes electron deficient.

Photosystem II

Oxidizes water to restore electrons to PSI; generates O2 and contributes to proton gradient.

Flow of electrons between PSII and PSI

PSII → plastoquinone → cytochrome bf → plastocyanin → PSI.

Chloroplast

Organelle where photosynthesis occurs in eukaryotes.

Stroma

Fluid within inner membrane; site of dark reactions.

Thylakoid membrane

Membrane system that houses photosystems, ETC, and ATP synthase; site of light reactions.

Granum

Stacks of thylakoids.

Stroma lamellae

Connections between grana; help organize photosynthetic machinery.

Chlorophylls

Primary photoreceptor molecules; have conjugated bonds that allow absorption of visible light.

Structure of chlorophyll a

Similar to heme but contains magnesium instead of iron.

Absorption spectrum of chlorophyll a

Strong absorption in the visible-light region, corresponding to wavelengths where sunlight is maximal.

Photoexcitation

Absorption of light boosts chlorophyll electron from ground to excited state.

Photoinduced charge separation

Key event in photosynthesis where excited electron is transferred to an acceptor molecule, creating D+ and A−.

Reaction center

The specialized site where photoinduced charge separation occurs (contains a "special pair" of chlorophylls).

Cyclic electron flow (bacterial)

Allows production of ATP without production of reducing power.

Two-photosystem model

Plants and cyanobacteria use PSII followed by PSI to generate both a proton gradient and NADPH.

Photosystem II function

Transfers electrons from water to plastoquinone; generates O2 and releases protons into the lumen.

Plastoquinone (Q)

Lipid-soluble mobile electron carrier that cycles between Q and QH2.

Water-oxidizing complex (WOC)

Cluster containing manganese ions that oxidizes water to O2 and provides electrons to PSII.

P680+

Strong oxidant in PSII that extracts electrons from water.

Proton gradient from PSII

Protons released into lumen + protons taken up for Q reduction → forms gradient.

Cytochrome bf complex

Connects PSII and PSI; performs Q cycle and pumps protons.

Plastocyanin (Pc)

Copper-containing mobile protein that transfers electrons to PSI.

Q cycle in plants

Process in cytochrome bf that increases proton pumping and electron transfer efficiency.

Photosystem I function

Uses light energy to produce reduced ferredoxin (Fd), which helps generate NADPH.

P700

Special chlorophyll pair in PSI absorbing maximally at 700 nm.