Photosynthesis 02: The Light-Harvesting Reactions Part 1

Two main stages of photosynthesis

The light-harvesting reactions and the Calvin cycle.

Alternative names for the light-harvesting reactions

Light reactions, light-dependent reactions, and electron transport chain.

Primary goal of the light-harvesting reactions

To capture sunlight and convert its energy into chemical energy stored as ATP and NADPH.

Location of the light-harvesting reactions

The thylakoid membranes of chloroplasts.

Limitation of ATP for long-term energy storage

It readily supplies immediate cellular energy but is chemically unstable and does not store well over long periods.

Dual nature of light

Light exhibits properties of both a wave and a particle.

Two primary wave properties of light

Wavelength and frequency.

Wavelength

The distance between two corresponding points on consecutive waves, such as crest to crest.

Frequency

The number of complete wave cycles or oscillations that pass a point per second.

Relationship between wavelength and light energy

Inversely proportional: shorter wavelengths carry higher energy, whereas longer wavelengths carry lower energy.

Visible light

The narrow band of the electromagnetic spectrum detectable by human vision, spanning approximately $400\text{--}710\,\text{nm}$.

Photon

A discrete quantum or particle of light energy.

Pigment absorption across the visible spectrum

Unequal; distinct photosynthetic pigments absorb specific wavelengths while reflecting or transmitting others.

Three major classes of photosynthetic pigments

Chlorophyll a, chlorophyll b, and carotenoids.

Wavelengths most strongly absorbed by chlorophyll

Blue and red light.

Wavelength most poorly absorbed by plant photosynthetic pigments

Green light.

Light color under which plant growth is most limited

Green light, because photosynthetic pigments absorb very little of it.

Mechanism of light absorption by pigment molecules

Absorbing a photon elevates an electron from its ground state to a higher energy level.

Excited electron

An electron that has absorbed energy and transitioned to an orbital of higher potential energy.

Form of energy held by an excited electron

Chemical potential energy.

Outcome when an excited electron relaxes to ground state

Its absorbed energy is released as heat/light or transferred to a neighboring molecule.

Antenna complex

A network of light-absorbing pigment molecules that channels excitation energy toward a reaction center.

Three possible fates of energy in excited chlorophyll

Fluorescence (with heat), resonance energy transfer, or oxidation-reduction (redox).

Fluorescence in photosynthesis

The radiative de-excitation of an electron, releasing the absorbed energy as light of a longer wavelength alongside heat.

Utility of fluorescence in driving photosynthesis

None; the released energy dissipates and is lost from the photosynthetic pathway.

Resonance energy transfer

The non-radiative relay of excitation energy from one pigment molecule to an adjacent one until it reaches the reaction center.

Electron movement during resonance energy transfer

No electrons are transferred; only excitation energy shifts between molecules.

Role of resonance in the light reactions

To funnel absorbed photon energy through antenna pigments directly to the reaction center.

Reaction center

A specialized chlorophyll complex that converts absorbed excitation energy into chemical energy by transferring a high-energy electron to an acceptor.

Redox event at the reaction center

The reaction center chlorophyll donates an excited electron to a primary electron acceptor.

Pheophytin

An organic molecule that serves as the primary electron acceptor, taking an excited electron from the reaction center.

Chemical state of the reaction center after electron release

It becomes oxidized.

Chemical state of pheophytin after taking an electron

It becomes reduced.

Difference between resonance transfer and redox in light capture

Resonance transfers excitation energy without relocating electrons, whereas redox transfers an actual high-energy electron to an acceptor.

Sequence of light energy capture in photosystems

Pigments absorb photons → electrons enter excited states → excitation energy moves via resonance through the antenna complex → energy reaches the reaction center → an electron is transferred to an acceptor.