The Light Reactions

Light Energy

Light travels in small packets of energy called photons, which behave both as waves and particles and have wavelength and amplitude.

Visible Light

Only certain wavelengths of light are visible to the human eye; these are known as visible light.

White Light Interaction

When white light strikes an object, some wavelengths are absorbed and others are reflected; the reflected light determines the color we see.

Pigments

Molecules that absorb certain wavelengths of light; the absorbed wavelengths are not visible, while reflected ones give color to objects.

Color Perception

The light that is reflected from an object enters the eye, allowing us to see its color.

Chlorophyll a

The primary photosynthetic pigment; absorbs blue/purple and orange/red light most effectively.

Chlorophyll b

An accessory pigment that absorbs blues, light greens, and some oranges, extending the range of light usable for photosynthesis.

Carotenoids

Yellow, orange, and brown accessory pigments that help chlorophyll capture light and protect it from excess light energy.

Location of Light Reactions

The light reactions occur in the thylakoid membranes of the chloroplast.

Purpose of Light Reactions

To convert light energy into chemical energy in the form of ATP and NADPH, releasing oxygen as a byproduct.

Excited Electrons

When photons strike chlorophyll molecules, electrons are excited to higher energy levels (orbitals).

Energy Release

Most excited electrons quickly return to their original orbital, releasing excess energy as heat or light.

Energy Transfer

In the thylakoid membrane, chlorophyll passes excess energy to nearby molecules, initiating reactions in the photosystems.

Photosystems

Complexes of pigments and proteins that capture light energy and convert it to chemical energy; consist of light-harvesting complexes and reaction centers.

Photosystem II (PSII)

The first photosystem (P680) to absorb light energy; it excites electrons and passes them to an electron acceptor.

Water Splitting

Water molecules are split to replace lost electrons in PSII, releasing O₂, protons (H⁺), and electrons.

Electron Transport Chain (ETC)

A series of proteins that pass electrons from PSII to PSI, using energy from falling electrons to pump H⁺ ions across the thylakoid membrane.

Chemiosmosis

The process of generating ATP as H⁺ ions flow back across the thylakoid membrane through ATP synthase.

Photosystem I (PSI)

The second photosystem (P700) absorbs light energy to re-excite electrons coming from the ETC; these electrons are used to form NADPH.

NADPH Formation

Excited electrons combine with NADP⁺ and H⁺ to form NADPH, a molecule that carries high-energy electrons to the Calvin cycle.

ATP Synthase

An enzyme in the thylakoid membrane that uses the energy of H⁺ ions moving down their gradient to synthesize ATP from ADP and Pᵢ.

Photophosphorylation

The process of forming ATP using light energy during photosynthesis, specifically during the light reactions.

Hydrogen Ion Gradient

The buildup of H⁺ ions inside the thylakoid lumen that powers ATP production through chemiosmosis.

Movement of ATP and NADPH

ATP and NADPH produced in the light reactions move from the thylakoid membrane into the stroma to power the Calvin cycle.

Inputs of Light Reactions

Sunlight, H₂O, ADP + P, and NADP⁺.

Outputs of Light Reactions

O₂, ATP, and NADPH.

Energy Flow Summary

Light energy → excited electrons → ETC → proton gradient → ATP and NADPH formation.

Oxygen Production

Oxygen is released as a byproduct of water splitting in Photosystem II.

Importance of Light Reactions

The light reactions capture and store solar energy in chemical form (ATP and NADPH) for use in the Calvin cycle.

Photosystem II Peak Absorption

PSII absorbs light best at 680 nm (red light).

Photosystem I Peak Absorption

PSI absorbs light best at 700 nm (far-red light).

Sequence of Energy Transfer

Light excites PSII → electrons move through ETC → PSI re-excites electrons → NADPH formed → ATP synthesized.

Where ATP Is Produced

Inside the thylakoid membrane through photophosphorylation.

Where NADPH Is Produced

In the stroma after PSI passes electrons to NADP⁺.

Role of Water

Provides electrons to PSII, protons for the proton gradient, and oxygen as a waste product.

Role of NADPH and ATP

Provide energy and reducing power to drive carbon fixation in the Calvin cycle.

Simplified Overview of the Light Reactions

Light + H₂O → O₂ + ATP + NADPH.

Why Light Reactions Are Important

They convert solar energy into chemical energy that can be used to build sugars during the Calvin cycle.