Photosynthesis - Light-Dependent Reactions

Photosynthesis

Overview

  • Photosynthesis involves reactions that capture light energy and produce carbohydrates.

  • It consists of over 100 chemical reactions essential for life.

Two Sets of Reactions

  • Light-Dependent Reactions (Light Reactions):

    • Traps solar energy to generate ATP and reduce NADP+ to NADPH.

    • O<em>2O<em>2 is produced as a byproduct from splitting H</em>2OH</em>2O.

  • Light-Independent Reactions (Calvin Cycle/"Dark" Reactions):

    • Uses ATP and NADPH to synthesize organic molecules like glucose from CO2CO_2.

    • These organic molecules are used to produce macromolecules such as carbohydrates (starch, cellulose).

Photosynthesis Equation

  • 6CO<em>2+6H</em>2O→LightC<em>6H</em>12O<em>6+6O</em>26CO<em>2 + 6H</em>2O \xrightarrow{\text{Light}} C<em>6H</em>{12}O<em>6 + 6O</em>2

  • Carbon dioxide + Water yields Sugar + Oxygen

NADPH

  • NADP++2e−→NADPHNADP^+ + 2e^- \rightarrow NADPH

  • NADPH is similar to NAD+/NADH but with a phosphate group.

  • It acts as an electron carrier.

Photosynthetic Organs: Leaves

  • Cuticle: Waxy outer layer that prevents water loss.

  • Vein (Vascular Bundle): Transports water and sugars.

  • Stomata: Openings for gas exchange (O<em>2O<em>2 out, CO</em>2CO</em>2 in).

  • Mesophyll Cells: Contain chloroplasts for photosynthesis.

    • Palisade: Tightly packed for maximum light absorption.

    • Spongy: Loosely packed with air spaces for gas exchange.

  • Chloroplast: Organelle containing pigments and proteins for photosynthesis.

Chloroplast Structure

  • Outer and Inner Membranes: Enclose the chloroplast.

  • Intermembrane Space: Space between the outer and inner membranes.

  • Stroma: Fluid-filled space around the thylakoids.

  • Thylakoid: Disc-like structure within the chloroplast.

  • Granum: Stack of thylakoids.

  • Lumen: Space inside the thylakoid.

Thylakoid Membrane

  • Location of light reactions.

  • Includes Photosystem II, Cytochrome complex, Photosystem I, and ATP synthase.

Absorption of Light Energy

  • Plants use pigments to absorb light energy.

  • Chlorophyll is the most abundant pigment.

  • Pigments are grouped in a photosystem within the thylakoid membrane.

  • The grouping of pigments is called the antenna complex.

Photosystems

  • Protein complexes in the thylakoid membrane containing pigment molecules.

  • The antenna complex includes chlorophyll a, chlorophyll b, carotene, and xanthophyll.

  • Each pigment absorbs light energy and transfers the energy to the reaction center: chlorophyll a + primary e- acceptor.

Electromagnetic Spectrum

  • The wavelengths that are reflected are the ones we see.

  • Chlorophyll a: Blue-green

  • Chlorophyll b: Yellow-green

  • XanthophyllPigments: Yellow

  • Beta-carotene: Orange

  • Anthocyanin: Red/purple

  • Chlorophyll absorbs red and blue light and reflects the green light.

Pigment Absorption Spectrum

  • Shows wavelengths absorbed and reflected by different pigments.

How Pigments Absorb Light Energy

  • Pigments have porphyrin rings with delocalized electrons.

  • When electrons absorb light, they jump to higher energy levels and can be oxidized.

Photosystem Components

  • Antenna Complex: Pigments that absorb and transfer energy.

  • Reaction Center: Chlorophyll a and primary electron acceptor.

Photosystem Summary

  • Energy is transferred from the antenna complex to the reaction center.

  • Electrons are passed down an electron transport chain.

Two Photosystems: II & I

  • Each photosystem is associated with an electron transport chain.

Differences: Photosystem I & II

  • Photosystem II (PS II):

    • Reaction center: Chlorophyll a P680 + pheophytin.

    • ETC after PS II creates an electrochemical gradient for ATP production.

  • Photosystem I (PS I):

    • Reaction center: Chlorophyll a P700 + iron-sulfur protein.

    • ETC after PS I transfers electrons to NADP+ to make NADPH.

  • P680 & P700 indicate the wavelengths of light absorbed best by chlorophyll.

ETC Components

  • Between PSII and PSI:

    • Plastoquinone (pq): Electron shuttle.

    • Cytochrome b6-f: H+H^+ pump for ATP production.

    • Plastocyanin (pc): Electron shuttle.

  • After PS I:

    • Ferredoxin (fd): Electron shuttle.

    • NADP+ Reductase: Enzyme that transfers electrons to NADP+ to make NADPH.

Light Reactions - PSII

  • A photon is absorbed by pigments, and energy is transferred to the reaction center.

  • Electrons in chlorophyll a P680 are excited and transferred to pheophytin.

  • P680 is left with a "hole".

  • Photolysis occurs: H<em>2OH<em>2O is split by Z protein into H+H^+, electrons, and O</em>2O</em>2.

  • Electrons replace those lost by P680.

  • H+H^+ contributes to the gradient inside the thylakoid lumen.

  • O2O_2 is released as waste.

Light Reactions – ETC

  • Electrons travel down the ETC, starting with plastoquinone.

  • Cytochrome b6-f pumps H+H^+ from the stroma into the thylakoid lumen, creating an electrochemical gradient.

  • ATP Synthase uses this gradient to make ATP.

  • Electrons pass to plastocyanin and then to Photosystem I to replace electrons lost by P700.

Light Reactions - PSI

  • Energy excites electrons in chlorophyll a P700, which jump to a higher energy level.

  • High energy electrons are transferred to the iron-sulfur protein.

  • P700 is left with a "hole".

  • Electrons from plastocyanin are passed to Photosystem I.

  • No photolysis occurs at PSI.

Light Reactions - ETC Again

  • Electrons pass down another ETC, starting with ferredoxin.

  • From ferredoxin, electrons pass to NADP+ Reductase, which gives the electrons to NADP+ to make NADPH.

  • Final products of the Light Reactions: ATP, NADPH, O2O_2 (waste).

Light Reactions – NADPH

  • NADP+ is converted to NADPH by NADPH Reductase.

  • This captures high-energy electrons in a mobile molecule (NADPH).

  • These electrons will be used in the Calvin Cycle to reduce CO2CO_2 to make sugars.

Light Reactions - ATP

  • Photophosphorylation uses light to release high energy electrons from chlorophyll a.

  • These electrons are used to create an electrochemical gradient that makes ATP via phosphorylation of ADP.

  • Two photophosphorylation pathways:

    • Noncyclic Photophosphorylation (NCPP): Produces ATP and NADPH in a 1:1 ratio.

    • Cyclic Photophosphorylation (CPP): Produces ATP only.

Noncyclic and Cyclic Photophosphorylation

  • Noncyclic photophosphorylation: Electrons pass through the Z scheme linearly from PS II to NADP+.

    • Net result for 2 electrons: 1 NADPH + 1 ATP. This is not enough ATP for light-independent reaction

  • Cyclic photophosphorylation: Electrons cycle from PS I back to the b6-f complex to generate a proton gradient for ATP synthesis.

    • No NADPH or O2O_2 is produced.