Understanding Light and Photosynthesis from Rhul.ac.uk

Learning Objectives

• By the end of this session, you should:
• Be able to understand the nature of light in the context of biology
• Be able to evaluate the different roles of excitation energy within biology
• Understand and evaluate the photosynthetic electron transport chain

Light and Photosynthesis Overview

· Photosynthesis is the process of the anabolic process of glucose biosynthesis, from the CO2 and H2O molecules. The energy which fuels this reaction is provided by light energy, which is converted into excitation energy, which is converted into chemical energy. 6CO2 + 6H2O > C6H1206 + 602

Autotrophy and Plants

· Terrestrial plant species are described as: Photoauto trophic Using photons Sourcing nutritional molecules By itself . So, they produce their own biological molecules sourcing energy from the absorption of photons of light.

Properties of Light

· Light can behave as both as continuous wave or as a discrete particle (I.E photon): · Acting as a wave when it propagates through space, air, liquid etc. · Acting as a particle when interacting with molecules that can absorb light (chromophores). Wavelength Amplitude

Electromagnetic Radiation Wavelengths

· All electromagnetic radiation (such as Infrared, Ultraviolet, and Visible Light) all have a distinct wavelength. · Visible light with a wavelength between 380nm to 700nm in length . As a photon of light's wavelength increases, the amount of energy within that photon decreases (I.E Short Wavelength = Higher Energy) 380nm 440nm 500nm 560nm 620nm 660nm 700nm Wavelength Increases/Energy Decreases

Absorbing Light by Chromophores

· As previously mentioned, molecules which are capable of absorbing light are known as chromophores. . Chromophores typically absorb a single narrow band of wavelengths of light, which changes due to: · The molecule's chemical structure · The physical environment it is currently within. CI -Z + .N. Malachite green (~621nm) ß-Carotene (~450nm)

Chlorophyll Light Absorption

. However, some chromophores, such as chlorophylls have large, complex structures which can absorb different wavelengths of light. . For example: Chlorophyll a can absorb light with wavelengths at ~440nm and ~662nm. · Wavelengths can be detected by several different techniques including HPLC-PDA and Absorbance Spectrophotometry N 2 ---- Mg Z Chlorophyll a

Excitation Energy

· When a chromophore absorbs a photon, an electron becomes excited. This energy 'moves' the electron into a higher energy state, and therefore has as much additional energy as the photon absorbed had. M AE = hv Photon E = hv - e Excitation by photon

Excited State Decay

. This excited state is very high energy and is therefore energetically unfavourable, and therefore the electron wants to lose this energy and 'decay' back to the ground state. . This returns the electron to a stable state, with the energy released in different forms. There are 5 different 'fates' for this energy, some useful, some dangerous to organic material.

Fate of Excitation Energy

Fates	Properties
Fluorescence	Release of the excitation energy as a photon
Non-photochemical Quenching	Release of the excitation energy as a heat
Photosynthesis	Uses the excitation energy to fuel the anabolic reactions of glucose biosynthesis
Triplet/Singlet State Formation	Dangerous, forming free radicals and reactive oxygen species
Excitation Transfer	Passing excitation energy to neighbouring chromophores

Fluorescence Process

. An excitation state can decay through the release of the energy back-out as a photon. . However, due to multiple different excited states existing within the chromophore, small bursts of vibrational energy are released as the excited electron moves down those intermediatory steps. . This reduces the overall amount of energy when the photon is released, resulting in a longer wavelength (therefore with less energy). . The wavelength of these fluorescent photons are related to the physical properties and are detectable. S1' S1 Excited State ENERGY absorption fluorescence HVEM nv Ex Incoming light Ground State

Photosynthesis: Light to Chemical Energy

1. 2. 680nm H+ PSII PQ bøf PSI C PC OEC H+ H+ 4x H+ 2x H2O O2 3. NADPH NADP + H+ 4. ADP + Pi ATP 700nm FNR Fd ATP Synthase

Photophosphorylation in PSII

680nm PSII OEC 2x H2O 4x H+ O2 · Light energy with a wavelength of ~680nm is absorbed by the chlorophyll within photosystem II (P680). This excites an electron, which is transferred to phenophytin, producing a P680+ . . This charged P680+ splits a water molecule into its constituent parts (2H+ + 1/2 O2 + 2e-). . One electron reduces P680+ back to P680 while the second electron is transferred through pheonphytin into plastoquinone (PQ). . The H+ ions remain the thylakoid membrane, forming an electrochemical gradient

Plastoquinone and Cytochrome b6f Complex

H+ PQ bof C H+ . The electrons transferred to plastoquinone (PQ) are used to bond with 2 H+ ions within the stroma, this reduces PQ into PQH2 storing the H+ ions. · PQH2 is then later oxidized back to PQ, with the electrons being transfer onto cytochrome bof complex and the H+ ions being pumped across to the thylakoid membrane. . These H+ ions are used to further generate the electro- chemical gradient across the thylakoid membrane.

PSI and NADPH Production

NADPH NADP + H+ FNR Fd PSI PC · The electrons are passed through plastocyanin (PC), into PSI where an additional excitation event occurs with the absorption of a photon of 700nm by the chlorophyll within the core of PSI, · This excited electron is passed onto ferredoxin (Fd) and then onto Ferredoxin-NADP+ reductase which catalyses the production of NADPH. · NADPH is used a number of anabolic pathways including cholesterol, steroid, ascorbic acid synthesis.

ATP Synthesis

ADP + Pi ATP ATP Synthase H+ . The H+ ions which have been pumped across the thylakoid membrane form a chemiosmotic gradient. This pressure of H+ ions is allowed to flow back through the membrane into the stroma of the chloroplast. . This flow of H+ ions pushes the mobile rotational mechanism round, allowing for the conformation shape change of the subunits and therefore the binding, synthesis and unbinding of the ADP and Pi into ATP.

Experimental Proof of ATP Synthesis

PH 7 1. pH 4 I pH 4 2. pH 4 . In 1966, Jagendorf showed that the synthesis of ATP within the chloroplasts was independent from light energy, but due to the chemiosmotic gradient across the thylakoid membrane 1) Isolated chloroplasts, with an internal pH of 7 were added to a solution of pH 4 and was left to incubate for several hours in the dark. 2) Flow of H+ ions across the membrane equalised the pH between the stroma and thylakoid space 3) Chloroplasts were added to a pH 8 solution, forming a chemiosmotic gradient 4) When ADP and pi were added to the solution, the pH of the solution decreased, and ATP was produced. ADP+Pi -> ATP 3. 4. pH 4 H+ pH 8

Learning Outcomes

• Now, you should:
• Be able to understand the nature of light in the context of biology
• Be able to evaluate the different roles of excitation energy within biology
• Understand and evaluate the photosynthetic electron transport chain