PhotosynthesisLight Dependent Reactions

Dr. Mark A. McGinleyHonors College and Department of Biological

SciencesTexas Tech University

Purpose of Photosynthesis

• Photosynthesis converts electromagnetic energy in light to potential energy (sometimes called chemical energy) stored in chemical bonds of glucose– The potential energy in glucose can be stored and

moved to allow it to be used when and where it is needed

Two Stages of Photosynthesis

• Convert electromagnetic energy in light to potential energy in ATP and NADPH– Light Dependent Reactions

• Convert potential energy in ATP and NADPH to potential energy stored in glucose– Light Independent Reactions

Where Does Photosynthesis Occur?

• Most photosynthesis takes places in leaves–Palisade and

spongy mesophyll cells•Organelles

known as chloroplasts

Cloroplast

Photosystem

Component of a Photosystem

• Antennae Pigments– Chlorophyll a, b and carotene– 200 – 300 per photosystem

• Reaction Center – Chlorophyll a

• Primary Electron Acceptor

Chlorophyll

What Happens in a Photosystem?• Antennae pigment absorbs a photon of light

energy– Electron excited to higher energy level• Potential energy in excited electron

– When electron falls back to resting stage, the energy that is released is used to excited an electron in an adjacent antennae pigment• resonance

What Happens in a Photosystem?

• Eventually energy released by one excited electron is used to excite an electron in the reaction center– Chlorophyll a

• This excited electron does not fall back to resting stage

• Instead, the excited electron (and its potential energy) moves to an adjacent molecule know as the Primary Electron acceptor– 1o electron acceptor has “extra” electron– Reaction center is missing one electron

What Happens in a Photosystem?Summary

• Light energy is converted into potential energy that is stored in an excited electron that is passed form the reaction center to the primary electron acceptor

• Energetic results– Light energy converted into potential energy in

excited electron

Why Is This Important

• Potential energy in excited electron can be used to do work!!!

Electron Flow

• Two patterns of electron flow– Cyclic electron flow– Non-cyclic electron flow

Cyclic Electron Flow

Electron Flow

• Excited electron in 1o electron acceptor moves to adjacent molecule

• Electron drops to lower energy level• Energy released • Energy used to actively transport H+ from

stroma into the thylakoid space– Causes a H+ concentration gradient

Cyclic Electron Flow

• As the name suggests, the excited electron is eventually returned to the reaction center chlorophyll that originally lost it.

Importance of the H+ Concentration Gradient

• The active transport of H+ inside of the thylakoid space produces a H+ concentration gradient

• This concentration gradient powers a process known as Chemiosmosis that converts ADP to ATP– Potential energy stored in the chemical bonds of

ATP

Chemiosmosis

Cyclic Electron FlowSummary

• Excited electron from 1o electron acceptor is moves from molecule to molecule and eventually returns to the reaction center that lost it

• Energetic Result– Potential energy stored in excited electron is

converted to potential energy stored in ATP

Photosystems

• Turns out there are two types of photosystems– Photosystem I and Photosystem II– Photosystems are named based on the order that

they were discovered• PS I discovered before PS II

• PS I and PS II differ slightly in the absorption spectra of the Chlorophyll a molecule– PS I P700– PS II P680

Photosystems

• Cyclic Flow– PS I

• Non-cyclic Flow– Both PS I and PS II

Non-cyclic Flow

Involves both photosystems. Begins in PS II and then involves PS I

Non-cyclic Flow

• Starts in PS II– Electron from P700 passed to 1o electron acceptor– excited electron undergoes electron flow• Similar to in cyclic flow• H+ concentration gradient powers chemiosmosis-> ATP

– excited electron does not return to P700 that lost it• Instead, that electron and its remaining potential energy is

transferred to P680 in the reaction center of PS I

Non-cyclic Flow

• In PS I, this electron that originated in PS II is re-excited to an even higher energy level– Excited electron undergoes a different pattern of

electron flow– Result of this flow is that energy in excited

electron is converted into potential energy in NADPH

Non-cyclic FlowDo You See the Problem??

• The electron from P700 in PS II used to help produce NADPH– Thus P700 is missing an electron

• P700 recovers its missing electron by taking an electron from water (H20)– H+ and O2 released

Non-cyclic Flow Summary

• Energy from two photons of light energy are converted into potential energy in ATP and NADPH

• Water is broken down to release H+ and O2

Non-cyclic Flow-

Light Dependent ReactionsSummary

• Both cyclic and non-cyclic flow are occurring simultaneously in the same chloroplasts

• Energetic result- light energy is converted into potential energy in ATP and NADPH

• Chemical result- H20 => H+ + O2

The Light Dependent Reactions

• Light dependent reactions involves molecules imbedded in the thylakoid membrane

• Allows the cell to precisely control the spatial organization of molecules– Electron transport and

resonance can not occur if molecules are floating in cytoplasm

What’s Next!

• The ATP and NADPH produced during the light dependent reactions will be used to power the light dependent reactions of photosynthesis