1

3/3/04 Photosynthesis: Light reactionsREQUIRED tutorial on photosynthesishttp://bioweb.wku.edu/courses/Biol120/images/Photosynthesis.asp

animation of electron transport in thylakoid membranehttp://instruct1.cit.cornell.edu/Courses/biomi290/MOVIES/OXYGENIC.HTML

• The vast majority of the energy consumed by livingorganisms stems from solar energy captured by the process ofphotosynthesis

• Only chemoautotrophs (aka chemolithotrophs) areindependent of this energy source

2

Of the sunlight that reaches the earth each day:• 1% is absorbed by photosynthetic organisms and transduced

into chemical energy• of the remaining 99%: 2/3 is absorbed by the earth and oceans

(heating the planet) and 1/3 is lost as light reflected back intospace

Often referred to as carbon dioxide fixation:

light energy6 CO2 + 6 H2O -------------> C6H12O6 + 6 O2

• The fixation of carbon is an endergonic (endothermic)reaction

• Estimates indicate that 1011 tons of carbon dioxide arefixed globally per year

• 1/3 is fixed in the oceans, primarily by photosyntheticmarine microorganisms

3

• cyanobacteria• other photosynthetic

bacteria• algae• plants

4

Two stages of photosynthesis in a green plant orcyanobacteria

6

7

Respiration as a series of energy transformations:• electrons released from carbohydrates(and other foodstuffs)

are transferred by a circuitous route to O2• O2 is reduced to H2O• the free energy released as electrons flow from a high energy

to a low energy state is transformed into a proton gradient• the potential energy in this proton gradient is then converted

into chemical energy in the form of ATP

Aspects of photosynthesis can be described in similar terms:• Photosynthesis also involves a series of oxidation-reduction

events• Flow of electrons is from water to CO2, reducing it to a

carbohydrate• light provides the energy required to move electrons from a

lower to a higher free energy state• energy interconversions carried out by chemiosmotic

mechanisms

8

Whether photosynthesis occurs in a cyanobacteria or a giantsequoia, some generalitites apply:• photosynthesis is associated with membranes• eukaryotic cells, photosynthesis occurs in the chloroplast• in prokaryotes, photosynthesis is associated with the plasma membrane

9

Thylakoids• flattened

membrane-boundsacs inside thechloroplast

• location ofchlorophyll

Chloroplasts possessthree membranebound aqueouscompartments• the intermembrane

space• the stroma -- inside

the inner membraneand outside thethylakoid sacs (likethe mt matrix)

• thylakoid space --inside the thylkoidsacs

10

Comparison of membrane components of the chloroplastand mt

Similar:• electron transport chains: entire set of proteins and

small molecules involved in the orderly sequence ofelectron transfers

• ATP synthase• inner membrane very impermeable and narrow inter-

membrane space

Different• photosystems: site where light energy is captured and

harnessed to drive the transfer of electrons• in chloroplasts the electron transport chains and ATP

synthase are located in the thylakoid membrane, not theinner membrane

11

The many reactions that occur during photosynthesis canbe grouped into two broad categories:

Photosynthetic electron-transfer reactions• AKA light reactions• energy derived from sunlight “energizes” an electron in

chlorophyll• chlorophyll obtains its electrons from water (generating

O2)• the energized electrons move along an electron

transport chain in the thylakoid membrane (analogousto mt electron transport)

• an H+ gradient is generated and drives the production ofATP

• NADP+ is reduced to NADPH

Carbon Fixation Reactions• also called Calvin cycle or dark reactions• the ATP and NADPH produced by the light reactions

serve as the source of energy and reducing power todrive the conversion of CO2 to carbohydrate

• reactions take begin in the stroma of the chloroplast andend in the cytosol

12

13

Nice tutorial onhttp://bioweb.wku.edu/courses/Biol120/images/Photosynthesis.aspphotosynthesis

http://www.biologie.uni-osnabrueck.de/Biophysik/Junge/overheads.html

14

Change in the redox potential during photosynthesis. The redox potential foreach molecule is indicated by its position along the vertical axis. Transferringelectron “up the scale” from H2O requires energy

• The net electron flow through the two photosystems is from water toNADP+ to form NADPH.

• In the process of electron transfer a H+ gradient is generated across thethylakoid membrane. ATP is synthesized by membrane bound ATPsynthase -- same process as in the mitochondria

Two different photosystems:Photosystem I:• chlorophyll molecules of the reaction center are a form of

chlorophyll a, known as P700• the P stands for pigment and the 700 designates the optimal

absorption maximum in nanometers (wavelength = 700 nm)

Photosystem II:• chlorophyll molecules of the reaction center are a form of

chlorophyll a, known as P680 (for its optimal absorption)

15

16

Figure 10.6 Interactions of light with matter in a chloroplast. The pigmentsof chloroplasts absorb blue and red light, the colors most effective inphotosynthesis. The pigments reflect or transmit green light, which is whyleaves appear green.

Maximal absorption by chlorophylls between 400-500 and 600-700nm

antenna complex on left

17

Photosystems consist of two closely linked components:

Photochemical reaction center:• complex of proteins and chlorophyll (transmembrane

protein-pigment complex)• converts light energy to chemical energy

Antenna complex:• pigment molecules that capture light energy and feed it

to the reaction center• cluster of several hundred chlorophyll molecules and

accessory pigments (collect light of other wavelengths)

18

When a chlorophyll molecule in the antenna complex isexcited, energy is rapidly transferred from one moleculeto the next until it reaches chlorophyll molecules in thereaction center

These chlorophyll molecules transfer the excited electronsto the electron transport chain

Reaction center:• acts as a trap for quanta of energy captured by the

pigment molecules in the antennae• excitation of the reaction center begins the actual

light reactions

Reaction center chlorophylls are in a differentenvironment from antennae chlorophylls and asborbenergy at a slightly lower level