Photosynthesis

The light reactions

Photosynthesis• One of the most important biochemical

process in plants.– Let’s not forget cell wall biosynthesis

and adaptation during plant development, growth, interaction with the environment, and disease defense.

• Among the most expensive biochemical processes in plant in terms of investment

• The biochemical process that has driven plant form and function

General overall reaction

6 CO2 + 6 H2O C6H12O6 + 6 O2 Carbon dioxide Water Carbohydrate Oxygen

Photosynthetic organisms use solar energy to synthesize carbon compounds that cannot be formed without the input of energy.

More specifically, light energy drives the synthesis of carbohydrates from carbon dioxide and water with the generation of oxygen.

Overall Perspective• Light reactions:

– Harvest light energy– Convert light energy

to chemical energy

• Dark Reactions:– Expend chemical energy– Fix Carbon [convert CO2

to organic form]

The sun emits a tremendous amount of energy, only some of

which is useable

But only a small fraction of the sun’s energy reaches the surface

• Solar spectrum and its relation to the absorption spectrum of chlorophyll

• Very little of the Sun’s energy gets to the ground (red line).– gets absorbed by water

vapor in the atmosphere

• The absorbance spectra of chlorophyll (green line).– Absorbs strongly in the

blue and red portion of the spectrum

– Green light is reflected and gives plants their color.

Photosynthetic pigments• Two types in plants:• Chlorophyll- a• Chlorophyll –b• Structure almost identical,

– Differ in the composition of a sidechain

– In a it is -CH3, in b it is CHO

• The different sidegroups 'tune' the absorption spectrum to slightly different wavelengths– light that is not significantly

absorbed by chlorophyll a, will instead be captured by chlorophyll b

Photosynthetic pigments• Chlorophyll has a

complex ring structure– The basic structure is a

porphyrin ring, co-coordinated to a central atom.

– This is very similar to the heme group of hemoglobin

• Ring contains loosely bound electrons – It is the part of the

molecule involved in electron transitions and redox reactions of photosynthesis

Photosynthetic pigments• When chlorophyll absorbs a

light particle (Proton)– Enters a higher excitation state– Becomes unstable, gives up

energy as heat– Enters lower excited state

• can be stable for a few nanoseconds

• This energy causes chemical reactions to occur

• These reactions are the fastest known to science!!!!

Photosynthesis• Takes place in complexes

containing light-harvesting antennas & photochemical reaction centers

• Antenna complex– Chemical oxidation &

reduction reactions leading to long term energy storage take place

• Antenna collects light and transfers its energy to the reaction center

• Chemical reactions store some of the energy by transferring electrons from chlorophyll to an electron acceptor molecule

Photosynthesis• An electron donor then

reduces the chlorophyll again• The transfer of energy to the

antenna is a purely physical phenomenon and involves no chemical changes

• Even in bright sunlight, a chlorophyll molecule absorbs only a few photons each second– Therefore, many chlorophyll

molecules send energy into a common reaction center

– The whole system is kept active most of the time

The chemical reaction of photosynthesis is driven by

light• The initial reaction of photosynthesis is:

– CO2 +H2O (CH2O) + O2 – Under optimal conditions (red light at 680 nm), the

photochemical yield is almost 100 %– However, the efficiency of converting light energy

to chemical energy is about 27 %• Very high for an energy conversion system

– Quantum efficiency: Measure of the fraction of absorbed photons that take part in photosynthesis

– Energy efficiency: Measure of how much energy in the absorbed photons is stored as chemical products

• ¼ energy from photons stored – the rest is converted to heat

Light drives the reduction of NADP & the formation of

ATP• Overall process of photosynthesis is a redox

chemical reaction– Electrons are removed from one chemical species

(oxidation) and added to another (reduction)

• Light reduces NADP, which serves as the reducing agent for carbon fixation during the dark reactions– ATP also formed during electron flow from water

to NADH and is also used during carbon fixation

• Thylakoid reactions: water oxidized to oxygen, NADP reduced and ATP is formed

• Stroma reactions: carbon fixation and reduction reactions

Oxygen-evolving organisms have two photosystems that operate in

series• Two photochemical complexes operate in

series to carry out the early energy storage reactions of photosynthesis

• Photosystem I: Absorbs far-red light (greater than 680 nm)– Produces a strong reductant, capable of reducing

NADP– Also produces a weak oxidant

• Photosystem II: Absorbs red light of 680 nm– Produces a strong oxidant, capable of oxidizing water– Also produces a weak reductant

• These two photosystems are linked by an electron transport chain

Oxygen-evolving organisms have two photosystems (PS) that operate in

series• Z (zigzag) scheme – the basis of understanding O2-evolution

• Far-Red light absorbed by PS-I makes strong oxidant (& weak reductant)

• Red light absorbed by PS-II makes strong reductant (& weak oxidant)

• PS-II oxidizes water and PS-I reduces NADP - P680 and P700 refers to wavelength of max absorption of reaction center chlorophylls

The chloroplast

A quick recap

The Chloroplast• Contain their own DNA and

protein-synthesizing machinery– Ribosomes, transfer

RNAs, nucleotides.– Thought to have evolved

from endosymbiotic bacteria.

– Divide by fusion– The DNA is in the form

of circular chromosomes, like bacteria

– DNA replication is independent from DNA replication in the nucleus

The Chloroplast• Membranes contain

chlophyll and it’s associated proteins– Site of

photosynthesis• Have inner & outer

membranes• 3rd membrane system

– Thylakoids• Stack of Thylakoids =

Granum• Surrounded by Stroma

– Works like mitochondria• During photosynthesis,

ATP from stroma provide the energy for the production of sugar molecules

The Chloroplast

PS-I and PS-II are spatially separated in the thylakoid

membrane • The PS-II reaction center is located mostly in Granum.– Stack of Thylakoids

• The PS-I reaction center is located in the Stroma & the edges of the Granum.– There is a cytochrome b6f

complex that connects the two photosystems that is evenly distributed between Granum and Stroma

• One or more of the electron carriers that function between the photosystems diffuses from the from the Granum to the Stroma.

Electron transfer

A step by step look

Oxygen-evolving organisms have two photosystems (PS) that operate in

seriesWhat is an electron transport chain?

A series of coupled oxidation/reduction reactions where electrons are passed from one membrane-bound protein/enzyme to another before being finally attached to a terminal electron acceptor (usually oxygen or NADPH).

Mechanisms of electron transfer

• The energy changes of electrons as they flow through the light reactions are analogous to the cartoon.

• The light reactions use solar power to generate ATP and NADPH which provide chemical energy and reducing power to the sugar making reactions

The transport chain• Z (zigzag) scheme – the

basis of understanding O2-evolution

• Far-red light absorbed by PS-I makes strong oxidant (& weak reductant)

• Red light absorbed by PS-II makes strong reductant (& weak oxidant)

• PS-II oxidizes water and PS-I reduces NADP - P680 and P700 refers to wavelength of max absorption of reaction center chlorophylls

The transport chain

• PS-II oxidizes water to O2 in the thylakoid lumen– Releases protons into the lumen

• Cytochrome b6f receives electrons from PS-II & delivers them to PS-I– Also transports additional protons into lumen from

stroma

The transport chain• PS-I reduces NADP to NADPH in the stroma

– Uses the action of:– Ferredoxin (Fd)– Ferredoxin-NADP reductase (FNR)

• ATP synthesis produces ATP as protons diffuse back through it from the lumen into the stroma

Summary of light reactions

• Photosynthesis (light reactions):– Storage of solar energy carried out by

plants• Absorbed photons excite chlorophyll

molecules– can dispose of this energy as:

• Heat• Fluorescence• Energy transfer• Photochemistry – the light reactions of

photosynthesis

• Absorption of light occurs in the thylakoid membranes of the chloroplast by chlorophyll a & b

Summary of light reactions

• Plants have two reaction centers:– PS-II

• Absorbs Red light – 680mn• makes strong reductant (& weak oxidant)• oxidizes 2 H2O molecules to 4 electrons, 4 protons

& 1 O2 molecule• Mostly found in Granum

– PS-I• Absorbs Far-Red light – 700nm• strong oxidant (& weak reductant)• PS-I reduces NADP to NADPH• Mostly found in Stroma

Summary of light reactions

• Excess light energy can damage photosynthetic systems– Several mechanisms occur to minimize such

damage•Some proteins made in chloroplasts act as

photoprotective agents to control excited state of chlorophyll molecules

• Chloroplasts contain DNA and encode and synthesize most of the proteins essential for photosynthesis– Others encoded by nuclear DNA

Summary of light reactions

• Chlorophylls made in a biosynthetic pathway involving more than 12 steps.

• Once synthesized, pigment proteins are assembled in the thylakoid membrane.

• Lastly:• The initial reaction of the light reaction is:

– CO2 +H2O (CH2O) + O2