photosynthesis.a2ppt

Can you label the plant cell?

The location and structure of chloroplasts

Figure 7.2

LEAF CROSS SECTION MESOPHYLL CELLLEAF

Chloroplast

Mesophyll

CHLOROPLAST Intermembrane space

Outermembrane

Innermembrane

ThylakoidcompartmentThylakoidStroma

Granum

StromaGrana

Stoma (pl. stomata)

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Week 10

A photosystem

Two photo systems named by time of discovery PS I and PS II PSI has a light absorption peak of 700nm and appears mainly on inter-granal lamellae PSII has a light absorption peak of 680nm and appears mainly on granal lamellae

Capture solar power in form of photons

Act as light harvesters

Accessory pigments make up antenna complex which absorbs photons of light energy

This energy is channeled to the reaction centre of each photo system

Light-absorbing molecules

Absorb some wavelengths and reflect others

Color you see are the wavelengths NOT absorbed

Wavelength (nanometers)

chlorophyll b

chlorophyll a

Q: what wavelengths are NOT absorbed?

ATP- ENERGY ‘CURRENCY’

An overview of photosynthesis

Figure 7.5

Light

Chloroplast

LIGHTREACTIONS

(in grana)

CALVINCYCLE

(in stroma)

Electrons

H2O

O2

CO2

NADP+

ADP+ P

Sugar

ATP

NADPH

• Each of the many photosystems consists of:

–an “antenna” of chlorophyll and other pigment molecules that absorb light

–a primary electron acceptor that receives excited electrons from the reaction-center chlorophyll

–P680 (in PS II)

–P700 (in PS I)

Figure 7.7C

Primaryelectron acceptor

Photon

Reaction center

PHOTOSYSTEM

Pigmentmoleculesof antenna

Fluorescence of isolated chlorophyll in solution. Excited electrons have no place to go.

Figure 7.7A

Heat

Photon(fluorescence)Photon

Chlorophyllmolecule

Figure 7.7B

Excitation of chlorophyll in a chloroplast

The electrons are then passed to other molecules in an electron transport chain

Primaryelectron acceptor

Othercompounds

Chlorophyllmolecule

Photon

The energy changes of electrons as they flow through the light reactions are analogous to the cartoon. As complicated as the scheme is, don’t lose track of its functions:The light reactions use solar power to generate ATP and NADPH which provide chemical energy and reducing power to the sugar making reactions.

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Week 10

(a) The distribution of photosystems on granal and intergranal lamellae; (b) Formation of ATP during non-cyclic photophosphorylation

(a)

(b)

Light reactions

Figure 7.8

Primaryelectron acceptor

Primaryelectron acceptor

Electron transport chain

Electron transport

Photons

PHOTOSYSTEM I

PHOTOSYSTEM II

Energy forsynthesis of

by chemiosmosis

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Week 10

The Z-scheme

Light strikes photo system II and the energy is absorbed and passed along until it reaches P680 chlorophyll.

The excited electron is passed to the primary electron acceptor. Photolysis in the thylakoid takes the electrons from water and replaces the P680 electrons that were passed to the primary electron acceptor. ( O2 is released as a waste product)

The electrons are passed to photo system I via the electron transport chain (ETC) and in the process used to pump protons across the thylakoid membrane into the lumen.

The stored energy in the proton gradient is used to produce ATP which is used later in the Calvin-Benson Cycle.

P700 chlorophyll then uses light to excite the electron to its second primary acceptor.

The electron is sent down another ETC and used to reduce NADP+ to NADPH.

The NADPH is then used later in the Calvin-Benson Cycle.

Where do the electrons come from that keep the light reactions running?

Photosystem II regains electrons by splitting water molecules and releasing oxygen The reaction center pigment (P680) that gave up

electrons gets replacement electrons Photosystem I receives electrons from the

bottom of the cascade of the ETC from PS II and passes it to the P700 chlorophyll

Light dependent phase

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Week 10

The Calvin cycle

LIGHT AS A LIMITING FACTOR