Prof. Dr. Samih Tamimi Bio 304101 Photosynthesis Biology 304101

Prof. Dr. Samih Tamimi Bio 304101

Photosynthesis

Biology 304101


The Two Stages of Photosynthesis: A Preview

Photosynthesis consists of two processes

The Light reactionsNEEDS LIGHTLight Dependent Reactions

The Calvin cycleA.k.a- Dark Reactions or Light Independent Reactions

DOES NOT NEED LIGHT


The Light Reactions

Occur in the grana (& thylakoids) Convert solar energy to

chemical energy Chlorophyll absorbs solar energy Split water release oxygen gas (a by-product) produce ATP (using chemiosmosis) Forms NADPH from NADP+ (an e-

acceptor) Temporarily stores high energy e-’s “Electron shuttle bus”


The Calvin Cycle

Occurs in the stromaForms SUGAR from carbon dioxide

Carbon fixation occurs (CO2 fixed carbon)

using ATP for energy and NADPH for reducing power (adding e-s to fixed carbon) Fixed carbon carbohydrate


An overview of photosynthesis

H2O CO2

Light

LIGHT REACTIONS

CALVINCYCLE

Chloroplast

[CH2O](sugar)

NADPH

NADP

ADP+ P

O2

ATP

G3P


Light Reactions (in detail)

The light reactions convert solar energy to the chemical energy of ATP and NADPH


RECALL:Color we SEE = color most reflected by

pigment; other colors (wavelengths) are absorbed BLACK all colors are reflected

Light

ReflectedLight

Chloroplast

Absorbedlight

Granum

Transmittedlight


SpectrophotometerMachine that sends light through pigments measures fraction of light transmitted and absorbed at each wavelength

Produces an absorption spectrum


An absorption spectrum graph plotting light absorption

versus wavelengthWhitelight

Refractingprism

Chlorophyllsolution

Photoelectrictube

Galvanometer

Slit moves topass lightof selectedwavelength

Greenlight

The high transmittance(low absorption)reading indicates thatchlorophyll absorbsvery little green light.

The low transmittance(high absorption) readingchlorophyll absorbs most blue light.

Bluelight

1

2 3

4

0 100

0 100


The absorption spectra of three types of pigments in chloroplasts


The action spectrum of a pigment Profiles the relative effectiveness of

different wavelengths of radiation in driving photosynthesis

Rat

e o

f p

ho

tosy

nth

esis

(me

asu

red

by

O2 r

ele

ase

)

Action spectrum. This graph plots the rate of photosynthesis versus wavelength. The resulting action spectrum resembles the absorption spectrum for chlorophyll a but does not match exactly (see part a). This is partly due to the absorption of light by accessory pigments such as chlorophyll b & carotenoids.


Chlorophyll aThe main photosynthetic pigment (primary pigment)

Accessory PigmentsAbsorb different Absorb different wavelengths of light wavelengths of light

pass energy to pass energy to chlorophyll achlorophyll a


Excitation of Chlorophyll by Light

When a pigment absorbs lightIt goes from a ground state (stable) to an excited state (unstable)


Photosystems


Primary electionacceptor

Photon

Thylakoid

Light-harvestingcomplexes

Reactioncenter

PhotosystemPhotosystem STROMA

Th

ylak

oid

mem

bra

ne

Transferof energy

Specialchlorophyll amolecules

Pigmentmolecules

THYLAKOID SPACE(INTERIOR OF THYLAKOID)

e–

When a reaction-center “special” chlorophyll a molecule absorbs energyAn electron

gets bumped up to a primary electron acceptor

http://www.bio.miami.edu/~cmallery/255/255phts/photosynthesis.swf


2 Different Photosystems

BOTH found in thylakoid membrane 2 types

photosystems II (PII) Uses chlorophyll a called P680 1ST photosystem in membrane

photosystems I (PI) Uses chlorophyll a called P700 2ND photosystem in membrane


2 types of e- flow

1.Non-cyclic photophosphorylation

1.Cyclic photophosphorylation


Noncyclic Electron Flow Steps1. PII excited e- to primary e- acceptor

2. Photolysis- water splits by enzyme e-s are replaced from lost chl a P680 H20 2 H+ + 2e- + ½ O2

(2 O’s combine and O2 is released)

3. Electron Transport Chain proteins in thylakoid membrane

pass e-s (become reduced) Flow of e-’s is exergonic releases

energy to make ATP Proteins used = Cytochromes, PC,

and PQ complexes


4. Chemiosomosis – the process that forms ATP during light reactions

Protons (H+) are pumped ACTIVELY into thylakoid space (lumen) from stroma durin electron transport

Protons (H+) from split water build up in thylakoid space (lumen) MORE acidic

H+’s then DIFFUSE down ATP synthase channels in stroma


Non cyclic light reactions & Chemiosmosis


Cyclic Electron Flow

Under certain conditions Photoexcited electrons take an

alternative path (shorter pathway)

Why use this pathway? Sugar production (Calvin Cycle)

uses a lot more ATP than NADPH Sometimes, autotrophs run low on

ATP needs to replenish ATP levels and uses cycle e- flow


Why is it “cyclic”?

This process is cyclic since electrons return to the reaction center.

An electron donor (i.e.-water) is NOT required and oxygen is NOT produced.


Cyclic Electron Flow Steps1. Photon hits PS1

2. E-s enter PSI P700 a primary e- acceptor

3. E-s travel BACK to P700 through FD cytochrome complex and PC ATP is produced using ATP synthase and H+ diffusion…NO NADPH!!!


In cyclic electron flow Only photosystem I is used Only ATP is produced NO NADPH


NON CYCLIC

CYCLIC


The Calvin cycle uses ATP and NADPH to convert CO2 to sugar

The Calvin cycleOccurs in the stroma


Recall…


?H

A

L

G

E + F

C D

M

B

I + J

K


?H

A

L

G

E + F

C D

M

B

I + J

K


The Calvin cycle has three phases

1. Carbon fixation2. Reduction3. Regeneration of the CO2 acceptor (RuBP)


The Calvin Cycle StepsCARBON FIXATION

1. CO2 enters cycle and attached to a 5-carbon sugar called ribulose bisphosphate (RuBP) forming 6-C molecule (unstable)

Enzyme RUBISCO catalyzes reaction

2. Unstable 6-C molecule immediately breaks down to two 3-C molecules called 3-phosphoglycerate (3-PGA)


REDUCTION

3. Each 3-phosphoglycerate (3-PGA) gets an additional phosphate from ATP (from LIGHT RXN) becomes 1,3 bis phosphoglycerate

4. NADPH reduces 1,3 bisphosphoglycerate to Glyceraldehyde-3-phosphate (G3P) G3P = a sugar that stores potential

energy Every 3 CO2 yields 6 G3P’s BUT only 1

can be counted in net gain for carbohydrate


REGENERATION OF CO2 ACCEPTOR (RuBP)

5. The C- skeletons of 5 G3P molecules are rearranged into 3 RuBP molecules

ATP is used !!!!


The Calvin cycle

Phase 1: Carbon fixation

Phase 2:Reduction

Phase 3:Regeneration ofthe CO2 acceptor(RuBP)

output

NOTE:

MORE ATP is

needed than

NADPH!!


Calvin Cycle Overview For 1 G3P molecule made 9 ATP molecules are used 6 NADPH molecules are used

G3P (starting material to make other organic molecules (glucose, starch, etc.)

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Prof. Dr. Samih Tamimi Bio 304101 Photosynthesis Biology 304101