Photosynthesis Dr.Samih Tamimi Biology 304101 Structures Photosynthesis occurs only in plants and a small number of single-celled organisms (like algae).

PhotosynthesisDr.Samih TamimiBiology 304101

Structures Photosynthesis occurs only in plants and a small

number of single-celled organisms (like algae). To be able to photosynthesize, you must have a

specific organelle: the Chloroplast.

Prof.DR. Samih Tamimi Bio304101

Chloroplasts

chloroplastsin plant cell

cross sectionof leafleaves

chloroplast

absorbsunlight & CO2

makeenergy & sugar

chloroplastscontain

chlorophyll

CO2

Prof.DR. Samih Tamimi Bio304101

• Chloroplasts– double membrane– stroma

• fluid-filled interior– thylakoid sacs– grana stacks

• Thylakoid membrane contains– chlorophyll molecules– electron transport chain– ATP synthase

• H+ gradient built up within thylakoid sac

Plant structure H+ H+

H+

H+

H+H+

H+ H+

H+H+

H+

outer membrane inner membrane

thylakoidgranum

stroma

thylakoid

chloroplast

ATP

Prof.DR. Samih Tamimi Bio304101

Photosynthesis

• Light reactions– light-dependent reactions– energy conversion reactions

• convert solar energy to chemical energy• ATP & NADPH

• Calvin cycle– light-independent reactions– sugar building reactions

• uses chemical energy (ATP & NADPH) to reduce CO2 & synthesize C6H12O6Prof.DR. Samih Tamimi Bio304101

A Look at Light• The spectrum of color

ROYGBIV

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Pigments of photosynthesis

• Chlorophylls & other pigments– embedded in thylakoid membrane– arranged in a “photosystem”

• collection of molecules– structure-function relationship

Prof.DR. Samih Tamimi Bio304101

• pigments:

• chlorophyll a

• accessory pigments

-energy-absorbing ring

-hydrocarbon tail

- carotenoids

- photoprotective

Photosynthesis – light absorption

- chlorophyll b

Light: absorption spectra• Photosynthesis gets energy by absorbing wavelengths of light

– chlorophyll a • absorbs best in red & blue wavelengths & least in green

– accessory pigments with different structures absorb light of different wavelengths

• chlorophyll b, carotenoids, xanthophylls

Prof.DR. Samih Tamimi Bio304101

Photosystems of photosynthesis

• 2 photosystems in thylakoid membrane– collections of chlorophyll molecules – act as light-gathering molecules– Photosystem II

• chlorophyll a• P680 = absorbs 680nm

wavelength red light – Photosystem I

• chlorophyll b• P700 = absorbs 700nm

wavelength red light

reactioncenter

antennapigmentsProf.DR. Samih Tamimi Bio304101

Photosynthesis – light absorption

light harvesting complex

• energy absorbed from light - to pigments

• to reaction center

- two special chlorophyll a

- 1° electron acceptor

• light harvesting complex & reaction center = photosystem (PS)

- proteins

Pigments are held by proteins in the thylakoid membranes

Photosynthesis – energy transfer

• PSII: absorbs 680 nm,

• PS I: absorbs 700 nm,

(less energy)

splits water, powerful ETC, ATP made

e- from PSII, short ETC, NADPH made

Photosynthesis – energy transfer

• e- from PS II electron transport chain (ETC) PS I

NADPH• e- from PS I 2nd ETC e- carrier: NADP+

• e- in PS II, from split H20

ETC of Photosynthesis

Photosystem II

Photosystem I

chlorophyll a

chlorophyll b

Prof.DR. Samih Tamimi Bio304101

1

ETC of Photosynthesis

ee

sun

Photosystem IIP680

chlorophyll a Prof.DR. Samih Tamimi Bio304101

12

ETC of Photosynthesis

Photosystem IIP680

chlorophyll a

OH H

H

H

e e

ee

e-

e-

H+H+

H+H+

H+H+

H+H+

H+H+

H+

H+H+

H+H+

H+H+

H+H+

H+H+

H+ATP

thylakoidchloroplast

H+

+H

OO

Plants SPLIT water!

fill the –e vacancy

Prof.DR. Samih Tamimi Bio304101

12

H+

H+

3

4

H+

ADP + Pi

H+H+

H+

H+ H+

H+H+H+

ee

e e

ATP

to Calvin Cycle

energy to buildcarbohydrates

H+H+

H+H+

H+H+

H+H+

H+H+

H+

H+H+

H+H+

H+H+

H+H+

H+H+

H+ATP

thylakoidchloroplast

Photosystem IIP680

chlorophyll a

ETC of Photosynthesis

ATP

Prof.DR. Samih Tamimi Bio304101

ee

e e

sun

5

Photosystem IIP680

chlorophyll a

Photosystem IP700

chlorophyll b

e e

ETC of Photosynthesis

e e fill th

e e– vacancy

Prof.DR. Samih Tamimi Bio304101

6

electron carrier

ee

e e

5sun

NADPH toCalvin Cycle

Photosystem IIP680

chlorophyll a

Photosystem IP700

chlorophyll b$$ in the bank…reducing power!

ETC of Photosynthesis

Prof.DR. Samih Tamimi Bio304101

split H2O

ETC of Photosynthesis

O

ATP

to Calvin CycleH+

H+

H+

H+

H+

H+

H+

H+

H+H+

H+

e e

e e

sun sun

Prof.DR. Samih Tamimi Bio304101

ETC of Photosynthesis• ETC uses light energy to produce

– ATP & NADPH• go to Calvin cycle

• PS II absorbs light– excited electron passes from chlorophyll to “primary

electron acceptor”– need to replace electron in chlorophyll– enzyme extracts electrons from H2O & supplies them

to chlorophyll• splits H2O• O combines with another O to form O2

• O2 released to atmosphere• and we breathe easier!

Prof.DR. Samih Tamimi Bio304101

Photophosphorylation

NONcyclicphotophosphorylatio

n

cyclicphotophosphorylatio

n

ATP

NADP

Prof.DR. Samih Tamimi Bio304101

Prof.DR. Samih Tamimi Bio304101

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

The Calvin cycleIs similar to the citric acid

cycle in mitochondriaOccurs in the stroma

Prof.DR. Samih Tamimi Bio304101

The Calvin cycle has three phases

1. Carbon fixation2. Reduction3. Regeneration of the CO2

acceptor (RuBP)

Prof.DR. Samih Tamimi Bio304101

The Calvin Cycle Steps

CARBON FIXATION1. CO2 enters cycle and attached to a 5-

carbon sugar called ribulose biphosphate (RuBP) forming 6-C molecule (unstable)

Enzyme RUBISCO catalyzes reaction

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

Prof.DR. Samih Tamimi Bio304101

REDUCTION3. Each 3-phosphoglycerate (3-PGA) gets an

additional phosphate from ATP (from LIGHT RXN) becomes 1,3 phosphoglycerate

4. NADPH reduces 1,3 phosphoglycerate 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 (GLUCOSE) production

Prof.DR. Samih Tamimi Bio304101

REGENERATION OF CO2 ACCEPTOR (RuBP)

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

– ATP is used !!!!

Prof.DR. Samih Tamimi Bio304101

The Calvin cycle

Phase 1: Carbon fixation

Phase 2:Reduction

Phase 3:Regeneration ofthe CO2 acceptor(RuBP)

Also known as PGAL

NOTE:

MORE ATP is

needed than

NADPH!!

Prof.DR. Samih Tamimi Bio304101

Calvin Cycle Overview

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

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

Prof.DR. Samih Tamimi Bio304101