Photosynthesis

PhotosynthesisPhotosynthesis• Light energyLight energy (kinetic) converted into

chemical energychemical energy (potential)• Anabolic, endergonic• 6CO2 + 6H2O C6H12O6 + 6O2

LightLight (chlorophyll + accessory pigments)(chlorophyll + accessory pigments)• LightLight = electromagnetic= electromagnetic radiation; wavesradiation; waves

PhotosynthesisPhotosynthesis• COCO22 and HH22OO are ‘energy-poor’

– Become energy-rich; C-H bond

• COCO22 is reduced; HH22OO is oxidized

• CO2 + H2 CH2O (glucose) + O2

2 Ways Organisms Get Energy2 Ways Organisms Get Energy

AutotrophAutotroph;

PhotoautotrophPhotoautotroph – – plants, algaeplants, algae

ChemoautotrophChemoautotroph – Bacteria

HeterotrophHeterotroph Photoheterotroph

Bacteria

ChemoheterotrophChemoheterotroph

Animals, Fungi

PhotosynthesisPhotosynthesis• 2 step process:2 step process:

– ‘‘Light reactions’Light reactions’; water ; water is split, ATP generated, H is split, ATP generated, H gatheredgathered

– Calvin CycleCalvin Cycle; CO; CO22

incorporated into other incorporated into other organic molecules; organic molecules; (‘carbon fixation’)(‘carbon fixation’)

Leaves Leaves

• ChlorophyllChlorophyll = 1o pigment

• Most chloroplasts are in the MesophyllMesophyll of leaf

• StomataStomata = pores in leaf (?)• Vascular bundles

– Transport water and glucose

ChloroplastsChloroplasts

• Divided into 3 areas:

1. 1. Intermembrane spaceIntermembrane space

2. ThylakoidThylakoid = membrane folded

into flattened discs; cristae

3. StromaStroma = viscous fluid around

grana (stacks of thylakoids);

matrix

Light reactions in thylakoid membrane, and thylakoid space (lumen) Calvin cycle in stromastroma

The Nature of The Nature of LightLight• Sun produces

electromagnetic energy by fusion of H

• Quantum mechanicsQuantum mechanics; duality of light– Wave-likeWave-like; frequency,

wavelength– Particle-likeParticle-like; photons

Nanometers; nm

•White lightWhite light ‘split’ by diffraction gradient (prism); ‘bent’

•Longer wavelengths = less energy; refracted more•Shorter wavelengths = more energy; refracted less

•Spectrum

LightLight

LightLight• Spectrophotometer Spectrophotometer • Measures light Measures light

transmittance/absorption at transmittance/absorption at adjustable frequenciesadjustable frequencies

• ? Wavelengths are absorbed? Wavelengths are absorbed• Absorbed waves are ‘useable’Absorbed waves are ‘useable’• ‘‘Green’ is reflected; Green Green’ is reflected; Green

must not be usedmust not be used

SpectrophotometerSpectrophotometer

Use of light in Use of light in PhotosynthesisPhotosynthesis

• Engelmann, 1883. (science)Engelmann, 1883. (science)• Used prism to expose algae Used prism to expose algae

to various wavelengths to various wavelengths • Measured growth of aerobic Measured growth of aerobic

bacteria (?) bacteria (?) • Measured rate of Measured rate of

photosynthesis by measuring photosynthesis by measuring the appearance of Othe appearance of O22

Process of ScienceProcess of Science

• Action spectrum (how much photosynthesis is occurring) does not match absorption spectrum (what wavelengths are being absorbed)– Absorption is also occurring between ‘blue’ and Absorption is also occurring between ‘blue’ and

‘red’‘red’

• What is going on?

• Must be more pigments than chlorophyll(s)Must be more pigments than chlorophyll(s)

PPiiggmmeennttss

• Chlorophyll a; b

• C – G

• Accessory pigments; – Carotenoids - yellows, oranges, oranges, redsreds– Keep chlorophyll from ODing

PigmentPigment = molecule that has color

Overview of PhotosynthesisOverview of Photosynthesis

• Light reactionsLight reactions: membrane-bound proteins– Noncyclic photophosphorylation

– Chemiosmosis

– Cyclic photophosphorylation

• Calvin cycleCalvin cycle – – chemical ‘soup’

Light ReactionsLight Reactions

• Production of ATPProduction of ATP

• Capture of H+Capture of H+

• Formation of OFormation of O22

PhotoexcitationPhotoexcitation of of ChlorophyllChlorophyll

e_

e_

Heat

Fluorescence: dinoflagellates

PhotonPhoton

ChlorophyllChlorophyll

PhotoexcitationPhotoexcitation of of ChlorophyllChlorophyll

• PhotosystemPhotosystem = cluster of pigment molecules– Chlorophyll a, b, and

accessory pigments

• Most pigment molecules are ‘antennae’– Pass excited energy to one

chlorophyll a

• Reaction centerReaction center - chlorophyll molecule

photosystem

Electron not allowed to drop back to ground

state

2 Types of Photosystems2 Types of Photosystems

• Named in order of discovery

• Photosystem IPhotosystem I; 700 nm (red)

• Photosystem IIPhotosystem II: 680 nm (red)

• Photosystem IIPhotosystem II is ‘beginning’

e- is excited to a higher level of

energy; picked up by electron

acceptor

Photosystem IIPhotosystem II • PS II receives light photons (from sun)PS II receives light photons (from sun)

• Electrons from chlorophyll become Electrons from chlorophyll become ‘excited’ ‘excited’

• Excited electrons picked up by 1Excited electrons picked up by 1oo acceptor moleculeacceptor molecule

• Excited electrons passed down ETC to Excited electrons passed down ETC to PS I (series of cytochromes)PS I (series of cytochromes)

Enzyme splits water releasing e- and

oxygen; e- goes to P680 replacing one lost from the photon

Photosystem IIPhotosystem II• Chemiosmotic generation of ATP

• PhotophosphorylationPhotophosphorylation• Electrons from water are extracted and

donated back to PS II (cycle)

• Water splits into H2 + O2 (waste)

e- passed down an ETC; generates ATP; by

chemiosmosis

e- travels ETC to Photosystem I; P700

boosts e- to its primary acceptor

e- passed from I down ETC to NADP; NADPH goes to Calvin

11oo Acceptor Acceptor

2 e2 e__

Reaction Reaction centercenter

HH22OO

OO22

Chemiosmosis

Photosystem IIP680P680

H+

2 e2 e__

2 e2 e__

2 e2 e__

2 e2 e__

Photosystem I

Photosystem I

e from e from PS IIPS II

2 e_

2 e2 e__

2 e2 e__

1o Acceptor

Fd

NADP+ NADP+ reductasereductase

NADP+NADP+

NADPHNADPH To CalvinHH++

Cyclic photophosphorylationCyclic photophosphorylation

Melvin CalvinMelvin Calvin• Used tracers to determine Used tracers to determine

carbon fixation and the carbon fixation and the sequence of events sequence of events – Lab next to atomic reactorLab next to atomic reactor

• Also used HAlso used H22 to discover to discover that water is the source of that water is the source of HH

Calvin CycleCalvin Cycle

• Similar to Kreb’sSimilar to Kreb’s• Ribulose Biphosphate (RuBP) is ‘fixed’ w/ Ribulose Biphosphate (RuBP) is ‘fixed’ w/

COCO22 (from ?) (from ?)• Uses Ribulose Bisphosphate carboxylase…Uses Ribulose Bisphosphate carboxylase…• RubiscoRubisco (#1 enzyme) (#1 enzyme)• CC33 plants (most plants) plants (most plants)

Calvin CycleCalvin Cycle• Three ‘phases’

• Carbon fixationCarbon fixation C added to RuBP

• ReductionReduction: addition of H from NADPH (light addition of H from NADPH (light reactions)reactions)

• Regeneration of RuBP:Regeneration of RuBP: CO2 acceptor

Rubisco

Calvin CycleCalvin Cycle

1. 33 RuBP (5C) + 3 3 CO2 (1C) = 1818C

2. 66 PGA (3C) = 1818 C + 6 6 H

3. 66 PGAL – 11G3P = 55 PGAL (3C);

55 PGAL (15C) = 33 RuBP (15C)

• Requires 2 cycles of Calvin to make 1 glucose (6C)

PhotorespiratioPhotorespirationn

• Hot, dry days (?)

• Stomata close

• No CO2

• Oxygen levels increase

• Rubisco adds O2 to Calvin instead of CO2

• Calvin shuts down

PhotorespirationPhotorespiration• Oxygen splits RuBP into CO2

– ‘Photo’ = light– Respiration = consumes oxygen

• No ATP

• No glucose

PhotorespirationPhotorespiration

RubiscoRubisco

Glucose

NOT glucose

PhotorespiratPhotorespirationion

• Two adaptations have evolved to solve the Two adaptations have evolved to solve the problem of photorespiration:problem of photorespiration:

• CC44 plant photosynthesisplant photosynthesis

• CAMCAM plant photosynthesisplant photosynthesis

CC4 4 PathwayPathway

• Hot, dry day; STOMATASTOMATA closed • CO2 fixed to a 3C compound (PEP) to

form oxaloacetate (OAA); 4C • PEP carboxylase (PEPCase)PEP carboxylase (PEPCase)• Strong affinity for COCO22

– Prevents RUBISCORUBISCO from fixing oxygen to RuBP

CC4 4 PathwayPathway

• Oxaloacetate (OAA) 4C converted to Malate

• Malate converted into CO2 + Pyruvate (3C)

• COCO22 Calvin

• Pyruvate (C3) C4 pathway

Plasmodesmata

CC4 4 PathwayPathway

• Bundle Sheath Cells:Bundle Sheath Cells:

– Adjacent to mesophyllmesophyll cells

– Calvin cycle

• Mesophyll cellsMesophyll cells

– C4 pathway

C4 plants

CAM PlantsCAM Plants

• Crassulacean Acid Metabolism

• Daytime = stomata closed

• Stomata open at night

• Pineapple, succulents

• Great water storage capability

–Waxy cuticle, thin, waxy leaves

CAM PlantsCAM Plants• C4 compounds made and

stored in vacuolesvacuoles at night when stomata are open

• During daylight, light reactions supply NADPH and ATP to run Calvin

• 4 C compounds release CO2 into Calvin

Differences In Types Of Photosynthesis

Differences In Types Of Photosynthesis

C3, C4, CAMC3, C4, CAM

• COCO22 fixed directly into Calvin in mesophyll cells.

• Bundle sheath cells not used for photosynthesis

• Stomata open in day.

• Most common; normal

CC33

• CO2 fixed into a 3C compound (OAA) by PEPCase in Mesophyll CellsMesophyll Cells

• Calvin takes place in separate Bundle Bundle SheathSheath cells;

• Stomata open in day

C4

• Stomata open at nightnight

• COCO22 fixed into PEP by PEPCase

• 4C stored in vacuolesvacuoles in mesophyll cells

• CO2 released into Calvin during day

CAMCAM

Compare and Contrast C4 and CAM Photosynthesis

• C4

• CO2 fixed into a 4C compound

• Physical separation

• CAM

• CO2 fixed into a 4C compound

• Temporal separation

SummarySummary

• 50% of sugar produced is used by the plant for respiration (growth/metabolism)

• Unused carbos transported and stored in non-photosynthetic parts; (roots, stem, flowers, fruits)– Large amount converted into

cellulosecellulose