3. Photosynthesis

Chapter 10

Photosynthesis converts light energy to the chemical energy of food

• _{Chloroplasts are organelles that are responsible}

for feeding the vast majority of organisms

• _{Leaves are the major locations of}

photosynthesis

– _{Their green color is from chlorophyll, the green}

pigment within chloroplasts

Chloroplasts

• _{Found mainly in cells of}

the mesophyll, the

interior tissue of the leaf

• _{A typical mesophyll cell}

has 30-40 chloroplasts

• _{Chlorophyll is in the}

membranes of thylakoids; thylakoids may be stacked in columns called grana

• _{Chloroplasts also contain}

Tracking Atoms Through Photosynthesis

• _{Photosynthesis can be summarized as:}

• _{Chloroplasts split water into hydrogen and oxygen,}

incorporating the electrons of hydrogen into sugar molecules

• _{Photosynthesis is a redox process in which water is oxidized}

and carbon dioxide is reduced

6 CO₂ + 12 H₂O + Light energy  C₆H₁₂O₆ + 6 O₂ + 6 H₂ O

Reactants:

Products:

6 CO₂ 12 H₂O

H₂O

LIGHT REACTIONS

Chloroplast Light

ATP NADPH

O₂

NADP+

CO₂

ADP P + i

CALVIN CYCLE

[CH₂O] (sugar)

The Two Stages of Photosynthesis

• _{The light reactions (the photo part) and Calvin cycle}

(the synthesis part)

• _{The light reactions (in the thylakoids) split water,}

release O₂, produce ATP and NADPH

• _{The Calvin cycle (in the stroma) forms sugar from}

The Nature of Sunlight

• _{Light is a form of electromagnetic energy}

– _{Like other electromagnetic energy, light travels in}

waves

• _{Light also behaves as though it consists of discrete}

Photosynthetic Pigments

• _{Pigments are substances that absorb visible light} • _{Different pigments absorb different wavelengths} • _{Wavelengths that are not absorbed are reflected}

or transmitted

• _{Leaves appear green because chlorophyll …does}

Meet the Spectrophotometer

• _{A spectrophotometer measures a pigment’s}

ability to absorb various wavelengths

• _{This machine sends light through pigments}

and measures the fraction of light transmitted at each wavelength

White light

Refracting prism

Chlorophyll solution

Photoelectric tube

Galvanometer

The high transmittance (low absorption)

reading indicates that chlorophyll absorbs very little green light. Green

light Slit moves to

pass light of selected wavelength

0 100

Absorption and action spectra

• _{The absorption}

spectrum of chlorophyll

a suggests that

violet-blue and red light work best for photosynthesis

– _{An absorption spectrum:}

plots a pigment’s light absorption versus

wavelength

– _{An action spectrum}

profiles the relative

effectiveness of different wavelengths of radiation in driving a process

Chlorophyll a

Chlorophyll b

Carotenoids

Wavelength of light (nm) Absorption spectra A b so rp ti o n o f li g h t b y ch lo ro p la st p ig m en ts

400 500 600 700

Photosynthetic Pigments

CH₃

CHO

in chlorophyll a

in chlorophyll b

Porphyrin ring: light-absorbing “head” of

molecule; note magnesium atom at center

Hydrocarbon tail: interacts with hydrophobic

regions of proteins inside

thylakoid membranes of chloroplasts; H atoms not shown

• _{Chlorophyll a is the main}

photosynthetic pigment

• _{Accessory pigments,}

such as chlorophyll b, broaden the spectrum used for photosynthesis

• _{Accessory pigments}

called carotenoids

Excitation of Chlorophyll by Light

• _{When a pigment absorbs light, it goes from a}

ground state to an unstable excited state

– _{When excited electrons fall back to the ground state,}

photons are given off, an afterglow called fluorescence

Excited state

Heat

Photon

(fluorescence) Ground

state

Chlorophyll molecule

Photon

Excitation of isolated chlorophyll molecule Fluorescence

E

n

er

g

y

o

f

el

ec

tr

o

Thylakoid Photon Light-harvesting complexes Photosystem Reaction center STROMA Primary electron acceptor e– Transfer of energy Special

chlorophyll a

molecules

Pigment molecules

THYLAKOID SPACE (INTERIOR OF THYLAKOID)

T h yl a ko id m em b ra n e

Photosystems

• _{A photosystem consists of a}

reaction center surrounded by light-harvesting

complexes

– _{The light-harvesting}

complexes (pigment

molecules bound to proteins) funnel the energy of photons to the reaction center

• A primary electron acceptor in the reaction center

accepts an excited electron from chlorophyll a

• _{Solar-powered transfer of an electron from a}

Two Types of Photosystems

• Photosystem II functions first and is best at absorbing a wavelength of 680 nm

• Photosystem I is best at absorbing a wavelength of 700 nm

Light P680 e– Photosystem II (PS II) Primary acceptor

[CH2O] (sugar)

NADPH ATP ADP CALVIN CYCLE LIGHT REACTIONS NADP+ Light

H2O CO2

E n er g y o f el e ct ro n s O2 e– e– +

2 H+ H2O

O₂

1_/2

Pq Cytochrome complex Pc ATP P700 e– Primary acceptor Photosystem I (PS I) e– e– NADP+ reductase Fd NADP+ NADPH

+ H+

+ 2 H+

Light

Noncyclic Electron Flow

• _{During the light reactions, there are two possible}

routes for electron flow: cyclic and noncyclic

– _{Noncyclic electron flow, the primary pathway, involves}

LE 10-14

ATP

Photosystem II

e–

e–

e– e

–

Mill makes

ATP

e–

e–

e–

Ph oto

n

Photosystem I

P h o

to

n

Chemiosmosis in Chloroplasts and Mitochondria

MITOCHONDRION STRUCTURE

Intermembrane space

Membrane _transportElectron

chain

Mitochondrion Chloroplast

CHLOROPLAST STRUCTURE

Thylakoid space

Stroma

ATP Matrix

ATP synthase Key

H+ _Diffusion

ADP + P

H+

i

Higher [H+_]

Lower [H+_]

• _{They use different sources of energy}

• Mitochondria transfer chemical energy from food to ATP; chloroplasts transform light energy into the

chemical energy of ATP

The Function of the Thylakoid Membrane

• _{Water is split by photosystem II on the side of the}

membrane facing the thylakoid space

• The diffusion of H+ _{from the thylakoid space back to}

the stroma powers ATP synthase

• _{ATP and NADPH are produced on the side facing the}

LE 10-17

STROMA

(Low H+ _{concentration)}

Light

Photosystem II Cytochrome_complex

2 H+

Light Photosystem I NADP+ reductase Fd Pc Pq

H₂O

O₂

+2 H+

1_/2

2 H+

NADP+ _{+ 2H}+

+ H+

NADPH

To Calvin

cycle THYLAKOID SPACE

(High H+ _{concentration)}

STROMA

(Low H+ _{concentration)}

Thylakoid membrane ATP synthase ATP ADP + P H+ i

[CH2O] (sugar)

O₂ NADPH ATP ADP NADP+ CO2

H2O

LIGHT REACTIONS

• _{http://www.science.smith.edu/departments/B}

iology/Bio231/ltrxn.html

Cyclic Electron Flow

• Cyclic electron flow uses only photosystem I and

produces only ATP

• Generates ATP, not NADPH- need more ATP than

NADPH in the Calvin cycle

Photosystem I

Photosystem II ATP

Pc Fd

Cytochrome complex

Pq

Primary acceptor

Fd

NADP+ reductase

NADP+

NADPH Primary

Biology Theater Presents…

End g,c tuPictionary time!!!

• _{ATP synthase} • _Chemiosmosis • _Thylakoid

• _Glycolysis

• _Photosystem

The Calvin Cycle

• _{Regenerates its starting material}

• Builds sugar by using energy from ATP and the reducing power of electrons carried by NADPH

• _{Carbon enters as CO2 and leaves as a sugar named}

glyceraldehyde-3-phospate (G3P)

• For net synthesis of one G3P, the cycle must take place three times, fixing three molecules of CO₂

• _{Three phases:}

– _{Carbon fixation (catalyzed by rubisco)} – _Reduction

LE 10-18_3

[CH2O] (sugar)

O2 NADPH ATP ADP NADP+ CO2

H2O

LIGHT REACTIONS CALVIN CYCLE Light Input CO₂ (Entering one at a time)

Rubisco

3 P P

Short-lived intermediate

Phase 1: Carbon fixation

6 P 3-Phosphoglycerate 6 ATP 6 ADP CALVIN CYCLE 3 P P Ribulose bisphosphate (RuBP) 3

6 NADP+

6 6 NADPH P_i 6 P 1,3-Bisphosphoglycerate P 6 P Glyceraldehyde-3-phosphate (G3P) P 1 G3P (a sugar) Output Phase 2: Reduction Glucose and other organic compounds 3 3 ADP ATP Phase 3: Regeneration of

the CO₂ acceptor

(RuBP) _{5 P}

• _{http://www.science.smith.edu/departments/B}

iology/Bio231/calvin.html

• _{http://highered.mcgraw-hill.com/sites/007096}

0526/student_view0/chapter5/animation_qui z_1.html

Pop bead time (or draw it out…)

• _{Take 15 popbeads of one color…those are}

your three RuBPs

• _{Take 3 popbeads of another..those are your}

three carbons from carbon dioxide

• _{Take three popbeads of another color to be}

Tradeoffs in Photosynthesis

• _{Dehydration is a problem for plants}

• On hot, dry days, plants close stomata

• _{The closing of stomata reduces access to CO2 and}

causes O₂ to build up

Photorespiration

• In most plants (C₃ plants), initial fixation of CO₂forms a three-carbon compound

• In photorespiration, rubisco adds O₂ to the Calvin cycle instead of CO₂

– This consumes O₂ and organic fuel and releases CO₂ without producing ATP or sugar

• _{On a hot, dry day it can drain as much as 50% of the}

C

Plants

• _{Minimize the cost of photorespiration by}

incorporating CO₂ into four-carbon compounds in mesophyll cells

• _{These four-carbon compounds are exported to}

bundle-sheath cells, where they release CO₂ used in the Calvin cycle

Photosynthetic cells of C₄ plant leaf Mesophyll cell Bundle-sheath cell Vein (vascular tissue) C₄ leaf anatomy

Stoma Bundle-_sheath cell

Pyruvate (3 C) CO₂ Sugar Vascular tissue CALVIN CYCLE

PEP (3 C)

ATP ADP

Malate (4 C) Oxaloacetate (4 C)

The C₄ pathway CO₂

PEP carboxylase

• _{CAM plants open}

their stomata at

night, incorporating CO₂ into organic

acids

• _{Stomata close}

during the day, and CO₂ is released

from organic acids and used in the

Calvin cycle

Bundle-sheath cell

Mesophyll

cell Organic acid

C₄ CO₂ CO₂ CALVIN CYCLE Sugarcane Pineapple Organic acids

release CO₂ to

Calvin cycle

CO₂ incorporated

into four-carbon organic acids (carbon fixation) Organic acid CAM CO₂ CO₂ CALVIN CYCLE Sugar

Spatial separation of steps Temporal separation of steps

Sugar

Day Night

LE 10-21

Light

CO₂

H₂O

Light reactions Calvin cycle

NADP+

RuBP

G3P ATP

Photosystem II Electron transport

chain

Photosystem I

O₂

Chloroplast

NADPH ADP + P_i

3-Phosphoglycerate

Starch (storage)