Chapter 10

Lecture #4 Date ________

• Chapter 10 -Photosynthesis

Photosynthesis in nature

• Autotrophs: biotic producers;1. Photoautotrophs: transform

sunlight energy into chemical energy by combining it with CO2 and H2O .

2. Chemoautotrophs; obtain inorganic nutrients and combine it with CO2 to make organic food.

• Heterotrophs: biotic consumers; obtains organic food by eating other organisms or their by-products (includes decomposers).

Photosynthesis

• Occurs in plants, algae, certain other protists, and some prokaryotes

These organisms use light energy to drive the synthesis of organic molecules from carbon dioxideand (in most cases) water. They feed not onlythemselves, but the entire living world. (a) Onland, plants are the predominant producers offood. In aquatic environments, photosyntheticorganisms include (b) multicellular algae, suchas this kelp; (c) some unicellular protists, suchas Euglena; (d) the prokaryotes calledcyanobacteria; and (e) other photosyntheticprokaryotes, such as these purple sulfurbacteria, which produce sulfur (sphericalglobules) (c, d, e: LMs). (a) Plants

(b) Multicellular algae

(c) Unicellular protist10 m

40 m(d) Cyanobacteria

1.5 m(e) Pruple sulfurbacteria

Figure 10.2

The chloroplast

Sites of photosynthesis• Organ: leaves (major site)

– Gas exchange: stomata

• Plant cell: mesophyll• Organelle: chloroplast

– Double membrane

– Thylakoids, grana, stroma

– Pigment: chlorophyll

Summary of Photosynthesis

6 CO2 + 12 H2O + Light energy C6H12O6 + 6 O2 + 6 H2O

• Photosynthesis is a redox process - water is oxidized, e- (along w/ H+) are transferred to CO2, reducing it to sugar

6 CO2 12 H2OReactants:

Products: C6H12O66 H2O 6 O2

Figure 10.4

Photosynthesis: an overview

2 major steps:

1. light reactions (“photo”)

– NADP+ (electron acceptor) to NADPH

– Photophosphorylation: ADP ---> ATP

2. Calvin cycle (“synthesis”)

– Carbon fixation: carbon into organics

The Nature of Sunlight

• Light– Is a form of electromagnetic energy, which

travels in waves

• Wavelength– Is the distance between the crests of waves– Determines the type of electromagnetic energy

The Nature of Sunlight

• The electromagnetic spectrum– Is the entire range of electromagnetic energy, or

radiation

• The visible light spectrum– Includes the colors of light we can see– Includes the wavelengths that drive

photosynthesis

The Nature of Sunlight

Gammarays X-rays UV Infrared

Micro-waves

Radiowaves

10–5 nm 10–3 nm 1 nm 103 nm 106 nm1 m

106 nm 103 m

380 450 500 550 600 650 700 750 nm

Visible light

Shorter wavelength

Higher energy

Longer wavelength

Lower energyFigure 10.6

The Electromagnetic Spectrum

The Nature of Sunlight

• Pigments– Are substances that

absorb visible light– The also reflect

light, which include the colors we see

LightReflectedLight

Chloroplast

Absorbedlight

Granum

Transmittedlight

Figure 10.7

Which Wavelengths of Light are Important for Photosynthesis?

Three different experiments helped reveal which wavelengths of light are photosynthetically important. The results are shown below.

EXPERIMENT

RESULTS

Ab

sorp

tion

of

ligh

t b

ych

loro

pla

st p

igm

en

ts

Chlorophyll a

(a) Absorption spectra. The three curves show the wavelengths of light best absorbed by three types of chloroplast pigments.

Wavelength of light (nm)

Chlorophyll b

Carotenoids

Which Wavelengths of Light are Important for Photosynthesis?

Rat

e o

f ph

otos

ynth

esis

(mea

sure

d by

O2 r

elea

se)

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 and carotenoids.

(b)

Which Wavelengths of Light are Important for Photosynthesis?

400 500 600 700

Aerobic bacteria

Filamentof alga

Engelmann‘s experiment. In 1883, Theodor W. Engelmann illuminated a filamentous alga with light that had been passed through a prism, exposing different segments of the alga to different wavelengths. He used aerobic bacteria, which concentrate near an oxygen source, to determine which segments of the alga were releasing the most O2 and thus photosynthesizing most.Bacteria congregated in greatest numbers around the parts of the alga illuminated with violet-blue or red light. Notice the close match of the bacterial distribution to the action spectrum in part b.

(c)

Light in the violet-blue and red portions of the spectrum are most effective in driving

photosynthesis.

CONCLUSION

Chlorophyll

• Chlorophyll a– Is the main

photosynthetic pigment

• Chlorophyll b– Is an accessory pigment

C

CH

CH2

CC

CC

C

CNNC

H3C

C

CC

C C

C

C

C

N

CC

C

C N

MgH

H3C

H

C CH2CH3

H

CH3C

HHCH2

CH2

CH2

H CH3

C O

O

O

O

O

CH3

CH3

CHO

in chlorophyll a

in chlorophyll b

Porphyrin ring:Light-absorbing“head” of moleculenote magnesiumatom at center

Hydrocarbon tail:interacts with hydrophobicregions of proteins insidethylakoid membranes ofchloroplasts: H atoms notshown

Figure 10.10

Photosystems

• Light harvesting units of the thylakoid membrane

• Composed mainly of protein and pigment antenna complexes

• Antenna pigment molecules are struck by photons

• Energy is passed to reaction centers (redox location)

• Excited e- from chlorophyll is trapped by a primary e- acceptor

Noncyclic electron flow• Photosystem II (P680):

– photons excite chlorophyll e- to an acceptor

– e- are replaced by splitting of H2O (release of O2)

– e-’s travel to Photosystem I down an electron transport chain (Pq~cytochromes~Pc)

– as e- fall, ADP ---> ATP (noncyclic photophosphorylation)

• Photosystem I (P700):

– “fallen” e- replace excited e- to primary e- acceptor

– 2nd ETC ( Fd~NADP+ reductase) transfers e- to NADP+ ---> NADPH (...to Calvin cycle…)

• These photosystems produce equal amounts of ATP and NADPH

Light reactions

www.johnkyrk.com/• This website has an excellent animation of the

process of photosynthesis. Please utilize this website to review the light reactions of photosynthesis.

The Calvin cycle

• 3 molecules of CO2 are ‘fixed’ into glyceraldehyde 3-phosphate (G3P)

• Phases:1. Carbon fixation - each CO2 is

attached to RuBP (rubisco enzyme)

2. Reduction - electrons from NADPH reduces to G3P; ATP used up

3. Regeneration - G3P rearranged to RuBP; ATP used; cycle continues

Light reactions

www.johnkyrk.com/• This website has an excellent animation of the

process of photosynthesis. Please utilize this website to review the dark reactions (Calvin Cycle) of photosynthesis.

Calvin Cycle, net synthesis

• For each G3P (and for 3 CO2)…….Consumption of 9 ATP’s & 6 NADPH(light reactions regenerate these molecules)

• G3P can then be used by the plant to make glucose and other organic compounds

Cyclic electron flow

• Alternative cycle when ATP is deficient

• Photosystem I used but not II; produces ATP but no NADPH

• Why? The Calvin cycle consumes more ATP than NADPH…….

• Cyclic photophosphorylation

The light reactions and chemiosmosis: the organization of the thylakoid membrane

LIGHTREACTOR

NADP+

ADP

ATP

NADPH

CALVINCYCLE

[CH2O] (sugar)STROMA(Low H+ concentration)

Photosystem II

LIGHT

H2O CO2

Cytochromecomplex

O2

H2O O21

1⁄2

2

Photosystem ILight

THYLAKOID SPACE(High H+ concentration)

STROMA(Low H+ concentration)

Thylakoidmembrane

ATPsynthase

PqPc

Fd

NADP+

reductase

NADPH + H+

NADP+ + 2H+

ToCalvincycle

ADP

PATP

3

H+

2 H++2 H+

2 H+

Figure 10.17

Alternative carbon fixation methods, I

• Photorespiration: hot/dry days; stomata close; CO2 decrease, O2 increase in leaves; O2 added to rubisco; no ATP or food generated

• Two Solutions…..• 1- C4 plants: two

photosynthetic cells, bundle-sheath & mesophyll; PEP carboxylase (instead of rubisco) fixes CO2 in mesophyll; new 4C molecule releases CO2 (grasses).

Alternative carbon fixation methods, II

• 2- CAM plants: open stomata during night, close during day (crassulacean acid metabolism); cacti, pineapples, etc.

A review of photosynthesis

Light reactions:• Are carried out by molecules in the thylakoid membranes• Convert light energy to the chemical energy of ATP and NADPH• Split H2O and release O2 to the atmosphere

Calvin cycle reactions:• Take place in the stroma• Use ATP and NADPH to convert CO2 to the sugar G3P• Return ADP, inorganic phosphate, and NADP+ to the light reactions

O2

CO2H2O

Light

Light reaction Calvin cycle

NADP+

ADP

ATP

NADPH

+ P 1

RuBP 3-Phosphoglycerate

Amino acidsFatty acids

Starch(storage)

Sucrose (export)

G3P

Photosystem IIElectron transport chain

Photosystem I

Chloroplast

Figure 10.21

A Comparison of Chemiosmosis in Chloroplasts and Mitochondria

• Chloroplasts and mitochondria– Generate ATP by the same basic mechanism:

chemiosmosis

– But use different sources of energy to accomplish this

• In both organelles– Redox reactions of electron transport chains generate a

H+ gradient across a membrane

• ATP synthase– Uses this proton-motive force to make ATP

A Comparison of Chemiosmosis in Chloroplasts and Mitochondria

• The spatial organization of chemiosmosis– Differs in

chloroplasts and mitochondria

Key

Higher [H+]Lower [H+]

Mitochondrion Chloroplast

MITOCHONDRIONSTRUCTURE

Intermembrancespace

Membrance

Matrix

Electrontransport

chain

H+ DiffusionThylakoidspace

Stroma

ATPH+

PADP+

ATPSynthase

CHLOROPLASTSTRUCTURE

Figure 10.16