Life on Earth is SOLAR poweredPhotosynthesis (Ps) nourishes almost all living organisms

Autotrophs - mainly Ps organisms (photoautotrophs) that make their own food (using sun E, CO2, and H2O)Also called producers of the biosphereExs = green plants and Ps protist groups (fig 10.2)

Heterotrophs - get E from organic compounds produced by other organismsAlso called consumers of the biosphereExs = fungi, animals, & many protist groups

Photosynthesis converts light E to chemical E of food

Introduction to Photosynthesis (181-200)

The Process That Feeds the Biosphere• Photosynthesis

– Is the process that converts solar (light) energy into chemical energy

• Plants and other autotrophs

– Are the producers of the biosphere

Plants are photoautotrophs

• They use the energy of sunlight to make organic molecules from water and carbon dioxide

Figure 10.1

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 protist 10 m

40 m(d) Cyanobacteria

1.5 m(e) Purple sulfurbacteria

Figure 10.2

Heterotrophs• Heterotrophs

– Obtain their organic material from other organisms

– Are the consumers of the biosphere

– Includes fungi, animals, many protist groups and many bacteria

Primarily found in leaves (mesophyll = main part of a leaf)

Stomata = regulated holes in leaves where gas exchange occurs (what gases does a plant need to exchange for Ps?)

Organelles enclosed by a double-membrane system (endosymbiosis)

Stroma = internal fluid-filled cavity

Thylakoids = system of interconnected membrane sacs (separates the stroma from the thylakoid space)

Grana = stacks of thylakoids

Chlorophyll = green pigment that absorbs light E = molecular bridge between sunlight and Ps activity

Molecules are embedded in the thylakoid membrane system

Chloroplasts – Sites of Ps within the cell

Chloroplasts: The Sites of Photosynthesis in Plants

• The leaves of plants

– Are the major sites of photosynthesis

Vein

Leaf cross section

Figure 10.3

Mesophyll

CO2 O2Stomata

Chloroplasts

• Chloroplasts

– Are the organelles in which photosynthesis occurs

– Contain thylakoids and grana

– Stroma is the fluid in the internal cavity

– Chlorophyll is imbedded in the thylakoid membranes

Chloroplast

Mesophyll

5 µm

Outermembrane

Intermembranespace

Innermembrane

Thylakoidspace

ThylakoidGranumStroma

1 µm

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Tracking Atoms Through Photosynthesis:

• Photosynthesis is summarized as6 CO2 + 12 H2O + Light energy C6H12O6 + 6 O2 + 6 H2 O

ORCO2 + H2O [CH2O] + O2

Overall Ps equation has been known since the 1800sThe equation for Ps (fig 10.4) = reverse of respirationBut carbohydrates are not made by simply reversing what happens in respirationBOTH processes occur in plant cells!

The Splitting of Water

• Chloroplasts split water into

– Hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules

6 CO2 12 H2OReactants:

Products: C6H12O66 H2O 6 O2

Figure 10.4

Photosynthesis as a Redox Process• Photosynthesis is a redox process

– Water is oxidized, carbon dioxide is reduced

Two stages of Ps (fig 10.5):

1. Light rxns: depend on light make ATP & NADPH and give off O2

NADPH = very similar in structure to NADH (just add a phosphate group to NADH) = the e- carrierPhotophosphorylation = how ATP is generated (using chemiosmosis again)

2. Calvin cycle: use ATP and NADPH to fix C from the atmosphere into organic compoundsCarbon fixation = initial incorporation of C into organic compounds

Two Stages of Photosynthesis

The Two Stages of Photosynthesis• Photosynthesis consists of two processes

– The light reactions

– The Calvin cycle

sunlight

light-dependentrxns

Calvin cycle

water

NADPH

NADP+

ADP + Pi

ATP

carbon dioxide

glucosePoxygen newwater

The Light Reactions• The light reactions

– Occur in the grana

– Split water, release oxygen, produce ATP, and form NADPH

The Calvin Cycle• The Calvin cycle

– Occurs in the stroma

– Forms sugar from carbon dioxide, using ATP for energy and NADPH for reducing power

An overview of photosynthesis

H2O CO2

Light

LIGHT REACTIONS

CALVINCYCLE

Chloroplast

[CH2O](sugar)

NADPH

NADP

ADP

+ P

O2Figure 10.5

ATP

Light Reactions• The light reactions convert solar energy to the

chemical energy of ATP and NADPH

Light• Light = electromagnetic energy, which travels in

waves

• Wavelength = distance between crests/troughs of waves (nm - km)

– Smaller wavelengths = stronger light waves

• Electromagnetic spectrum (fig 10.6) = entire range of light

– Visible light (380-750 nm) important to biological systems

• Different pigments absorb different wavelengths and reflect others (what we see that makes them colored)

– What wavelength of light do plants reflect?

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 electromagnetic spectrum• The electromagnetic spectrum

– Is the entire range of electromagnetic energy, or radiation

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 visible light spectrum• The visible light spectrum

– Includes the colors of light we can see

– Includes the wavelengths that drive photosynthesis

Photosynthetic Pigments

• Photosynthetic pigments absorb specific wavelenths of light

• Absorption spectrum = a pigment’s light absorption vs. wavelength

• Spectrophotometer = instrument that measures absorbance of specific wavelengths (fig 10.8)

• Beam of light sent through solution fraction of light transmitted at each wavelength measured

Photosynthetic Pigments: Light Receptors

• Photosynthetic Pigments

– Are substances that absorb specific wavelengths within the visible light spectrum

Pigments– Reflect some light, which include the colors we

seeLight

ReflectedLight

Chloroplast

Absorbedlight

Granum

Transmittedlight

Figure 10.7

The spectrophotometer• The spectrophotometer

– Is a machine that sends light through pigments and measures the fraction of light transmitted at each wavelength

Transmitted light is NOT absorbed by that particular pigment

An absorption spectrum• An absorption spectrum

– Is a graph plotting light absorption versus wavelength

Figure 10.8

Whitelight

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

40 100

0 100

Photosynthetic Pigments• Chlorophyll a (fig 10.10) absorption spectrum (fig 10.9a)

• Chlorophyll b = accessory pigment similar to chl. a

• When chlorophyll pigment absorbs light energy boosts an e- to an orbital of higher energy level (pigment is in its excited state)

• If chlorophyll is isolated from chloroplast (fig 10.11) fluoresces (emits light) in red-orange end of spectrum (E given off as heat)

• Carotenoids = other accessory pigments (hydrocarbons) reflecting various shades of orange/yellow/red (fig 10.9a)

• Most important function = photoprotection (absorb & dissipate excess light E)

Pigment Absorption Spectra• The absorption spectra of chloroplast pigments

– Provide clues to the relative effectiveness of different wavelengths for driving photosynthesis

Absorption spectra of three pigments in chloroplasts

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

EXPERIMENT

RESULTSA

bso

rptio

n o

f lig

ht

by

chlo

rop

last

pig

me

nts

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

Figure 10.9

The action spectrum for photosynthesis• Profiles the relative effectiveness of different

wavelengths of radiation in driving photosynthesisR

ate

of

phot

osyn

thes

is(m

easu

red

by O

2 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)

The action spectrum for photosynthesis• Was first demonstrated by Theodor W. Engelmann

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

Types of 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

Other Pigments• Other accessory pigments

– Absorb different wavelengths of light and pass the energy to chlorophyll a

Excitation of Chlorophyll by Light• When a pigment absorbs light

– It goes from a ground state to an excited state, which is unstable

Excitedstate

Ene

rgy

of e

lect

ion

Heat

Photon(fluorescence)

Chlorophyllmolecule

GroundstatePhoton

e–

Figure 10.11 A

Chlorophyll absorbs energy• If an isolated solution of

chlorophyll is illuminated

– It will fluoresce, giving off light and heat

– The excited electron drops back to the ground-state orbital.

Figure 10.11 B

• Tomorrow, we will start with the different types of photosynthetic pigments, and which wavelengths of light each absorbs.

• We will also discuss the light reaction portion of photosynthesis. The light reaction produces ATP and NADPH which go to power the fixation and reduction of carbon dioxide into sugar by the Calvin Cycle.