PhotosynthesisChapter 10

A. P. Biology

Liberty Senior High School

Mr. Knowles

Question:

Does eating carrots really improve your

vision?

• Photosynthesis– Occurs in plants, algae, certain other protists, and

some prokaryotes (photoautotrophs).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

• Plants are photoautotrophs– They use the energy of sunlight to make organic

molecules from water and carbon dioxide

Figure 10.1

Chloroplasts: The Sites of Photosynthesis in Plants

• Leaves are the major sites of photosynthesis

Vein

Leaf cross section

Figure 10.3

Mesophyll

CO2 O2Stomata

• Chloroplasts–Are the organelles in which photosynthesis

occurs; contain thylakoids and grana

Chloroplast

Mesophyll

5 µm

Outermembrane

Intermembranespace

Innermembrane

Thylakoidspace

ThylakoidGranumStroma

1 µm

Chloroplasts

Photosynthesis• < 1.0 % of sun’s energy that strikes Earth is

converted into 150 billion metric tons of sugar.• In cellular respiration, electrons are transferred

from sugar to oxygen; they lose potential energy to make ATP.

• In photosynthesis, electrons from water gain potential energy as they are transferred to carbon dioxide to make sugar; the energy is provided by light.

C6H12O6 + O2

CO2 + H2OFre

e E

ner

gy

G

Rxn Time

Historical Perspective• 17th Century: van Helmont measured

willow tree growth; compared mass of tree to mass of soil.

• Late 18th Century: J. Priestly “restores” air in a vacuum with mint.

• 18th Century: Ingenhousz found air only restored in the presence of sunlight; hypothesized CO2 C + O2

Role of Water• 1930’s: Van Niel studied purple sulfur

bacteria that performed photosynthesis w/o water:

CO2 + 2 H2S + Light Energy (CH2O) + H2O +2 S

• The H2S serves an electron donor.• H2O serves as donor in green plants.

Role of Water• In the 1950’s: used 18O ( an isotope

of oxygen ) in water to trace oxygen in the reaction.

• 6 CO2 + 12 H218O + Light Energy

C6H12O6 + 6 18O2 + 6 H2O

The Role of Light • In 1900’s, F. F. Blackman determined that

photosynthesis occurred in two steps.• Initial set of reactions were dependent on light-

“Light Reactions” or Light-Requiring Reactions.• A second set of reactions were independent of

light-”Dark Reactions” or Non-Light Requiring Reactions- affected by temperature.

• Dark Reactions use enzymes.

The Splitting of Water• Chloroplasts split water into– Hydrogen and oxygen, incorporating the electrons of

hydrogen into sugar molecules

6 CO212 H2OReactants:

Products: C6H12O66 H2O 6 O2

Figure 10.4

The Biophysics of Light• Light consists of units of energy-

Photons.• Not all photons have the same amount

of energy.• Photons travel in waves; higher the

energy the shorter the wavelength (measured in nm, lambda).

• 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

– Reflect light, which include the colors we see

Light

ReflectedLight

Chloroplast

Absorbedlight

Granum

Transmittedlight

Figure 10.7

Absorption Spectrum

• The chemical nature of the molecule light hits determines if the energy is absorbed.

• Each molecule has a characteristic range of photons – absorption spectrum – it can absorb.

• Spectrophotometer– Is a machine that sends light through pigments and

measures the fraction of light transmitted at each wavelength.

• An absorption spectrum (with a spectrophotometer)– 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

4

0 100

0 100

Energy Absorption• Pigments are molecules that absorb visible

light well.

• Two kinds of photosynthetic pigments:

Carotenoids• Carotenoids- yellow and orange pigments

that are highly efficient at absorbing a broad range of energies. Ex. Beta-carotene in carrots.

• Ex. Xanthophylls are an oxygenated form of carotene. These are accessory pigments.

• Other carotenoids are found in petals, birds and shrimp

• Accessory pigments– Absorb different wavelengths of light and pass the

energy to chlorophyll a

Beta-Carotene

Chlorophylls

•Cholorophylls- chlorophyll a and b- absorb narrow ranges of spectrum but highly efficient. Chlorophyll a is the major photosynthetic pigment.

• Chlorophyll a– Is the main

photosynthetic pigment

• Chlorophyll b– Is an accessory

pigment

C

CH

CH2

CC

CC

C

CNNC

H3C

C

C

C

C C

C

C

C

N

CC

C

C N

MgH

H3C

H

C CH2CH3

H

CH3C

HH

CH2

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

• The absorption spectra of three types of pigments in chloroplasts

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

EXPERIMENT

RESULTSA

bsor

ptio

n of

ligh

t by

chlo

ropl

ast p

igm

ents

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 of a pigment– Profiles the relative effectiveness of different

wavelengths of radiation in driving photosynthesis

Rat

e of

pho

tosy

nthe

sis

(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)

• 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

Excitation of Chlorophyll by Light• When a pigment absorbs light– It goes from a ground state to an excited state, which

is unstableExcitedstate

Ene

rgy

of e

lect

i on

Heat

Photon(fluorescence)

Chlorophyllmolecule

GroundstatePhoton

e–

Figure 10.11 A

• If an isolated solution of chlorophyll is illuminated– It will fluoresce, giving off light and heat

Figure 10.11 B