copyright © 2005 pearson education, inc. publishing as benjamin cummings topic 3.8 photosynthesis...

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

Topic 3.8 Photosynthesis

Life on Earth is solar powered!

3.8.1 State that photosynthesis involves the conversion of light energy into chemical energy.


Topic 3.8 Photosynthesis

Photosynthesis is the process that converts solar energy into chemical energy.

3.8.1 State that photosynthesis involves the conversion of light energy into chemical energy.

Plants

Unicellular protist

Multicellular algae Cyanobacteria

Purple sulfurbacteria

10 µm

1.5 µm

40 µm

Photosynthesis occurs in plants, algae, certain other protists, and some prokaryotes


• Chloroplasts are organelles that are responsible for feeding the vast majority of organisms

3.8.3 State that chlorophyll is the main photosynthetic pigment.


Chloroplasts: The Sites of Photosynthesis in Plants

• Leaves are the major locations of photosynthesis

• Their green color is from chlorophyll, the green pigment within chloroplasts

• Light energy absorbed by chlorophyll drives the synthesis of organic molecules in the chloroplast

• Through microscopic pores called stomata, CO2 enters the leaf and O2 exits



• Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf

• A typical mesophyll cell has 30-40 chloroplasts

• The chlorophyll is in the membranes of thylakoids (connected sacs in the chloroplast); thylakoids may be stacked in columns called grana

• Chloroplasts also contain stroma, a dense fluid


Leaf cross sectionVein

Mesophyll

Stomata CO2O2

Mesophyll cellChloroplast

5 µm

Outermembrane

Intermembranespace

Innermembrane

Thylakoidspace

Thylakoid

GranumStroma

1 µm


• Photosynthesis can be summarized as the following equation:

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

SC.912.L.18.7 Identify the reactants, products, and basic functions of photosynthesis


• Chloroplasts split water into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules

3.8.5 State that light energy is used to produce ATP, and to split water molecules (photolysis) to form oxygen and hydrogen.

Reactants:

Products:

6 CO2 12 H2O

C6H12O6 6 H2O 6 O2

SC.912.L.18.7 Identify the reactants, products, and basic functions of photosynthesis


The Two Stages of Photosynthesis: A Preview

• Photosynthesis consists of 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 form NADPH

3.8.5 State that light energy is used to produce ATP, and to split water molecules (photolysis) to form oxygen and hydrogen.3.8.6 State that ATP and hydrogen (derived from the photolysis of water) are used to fix carbon dioxide to make organic molecules.


The Two Stages of Photosynthesis: A Preview

• The Calvin cycle (in the stroma) forms sugar from CO2, using ATP and NADPH

• The Calvin cycle begins with carbon fixation, incorporating CO2 into organic molecules

3.8.5 State that light energy is used to produce ATP, and to split water molecules (photolysis) to form oxygen and hydrogen.3.8.6 State that ATP and hydrogen (derived from the photolysis of water) are used to fix carbon dioxide to make organic molecules.

LE 10-5_1

H2O

LIGHTREACTIONS

Chloroplast

Light

LE 10-5_2

H2O

LIGHTREACTIONS

Chloroplast

Light

ATP

NADPH

O2

LE 10-5_3

H2O

LIGHTREACTIONS

Chloroplast

Light

ATP

NADPH

O2

NADP+

CO2

ADPP+ i

CALVINCYCLE

[CH2O](sugar)


The light reactions convert solar energy to the chemical energy of ATP and NADPH

• Chloroplasts are solar-powered chemical factories

• Their thylakoids transform light energy into the chemical energy of ATP and NADPH


The Nature of Sunlight

• Light is a form of electromagnetic energy, also called electromagnetic radiation

• Like other electromagnetic energy, light travels in rhythmic waves

• Wavelength = distance between crests of waves

• Wavelength determines the type of electromagnetic energy

• Light also behaves as though it consists of discrete particles, called photons


• The electromagnetic spectrum is the entire range of electromagnetic energy, or radiation

• Visible light consists of colors we can see, including wavelengths that drive photosynthesis

3.8.2 State that light from the sun is composed of a range of wavelengths (colours).

Visible light

Gammarays

X-rays UV InfraredMicro-waves

Radiowaves

10–5 nm 10–3 nm 1 nm 103 nm 106 nm1 m

(109 nm) 103 m

380 450 500 550 600 650 700 750 nm

Longer wavelength

Lower energy

Shorter wavelength

Higher energy


Photosynthetic Pigments: The Light Receptors

• 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 reflects and transmits green light

LE 10-7

Chloroplast

Light

Reflected light

Absorbed light

Transmitted light

Granum


• 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

LE 10-8a

Whitelight

Refractingprism

Chlorophyllsolution

Photoelectrictube

Galvanometer

The high transmittance (low absorption) reading indicates that chlorophyll absorbs very little green light.

Greenlight

Slit moves to pass light of selected wavelength

0 100


• An absorption spectrum is a graph plotting a pigment’s light absorption versus wavelength

• The absorption spectrum of chlorophyll a suggests that violet-blue and red light work best for photosynthesis

8.2.7 Explain the relationship between the action spectrum and the absorption spectrum of photosynthetic pigments in green plants.

Chlorophyll a

Chlorophyll b

Carotenoids

Wavelength of light (nm)

Absorption spectra

Ab

sorp

tio

n o

f lig

ht

by

chlo

rop

last

pig

men

ts

400 500 600 700

3.8.4 Outline the differences in absorption of red, blue and green light by chlorophyll


• An action spectrum profiles the relative effectiveness of different wavelengths of radiation in driving a process

8.2.7 Explain the relationship between the action spectrum and the absorption spectrum of photosynthetic pigments in green plants.

Action spectrum

Rat

e o

f p

ho

to-

syn

thes

is (

mea

sure

db

y O

2 re

leas

e)


• The action spectrum of photosynthesis was first demonstrated in 1883 by Thomas Engelmann

• In his experiment, he exposed different segments of a filamentous alga to different wavelengths

• Areas receiving wavelengths favorable to photosynthesis produced excess O2

• He used aerobic bacteria clustered along the alga as a measure of O2 production

LE 10-9c

Engelmann’s experiment

400 500 600 700

Aerobic bacteria

Filament of algae


• Chlorophyll a is the main photosynthetic pigment

• Accessory pigments, such as chlorophyll b, broaden the spectrum used for photosynthesis

• Accessory pigments called carotenoids absorb excessive light that would damage chlorophyll


Excitation of Chlorophyll by Light

• When a pigment absorbs light, it goes from a ground state to an excited state, which is unstable

• When excited electrons fall back to the ground state, photons are given off, an afterglow called fluorescence

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

Excitedstate

Heat

Photon(fluorescence)

GroundstateChlorophyll

molecule

Photon

Excitation of isolated chlorophyll molecule Fluorescence

En

erg

y o

f el

ectr

on

e–


A Photosystem: A Reaction Center Associated with Light-Harvesting Complexes

• 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 chlorophyll a molecule to the primary electron acceptor is the first step of the light reactions

LE 10-12

Thylakoid

Photon

Light-harvestingcomplexes

Photosystem

Reactioncenter

STROMA

Primary electronacceptor

e–

Transferof energy

Specialchlorophyll amolecules

Pigmentmolecules

THYLAKOID SPACE(INTERIOR OF THYLAKOID)

Th

ylak

oid

mem

bra

ne


• There are two types of photosystems in the thylakoid membrane

• Photosystem II functions first (the numbers reflect order of discovery) and is best at absorbing a wavelength of 680 nm

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

• The two photosystems work together to use light energy to generate ATP and NADPH

3.8.5 State that light energy is used to produce ATP, and to split water molecules (photolysis) to form oxygen and hydrogen.

ATP

Photosystem II

e–

e–

e–e–

MillmakesATP

e–

e–

e–

Ph

oto

n

Photosystem I

Ph

oto

n

NADPH

8.2.3 Explain the light-dependent reactions.

STROMA(Low H+ concentration)

Light

Photosystem IICytochrome

complex

2 H+

Light

Photosystem I

NADP+

reductase

Fd

PcPq

H2O O2

+2 H+

1/2

2 H+

NADP+ + 2H+

+ H+NADPH

ToCalvincycle

THYLAKOID SPACE(High H+ concentration)

STROMA(Low H+ concentration)

Thylakoidmembrane ATP

synthase

ATP

ADP+P

H+i

[CH2O] (sugar)O2

NADPH

ATP

ADP

NADP+

CO2H2O

LIGHTREACTIONS

CALVINCYCLE

Light


The Importance of Photosynthesis: A Review

• The energy entering chloroplasts as sunlight gets stored as chemical energy in organic compounds

• Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules of cells

• In addition to food production, photosynthesis produces the oxygen in our atmosphere

LE 10-21

Light

CO2H2O

Light reactions Calvin cycle

NADP+

RuBP

G3PATP

Photosystem IIElectron transport

chainPhotosystem I

O2

Chloroplast

NADPH

ADP+ P i

3-Phosphoglycerate

Starch(storage)

Amino acidsFatty acids

Sucrose (export)

copyright © 2005 pearson education, inc. publishing as benjamin cummings topic 3.8 photosynthesis...

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