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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.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Chloroplasts are organelles that are responsible for feeding the vast majority of organisms
3.8.3 State that chlorophyll is the main photosynthetic pigment.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
3.8.3 State that chlorophyll is the main photosynthetic pigment.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 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
3.8.3 State that chlorophyll is the main photosynthetic pigment.
Leaf cross sectionVein
Mesophyll
Stomata CO2O2
Mesophyll cellChloroplast
5 µm
Outermembrane
Intermembranespace
Innermembrane
Thylakoidspace
Thylakoid
GranumStroma
1 µm
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 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)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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–
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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)
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