Teresa Audesirk • Gerald Audesirk • Bruce E. Byers
Biology: Life on Earth
Eighth Edition
Lecture for Chapter 7
Capturing Solar Energy:
Photosynthesis
Copyright © 2008 Pearson Prentice Hall, Inc.
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
The ability to capture sunlight energy and
convert it to chemical energy.
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6CO2 + 6H2O + light energy  C6H12O6 + 6O2
carbon water
dioxide
sunlight
glucose
(sugar)
oxygen


Plants, algae, and some prokaryotes
Are autotrophs (“self- feeders”)
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

Are interconnected
Water, CO2, sugar, and O2 are used or
produced as byproducts in both processes

Leaves

Chloroplasts


Flattened leaf shape exposes large surface area
to catch sunlight
Epidermis


upper and lower leaf surfaces
Cuticle


waxy, waterproof outer surface
reduces water evaporation

Stomata


Mesophyll


adjustable pores allow for entry of air with CO2
inner cell layers that contain majority of chloroplasts
Vascular bundles (veins)

supply water and minerals to the leaf while carrying
sugars away from the leaf

Chloroplasts


Stroma


bounded by a double membrane composed of inner
and outer membranes
semi-fluid medium within the inner membrane
Thylakoids


disk-shaped sacs found within the stroma
in stacks called grana

2 sets of chemical
reactions occur in the:
1.
Thylakoid membranes
2.
Stroma

Pigment molecules (e.g. chlorophyll) of the
thylakoids capture sunlight energy

Sunlight energy is converted to the energy
carrier molecules ATP and NADPH

Oxygen is released as a by-product

Enzymes in stroma synthesize glucose
and other organic molecules using the
chemical energy stored in ATP and
NADPH

What two energy carrying molecules are used
to store captured sunlight energy during lightdependent reactions?

Sun radiates electromagnetic energy

Photons (basic unit of light)


packets of energy with different energy levels
 short-wavelength photons are very energetic
 longer-wavelength photons have lower
energies
Visible light is radiation falling between
400-750 nanometers of wavelength
Light Captured by Pigments

Absorption of certain wavelengths
 light is “trapped”

Reflection of certain wavelengths
 light bounces back

Transmission of certain wavelengths
 light passes through

Absorbed light drives biological processes when it is
converted to chemical energy

Pigment absorbs visible light

Common pigments:

Chlorophyll a and b
 absorb violet, blue, and red light but reflect green light (hence
they appear green)

Carotenoids
 absorb blue and green light but reflect yellow, orange, or red
(hence they appear yellow-orange)
 Are accessory pigments
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
Photosystems within thylakoids
 Assemblies of proteins, chlorophyll, &
accessory pigments

Two Photosystems
 PSII (comes 1st) and PSI (comes 2nd)

Each Photosystem is associated with a
chain of electron carriers
Steps of the light reactions:
1.
Accessory pigments in Photosystems absorb light and
pass energy to reaction centers containing chlorophyll
2.
Reaction centers receive energized electrons…
3.
Energized electrons then passed down a series of
electron carrier molecules (Electron Transport Chain)
4.
Energy released from passed electrons used to
synthesize ATP from ADP and phosphate
5.
Energized electrons also used to make NADPH from
(NADP+) + (H+)

Electrons from PSII flow one-way into PS I

PSII – produces ATP

PSI – produces NADPH

May be used by plant or released into
atmosphere



NADPH and ATP from light-dependent rxns
used to power glucose synthesis
Light not directly necessary for lightindependent rxns if ATP & NADPH available
Light-independent rxns called the CalvinBenson Cycle or C3 Cycle



6 CO2 molecules used to synthesize 1 glucose
(C6H12O6)
CO2 is captured and linked to ribulose
bisphosphate (RuBP)
ATP and NADPH from light dependent rxns
used to power C3 reactions

“Photo”


“Synthesis”



capture of light energy (light dependent rxns)
glucose synthesis (light-independent rxns)
Light dependent rxns produce ATP and NADPH
which is used to drive light-independent rxns
Depleted carriers (ADP and NADP+) return to lightdependent rxns for recharging

The ideal leaf:


Large surface area
to intercept
sunlight
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Very porous to
allow for CO2
entry from air
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
Problem:


Substantial leaf porosity leads to substantial water
evaporation, causing dehydration stress on the plant
Plants evolved waterproof coating and
adjustable pores (stomata) for CO2 entry


When stomata close, CO2 levels drop and O2 levels
rise
Photorespiration occurs

Carbon fixing enzyme combines O2 instead of CO2 with
RuBP

Photorespiration:
 O2 is used up as CO2 is generated
 No useful cellular energy made
 No glucose produced
 Photorespiration is unproductive and
wasteful

Hot, dry weather causes stomata to stay closed

O2 levels rise as CO levels fall inside leaf


Photorespiration very common under such
conditions
Plants may die from lack of glucose synthesis
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
“C4 plants” have chloroplasts in bundle sheath
cells and mesophyll cells

Bundle sheath cells surround vascular
bundles deep within mesophyll

C3 plants lack bundle sheath cell
chloroplasts

C4 plants utilize the C4 pathway

Two-stage carbon fixation pathway

Takes CO2 to chloroplasts in bundle
sheath cells



C4 pathway uses up more energy than C3
pathway
C3 plants thrive where water is abundant or if light
levels are low (cool, wet, and cloudy climates)
 Ex. : most trees, wheat, oats, rice, Kentucky
bluegrass
C4 plants thrive when light is abundant but water is
scarce (deserts and hot climates)
 Ex. : corn, sugarcane, sorghum, crabgrass, some
thistles