Transcript video slide

Introduction to Photosynthesis (181-200)
Life on Earth is SOLAR powered
Photosynthesis (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 biosphere
Exs = green plants and Ps protist groups (fig 10.2)
Heterotrophs - get E from organic compounds produced by
other organisms
Also called consumers of the biosphere
Exs = fungi, animals, & many protist groups
Photosynthesis converts light E to chemical E of food
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 dioxide
and (in most cases) water. They feed not only
themselves, but the entire living world. (a) On
land, plants are the predominant producers of
food. In aquatic environments, photosynthetic
organisms include (b) multicellular algae, such
as this kelp; (c) some unicellular protists, such
as Euglena; (d) the prokaryotes called
cyanobacteria; and (e) other photosynthetic
prokaryotes, such as these purple sulfur
(a) Plants
bacteria, which produce sulfur (spherical
globules) (c, d, e: LMs).
(c) Unicellular protist 10 m
(e) Purple sulfur
bacteria
Figure 10.2
(b) Multicellular algae
(d) Cyanobacteria
40 m
1.5 m
Heterotrophs
• Heterotrophs
– Obtain their organic material from other
organisms
– Are the consumers of the biosphere
– Includes fungi, animals, many protist groups
and many bacteria
Chloroplasts – Sites of Ps within the cell
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: The Sites of Photosynthesis in Plants
• The leaves of plants
– Are the major sites of photosynthesis
Leaf cross section
Vein
Mesophyll
Stomata
Figure 10.3
CO2
O2
Chloroplasts
• Chloroplasts
–
Are the organelles in
which photosynthesis
occurs
–
Contain thylakoids
and grana
–
Stroma is the fluid in
the internal cavity
Mesophyll
Chloroplast
5 µm
Outer
membrane
Stroma Granum
–
Thylakoid Thylakoid
space
Intermembrane
space
Inner
membrane
Chlorophyll is
imbedded in the
thylakoid membranes
1 µm
Tracking Atoms Through Photosynthesis:
• Photosynthesis is summarized as
6 CO2 + 12 H2O + Light energy  C6H12O6 + 6 O2 + 6 H2 O
OR
CO2 + H2O  [CH2O] + O2
Overall Ps equation has been known since the 1800s
The equation for Ps (fig 10.4) = reverse of respiration
But carbohydrates are not made by simply reversing what
happens in respiration
BOTH processes occur in plant cells!
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Splitting of Water
• Chloroplasts split water into
– Hydrogen and oxygen, incorporating the
electrons of hydrogen into sugar molecules
Reactants:
Products:
Figure 10.4
12 H2O
6 CO2
C6H12O6
6
H2O
6
O2
Photosynthesis as a Redox Process
• Photosynthesis is a redox process
– Water is oxidized, carbon dioxide is reduced
Two Stages of Photosynthesis
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- carrier
Photophosphorylation = how ATP is generated (using
chemiosmosis again)
2. Calvin cycle: use ATP and NADPH to fix C from the
atmosphere into organic compounds
Carbon fixation = initial incorporation of C into organic
compounds
The Two Stages of Photosynthesis
• Photosynthesis consists of two processes
– The light reactions
– The Calvin cycle
sunlight
water
carbon dioxide
ATP
ADP + Pi
light-dependent
rxns
Calvin cycle
NADPH
NADP+
oxygen
P
glucose
new
water
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
NADP 
ADP
+ P
LIGHT
REACTIONS
CALVIN
CYCLE
ATP
NADPH
Chloroplast
Figure 10.5
O2
[CH2O]
(sugar)
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
10–5 nm
10–3 nm
Gamma
rays
X-rays
UV
1m
106 nm
106 nm
103 nm
1 nm
Infrared
Microwaves
103 m
Radio
waves
Visible light
380
450
500
Shorter wavelength
Figure 10.6
Higher energy
550
600
650
700
Longer wavelength
Lower energy
750 nm
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
see
Light
Reflected
Light
Chloroplast
Absorbed
light
Granum
Transmitted
light
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
Refracting Chlorophyll
prism
solution
White
light
2
Photoelectric
tube
Galvanometer
3
1
0
100
4
Slit moves to Green
pass light
light
of selected
wavelength
The high transmittance
(low absorption)
reading indicates that
chlorophyll absorbs
very little green light.
0
Figure 10.8
Blue
light
100
The low transmittance
(high absorption) reading
chlorophyll absorbs most blue light.
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
RESULTS
Absorption of light by
chloroplast pigments
Chlorophyll a
Chlorophyll b
Carotenoids
Wavelength of light (nm)
(a) Absorption spectra. The three curves show the wavelengths of light best absorbed by
three types of chloroplast pigments.
Figure 10.9
The action spectrum for photosynthesis
Rate of photosynthesis
(measured by O2 release)
• Profiles the relative effectiveness of different
wavelengths of radiation in driving photosynthesis
(b) 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.
The action spectrum for photosynthesis
• Was first demonstrated by Theodor W. Engelmann
Aerobic bacteria
Filament
of alga
500
600
700
400
(c) 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.
CONCLUSION
photosynthesis.
Light in the violet-blue and red portions of the spectrum are most effective in driving
Types of Chlorophyll
• Chlorophyll a
–
Is the main
photosynthetic pigment
CH3
in chlorophyll a
CHO
in chlorophyll b
CH2
• Chlorophyll b
CH
C
H3C
C
H
C
C
C
–
Is an accessory pigment
C
C
N
C
N
C
Mg
N
C
C
C
H
C
N
C
H3C
CH3
H
CH2
H
H
C
C
C
O
C
H
C
CH3
CH3
Porphyrin ring:
Light-absorbing
“head” of molecule
note magnesium
atom at center
C
O
O
CH2
C
C
CH2
C
O
O
CH3
CH2
Figure 10.10
Hydrocarbon tail:
interacts with hydrophobic
regions of proteins inside
thylakoid membranes of
chloroplasts: H atoms not
shown
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
e–
Excited
state
Heat
Photon
(fluorescence)
Photon
Figure 10.11 A
Chlorophyll
molecule
Ground
state
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.