Photosynthesis
CHAPTER 7
Photosynthetic Organisms
 All life on Earth depends on solar energy
 Photosynthetic organisms (algae, plants, and
cyanobacteria) transform solar energy into the chemical
energy of carbohydrates
 Called autotrophs because they produce their own
food.
 Photosynthesis:
 A process that captures solar energy
 Transforms solar energy into chemical energy
 Energy ends up stored in a carbohydrate
 Photosynthesizers produce food energy
 Feed themselves as well as heterotrophs
Photosynthetic Organisms
mosses
trees
kelp
Euglena
garden plants
cyanobacteria
diatoms
Photosynthetic Organisms
 Photosynthesis takes place in the green portions of
plants




Leaf of flowering plant contains mesophyll tissue
Cells containing chloroplasts are specialized to carry out
photosynthesis
~ about half a million chloroplasts/mm2 of leaf surface
Color of leaf due to green pigment chlorophyll
6CO2 + 6H2O + light energy  C6H12O6 + 6O2
5
Photosynthesis
 The raw materials for photosynthesis are carbon dioxide
and water




Roots absorb water that moves up vascular tissue
Carbon dioxide enters a leaf through small openings called
stoma/stomata and diffuses into chloroplasts in mesophyll cells
In stroma, CO2 is combined with H2O to form C6H12O6 (sugar)
Energy supplied by light

Chlorophyll and other pigments absorb solar energy and energize
electrons prior to reduction of CO2 to a carbohydrate
Leaves and Photosynthesis
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cuticle
upper
epidermis
Leaf cross section
mesophyll
lower
epidermis
CO2
O2
leaf vein
outer membrane
stoma
inner membrane
stroma
stroma
granum
Chloroplast
thylakoid space
thylakoid membrane
Grana
independent thylakoid
in a granum
overlapping thylakoid
in a granum
© Dr. George Chapman/Visuals Unlimited
37,000
The Process of Photosynthesis
 Light Reactions – take place only in the presence of light
 Energy-capturing reactions
 Chlorophyll absorbs solar energy
 This energizes electrons
 Electrons move down an electron transport chain


Pumps H+ into thylakoids
Used to make ATP out of ADP and NADPH out of NADP
 Calvin Cycle Reactions – take place in the stroma
 CO2 is reduced to a carbohydrate
 Use ATP and NADPH to produce carbohydrate
Photosynthesis Releases Oxygen
Overview of Photosynthesis
thylakoid
membrane
Overview of Photosynthesis
solar
energy
thylakoid
membrane
Overview of Photosynthesis
H2O
solar
energy
ADP +
NADP+
Light
reactions
thylakoid
membrane
Overview of Photosynthesis
H2O
solar
energy
ADP +
NADP+
Light
reactions
NADPH
ATP
thylakoid
membrane
O2
Overview of Photosynthesis
CO2
H2O
solar
energy
ADP + P
NADP+
Light
reactions
Calvin
cycle
reactions
NADPH
ATP
stroma
thylakoid
membrane
O2
CH2O
Plants as Solar Energy Converters
 Pigments:


Chemicals that absorb certain wavelengths of light
Wavelengths that are not absorbed are reflected/transmitted
 Absorption Spectrum



Pigments found in chlorophyll absorb various portions of visible light
Graph showing relative absorption of the various colors of the
rainbow
Chlorophyll is green because it absorbs much of the reds and blues of
white light
Photosynthetic Pigments and
Photosynthesis
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Increasing wavelength
chlorophyll a
chlorophyll b
carotenoids
Gamma
rays
X rays
UV
Micro- Radio
Infrared waves waves
visible light
500
600
Wavelengths (nm)
a. The electromagnetic spectrum includes visible light.
750
Relative Absorption
Increasing energy
380
500
600
Wavelengths (nm)
b. Absorption spectrum of photosynthetic pigments.
750
Photosynthetic Pigments
 Plants contain 3 major groups of pigments
 Chlorophyll a
 Chlorophyll b
 Carotenoids (accessory pigment)

Red, orange and yellow
Plants as Solar Energy Converters
 The light reactions consist of two alternate electron
pathways:

Noncyclic pathway

Cyclic pathway
 Capture light energy with photosystems

Pigment complex helps collect solar energy like an antenna

Occur in the thylakoid membranes
 Both pathways produce ATP
 The noncyclic pathway also produces NADPH
Photosystems
 reaction center containing chlorophyll a and
“primary electron acceptor”
 surrounded by a light-harvesting complex of other
pigments and proteins (chlorophyll b, carotenoids)
 act as “antenna” to collect light energy → chlorophyll
a → “primary electron acceptor”


Photosystem I (PS I) reaction center absorption peak at
700 nm (P700)
Photosystem II (PS II) reaction center absorption peak at
680 nm (P680)
Plants as Solar Energy Converters
 Noncyclic pathway
 Takes place in the thylakoid membrane
 PS II captures light energy
 Causes an electron to be ejected from the reaction center
(chlorophyll a)
 Electron travels down electron transport chain to PS I
 Replaced with an electron from water, which is split to form O2
and H+ (photolysis)
 This causes H+ to accumulate in thylakoid chambers
 The H+ gradient is used to produce ATP
 PS I captures light energy and ejects an electron
 The electron is transferred permanently to a molecule of NADP+
 Causes NADPH production
Noncyclic Electron Pathway
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H2 O
CO2
solar
energy
ADP+ P
NADP+
Light
reactions
sun
Calvin
cycle
sun
NADPH
ATP
thylakoid
membrane
energy level
electron
acceptor
electron
acceptor
O
CH2O
e–
e–
e–
e–
NADP+
H+
ATP
e–
NADPH
e–
reaction center
reaction center
pigment
complex
pigment
complex
Photosystem I
e–
Photosystem II
CO2
H2O
CH2O
Calvin cycle
reactions
2H+
1
– O2
2
22
Organization of a Thylakoid
H2O
CO2
solar
energy
ADP +
P
NADP +
Light
reactions
Calvin
cycle
reactions
NADPH
ATP
thylakoid membrane
thylakoid
membrane
thylakoid space
O2
CH2O
granum
photosystem II
electron transport
chain
H+
stroma
photosystem I
H+
NADP
reductase
Pq
ee-
e-
NADP+
NADPH
e-
eH+
H+
H2 O
2
H+
+
1
2
H+
H+
H+
H+
H+
H+
H+
O2
ATP synthase
H+
H+
H+
H+
Thylakoid
space
H+
ATP
H+
H+
chemiosmosis
P
Stroma
+ ADP
23
thylakoid
Tropical Rain Forest Destruction and Climate Change
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Mean Global Temperature Change (0c)
5.5
4.5
3.5
maximum likely increase
most probable temperature
increase for 2 × CO2
2.5
1.5
Minimum likely increase
0.5
–0.5
1860
1940
2020
Year
2060
2100
24
Cyclic photophosphorylation
 Sole purpose is to produce ATP
 Occurs when the chloroplasts need to resupply ATP
energy during photosynthesis
The Calvin Cycle
 A series of reactions that occur after the light
reactions (but occurs only in the light)
 Calvin cycle produces carbohydrates (C6H12O6)
 Named after Melvin Calvin
 3 CO2 enters the stomates of the leaf and becomes
incorporated into 1 G3P

1 cyclic takes 1 CO2, 3 turns are needed to make one G3P
Plants as Carbon Dioxide Fixers
 Importance of the Calvin Cycle:

G3P (glyceraldehyde-3-phosphate) can be converted to
many other molecules

The hydrocarbon skeleton of G3P can form

Fatty acids and glycerol to make plant oils

Glucose phosphate (simple sugar)

Fructose (which with glucose = sucrose)

Starch and cellulose

Amino acids
Fate of G3P
G3P
fatty acid
synthesis
glucose
phosphate
amino acid
synthesis
+
fructose
phosphate
Sucrose (in leaves,
fruits, and seeds)
Starch (in roots
and seeds)
Cellulose (in trunks,
roots, and branches)
© Herman Eisenbeiss/Photo Researchers, Inc.
Other Types of Photosynthesis
 In hot, dry climates
 Stomata must close to avoid wilting
 CO2 decreases and O2 increases
 O2 starts combining with RuBP, leading to the production of CO2
 This is called photorespiration
 C4 plants solve the problem of photorespiration
 Fix CO2 to PEP (a C3 molecule)
 The result is oxaloacetate, a C4 molecule
 In hot & dry climates



C4 plants avoid photorespiration
Net productivity is about 2-3 times greater than C3 plants
In cool, moist environments, C4 plants can’t compete with C3 plants
Chloroplast Distribution in C4 vs. C3 Plants
C3 Plant
C4 Plant
mesophyll
cells
bundle sheath
cell
vein
stoma
bundle sheath
cell
vein
stoma
CO2 Fixation in C3 and C4 Plants
CO2
RuBP
Calvin
cycle
3PG
G3P
mesophyll cell
a. CO2 fixation in a C3 plant, wildflowers
CO2
mesophyll C
4
cell
bundle
sheath
cell
CO2
Calvin
cycle
G3P
b. CO2 fixation in a C4 plant, corn, sugar cane
Other Types of Photosynthesis
 CAM Photosynthesis
 Crassulacean-Acid Metabolism
 CAM plants partition carbon fixation by time
During the night
 CAM plants fix CO2
 Form C4 molecules
 Stored in large vacuoles
 During daylight
 NADPH and ATP are available
 Stomata are closed for water conservation
 C4 molecules release CO2 to Calvin cycle

CO2 Fixation in a CAM Plant
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
night
CO
2
C4
day
CO
2
Calvin
cycle
G3P
CO2 fixation in a CAM plant, pineapple, Ananas
comosus
© S. Alden/PhotoLink/Getty Images.
34
Other Types of Photosynthesis
 Each method of photosynthesis has advantages and
disadvantages

Depends on the climate
 C4 plants most adapted to:
 High light intensities
 High temperatures
 Limited rainfall
 C3 plants better adapted to
 Cold (below 25°C)
 High moisture
 CAM plants are better adapted to extreme aridity
 CAM occurs in 23 families of flowering plants
 Also found among nonflowering plants