Transcript Photosynthesis - Mrs. Wardle's Teacher Page | Detroit

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Introduction
 Autotrophs: any organism that can make their own
food.
 Photoautotrophs : use light energy
 Chemoautotrophs change inorganic chemicals such as
hydrogen sulfate into chemical energy
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Important structures
•Chloroplast: Site of
photosynthesis in eukaryotic
cells.
•Thylakoids: Disk shaped
membranes containing
photosynthetic pigments. Site of
light dependent reactions.
•Grana: Stacks of thylakoids.
•Stroma: Fluid filled space
surrounding grana. Site of light
independent reactions.
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Properties of Light
 White light from the sun is composed of a range of
wavelengths.
 Chlorophyll is the main pigment that absorbs
sunlight.
 Chlorophyll absorbs blue light and to a lesser extent,
red light.
 It reflects green light hence its intense green color.
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What do chlorophyll and
fireworks have in common?
 Fireworks are made up of various metal salts that go through oxidation
and reduction reactions, producing great heat that causes electrons to
move from their usual level around the nucleus (ground state) up to a
higher position further away from the nucleus (excited state). The
energy from heat has been imparted to the electron(s), which now has
more potential energy. These excited electrons move almost
instantaneously back down to their ground state, releasing their stored
potential energy in the form of light. The greater the electrons
movement away from the nucleus to the excited state, the more
potential energy it has and the more energy it will release.
 It should be pointed out that chlorophyll absorbs light energy,
specifically its electrons, and this energy is used to pump protons
instead of producing light. This proton gradient in turn is used to
produce ATP, which along with other energized electrons are transferred
to the Calvin cycle.
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Suppose a large meteor hit the earth. How
could smoke and soot in the atmosphere
wipe out life far beyond the area of direct
impact?
 Because any particles in the atmosphere can block the light from the
sun and reduce the excitation of electrons in chlorophyll molecules,
photosynthesis depends on a relatively clean atmosphere. Any
reduction in the available sunlight can have serious effects on plants.
Scientists believe that if a large meteor hit the earth—as one did
when the dinosaurs were wiped out 65 million years ago—smoke,
soot, and dust in the atmosphere could block sunlight to such an
extent that plants in the region, or even possibly all of the plants on
earth, could not conduct photosynthesis at high enough levels to
survive. And when plants die off, all of the animals and other species
that rely on them for energy die as well. As dire as it sounds, all life on
earth is completely dependent on the continued excitation of
electrons by sunlight.
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Chloroplast
two outer
membranes
thylakoid
membrane
system
Organelle of
photosynthesis in
plants and algae
stroma
thylakoid compartment
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a A look
inside the
leaf
b One of the
photosynthetic
cells inside leaf
leaf’s upper
epidermis
photosynthetic
cell in leaf
leaf vein
leaf’s lower
epidermis
Leaf Structure
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Photosynthesis Equation
LIGHT ENERGY
6H2O + 6CO2
water carbon
dioxide
6O2 + C6H12O6
oxygen glucose
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Photosynthesis
 Two stages:
 light-dependent reactions require light
to work
 light-independent reactions do not
require light
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Two Steps in Photosynthesis
sunlight
H2 O
Where the two
stages of
photosynthesis
occur inside the
chloroplast
O2
lightdependent
reactions
CO2
NADPH, ATP
lightindependent
NADP+, ADP reactions
sugars
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Photosynthesis: LightDependent
 Photosystems - pigments surrounding a central
chlorophyll a molecule....the reaction center
 Each pigment absorbs a different wavelength of light
& transfers its energy to the reaction center which in
turns energizes an electron.
 Only the chlorophyll that is the reaction center can
give the energized electrons to the electron
acceptors! The energized electron is then used
elsewhere to make ATP or NADPH
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Two Potential Fates of Excited
Electrons
1. Electron returns to resting, unexcited
state.
2. Excited electrons are passed to other
atoms.
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Photosynthesis: Light-Dependent
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The Passing of Electrons in
Their Excited State


Chief way energy moves through cells
Molecules that gain electrons always carry
greater energy than before receiving them

Can view this as passing of potential energy
from molecule to molecule
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Photosynthesis: LightDependent
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Photosynthesis: LightDependent
 As these pigments absorb photons from the sunlight that
hits the leaves, electrons in the pigments become excited
and then return to their resting state.
 As the electrons return to their resting state, energy (but
not the electrons) is transferred to neighboring pigment
molecules.
 This process continues until the transferred energy from
many pigment molecules excites the electrons in a
chlorophyll a molecule at the center of the photosystem.
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Photosynthesis: LightDependent
 This is where the electron journey begins.
 The special chlorophyll a continually loses its excited
electrons to a nearby molecule, called the primary
electron acceptor, which acts like an electron vacuum.
 Why must plants get water for photosynthesis to
occur?
 As electrons keep getting taken away from the special
chlorophyll a molecule, the electrons must be
replaced. The replacement electrons come from water.
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Photosynthesis: LightDependent
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Photosynthesis: LightDependent
 Think of a pump pushing water into an elevated tank,
creating a store of potential energy that can run out of
the tank with great force and kinetic energy, which can be
harnessed to do work, such as moving a large paddle
wheel.
 Similarly, the protons eventually rush out of the thylakoid
sacs with great force—and that force is harnessed to
build energy-storing ATP molecules, one of the two
products of the “photo” portion of photosynthesis.
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Photosynthesis: LightDependent
 Product #1 of the “Photo” Portion of
Photosynthesis: ATP
 Product #2: NADPH
 Product #3 (waste): O2
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Photosynthesis: LightinDependent
 Also known as the Calvin Cycle
 They occur whether or not light is present.
 Occur in the stroma of the chloroplast.
 The purpose of the reactions is to take the energy
from ATP and energized ions from NADPH and add
them to carbon dioxide to make glucose or sugar.
 The reactions reduce carbon dioxide by adding
energize electrons and protons to it and removing
one oxygen atom. This effectively converts the
carbon dioxide into CH2O.
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Photosynthesis: LightinDependent
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Photosynthesis: LightinDependent
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Factors that Affect Rates
 3 factors can limit the speed of
photosynthesis: light intensity,
CO2 concentration,
temperature
 Without enough light, a plant
cannot photosynthesize very
quickly, even if there is plenty
of water and CO2 .
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Factors that Affect Rates
 Sometimes
photosynthesis is limited
by the concentration of
carbon dioxide in the air.
 Even if there is plenty of
light, a plant cannot
photosynthesize if there is
insufficient carbon
dioxide.
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Factors that Affect Rates
 If it gets too cold, the
rate of photosynthesis
will decrease. Plants
cannot photosynthesize
if it gets too hot.
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Summary of Photosynthesis
sunlight
Light
Dependent
Reactions 12H2O
ADP + Pi
6CO2
Light
Independent
Reactions
6 RuBP
6O2
ATP
NADPH NADP+
CalvinBenson
cycle
12 PGAL
6H2O
P
phosphorylated glucose
end products (e.g., sucrose, starch, cellulose)
Linked Processes
Photosynthesis
 Energy-storing pathway
 Releases oxygen
 Requires carbon dioxide
Aerobic Respiration
 Energy-releasing
pathway
 Requires oxygen
 Releases carbon dioxide
Photosynthesis: The Movie
 Animation
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The battle against world hunger can
use plants adapted to water scarcity.
Stomata
Pores for gas exchange
How to get CO2 when stomata are
shut?
 Primary sites for gas exchange in plants
 CO2 for photosynthesis
 O2 generated as a by-product in photosynthesis exits
 water vapor evaporates
 Closed stomata:
 prevents water evaporation 
 O2 cannot be released from the chloroplasts 
 CO2 cannot enter them 
 No carbon = no Calvin cycle
 plant growth comes to a standstill; crops fail
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Evolutionary Adaptations
 Some plants are able to thrive in hot, dry conditions.
 Evolutionary adaptations – along with recent
agricultural (technological) advances – help battle
world hunger
 plants close their stomata to combat water loss
through evaporation
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C4 Photosynthesis
 C4 plants produce an enzyme that is better at
attracting carbon (in addition to RUBISCO) called PEP
Carboxylase
 allows CO2 to be taken into the plant very quickly
 "delivers" the CO2 directly to RUBISCO
 Called C4 because the CO2 is first incorporated into a
4-carbon compound
 Stomata are open during the day
C4 Photosynthesis
 Adaptive Value:
 photosynthesizes faster than C3 plants under high
light intensity and high temperatures
 better water use efficiency because PEP Carboxylase
brings in CO2 faster  doesn’t need to keep stomata
open as much
 C4 plants include several thousand species in at
least 19 plant families, including corn & sugarcane
 C4 photosynthesis also adds additional energy
expense; outcompeted in mild climates by C3
plants.
C4 Photosynthesis
CAM Photosynthesis
 CAM = Crassulacean Acid Metabolism
 after plant family in which it was first found
(Crassulaceae)
 because CO2 is stored as an acid before use in
photosynthesis
 Stomata open at night (evaporation rates usually
lower), closed during day
 CO2 converted to acid, stored @ night
 daytime:


acid broken down
CO2 released to RUBISCO for photosynthesis
CAM Photosynthesis
 Adaptive Value:
 better water use efficiency than C3s under arid conditions


open stomata @ night when transpiration rates are lower
no sunlight, lower temperatures, lower wind speeds, etc.
 May CAM-idle under extremely arid conditions
 leave stomata closed night and day
 O2 given off in photosynthesis is used for respiration
 CO2 given off in respiration is used for photosynthesis
 kind of like a perpetual energy machine, but plant cannot CAMidle forever
 allows plant to survive dry spells, recover very quickly when water
is available again (unlike plants that drop their leaves and twigs
and go dormant during dry spells)
CAM Photosynthesis
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All Three Photosynthetic Pathways
Evolutionary Adaptations & World Hunger
 C4 and CAM photosynthesis originally evolved
as successful adaptations to hot/dry regions
 Researchers are now using these adaptations to
fight world hunger.
 several genes that code for the C4
photosynthesis enzymes have been
introduced from corn into rice
 once in rice, genes increase rice plant’s ability
to photosynthesize  higher growth rates
and food yields