Download Ch. 07 PhotoSynthesis: Using Light to Make Food

Chapter 7
Photosynthesis: Using Light to
Make Food
Biology and Society: Plant Power for Power Plants
• On a global scale the productivity of photosynthesis is
astounding (=160 billion metric tons)
• An “energy plantation (에너지농장)”
is a renewable energy source (willow
(버드나무) is being tested as an energy
source)
• The trees are cut once every three
years, and harvested wood is sent to
power plants to generate electricity
© Jong B. Lee, Ph.D.
• Burning wood for energy has
advantages over burning fossil fuels
RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved.
The Basics of Photosynthesis
Chloroplasts: Sites of Photosynthesis
• Almost all plants are photosynthetic autotrophs, as are
some bacteria and protists.
• Photosynthesis
– They generate their own organic matter through
photosynthesis.
– Occurs in chloroplasts.
– Leaves have the most chloroplasts and are the major
sites of photosynthesis
– More specifically, the green color in plants comes
from pigment molecules in the chloroplasts called
chlorophyll
– The chlorophyll molecules that capture light energy
are built into the thylakoid membrane
RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved.
RPTSE Biology – Fall 2015, Dr. Jong B. Lee
RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved.
1
The Overall Equation for Photosynthesis
• Chloroplasts contain stroma, a thick fluid.
: Thylakoids, disk-like membranous sacs,
are suspended in the stroma
• CO2 enters, and O2 exits, by way of tiny pores called
stomata, which are found on lower epidermis of leaves
• Water is mainly absorbed by the plant’s roots, then travels
via veins to the leaves
RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved.
• Photosynthesis takes “exhaust” of cellular respiration
and rearranges its atoms to produce food and oxygen
RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved.
In photosynthesis
A Photosynthesis Road Map
• It is a chemical transformation that requires much energy,
and sunlight provides the energy
• Photosynthesis is
composed of two
processes:
– Electrons are boosted “uphill” and the energized
electrons are added to carbon dioxide to make sugar
– This requires the chloroplast to actually split water
molecules into hydrogen and oxygen (It takes a lot of
energy to split water. the oxygen escapes into the
atmosphere as O2 )
– Hydrogen is moved along with the electrons, so the
redox process takes hydrogen transfer from water to
carbon dioxide to form sugar
RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved.
RPTSE Biology – Fall 2015, Dr. Jong B. Lee
– The light reactions
convert solar energy to
chemical energy, ATP
& NADPH.
– The Calvin cycle
makes sugar from
carbon dioxide.
(Using ATP & NADPH)
RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved.
2
The Light Reactions: Converting Solar Energy to
Chemical Energy
The Nature of Sunlight
• Sunlight is a type of energy called radiation or
electromagnetic energy
• The full range of radiation is called the electro-magnetic
spectrum
– Electromagnetic energy travels through space as
rhythmic waves
– The distance between the crests of two adjacent
waves is called wavelength
RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved.
RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved.
The Process of Science: What Colors of Light Drive
Photosynthesis?
• In 1883, German biologist Theodor Engelmann performed
an experiment using bacteria and algae and determined
that certain types of light drive photosynthesis.
And then, Experiment
Results ?
– Observation: certain bacteria tend to cluster in areas
with higher oxygen concentrations
– Question: which wavelengths of light are best for
promoting photosynthesis?
– Hypothesis: oxygen-seeking bacteria would
congregate near regions of algae undergoing the most
photosynthesis
RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved.
RPTSE Biology – Fall 2015, Dr. Jong B. Lee
RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved.
3
Chloroplast Pigments
• Chloroplasts absorb selected wavelengths of light that
drive photosynthesis.
우리가 잎을 보고 녹색으로 느끼는
것은 엽록체의 색소 (pigments)가
다른 색 (blue-violet and redorange)의 빛은 흡수시키지만
녹색은 반사하기 때문이다.
에너지는 파괴되지 않는 것이기
때문에 어떤 색소가 어느 특정
파장의 빛을 흡수하면, 그 에너지는
다른 에너지 형태로 전환된다.
RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved.
• Chloroplasts contain several pigments: all of these
chloroplast pigments are built into the thylakoid membranes
– Chlorophyll a: is the pigment that directly participate in the
light reaction, and absorbs mainly blue-violet and red light
– Chlorophyll b: absorbs blue and orange light, does not
participate directly in the light reactions, but broadens the
range of light that a plant can use by conveying absorbed
energy to chlorophyll a.
– Carotenoids: absorbs mainly blue-green light, some pass
energy to chlorophyll a, some have a protective function
(absorb and dissipate excessive light energy that would
otherwise damage chlorophyll)
RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved.
How Photosystems Harvest Light Energy
• When a pigment molecule absorbs a photon, one of the
pigment’s electrons gains energy: electrons has been raised
from a ground state to an excited state
• The excited state is very unstable, and generally the
electron loses the excess energy and falls back to its ground
state almost immediately.
• Colors of fall foliage: due to decreases in green
chlorophyll, allowing the yellow-orange hues of longerlasting carotenoids to show through
RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved.
RPTSE Biology – Fall 2015, Dr. Jong B. Lee
• Some pigments, including isolated chlorophyll that has
been extracted from chloroplasts, emit light as well as heat
after absorbing photons
RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved.
4
Photosystems (광계)
• Chlorophyll molecules absorb photons
- Electrons in the pigment gain energy
- Light-excited chlorophyll behaves very
differently in an intact chloroplast
- In its native habitat, chlorophyll
is organized with other molecules into photosystems
• The excited electrons quickly fall back down to their
ground state, releasing energy in the form of fluorescence
light
RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved.
• A photosystem is an organized group of chlorophyll and
other molecules.
- Each photosystem has a cluster of a few hundred
pigment molecules including chlorophyll a and b and
carontenoids: This cluster of molecules functions as a
light-gathering antenna
RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved.
- When a photon strikes one pigment molecule, the energy
jump from pigment to pigment until it arrives at the
reaction center of the photosystem.
Reaction center: chlorophyll a + primary electron acceptor
- This primary electron acceptor traps the light-excited
electron from the reaction center chlorophyll
- Another team of molecules
in thylakoid membrane then
uses that trapped energy
to make ATP and NANPH
RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved.
RPTSE Biology – Fall 2015, Dr. Jong B. Lee
RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved.
5
How the Light Reactions Generate ATP and NADPH
• Two types of photosystems cooperate in the light reactions.
- Water-splitting
photosystem (PSII): uses
light energy to extract
electrons from H2O and
releases O2
PSI: P700
How to release oxygen?
- NADPH-producing
photosystem (PSI):
produces NADPH by
transferring light-excited
electrons from chlorophyll
to NADP+
RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved.
RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved.
PSII: P680
Z scheme: non-cyclic event
• An electron transport chain
- Connects the two photosystems.
- Releases energy and acts as a proton pumping machine
- Chloroplast uses proton gradient energy to make ATP.
• The mechanism of ATP production during light reactions
is very similar to the ATP production in cellular respiration
- An electron transport chain pumps hydrogen ions (H+)
across a membrane
(inner membrane of mitochondria in cellular respiration
vs thylakoid membrane in photosynthesis)
- ATP synthases use the energy stored by the H+ gradient
Difference:
i) food provides the high-energy electrons
vs light-excited electrons provides the energy
ii) topology of ATP synthase
RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved.
RPTSE Biology – Fall 2015, Dr. Jong B. Lee
RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved.
6
The Calvin Cycle: Making Sugar from Carbon
Dioxide
Enzyme: rubisco
• The Calvin cycle
Ribulose
biphosphate
– Functions like a
sugar factory
within a
chloroplast.
– Regenerates the
starting material
with each turn.
RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved.
Water-Saving Adaptations of C4 and CAM Plants
• C3 plant : because the first organic compound produced
is the three-carbon compound (3-PGA)
- Are very common and widely distributed (soybeans,
oats, wheat, and rice), Use CO2 directly from the air.
- Dry weather can reduce the rate of photosynthesis and
decrease crop productivity
: On a hot, dry day, plants close their stomata. Closing
stomata is an adaptation that reduces water loss, but it
also prevents CO2 from entering the leaf
- As a result, CO2 levels can get very low in the leaf, and
sugar production ceases, at least in C3 plants
RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved.
RPTSE Biology – Fall 2015, Dr. Jong B. Lee
• The Calvin cycle
- Regenerates the starting material with each turn.
Therefore, it is called “a cycle”
- Input : CO2 (carbon source), ATP (energy source),
NADPH (high-energy electron source)
- The first organic molecule after CO2 fixation:
3-PGA (3-phospho-glyceric acid)
: Enzyme called “rubisco”: RuBP + CO2 → two 3-PGA
- Output : glyceraldehyde 3-phosphate (G3P)
RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved.
C4 plants : 옥수수 ,
사탕수수, 귀리...
• When the weather is hot and dry, a C4 plant keeps its
stomata closed most of time, thus conserving water. At the
same time, it continues making sugars by photosynthesis
• A C4 plant has an enzyme that incorporate carbon from CO2
into four-carbon compound (oxaloacetic acid) instead of
into 3-PGA
PEP carboxylase : PEP (C3) + CO2 → oxaloacetic acid(C4)
• This enzyme has an intense affinity for CO2 and can
continue to mine CO2 from the air spaces of the leaf even
when the stomata are closed
The four-carbon compound acts as a carbon shuttle; it
donates the CO2 to 3-PGA (Calvin cycle) in a nearby cell,
which therefore keeps on making sugars even though the
plant’s stomata is closed
RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved.
7
- Two steps are separated spatially: they are separated into
two cell types (mesophycells & bundle sheath cells)
- PEP carboxylase는 Rubisco 비하여 CO2에 대한 친화력이
매우 강하다.  주변 CO2 농도가 작아도 bundle sheath cells로
CO2 를 농축시킬수 있다
CAM plants ; 다육성 식물 (선인장.파인애플..)
• They open their stomata only at night to conserve water.
: They are adapted to very dry climates (pineapples, many
cacti, and most succulent plants)
• When CO2 enter leaf, it is incorporated into a four-carbon
compound  The four-carbon compound banks CO2 at
night (in vacuoles) and release it to the Calvin cycle
during the day
• This keeps photosynthesis operating during the day, even
though the leaf admits no more CO2 because stomata are
closed. The two steps are separated in time but not space.
RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved.
RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved.
The Environmental Impact of Photosynthesis
Hot and dry regions of tropics:
Corn, sorghum, sugarcane etc
RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved.
RPTSE Biology – Fall 2015, Dr. Jong B. Lee
Extremely dry areas:
Cactus, pineapple etc
RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved.
8
How Photosynthesis Moderates Global Warming
• Greenhouses used to grow plant indoors
• CO2 & other gases trap sunlight that warms the air inside.
 “greenhouse effect” 온실효과
Greenhouse gases
• The gases in the atmosphere absorb heat radiation:
Water vapor, CO2, CH4, CFC4 (chlorofluorocarbons: Freon)
– CO2 is one of the most important greenhouse gases
– Most of that carbon returns to the atmosphere via
cellular respiration, the action of decomposers, and fires
– But a substantial amount of CO2 remains locked in large
tracts of forests
– The rise in atmospheric CO2 levels during last century
coincided with widespread deforestration.
RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved.
• CO2 concentration at air and global warning
– CO2 concentration: 280 ppm (0.03%) before 1850
– 380 ppm (2006)
– Mostly from the combustion of carbon-based fossil
fuels (coal, oil, and gasoline)
– Increasing concentrations of CO2 have been linked
to global warming, a slow but steady rise in Earth’s
surface temperature
– What can be done to slow this increase in
atmosphere?  “ Energy plantation”
RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved.
RPTSE Biology – Fall 2015, Dr. Jong B. Lee
RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved.
Evolution Connection:
The Oxygen Revolution
• The atmospheric oxygen we breathe is a by-product of
photosynthesis.
• Cyanobacteria were the first organisms to carry out
photosynthesis.
• The production of oxygen changed the Earth forever.
The “oxygen revolution”
was a major episode in
the history of life on
Earth.
RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved.
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