Chapter 7
How Cells
Acquire Energy
Autotrophs
• Self nourishing
• Obtain carbon from carbon dioxide
• Photosynthetic autotrophs (plants,
protistians, and bacterial membranes)
harness light energy
• Chemosynthetic autotrophs (few bacteria)
extract energy from chemical reactions
involving inorganic substances (e.g.Sulfur
compounds)
Heterotrophs
• Obtain carbon and energy from the
autotrophs
• Include protistans, bacteria, animals, and
fungi
• Carbon and energy enter the web of life by
photosynthesis and in turn are released by
glycolysis and aerobic respiration
Overview of Photosynthesis
• A. Photosynthesis Transforms Solar Energy
B. Organic molecules built by photosynthesis provide
both the building blocks and energy for cells.
C. Plants use the raw materials: carbon dioxide and
water
D. Chloroplasts carry out photosynthesis
• 2 stages of photosynthesis takes place in the
chloroplast
– Which has two layers (membranes) stroma and
thylakoids
• E. Chlorophylls and other pigments involved in
absorption of solar energy reside within thylakoid
membranes of chloroplasts
Plants as Solar Energy Converters
• A. Solar Radiation - Only 42% of solar radiation that
hits the earth’s atmosphere reaches surface; most is
visible light.
Continue…
• B. Photosynthetic Pigments - Pigments found in
chlorophyll absorb various portions of visible light;
absorption spectrum.
• 1. Two major photosynthetic pigments are chlorophyll
a and chlorophyll b.
2. Both chlorophylls absorb violet, blue, and red
wavelengths best.
3. Very little green light is absorbed; most is reflected
back; this is why leaves appear green.
4. Carotenoids are yellow-orange pigments which
absorb light in violet, blue, and green regions.
5. When chlorophyll breaks down in fall, the yelloworange pigments in leaves show through.
Secondary Pigments
• Carontenoid: absorbs blue-green
wavelengths but reflect yellow, orange,
and red
• Anthocyanins: Pigments found in flowers
• Phycobilins: are the blue and red pigments
of red algae and cyanobacteria
Properties of Light
• Electromagnetic Spectrum: light energy
travels in waves through space from
gamma rays to radio waves
• The shorter the wavelength the more
energy –Example Sun’s radiation
Continue…
• Photoautotrophs use a small range (400750 nm) of wavelength for photosynthesis
-- which is the range for visible light
• Light energy is packaged as photons,
which vary in energy as a function of
wavelength
– Blue violet light : most energetic
– Red light: least energetic
Where are photosynthetic pigments
located?
• Photosynthetic bacteria pigment is found
at the plasma membrane
• In thylakoid membrane systems of
cholorplast the pigments are organized in
clusters called photosystems consisting of
200 to 300 pigment molecules
Things that Don’t need Glucose
• Light-dependent reactions convert light
energy to chemical energy (which is then
stored into ATP)
– Liberated electrons are picked up by NADPH
• Light-independent reaction assemble
sugars and other organic molecules using
ATP, NADPH, and CO2
12 H20 + 6CO2  602 + C6H12O6 + 6H20
Light Dependent Reactions
• Light Dependent Reactions occur in the
thylakoid
– Thylakoid are folded into grana (stacks of
disks) and channels
– The interior spaces of the thylakoid disks and
channels are coninuous and are filled with H+
needed for ATP synthesis
• Carbohydrates formation occurs in the
stroma (semifluid) area that surrounds the
grana
Light Dependent Reactions
• First reaction of photosynthesis
• Three events occur
• 1- Pigment absorb sunlight energy and
give up excited electrons
• 2- Electron and hydrogen transfer lead to
ATP and NADPH formation
• 3- Pigments that gave up the electrons in
the first place get electron replacements
What Happens to the Absorbed
Energy?
• The pigments “harvest” photon energy
from sunlight
– Absorbed photons of energy boost electrons
to a higher level.
– Electrons return to lower level
– Released energy is trapped by cholorphylls
located in the photsystem’s reaction center
– The trapped energy is then used to transfer a
chlorophyll electron to an acceptor molecule
Electron Transfer Chain
• Is an organized array of enzymes,
coenzymes, and other proteins embedded
in or anchored to a cell membrane
– Accept electrons which are then processed
through a step-by-step array to produce ATP
and NADPH
Cyclic Pathway
• Oldest mean of ATP production being
used by early bacteria
– Excited electrons leave the P700 reaction
center, pass through an electron transport
system, and then return to the original
photosystem I
– Energy associated with the electron flow
drives the formation of ATP from ADP
– Figure 7.12
Noncyclic Pathway
SPLITS WATER, PRODUCES NADPH & ATP
• ATP Formation transfer through two
photosystems and two electron transport
systems (ETS) in the thylakoid membrane
– Boosted electrons moves through a transport
system that releases energy for
ADP + PiATP
– Electrons fills “hole” left by electron boost in
P700 of photosystem I
Continue…
– Electron from photolysis of water fills “electron
hole” left in p689 and produces oxygen
byproduct
• Pathway continues when chlorophyll P700
of photosystemI is absorbs energy
– Energy hole is filled by elctron from P680
– Boosted electron from P700 passes to
acceptor, then ETS is finally joins NADP to
form NADPH (which along with the ATP can
be used in synthesis of organic compounds
• Turn to page 123 (Figure 7.13)
• Hyperlink\Light reactions.mht
• Watch movie here…
• You can watch this movie going to my
portaportal.com website
• Guest Name Mssweikle
• Go to the AP Biology Folder
• Find Photosynthesis movie
The New Atmosphere
• Oxygen is a by-product of the noncyclic
pathway
• Beginning about 1.5 billion years ago,
large amounts of oxygen began
accumulating in the atmosphere, which at
the time was oxygen-free
Light-Independent Reactions
• These reactions (Calvin-Benson Cycle) are the
“synthesis” of phytosynthesis
– Handout: Slide 27
• The participants and their roles in the synthesis
of carbohydrates are
– ATP, which provides energy
– NADPH provides the hydrogen atoms and electrons
– Atmosphere provides the Carbon dioxide
• The reactions are dependent on sunlight
Fixation of Carbon Dioxide
1. CO2 fixation is the attachment of CO2 to
an organic compound called RuBP.
2. RuBP (ribulose bisphosphate) is a fivecarbon molecule that combines with
carbon dioxide.
3. The enzyme RuBP carboxylase (rubisco)
speeds this reaction; this enzyme
comprises 20–50% of theprotein content
of chloroplasts, probably since it is a slow
enzyme.
Reduction of Carbon dioxide
1. With reduction of carbon dioxide, a PGA (3phosphoglycerate[C3]) molecule forms.
2. Each of two PGA molecules undergoes
reduction to PGAL in two steps.
3. Light-dependent reactions provide NADPH
(electrons) and ATP (energy) to reduce PGA to
PGAL.
Regeneration of RuBP
• 1. Every three turns of Calvin cycle, five
molecules of PGAL are used to re-form
three molecules of RuBP.
• 2. Every three turns of Calvin cycle, there
is net gain of one PGAL molecule; five
PGAL regenerate threemolecules of
RuBP.
Importance of the Calvin Cycle
1. PGAL, the product of the Calvin Cycle can be
converted into all sorts of other molecules.
2. Glucose phosphate is one result of PGAL
metabolism; it is a common energy molecule
3. Glucose phosphate is combined with fructose
to form sucrose used by plants.
4. Glucose phosphate is the starting pint for
synthesis of starch and cellulose.
5. The hydrocarbon skeleton of PGAL is used to
form fatty acids and glycerol; the addition of
nitrogen forms various amino acids.
Calvin Cycle
How Do Plants Build Glucose?
• Each PGA then receives a phosphate
group from ATP plus H+ and electrons
from NADPH to form PGAL
– Most PGAL molecules will continue in the
cycle to fix more carbon dioxide, but two
PGAL join to form a sugar phosphate, which
will be modified to sucrose, starch, and
cellulose.
– Final Tally:
Continue…
• Sugar phosphate are used as cellular fuel
and as building blocks in synthesis of
sucrose or starch.
– Sucrose is the most easily transportable
– Starch is the main storage form, but will be
converted by to sucrose for distribution to
leaves, stems, and roots
– Photosynthesis yields intermediates and
products that can be used in lipid and amino
acid synthesis
Factors that affect
photosynthesis
1.
2.
3.
4.
5.
Light Quality (color)
Light intensity
Light Period
Carbon Dioxide Availability
Water Availability
How gases enter and leave
plants?
C3 Plants
• 1. The Calvin Cycle is the MOST Common
Pathway for Carbon Fixation. Plant
Species that fix Carbon EXCLUSIVELY
through the Calvin Cycle are known as C3
PLANTS.
• 2. Other Plant Species Fix Carbon through
alternative Pathways and then Release it
to enter the Calvin Cycle.
Continue…
• 3. When a plant's Stomata are partly
CLOSED, the level of CO2 FALLS (Used
in Calvin Cycle), and the Level of O2
RISES (as Light reactions Split Water
Molecules).
• 4. A LOW CO2 and HIGH O2 Level
inhibits Carbon Fixing by the Calvin Cycle.
Plants with alternative pathways of Carbon
fixing have Evolved ways to deal with this
problem.
C4 Plants
• C4 PLANTS - Allows certain plants to fix
CO2 into FOUR-Carbon Compounds.
During the Hottest part of the day, C4
plants have their Stomata Partially Closed.
C4 plants include corn, sugar cane and
crabgrass. Such plants Lose only about
Half as much Water as C3 plants when
producing the same amount of
Carbohydrate.
CAM Plants
• Cactus, pineapples, and other succlents have
different adaptations to Hot, Dry Climates. They Fix
Carbon through a pathway called CAM.
• Plants that use the CAM Pathway Open their
Stomata at NIGHT and Close during the DAY, the
opposite of what other plants do. At NIGHT, CAM
Plants take in CO2 and fix into Organic Compounds.
• During the DAY, CO2 is released from these
Compounds and enters the Calvin Cycle.
• Because CAM Plants have their Stomata open at
night, they grow very Slowly, But they lose LESS
Water than C3 or C4 Plants.
Ocean Photoautotrophs
• The ocean host a vast number of
photoautotrophic prokaryotic cells and
protistans
– They shape the global climate