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Ch. 9 Cellular Respiration and Fermentation & Ch.10 Photosynthesis -…
Ch. 9 Cellular Respiration and Fermentation & Ch.10 Photosynthesis
Catabolic Pathways
The chemical elements essential to life are
recycled
Photosynthesis uses CO2 and H2O to make organic
molecules and O2
Cellular respiration uses O2 and organic molecules
to make ATP; CO2 and H2O are produced as waste
Catabolic pathways release stored energy by
breaking down complex molecules
Catabolic Pathways and Production of ATP
The breakdown of organic molecules is exergonic
Fermentation
is a partial degradation of sugars
that occurs without oxygen
Aerobic respiration
consumes organic molecules
and oxygen and yields ATP
Cellular Respiration
- is aerobic & catabolic
C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + Energy (ATP + heat)
Anaerobic respiratio
n - A catabolic pathway in which inorganic molecules other than oxygen accept electrons at the “downhill” end of electron transport chains
Redox Reactions: Oxidation and Reduction
Chemical reactions that transfer electrons between
reactants are called oxidation-reduction reactions,
or
redox reactions
In redox reactions, the loss of electrons from a
substance is called
oxidation
The addition of electrons to a substance is called
reduction
(the amount of positive charge is
reduced)
Oxidation and reduction always go hand in hand
The electron donor is called the
reducing agent
The electron acceptor is called the
oxidizing
agent
Oxidation of Organic Fuel Molecules During
Cellular Respiration
During cellular respiration, fuel molecules (such as
glucose) are oxidized, and O2 is reduced
Organic molecules with an abundance of hydrogen
are excellent sources of high-energy electrons
Cellular respiration is a redox process; energy is
released as hydrogen and electrons are transferred
to O2 atoms
Stepwise Energy Harvest via NAD+ and the
Electron Transport Chain
Each electron travels with a proton thus, as a
hydrogen atom
Hydrogen atoms are usually first passed to electron
carriers, rather than directly to O2
NAD+
Nicotinamide adenine dinucleotide,
NAD+
, is a
coenzyme that functions as an electron carrier
As an electron acceptor, NAD+ functions as an
oxidizing agent during cellular respiration
Each NADH (the reduced form of NAD+) represents
stored energy that is tapped to synthesize ATP
An
electron transport chain
consists of a series of
molecules built into the inner membrane of the
mitochondria (or plasma membrane of prokaryotes)
The Stages of Cellular Respiration: A Preview
Harvesting energy from glucose by cellular
respiration has three stages
Glycolysi
s breaks down glucose into two
molecules of pyruvate
Pyruvate oxidation and the
citric acid cycle
complete the breakdown of glucose to CO2
During
oxidative phosphorylation
the electron
transfer chain and chemiosmosis facilitate
synthesis of most of the cell’s ATP
For each molecule of glucose degraded to CO2 and
H2O by cellular respiration, up to 32 molecules of
ATP are produced
Catabolic pathways yield energy
by oxidizing organic fuels
The Citric Acid Cycle
Oxidation of Pyruvate to Acetyl CoA
Pyruvate is converted to acetyl coenzyme A (acetyl
CoA) before entering the citric acid cycle
Pyruvate dehydrogenase catalyzes three reactions
Oxidation of pyruvate’s carboxyl group, releasing
the first CO2 of cellular respiration
Reduction of NAD+ to NADH
Combination of the remaining two-carbon fragment
with coenzyme A to form acetyl CoA
The
Citric Acid Cycle
The citric acid cycle, also called the
Krebs cycle
,
oxidizes organic fuel derived from pyruvate,
generating 1 ATP, 3 NADH, and 1 FADH2 per turn
Another 2 CO2 are produced as a waste product
2 pyruvate are produced per glucose, the
cycle runs twice per glucose molecule consumed
The citric acid cycle has eight steps, each
catalyzed by a specific enzyme
First the acetyl group of acetyl CoA joins the cycle
by combining with oxaloacetate, forming citrate
The next seven steps decompose the citrate back
to oxaloacetate, making the process a cycle
The NADH and FADH2 produced by the cycle carry
electrons to the electron transport chain
After pyruvate is oxidized, the
citric acid cycle completes the energy-yielding
oxidation of organic molecules
Chemiosmosis couples
Electron Transport to ATP synthesis
NADH and FADH2 donate electrons to the electron
transport chain, which powers ATP synthesis via
oxidative phosphorylation
The Pathway of Electron Transport
NADH and FADH2 donate electrons to different
electron acceptors early in the chain
Electrons are passed through a number of carrier
molecules including several
cytochromes
(proteins with heme groups containing an iron
atom)
Electrons drop in free energy as they are
transferred down the chain, finally passing to O2 to
form H2O
ETC breaks the large free-
energy drop from glucose to O2 into smaller steps,
releasing energy in manageable amounts
No ATP is produced directly by the chain
During oxidative
phosphorylation, chemiosmosis couples
electron transport to ATP synthesis
Chemiosmosis: The Energy-Coupling
Mechanism
The energy released as electrons are passed down
the electron transport chain is used to pump H+
from the mitochondrial matrix to the intermembrane
space
H+ then moves down its concentration gradient
back across the membrane, passing through the
protein complex ATP synthase
H+ moves into binding sites on the rotor of ATP
synthase, causing it to spin in a way that catalyzes
phosphorylation of ADP to ATP
This is an example of
chemiosmosi
s, the use of
energy in a H+ gradient to drive cellular work
The H+ gradient is referred to as a
proton-motive
force
, emphasizing its capacity to do work
An Accounting of ATP Production by Cellular
Respiration
During cellular respiration, most energy flows in this
sequence:
glucose → NADH → electron transport chain →
proton-motive force → ATP
About 34% of the energy in a glucose molecule is
transferred to ATP, making about 32 ATP
The rest of the energy is lost as heat
Fermentation and Anaerobic
Respiration
Fermentation and anaerobic
respiration enable cells to produce ATP without
the use of oxygen
Types of Fermentation
Alcohol fermentation
In alcohol fermentation, pyruvate is converted to
ethanol in two steps
The first step releases CO2 from pyruvate
The second step produces NAD+ and ethanol
Ex: Alcohol fermentation by yeast is used in brewing,
winemaking, and baking
Lactic acid fermentation
In lactic acid fermentation, pyruvate is reduced
directly by NADH to form lactate and NAD+
There is no release of CO2
Ex: Lactic acid fermentation by fungi and bacteria is
used to make cheese and yogurt
Fermentation is an extension of glycolysis that
oxidizes NADH by transferring electrons to
pyruvate or its derivatives
Comparing Fermentation with Anaerobic and
Aerobic Respiration
Some similarities
All use glycolysis (net ATP = 2) to oxidize glucose
and harvest the chemical energy of food
In all three, NAD+ is the oxidizing agent that accepts
electrons during glycolysis
Some differences
Obligate anaerobe
s carry out fermentation or
anaerobic respiration and cannot survive in the
presence of O2
Yeast and many bacteria are
facultative
anaerobes
, meaning that they can survive using
either fermentation or cellular respiration
Photosynthesis Feeds the
Biosphere
Photosynthesis
is the process that converts solar
energy into chemical energy within chloroplasts
Producers
Autotrophs
are “self-feeders” that sustain
themselves without eating anything derived from
other organisms
They
produce organic molecules from CO2 and other
inorganic molecule
Photosynthesis also occurs in algae, certain other
protists, and some prokaryotes
Consumers
Heterotrophs obtain organic material from other
organisms
Decomposers, eat dead organic material or feces
Photosynthesis Converts Light
Energy to the Chemical Energy of Food
Plants and other photosynthetic organisms contain
organelles called chloroplasts
Chloroplasts: The Sites of Photosynthesis
in Plants
Chloroplasts are found mainly in cells of the
mesophyll
, the interior tissue of the leaf
CO2 enters and O2 exits the leaf through
microscopic pores called
stomata
A chloroplast has an envelope of two membranes
surrounding a dense fluid called the
stroma
Thylakoids
are connected sacs in the chloroplast
that compose a third membrane system
Chlorophyll
, the pigment that gives leaves their
green color, resides in the thylakoid membranes
Tracking Atoms Through Photosynthesis:
Scientific Inquiry
Photosynthesis Equation
6 CO2 + 12 H2O + Light energy → C6H12O6 + 6 O2 + 6
H2O
The Splitting of Water
Chloroplasts split H2O into hydrogen and oxygen,
incorporating the electrons of hydrogen into sugar
molecules and releasing O2 as a by-product
Photosynthesis as a Redox Process
Photosynthesis is a redox process in which H2O is
oxidized and CO2 is reduced
Photosynthesis reverses the direction of electron
flow compared to respiration
It is an endergonic process; the
energy boost is provided by light
The overall chemical change during photosynthesis
is the reverse of cellular respiration
The Two Stages of Photosynthesis: A Preview
The
light reactions
(in the thylakoids)
– Split H2O, providing electrons and protons (H+)
– Release O2 as a by-product
– Reduce the electron acceptor NADP+ to NADPH
– Generate ATP from ADP by photophosphorylation
The Calvin cycle (in the stroma)
The Calvin cycle
makes sugar from
CO2, using the ATP and NADPH generated during
the light reactions
The Calvin cycle begins with
carbon fixation,
incorporating CO2 into organic molecules
It then reduces fixed carbon to carbohydrate by
transferring electrons from NADPH
The Light Reactions
The light reactions convert
solar energy to the chemical energy of ATP and
NADPH
The Nature of Sunlight
Light is electromagnetic energy, also called
electromagnetic radiation
Wavelength
is a measure of the distance between
crests of electromagnetic waves
The
electromagnetic spectrum
is the entire range
of electromagnetic energy, or radiation
Visible light
(wavelengths 380 nm to 740 nm)
drives photosynthesis and produces the colors
seen by the human eye
Light also behaves as though it consists of discrete
particles, called
photons
Photosynthetic Pigments: The Light Receptors
Pigments are substances that absorb visible light
Wavelengths that are not absorbed are reflected or
transmitted
Ex: most leaves appear green because
chlorophyll absorbs violet-blue and red light while
reflecting and transmitting green light
A
spectrophotometer
measures a pigment’s
ability to absorb various wavelengths
It sends light through pigments and measures the
fraction of light transmitted at each wavelength
An
absorption spectrum
is a graph plotting a
pigment’s light absorption versus wavelength
Three types of pigments in chloroplasts
–
Chlorophyll a
, the key light-capturing pigment that
participates directly in light reactions
–
Chlorophyll b
, an accessory pigment
–
Carotenoids
, a separate group of accessory
pigments
Are
yellow or orange because they absorb violet and
blue-green light
Excitation of Chlorophyll by Light
When a pigment molecule absorbs light, one of its
electrons goes from a ground state to an excited
state, which is unstable
In isolation, excited electrons fall back to the
ground state, releasing excess energy as heat or
light, an afterglow called fluorescence
A Photosystem: A Reaction-Center Complex
Associated with Light-Harvesting Complexes
A
photosystem
consists of a reaction-center
complex surrounded by light-harvesting complexes
The
reaction-center complex
is an association of
proteins holding a special pair of chlorophyll a
molecules and a primary electron acceptor
The
light-harvesting complex
consists of various
pigment molecules bound to proteins
Light-harvesting complexes transfer the energy of
photons to the chlorophyll a molecules in the
reaction-center complex
A
primary electron accepto
r in the reaction center
accepts excited electrons and is reduced as a
result
Two types of photosystems in the
thylakoid membrane, numbered in order of their
discovery
Photosystem II (PS II)
is called P680 because its
reaction-center chlorophyll a is best at absorbing
light with a wavelength of 680 nm
Photosystem I (PS I)
is called P700 because its
reaction-center chlorophyll a is best at absorbing
light with a wavelength of 700 nm
Linear Electron Flow
Linear electron flow
, the primary pathway,
involves both photosystems and produces ATP and
NADPH using light energy
There are eight steps in linear electron flow
Cyclic Electron Flow
In
cyclic electron flow
, photoexcited electrons
cycle back from Fd to the cytochrome complex
instead of being transferred to NADP+
Electrons are passed to a P700 chlorophyll in the
PS I reaction center via the plastocyanin molecule
(Pc)
Uses only photosystem I
It produces ATP, but no NADPH or oxygen
A Comparison of Chemiosmosis in
Chloroplasts and Mitochondria
Chloroplasts and mitochondria both generate ATP
by chemiosmosis
Some of the electron carriers, including iron-
containing proteins called cytochromes, are very
similar in mitochondria and chloroplasts
The ATP synthase complexes are also very similar
Photophosphorylation differs from oxidative
phosphorylation
In chloroplasts, high energy electrons drop down the
transport chain from water, while in mitochondria,
they are extracted from organic molecules
Mitochondria transfer chemical energy from food to
ATP; chloroplasts transform light energy into the
chemical energy of ATP
The Calvin Cycle
The Calvin cycle uses the
chemical energy of ATP and NADPH to reduce
CO2 to sugar
The Calvin cycle, like the citric acid cycle,
regenerates its starting material after molecules
enter and leave the cycle
Is anabolic; it builds sugar from
smaller molecules by using ATP and the reducing
power of electrons carried by NADPH
Carbon enters the cycle as CO2 and leaves as a
sugar named
glyceraldehyde 3-phospate (G3P)
The Calvin cycle has three phases
Phase 1: Carbon fixation
The binding of CO2 to a five-carbon sugar named
ribulose bisphosphate (RuBP) is catalyzed by
RuBP carboxylase-oxygenase, or
rubisco
The six-carbon intermediate molecule is
immediately split into two molecules of 3-
phosphoglycerate (for each CO2 fixed)
Phase 2: Reduction
For every three CO2 molecules that enter the cycle,
six molecules of G3P are formed
Each molecule of 3-phosphoglycerate is altered
through phosphorylation by six ATP and reduction
by six NADPH to ultimately produce a G3P sugar
Phase 3: Regeneration of the CO2 acceptor
(RuBP)
The remaining five molecules of G3P are
rearranged in a complex series of reactions yielding
three molecules of RuBP
Three additional molecules of ATP are used to
facilitate the regeneration of RuBP
Alternative Mechanisms of
Carbon Fixation
Alternative mechanisms of
carbon fixation have evolved in hot, arid
climates
Plants have metabolic adaptations to help conserve
water; but these adaptations often involve trade-
offs
Photorespiration: An Evolutionary Relic?
Most plants are
C3
plants, in which the initial
fixation of CO2, via rubisco, forms a three-carbon
compound (3-phosphoglycerate)
In
photorespiration
, rubisco binds with O2 instead
of CO2, producing a two-carbon compound
Is costly because it consumes O2
and organic fuel without producing any ATP or
sugar
Rubisco first evolved at a time when the atmosphere
had far less O2 and more CO2
C4
plants
C4
plants minimize the cost of photorespiration by
incorporating CO2 into a four-carbon compound as
the first product of the Calvin cycle
Important agricultural examples include sugarcane
and corn