Photosynthesis
and
Cellular Respiration
EK2A2: Organisms capture and store
free energy for use in biological
processes.
Redox Reactions Vocabulary
• Redox reaction: refers to both reactions together; because an electron
transfer requires both a donor and an acceptor, oxidation and reduction go
together
• Oxidation: refers to the loss of electros
• Reduction: refers to the gain of electrons (e- = negative  reduce charge)
• Oxidizing agent: causes the oxidation; electron acceptor; must receive efor a loss to happen
• Reducing agent: causes the reduction; electron donor; must give e- for a
gain to happen
• OILRIG: oxidation is loss, reduction is gain
• LEO says GER: loss of electrons oxidation, gain of electrons reduction
Why do redox reactions matter to organisms?
• The transfer of electrons during chemical reactions releases energy
stored in organic molecules
• This released energy is ultimately used to synthesize ATP
• EX: Cellular Respiration
• Glucose is oxidized and O2 is reduced
• Electrons lose potential eneregy as they move towards O2 e- move towards
O2 because it is so electronegative (wants electrons to become more stable)
energy is released
becomes oxidized
becomes reduced
Pair Share
• 45 seconds: Redox Rxns:
1. Explain the difference between oxidation and reductions
2. What is the difference between oxidizing agent and reducing agent
3. Why are redox rxns important to living things
4. What is the oxidizing agent in cellular respiration?
• 30 seconds: Overview of Cellular Respiration:
1. What are the three main steps of cellular respiration?
2. Where do these steps take place?
3. What are the reactants and products of cellular respiration?
4. What is the purpose of cellular respiration?
Overview of Cellular Respiration
1.Glycolysis
2.Citric acid cycle (aka Krebs cycle)
3.Oxidative phosphorylation:
Electron transport chain and
chemiosmosis
Purpose of Cellular Respiration
• Break down organic molecules (usually glucose) to form ATP (ADPATP)
• Two main ways to form ATP
1. Oxidative phosphorylation: 90% of ATP made this way; powered by redox
reactions of the electron transport chain; inorganic phosphate is added to
ADP by ATP synthase (driven by the chemical gradient created by the ETC)
2. Substrate-level phosphorylation: enzyme transfers a phosphate group
from a substrate to ADP; happens during glycolsis and citric acid cycle
Enzyme
Enzyme
ADP
P
Substrate
+
ATP
An overview of Cellular Respiration Basics
Electrons carried
via NADH and
FADH2
Electrons
carried
via NADH
Krebs
Cycle
Glycolysis
Pyruvate
Glucose
Oxidative
phosphorylation:
electron transport
and
chemiosmosis
Mitochondrion
Cytosol
ATP
ATP
Substrate-level
phosphorylation
Substrate-level
phosphorylation
ATP
Oxidative
phosphorylation
NAD+ (Nicotineamide adenine dinuclotide)
• Coenzyme that acts as an electron acceptor
• Functions as an oxidizing agent during respiration
• Traps electrons from glucose and other organic molecules with the
help of the enzyme dehydrogenase
• Enzyme removes a pair of hydrogen atoms (2 electrons and 2
protons) from the substrate  delivers 2 electrons and 1 proton to its
coenzyme (NAD+); other H+ is released into the surrounding solution
(this H+ will become important later)
• Each NADH represents stored energy that is used to synthesize ATP
Dehydrogenase
Details of Glycolysis
• Glycolysis means “sugar splitting;”
glucose is broken into two molecules
of pyruvate
• Occurs in the cytoplasm and has two
major phases:
• Energy investing phase –
activation energy is needed to get
the exergonic reaction going
(2ATP molecules)
• Energy payoff phase – ATP and
NADH is created
• Glucose + 2 ATP (activation energy)
pyruvate (Krebs cycle) + ATP
(substrate-level) + 2NADH (carries
electrons to ETC) + 2 H+ +2H2O
Energy investment
phase
Glucose
2 ADP + 2 P
2 ATP
used
4 ATP
formed
Energy payoff
phase
4 ADP + 4 P
2 NAD+ + 4 e– + 4 H+
2 NADH + 2 H+
2 Pyruvate + 2 H2O
Net
Glucose
4 ATP formed – 2 ATP used
2 NAD+ + 4 e– + 4 H+
2 Pyruvate + 2 H2O
2 ATP
2 NADH + 2 H+
The
energy
input
and
output
of
glycolysis
Energy investment
phase
Glucose
2 ADP + 2 P
2 ATP
used
4 ATP
formed
Energy payoff
phase
4 ADP + 4 P
2 NAD+ + 4 e– + 4 H+
2 NADH + 2 H+
2 Pyruvate + 2 H2O
Net
Glucose
4 ATP formed – 2 ATP used
2 NAD+ + 4 e– + 4 H+
2 Pyruvate + 2 H2O
2 ATP
2 NADH + 2 H+
A
closer
look
at
glycolysis
Glucose
ATP
1
Hexokinase
ADP
Glucose-6-phosphate
2
Phosphoglucoisomerase
Fructose-6-phosphate
Glucose-6-phosphate
2
Phosphoglucoisomerase
Fructose-6-phosphate
Glucose
ATP
1
Hexokinase
ADP
Fructose-6-phosphate
Glucose-6-phosphate
2
Phosphoglucoisomerase
ATP
3
Phosphofructokinase:
Fructose-6-phosphate
allosteric enzyme
ATP
3
Phosphofructokinase
ADP
ADP
Fructose1, 6-bisphosphate
Fructose1, 6-bisphosphate
Glucose
ATP
1
Hexokinase
ADP
Glucose-6-phosphate
2
Phosphoglucoisomerase
Fructose1, 6-bisphosphate
4
Fructose-6-phosphate
ATP
Aldolase
3
Phosphofructokinase
ADP
5
Isomerase
Fructose1, 6-bisphosphate
4
Aldolase
5
Isomerase
Dihydroxyacetone
phosphate
Dihydroxyacetone
phosphate
Glyceraldehyde3-phosphate
Glyceraldehyde3-phosphate
2 NAD+
2 NADH
+ 2 H+
6
Triose phosphate
dehydrogenase
2 Pi
2 1, 3-Bisphosphoglycerate
2 ADP
7
Phosphoglycerokinase
2 ATP
2 1, 3-Bisphosphoglycerate
2 ADP
2
3-Phosphoglycerate
2 ATP
2
7
Phosphoglycerokinase
3-Phosphoglycerate
Fig. 9-9-7
2 NAD+
2 NADH
+ 2 H+
6
Triose phosphate
dehydrogenase
2 Pi
2 1, 3-Bisphosphoglycerate
2 ADP
7 Phosphoglycerokinase
2 ATP
2
3-Phosphoglycerate
8
2
3-Phosphoglycerate
Phosphoglyceromutase
2
8
Phosphoglyceromutase
2-Phosphoglycerate
2
2-Phosphoglycerate
2 NAD+
2 NADH
+ 2 H+
6
Triose phosphate
dehydrogenase
2 Pi
2 1, 3-Bisphosphoglycerate
2 ADP
7 Phosphoglycerokinase
2 ATP
2
3-Phosphoglycerate
2
2-Phosphoglycerate
8
Phosphoglyceromutase
9
2
2 H2O
2-Phosphoglycerate
Enolase
9
Enolase
2 H2O
2
Phosphoenolpyruvate
2
Phosphoenolpyruvate
2 NAD+
6
Triose phosphate
dehydrogenase
2 Pi
2 NADH
+ 2 H+
2 1, 3-Bisphosphoglycerate
2 ADP
7 Phosphoglycerokinase
2 ATP
2
Phosphoenolpyruvate
2 ADP
2
3-Phosphoglycerate
8
Phosphoglyceromutase
2 ATP
2
10
Pyruvate
kinase
2-Phosphoglycerate
9
2 H2O
Enolase
2 Phosphoenolpyruvate
2 ADP
10
Pyruvate kinase
2 ATP
2
2
Pyruvate
Pyruvate
Details of the citric acid cycle
• If O2 is present,
pyruvate from
glycolysis enters the
mitochondria
• Pyruvate is then
converted into Acetyl
CoA
• In this process
CO2 is released
into the
atmosphere and
more NADH is
formed (this will
carry e- to the
ETC)
CYTOSOL
MITOCHONDRION
NAD+
NADH + H+
2
1
3
Acetyl CoA
Pyruvate
Transport protein
CO2
Coenzyme A
Details of the citric acid cycle (continued)
• Citric acid cycle (aka Krebs cycle) take place
within the mitochondrial matrix
• Cycle oxidizes organic fuel derived from
pyruvate, generating 1 ATP, 3 NADH, and 1
FADH2 per turn (two turns per glucose
molecule from glycolysis)
• Has eight steps, each catalyzed by a specific
enzyme
• NADH and FADH2 produced by the Krebs
cycle carry electrons extracted from food to
the electron transport chain in the
mitochondrial cristae membrane
• 2 additional molecules of CO2 are released
(three total; 1 from pyruvate  acetyl Co A)
Pyruvate
CO2
NAD+
CoA
NADH
+ H+
Acetyl CoA
CoA
CoA
Citric
acid
cycle
FADH2
2 CO2
3 NAD+
3 NADH
FAD
+ 3 H+
ADP + P i
ATP
Acetyl CoA
CoA—SH
NADH +
+H
H2 O
1
NAD+
8
Oxaloacetate
OAA
2
Malate
Citrate
Isocitrate
NAD+
Citric
Acid
Cycle
7
H2O
Fumarate
6
3
CO2
CoA—SH
4
CoA—SH
5
FADH2
NAD+
FAD
Succinate
GTP GDP
ADP
ATP
NADH
+ H+
Pi
Succinyl
CoA
NADH
+ H+
-Ketoglutarate
CO2
Pair Share
•At the end of glycolysis and
the citric acid cycle…where is
the energy that was originally
in the glucose molecule???
Let’s look at the energy so far
• Following glycolysis and the citric acid cycle we have a total
of 4 ATP molecules that have been created via substratelevel phosphorylation
• 2 from glycolysis and 2 from Krebs
• Most of the energy extracted from the glucose molecule is
currently stored in the Coenzyme carries NADH and FADH2
• Some of the energy has been lost as heat
• Energy in the NADH and FADH2 is now going to be used in
the third step of cellular respiration (electron transport
chain)
What exactly is the “Electron Transport
Chain?
NADH
• Collection of molecules
embedded in the inner
membrane of mitochondria
in eukaryotic cells
(prokaryotes = plasma
membrane)
• Most components are
proteins that exist in
multiprotein complexes
numbered I through IV
50
2 e–
NAD+
FADH2
2 e–
40

FMN
FAD
Multiprotein
complexes
FAD

Fe•S
Fe•S
Q

Cyt b
30
Fe•S
Cyt c1
IV
Cyt c
Cyt a
Cyt a3
20
10
0
2 e–
(from NADH
or FADH2)
2 H+ + 1/2
O2
H2O
waste
Electron Transport Chain
• Electron carries alternate between reduced
and oxidized states; as electrons move down
the chain energy is released
• Electrons removed from glucose by NAD+ are
transferred from NADH to the first molecule of
the electron transport chain in complex I
• The last electron acceptor passes its electrons
to O2 (which is known as the terminal electron
acceptor)
• O2 picks up a pair of H+ (hydrogen ions) from
the aqueous solution to form H2O
• ETC does not generate ATP; purpose is to break
release of energy into smaller manageable
amounts
NADH
50
2 e–
NAD+
FADH2
2 e–
40

FMN
FAD
Multiprotein
complexes
FAD
Fe•S 
Fe•S
Q

Cyt b
30
Fe•S
Cyt c1
I
V
Cyt c
Cyt a
Cyt a3
20
10
2 e–
(from NADH
or FADH2)
0
2 H+ + 1/2 O2
H2O
waste
Chemiosmosis
• Chemiosmosis: energy-coupling mechanisms that
uses energy stored in the form of an H+ gradient
across a membrane to drive cellular work
• In cellular respiration, H+ gradient is established by
the electron transport chain, at certain steps along
the ETC electron transfer causes H+ to be moved from
the inside of the mitochondrial matrix to the inner
membrane space; creates a proton H+ gradient
ATP Synthase
• In addition to the proteins that make up the ETC there are also protein
complexes called ATP synthases
• ATP synthase: enzyme that makes ATP from ADP and inorganic phosphate;
uses energy from the hydrogen gradient created by the electron transport
chain
• The exergonic flow of H+ ions through ATP synthase drives phosphorylation of
ATP
• This is an example of chemiosmosis, the use of energy in a H+ gradient to drive
work (in this case ATP synthesis)
• The energy stored in the proton gradient couples with the redox rxn of the ETC
to make ATP (oxidative phosphorylation)
• The H+ gradient is a proton-motive force
Chemiosmosis: Energy Coupling - couples the electron transport chain to ATP synthesis
H+
H+
H+
H+
Protein complex
of electron
carriers
Cyt c
V
Q


ATP
synthase

FADH2
NADH
2 H+ + 1/2O2
H2O
FAD
NAD+
ADP + P i
(carrying electrons
from food)
ATP
H+
1 Electron transport chain: redox
Oxidative phosphorylation
2 Chemiosmosis
Energy Flow through Cellular Respiration: A Summary
Glucose  NADH  electron transport chain  proton-motive force  ATP
• ATP yield per glucose
molecule oxidized is not an
exact number; but it is
between 36-38 ATP
molecules/ glucose
molecule depending on
efficiency of cellular
respiration
• 4 ATP by substratelevel phosphorylation
• 32-34 ATP by oxidative
phospohyalation
Electron shuttles
span membrane
CYTOSOL
2 NADH
Glycolysis
Glucose
2
Pyruvate
MITOCHONDRION
2 NADH
or
2 FADH2
6 NADH
2 NADH
2
Acetyl
CoA
+ 2 ATP
Citric
Acid
Cycle
+ 2 ATP
Maximum per glucose:
About
36 or 38 ATP
2 FADH2
Oxidative
phosphorylation:
electron transport
chemiosmosis
+ about 32 or 34 ATP
Fermentation and anaerobic respiration enable
cells to produce ATP without the use of oxygen
• Without the electronegative oxygen to pull
electrons down the transport chain, oxidative
phosphorylation stops
• Two general mechanisms can oxidize organic fuel
and generate ATP w/o O2: anaerobic respiration
and fermentation
Anaerobic Respiration
• Other less electronegative substances can be
used in place of O2 as the final electron acceptor
in an electron transport chain
• EX: “sulfate-reducing” marine bacteria use
sulfate ion (SO42-) at the end of their ETC; rather
than H2O being the end product H2S is the by
product.
Fermentation
• A way of harvesting chemical energy without oxygen or
any ETC
• Recall ATP is created in glycolysis
• Glucose + 2 ATP (activation energy) pyruvate (Krebs cycle)
+ ATP (substrate-level) + 2NADH (carries electrons to ETC) + 2
H+ +2H2O
• In the case of fermentation the Krebs cycle and ETC do
not exist…for the organisms to continue fermentation
there must be a way to recycle NADH back to NAD+
• Organisms has developed two ways to do this: alcohol
fermentation and lactic acid fermentation
Two Types of Fermentation
• Alcoholic Fermentation:
• Pyruvate is converted to ethanol in two steps
1. Releases carbon dioxide from the pyruvate  produces
acetaldehyde
2. Acetaldehyde is reduced by NADH to ethanol (this step
generates the supply of NAD+ needed for the continuation of
glycolysis
• Lactic Acid Fermentation:
• Pyruvate is reduced directly by NADH to form lactate as an end
product, with no release of CO2
• Human muscle cells make ATP by lactic acid fermentiaotn with
oxygen is scarce
Photosynthesis as a Redox Process
• During cellular respiration, energy is released from sugar when
electrons associated with hydrogen are transported by carriers
to oxygen, forming water as a by-product. Electrons lose
potential energy as they “fall” down the electron transport chain
toward electronegative oxygen, and the mitochondrion harness
that energy to synthesize ATP
• Photosynthesis reverses the direction of electron flow: water is
split, and electrons are transferred along with hydrogen ions
from the water to carbon dioxide, reducing it to sugar
• Because the electrons increase in potential energy as they move
from water to sugar, this process requires energy
(endergonic)…the energy is provided by light
Photosynthesis Preview
• The equation of photosynthesis is relatively simply…but the reality is the
actual process is not simple…actually two main steps with multiple steps
within each
1. Light Reaction
• Photo part of the reaction; takes place in the thylakoid
• Light energy is absorbed and converted into a chemical form (ATP
and NADPH)
2. Dark Reaction
• Synthesis part of the reaction; takes place in the stroma
• Chemical energy from the light reaction is used to to make sugar
More details of the Light Reaction
• Water is split  provides electrons and protons  O2 is given off as a
by-product
• Light absorbed by chlorophyll drives a transfer of the electrons and
hydrogen ions to an electron acceptor (in photosythesis the electron
acceptor is NADP+ (nicotinamide adenine dinucleotide phosphate)
• Light reaction reduces NADP+ to NADPH by adding a pair of electrons
along with an H+
• Light reaction also generates ATP using chemiosmosis to power the
addition of a phosphate group to ADP  this process is called
photophosphorylation
More details of the Calvin Cycle
• Cycle begins with incorporating CO2 from the air into organic
molecules already present in the chloroplast; this process is called
carbon fixation
• Calvin cycle then reduces the fixed carbon to carbohydrates by adding
electrons; the reducing power is provided by NADPH (from the light
reaction)
• To convert CO2 to C6H12O6 the Calvin cycle requires chemical energy in
the form of ATP (from the light reaction)
Fig. 10-5-4
CO2
H2O
Light
NADP+
ADP
+ P
i
Light
Reactions
Calvin
Cycle
ATP
NADPH
Chloroplast
O2
[CH2O]
(sugar)
Light Reaction EVEN MORE DETAILS
e–
Energy of electron
• Chlorophyll is a pigment located
inside photosystems
• Photosystems are proteins embedded
in the thylakoid’s membrane
• There are two photosystems
associated with photosynthesis:
Photosystem I and Photosystem II
• When chlorophyll inside Photosystem
II absorbs light it is excited due to one
of its electrons being elevated to an
orbital where it has more potential
energy
Excited
state
Heat
Photon
(fluorescence)
Photon
Chlorophyll
molecule
Ground
state
(a) Excitation of isolated chlorophyll molecule
Photosystem II
Light-harvesting
complexes
Thylakoid membrane
• Photosystem consists of a
reaction-center complex and lightharvesting complexes
• Light-harvesting complexes funnel
the energy of light to the reaction
center
• A primary electron acceptor in
the reaction center accepts an
excited electron from chlorophyll
• Enzyme splits water into two
electrons, two hydrogen ions and
an oxygen molecules (O2) is
released and electrons are
dumped into the electron
transport chain between PSII and
PSI
Photosystem
Photon
Reaction-center
complex
STROMA
Primary
electron
acceptor
e–
Transfer
of energy
Special pair of
chlorophyll a
molecules
Pigment
molecules
THYLAKOID SPACE
(INTERIOR OF THYLAKOID)
H2O
CO2
Primary
acceptor
Primary
acceptor
H2O
O2
Fd
Pq
NADP+
reductase
Cytochrome
complex
Pc
ATP
Photosystem II
O2
Photosystem I
NADP+
+ H+
NADPH
Photosystem I
• The exergonic fall of electrons to a lower energy level provides energy for the
synthesis of ATP
• As the electrons pass through certain parts of the electron transport chain,
protons are pumpped across the membrane
• The proton gradient created is used in chemiosmosis to generate ATP via ATP
synthase
• At the same time PSI absorbs light energy in the light-harvesting complex and
transfers it to the PSI reaction-center complex, the energy travels down a
second electron transport chain and is stored in NADPH
• ATP and NADPH can not be used in the Calvin cycle to reduce CO2 into
glucose
Summary of the complex light reaction
• Light is absorbed by
chylorphyll, with the help of
photosystems I and II, the
light energy travels down two
electron transport chains, the
first electron transport chain
generates ATP, the second
electron transport chain
generates NADPH
• Both NADPH and ATP are
ready to help the Calvin cycle
e–
ATP
e–
e–
NADPH
e–
e–
e–
Mill
makes
ATP
e–
Photosystem II
Photosystem I
Fig. 10-16
Mitochondrion
Chloroplast
MITOCHONDRION
STRUCTURE
CHLOROPLAST
STRUCTURE
H+
Intermembrane
space
Inner
membrane
Diffusion
Electron
transport
chain
Thylakoid
space
Thylakoid
membrane
ATP
synthase
Stroma
Matrix
Key
ADP + P
[H+]
Higher
Lower [H+]
i
H+
ATP
Fig. 10-17
STROMA
(low H+ concentration)
Cytochrome
complex
Photosystem II
4 H+
Light
Photosystem I
Light
Fd
NADP+
reductase
H2O
THYLAKOID SPACE
(high H+ concentration)
1
e–
Pc
2
1/
2
NADP+ + H+
NADPH
Pq
e–
3
O2
+2 H+
4 H+
To
Calvin
Cycle
Thylakoid
membrane
STROMA
(low H+ concentration)
ATP
synthase
ADP
+
Pi
ATP
H+
Dark Reaction: EVEN MORE DETAILS
• Calvin cycle is anabolic, building carbohydrate from smaller molecules
and consuming energy
• Carbon enters the Calvin cycle in the for of CO2 and leaves as sugar
• The cycle spends ATP as an energy source and consumes NADPH as
reducing power for adding high-energy electrons to make the sugar
• Calvin cycle is broken into three phases:
1. Phase 1: Carbon fixation
2. Phase 2: Reduction
3. Phase 3: Regeneration of the Co2 acceptor (RuBP)
Fig. 10-18-3
Input
3
CO2
(Entering one
at a time)
Phase 1: Carbon fixation
Rubisco
3 P
Short-lived
intermediate
3 P
Ribulose bisphosphate
(RuBP)
P
6
P
3-Phosphoglycerate
P
6
ATP
6 ADP
3 ADP
3
Calvin
Cycle
6 P
P
1,3-Bisphosphoglycerate
ATP
6 NADPH
Phase 3:
Regeneration of
the CO2 acceptor
(RuBP)
6 NADP+
6 Pi
P
5
G3P
6
P
Glyceraldehyde-3-phosphate
(G3P)
1
Output
P
G3P
(a sugar)
Glucose and
other organic
compounds
Phase 2:
Reduction
Fig. 10-21
H2O
CO2
Light
NADP+
ADP
+ P
i
Light
Reactions:
Photosystem II
Electron transport chain
Photosystem I
Electron transport chain
RuBP
ATP
NADPH
3-Phosphoglycerate
Calvin
Cycle
G3P
Starch
(storage)
Chloroplast
O2
Sucrose (export)