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UNIT 3:
Part 2
Cellular Respiration and
photosynthesis
Hillis Textbook Chapter 6
 Cellular
respiration is a major catabolic pathway.
Glucose is oxidized:
carbohydra te  6O2  6CO2  6H 2O  chemical energy
 Photosynthesis
is a major anabolic pathway. Light
energy is converted to chemical energy:
6CO2  6H 2O  light energy  6O2  carbohydra te
Cellular Respiration is AEROBIC (uses oxygen)
 A lot of energy is released when reduced
molecules with many C—C and C—H bonds are
fully oxidized to CO2.
 Oxidation
occurs in a series of small steps in three
pathways, followed by generation of ATP:
1. Glycolysis
2. Pyruvate Oxidation
3. Citric Acid Cycle
4. Electron Transport Chain
CELLULAR RESPIRATION:
REACTANTS: Glucose and oxygen
LEARN THE
NAMES OF THE
STEPS AND
WHERE IT
TAKES PLACE!
Oxygen
ATP
PRODUCTS: Carbon Dioxide,
water and ATP
STEP ONE: GLYCOLYSIS
Glycolysis: ten total reactions.
 Takes place in the cytosol.
 Starts with glucose
 Final
products:
2 molecules of NADH
2 molecules of ATP
2 molecules of pyruvate
(pyruvic acid)
These ATP molecules were
produced, however they don’t
count because we USED two
molecules at the beginning
STEP TWO: PYRUVATE OXIDATION
Pyruvate Oxidation:
 Happens in the mitochondria
 Starts with TWO separate pyruvates from glycolysis
 Products: CO2 and acetate; acetate is then bound
to coenzyme A (CoA)
2
pyruvate
in
Results in:
2 CO2 and 2
Acetyl CoA
total
STEP THREE: CITRIC ACID CYCLE
 Citric
Acid Cycle:
 Takes place in the mitochondrial matrix
 8 reactions
 Starts with the two Acetyl CoA produced by
pyruvate oxidation
 So, the cycle operates twice for every ONE
glucose molecule that enters glycolysis
 Each acetyl group is oxidized to two CO2.
 Oxaloacetate is regenerated in the last step to be
re-used again when another acetyl CoA comes
along.
 Energy
carriers are produced: 6 NADH, 2 FADH2, 2 GTP
STEP THREE: CITRIC ACID CYCLE
Every glucose forms
TWO acetyl CoA from
pyruvate…
End result = 4
CO2, 6 NADH, 2
FADH2 and 2 GTP
ALERT!!!
The Citric Acid
Cycle is also known
as The Kreb’s Cycle
and the TCA cycle
STEP FOUR: ELECTRON TRANSPORT CHAIN
Electron transport/ATP Synthesis:
 NADH is reoxidized to NAD+ and O2 is reduced to H2O in a
series of steps.
 Respiratory chain—series of redox carrier proteins embedded
in the inner mitochondrial membrane.
 Electron transport—electrons from the oxidation of NADH and
FADH2 pass from one carrier to the next in the chain.
STEP FOUR: ELECTRON TRANSPORT CHAIN
 The
oxidation reactions are exergonic; the
energy is used to actively transport H+ ions out of
the mitochondrial matrix, setting up a proton
gradient.
 ATP synthase in the membrane uses the H+
gradient to synthesize ATP by chemiosmosis.
 About 32 molecules of ATP are produced for
each fully oxidized glucose.
 The
role of O2: most of the ATP produced is
formed by oxidative phosphorylation, which is
due to the reoxidation of NADH.
Under anaerobic conditions (NO OXYGEN IS AVAILABLE), NADH is reoxidized by
fermentation. The overall yield of ATP is only two—the ATP made in glycolysis.
Lactic acid fermentation:
End product is lactic acid (lactate).
Alcoholic fermentation:
End product is ethyl alcohol (ethanol).
 Metabolic
pathways are
linked.
 Carbon skeletons
(molecules with
covalently linked
carbon atoms) can
enter catabolic or
anabolic
pathways.
How do you
think prokaryotes
perform
respiration?
Photosynthesis involves two pathways:
1. Light reactions convert light energy into chemical
energy (in ATP and the reduced electron carrier
NADPH).
2. Carbon-fixation reactions use the ATP and NADPH,
along with CO2, to produce carbohydrates.
LIGHT REACTIONS:
 Light
is a form of electromagnetic radiation,
which travels as a wave but also behaves as
particles (photons).
 Photons can be absorbed by a molecule, adding
energy to the molecule—it moves to an excited
state.
 In
plants, two chlorophylls absorb light energy
chlorophyll a and chlorophyll b.
LIGHT REACTIONS:
The light
reactions use
CHLOROPHYLL
to trap energy
from the sun!
That is why they
are considered
“light” reactions.
Chlorophyll molecule
LIGHT REACTIONS:



When chlorophyll (Chl) absorbs light, it enters an excited state
(Chl*), then rapidly returns to ground state, releasing an
excited electron.
Chl* gives the excited electron to an acceptor and becomes
oxidized to Chl+.
The acceptor molecule is reduced.

Chl *  acceptor  Chl  acceptor



The electron acceptor is first in an electron transport system in
the thylakoid membrane.
Final electron acceptor is NADP+, which gets reduced:



NADP  H  2e  NADPH

ATP is produced chemiosmotically during electron transport
(photophosphorylation).
LIGHT REACTIONS:
Two photosystems:
•Photosystem I absorbs light energy at 700 nm, passes an
excited electron to NADP+, reducing it to NADPH.
• Photosystem II absorbs light energy at
680 nm, produces ATP, and oxidizes water molecules.
CALVIN CYCLE:
 The
Calvin
cycle: CO2
fixation. It
occurs in the
stroma of the
chloroplast.
 Each reaction
is catalyzed by
a specific
enzyme.
1. Fixation of CO2:
 CO2 is added to ribulose 1,5-bisphosphate (RuBP).
 Ribulose bisphosphate carboxylase/oxygenase
(rubisco) catalyzes the reaction.
 A 6-carbon molecule results, which quickly breaks
into two 3-carbon molecules: 3-phosphoglycerate
(3PG).
2. 3PG is reduced to form glyceraldehyde 3phosphate (G3P).
3. The CO2 acceptor, RuBP, is regenerated from
G3P.
 When glucose accumulates, it is linked to form
starch, a storage carbohydrate.
The C—H bonds generated by the Calvin cycle
provide almost all the energy for life on Earth!
 Photosynthetic organisms (autotrophs) use most
of this energy to support their own growth and
reproduction.
 Heterotrophs cannot photosynthesize and
depend on autotrophs for chemical energy.
Do we rely on plants?
Do plants rely on us?
ALCOHOL PRODUCTION USING RESPIRATION:
 Catabolism of the beet sugar is a cellular process,
so living yeast cells must be present.
 With air (O2) yeasts used aerobic metabolism to
fully oxidize glucose to CO2.
 Without air, yeasts used alcoholic fermentation,
producing ethanol, less CO2, and less energy
(slower growth).
PATHWAYS THAT HARVEST ENERGY
RESPIRATION OVERVIEW:
RESPIRATION OVERVIEW:
GLYCOLYSIS
RESPIRATION OVERVIEW:
PYRUVATE OXIDATION
FOR EACH PYRUVATE
KREB’S CYCLE
FOR EACH
ACETYL CoA
ELECTRON TRANSPORT
CHAIN
USES ALL THE ENERGY
CARRIERS AND
OXYGEN TO MAKE ATP
Photosynthesis Overview:
Photosynthesis Overview:
Light Reactions in the
thylakoid
membranes
Calvin Cycle in the
stroma