CELLULAR RESPIRATION:
Harvesting Chemical Energy
Chapter 9
CATABOLIC PATHWAYS
• Complex molecules that are high in
potential energy are broken down into
smaller waste products that have less
energy
• Some of this released energy can later
do work, but most is given off as heat
• Two major catabolic pathways
– Aerobic Respiration
– Anaerobic respiration (fermentation)
AEROBIC RESPIRATION
C6H1206 + 602
6CO2 + 6H20 + energy (ATP and heat)
• Aerobic (uses oxygen) respiration
• Exergonic
• ΔG = -686 kcal/mole of glucose
• Big picture – chop up glucose and
make ATP
• Transfer energy in glucose to ATP
• Oxidation of glucose by oxygen
ATP (ADENOSINE TRIPHOSPHATE)
• The last phosphate of ATP can be removed
by enzymes and added to another molecule.
• This turns ATP into ADP (adenosine
diphosphate).
• Molecules that receive a phosphate group
have been phosphorylated.
• This makes the molecule change shape, which
allows the molecule to do work.
• After the work is done, the phosphate group
is released.
Figure 9.2 A review of how ATP drives cellular work
REDOX REACTIONS
• Oxidation - loss of electrons
• Reduction - gain of electrons
• In respiration, transferring
electrons releases energy to make
ATP
• An e- loses potential energy when it
moves from a less electronegative
atom toward a more electronegative
atom.
• In respiration, hydrogen’s electrons
are transferred to oxygen (the fall
of electrons), which liberates
energy.
NAD+
• Hydrogen atoms are removed gradually
from glucose.
• They are transferred to oxygen by a
coenzyme called NAD+ (nicotinamide
adenine dinucleotide).
• Dehydrogenase enzymes remove a pair
of hydrogen atoms (2 e- and 2
protons) from sugar.
– Remember, protons (H+) are hydrogen
cations or an H atom without its electron
• The enzyme delivers 1 proton and 2
e- to its coenzyme NAD+ making
NADH.
• The remaining proton (H+)is
released into surrounding solution.
• The e- lose very little energy in this
transfer.
Figure 9.4 NAD+ as an electron shuttle
The three metabolic stages of
respiration:
1. Glycolysis
2. The Kreb’s cycle
3. The electron transport chain and
oxidative phosphorylation
Figure 9.6 An overview of cellular respiration (Layer 3)
GLYCOLYSIS: “splitting of sugar”
• Occurs in cytoplasm
• Series of 10 steps, each with its own
enzyme
• No oxygen needed (anaerobic)
• Needs 2 ATP to start process
• Makes 4 ATP by substrate-level
phopsphorylation (when an enzyme
removes a phosphate from a substrate to
make ATP)
• Transfers electrons and H+ to NAD+ to
make 2 NADH (to go to ETC)
Figure 9.7 Substrate-level phosphorylation
• By the end, one glucose molecule
will been broken in half to form two
3-carbon molecules of pyruvate.
• Only if oxygen is present, puruvate
moves into the Kreb’s cycle (Citric
Acid Cycle) to continue aerobic
respiration.
Figure 9.8 The energy input and output of glycolysis
Figure 9.9 A closer look at glycolysis: energy investment phase
Figure 9.9 A closer look at glycolysis: energy payoff phase
Pyruvate converts to acetyl CoA
• Pyruvate enters mitochondria
• Pyruvate loses CO2, and the
resulting 2-carbon compound is
oxidized making acetate.
• The e- and H+ are transferred to
NAD+ to make NADH (to go to ETC)
• Coenzyme A (a vitamin B derivative)
attaches to acetate making acetyl
CoA
Figure 9.10 Conversion of pyruvate to acetyl CoA, the junction between glycolysis
and the Krebs cycle
THE KREB’S CYCLE
• Acetyl CoA combines with a 4carbon molecule
• This molecule is oxidized over a
series of steps that are cyclic
• e- and H+ are transferred to NAD+
and FAD+ to make 3 NADH and 1
FADH2 (flavin adenine dinucleotide).
• 2 molecules of CO2 are given off
• 1 ATP is made by substrate-level
phosphorylation
• Only 2 carbons can go through the
cycle at one time so the cycle must
“turn” twice to oxidize both
pyruvates.
• CO2 diffuses out of cell, into blood,
and is exhaled.
• NADH and FADH2 take their
electrons to the electron transport
chain (ETC)
Figure 9.12 A summary of the Krebs cycle
Figure 9.11 A closer look at the Krebs cycle
ELECTRON TRANSPORT CHAIN
• Made up of a chain of molecules
embedded in the inner membrane of
mitochondria
• Mostly proteins with prosthetic
groups that can easily donate and
accept e- (redox) – many are
cytochromes with heme groups (Fe)
• NADH transfers e- to first
molecule and FADH2 transfers e- to
a lower molecule.
• e- move down the chain via redox
reactions
• They move down the ETC because
oxygen is electronegative and pulls
the e- along
• Oxygen captures the e- at bottom
and along with 2 H+ (from solution)
forming water
• The energy released by falling ecauses H+ to be pumped out into
intermembrane space.
• H+ move back into mitochondria by
diffusion (a proton-motive force) only
through a protein called ATP synthase
(oxidative phosphorylation)
• These protons change ATP synthase’s
shape so that it acts as an active site for
Pi and ADP to make ATP.
• Each NADH eventually yields ~3 ATP.
• Each FADH2 eventually yields ~2 ATP.
Figure 9.13 Free-energy change during electron transport
Figure 9.14 ATP synthase, a molecular mill
Figure 9.15 Chemiosmosis couples the electron transport chain to ATP synthesis
SUMMARY OF AEROBIC
RESPIRATION
• Approximately 38 ATP’s made from
one glucose
• About 60% of energy from glucose
is “lost” as heat
• This heat helps to keep our warm
body temperature
Figure 9.16 Review: how each molecule of glucose yields many ATP molecules
during cellular respiration
ANAEROBIC RESPIRATION
(FERMENTATION)
• NO oxygen = anaerobic = no Kreb’s
– Alcohol fermentation (yeast)
• Pyruvate is converted to ethanol
– Lactic acid fermentation (humans)
• Pyruvate is converted to lactic acid
• 2 ATP and 2 NAD+ are made
• Makes NAD+ so glycolysis can continue –
otherwise NADH has no where to go
(without oxygen at bottom of ETC) and is
not converted back to NAD+.
Figure 9.17a Fermentation
Figure 9.x2 Fermentation
Figure 9.17b Fermentation
Figure 9.18 Pyruvate as a key juncture in catabolism
• Facultative anaerobes – organisms
that make ATP through
fermentation if no oxygen and
through respiration if oxygen is
present (ex. yeast and some
bacteria)
Evolutionary Significance of
Glycolysis
• No oxygen required (early earth had no
oxygen in atmosphere)
• No mitochondria required (prokaryotes
do not have)
• Most common metabolic pathway
Versatility of Respiration
• Proteins and lipids enter at
different locations than glucose
• Intermediates of respiration can be
used to make other necessities (like
amino acids)
• Intermediates and products of
respiration inhibit enzymes to slow
respiration down.
Figure 9.19 The catabolism of various food molecules
Figure 9.20 The control of cellular respiration
PHOTOSYNTHESIS
Chapter 10
BASIC VOCABULARY
Autotrophs – producers; make
their own “food”
Heterotrophs – consumers;
cannot make own food
LEAF STRUCTURE
Stomata (stoma) – microscopic pores that allow
water, carbon dioxide and oxygen to move into/out
of leaf
Chloroplasts – organelle that performs
photosynthesis
Found mainly in mesophyll – the tissue of the
interior leaf
Contain chlorophyll (green pigment)
Stroma – dense fluid in chloroplast
Thylakoid membrane – inner membrane of
chloroplast
Grana (granum) – stacks of thylakoid membrane
Figure 10.2 Focusing in on the location of photosynthesis in a plant
PHOTOSYNTHESIS SUMMARY
6CO2 + 6H20 + light energy
C6H12O6 + 6O2
Converting light energy into chemical
energy (using sunlight to make sugar)
Oxygen comes from water, not CO2
Two parts:
Light Reactions
The Calvin Cycle (Dark Reactions or
Light Independent)
Figure 10.3 Tracking atoms through photosynthesis
Figure 10.4 An overview of photosynthesis: cooperation of the light reactions and the
Calvin cycle
LIGHT
Photons – discrete packets of light
energy
Chlorophyll a – (blue-green)only
pigment that is directly used in light
reactions
Chlorophyll b – (yellow-green)
accessory pigment
Carotenoids - (yellow-orange)
Figure 10.6 Why leaves are green: interaction of light with chloroplasts
Figure 10.8 Evidence that chloroplast pigments participate in photosynthesis:
absorption and action spectra for photosynthesis in an alga
PHOTOEXCITAION
When photons hit chlorophyll
and other pigments, electrons
are excited to an orbital of
higher energy
In solution when the excited
electrons fall, they give off
energy (a photon) and
fluoresce
Figure 10.9 Location and structure of chlorophyll molecules in plants
LIGHT REACTIONS
Photosystems:
Made of proteins and other
molecules surrounding
chlorophyll a
Contain a primary electron
acceptor
Photosystem I – P700
Photosytem II – P680
Figure 10.11 How a photosystem harvests light
Require light to occur
Two pathways:
Noncyclic (predominant
route)
Cyclic
Noncyclic animation
Another animation
NONCYCLIC ELECTRON FLOW
Photosystem II absorbs light
Two electrons excited and captured
by primary electron acceptor
“Hole” in photosystem II is filled by
2 electrons that come from the
splitting of water
H2O
2H+ + ½ O2 + 2e-
Oxygen is released
Excited electrons pass from
primary electron acceptor down
an electron transport chain to
photosystem I (filling its “hole”)
ATP is made by
photophosphorylation as
electrons fall down ETC
Photons excite 2 electrons from
Photosystem I and are captured by
its primary electron acceptor
Electrons then move down another
ETC to ferredoxin (Fd)
Fd gives electrons to NADP+
(nicotinamide dinucleotide
phosphate) making NADPH
The enzyme that helps this transfer
of e- is called NADP+ reductase
Figure 10.12 How noncyclic electron flow during the light reactions generates ATP
and NADPH
Figure 10.13 A mechanical analogy for the light reactions
Figure 10.14 Cyclic electron flow
CYCLIC ELECTRON FLOW
Only Photosystem I is used
Fd passes electrons back to
Photosystem I via ETC
Some ATP made
No NADPH made
No oxygen released
Used when cell needs more
ATP than NADPH
ETC
MITOCHONDRIA CHLOROPLAST
Food (chemical
energy) to ATP
(chemical energy)
ATP synthase
Pumps H+ into
intermembrane
space
Light energy to
ATP (chemical
energy)
ATP synthase
Pumps H+ into
thylakoid space
Figure 10.15 Comparison of chemiosmosis in mitochondria and chloroplasts
Figure 10.17 The Calvin Cycle
CALVIN CYCLE
Also called Dark Reactions because
light is not needed; however products
from light reactions are needed.
Carbon Fixation – initial incorporation
of carbon into organic molecules
CO2 attaches to a 5-carbon sugar called
ribulose bisphosphate (RuBP)
The enzyme that catalyzes this is called
rubisco
Calvin cycle animation
Immediately splits into two 3-carbon molecules
called 3-phosphoglycerate
3-phosphoglycerate is phosphorylated by ATP
(from light reactions) making
1,3-bisphosphoglycerate
1,3-bisphosphoglycerate is reduced by taking
electrons from NADPH making glyceraldehyde
3-phosphate (G3P)
One G3P molecule leaves cycle to be used by
plant
The remaining G3P’s are converted into RUBP in
several steps and by getting phosphorylated by
ATP
Recall, G3P is the sugar formed
by splitting glucose in glycolysis
G3P can be made into glucose,
sucrose, cellulose etc. by plant
C3 PLANTS – have a problem
Examples : rice, wheat, and soy beans
Problem - produce less food when
stomata are closed during hot days
because low CO2 starves Calvin Cycle and
rubisco can accept O2 instead of CO2
High oxygen levels = O2 passed to RUBP
(not CO2) and Calvin cycle stops
When this oxygen made product splits, it
makes a molecule that is broken down by
releasing CO2
This process is called photorespiration.
Occurs during daylight (photo)
Uses O2 and makes CO2 (respiration)
NO ATP made (unlike respiration) and
NO food made
Early earth had low O2 when first plants
appeared so this would not have mattered
as much
Photorespiration drains away as much as
50% of carbon fixed by Calvin Cycle in many
plants.
C4 PLANTS – have a solution
Examples: sugarcane, corn and
grasses
Leaves contain bundle-sheath cells
and mesophyll cells
Bundle sheath surrounds veins of
leaf (location of Calvin cycle)
Mesophyll – between bundle and
surface
In mesophyll cells: CO2 fixed to
phosphoenolpyruvate (PEP)
PEP carboxylase is the enzyme that does this
PEP carboxylase has higher affinity for CO2
than rubisco so less danger of O2 interfering
The fixed CO2 is then taken to Calvin cycle (in
bundle-sheath) as part of a 4-carbon molecule
(malate)
Malate gives CO2 to Calvin cycle
Figure 10.18 C4 leaf anatomy and the C4 pathway
CAM PLANTS – have another solution
(crassulacean acid metabolism)
Examples: succulent plants
(pineapples and cacti etc.)
Open stomata at night and close
during day
At night CO2 is fixed into organic
acids in mesophyll and then taken
to Calvin cycle (also in mesophyll)
during day.
Figure 10.19 C4 and CAM photosynthesis compared
PHOTOSYNTHESIS FACTS
50% of organic material made is
used by plant in respiration
Organic molecules often leave
leaves as sucrose
Large amounts of cellulose are
made (for cell walls)
“And no process is more important
than photosynthesis to the welfare
of life on Earth.”
(Campbell and Reece, 2005)
Figure 10.20 A review of photosynthesis
Carbon Cycle
Carbon Cycle
Human Impact on C Cycle
Climate change = global warming
Increased levels of CO2 (and some
other gases) increase Greenhouse
effect (traps heat)
Increased Greenhouse Effect equals
warming earth
How does deforestation and burning
fossil fuels impact the carbon cycle?