Transcript Cell Respiration notes

Cell Respiration
Topic 3.7 Cell Respiration
Topic 8.1 Cell Respiration
Cell Respiration

Who does it?
– All living things (including plants!)

What is it?
– Carbohydrates and O2 are used to make ATP
(energy). CO2 and H20 are waste products.
– The opposite of photosynthesis.
– Involves three steps: glycolysis, kreb’s cycle, and
electron transport chain.

Where does it occur?
– The cytoplasm and the mitochondria of the cell
Mitochondria
Cell Respiration
C6H12O6 + 6O2 6CO2 + 6H20 + ATP
 Glucose+ oxygen carbon dioxide +
water + energy

Redox Reactions
Redox-reaction, or an oxidationreduction reaction, is the movement of
electrons from one molecule to another.
 Because an electron transfer requires both
a donor and acceptor, oxidation and
reduction always go together.
 Cellular respiration is an example of a
redox-reaction

– “fall” of electrons, with energy released in
small amounts that can be stored in ATP
Redox Reactions

Oxidation
– The loss of electrons from one substance
– Glucose loses electrons (in H atoms) and
becomes oxidized

Reduction
– The addition of elections to another substance
– O2 gains electrons (in H atoms) and becomes
reduced
Cell Respiration occurs in three
main stages
1.
Glycolysis
Occurs in the cytoplasm; glucose is broken down to
two pyruvate molecules; provides energy for ETC
2.
The citric acid cycle (Kreb’s cycle)
Takes place in the matrix of the mitochondria;
further breaks down pyruvate to carbon dioxide;
provides energy for ETC
3.
Oxidative phosphorylation (Electron Transport
Chain)
Takes place in the cristae of the mitochondria. Also
known as chemiosmosis; NADH and FADH2 made in
glycolysis and Kreb’s shuttle electrons and H+ to
make ATP.
Glycolysis
Means “splitting sugar”
Begins with a single molecule of glucose (6-C)
and concludes with two molecules of another
organic compound, called pyruvate (3-C).
 A net gain of 2 NADH molecules and 2 ATP
molecules
 ATP can be used by cell immediately; NADH
must pass down the ETC in mitochondria
 Substrate-level phophorylation occurs


– An enzyme transfers a phosphate group from a
substrate molecule directly to ADP, forming ATP
Glycolysis
9 Steps (Figure 6.7C)
Steps 1-3: A sequence of three chemical
reactions converts glucose to a molecule of
fructose using 2 ATP.
 Step 4: Fructose splits into two G3P molecules
 Step 5: G3P gets oxidized and NAD+ is reduced
to NADH
 Steps 6-9: specific enzymes make four
molecules of ATP by substrate-level
phosphorylation. Water gets produced as a byproduct


Glycolysis
2 ATP produced account only for 5% of
the energy that a cell can harvest from a
glucose molecule.
 2 NADH account for another 16%, but
there stored energy is not available for
use in the absence of O2.

Pyruvate chemical “grooming”
As pyruvate forms at the end of glycolysis,
it is transported from the cytoplasm into
the mitochondria
 Pyruvate does not enter the Kreb’s Cycle
as itself.
 It undergoes major chemical “grooming”

Pyruvate chemical “grooming”

A large, multienzyme complex catalyzes
three reactions:
1. A carbon atom is removed from pyruvate and
released in CO2
2. The two-carbon compound remaining is oxidized
while a molecule of NAD+ is reduced to NADH
3. A compound called coenzyme A, derived from a B
vitamin, joins with the two-carbon group to form a
molecule called acetyl coenzyme A:


Abbreviated acetyl CoA, is a high-energy fuel molecule for
the Kreb’s Cycle
For each molecule of glucose that enters glycolysis, two
molecules of acetyl CoA are produced and enter the Kreb’s
cycle.
Pyruvate chemical “grooming”

Overview:
Kreb’s Cycle
– Called Krebs in honor of Hans Krebs, German-British
researcher who worked out much of this cyclic phase
of cellular respiration in the 1930s.
– Only the two-carbon acetyl part of the acetyl CoA
molecule actually participates in the citric acid cycle.
– Coenzyme A helps the acetyl group enter the cycle and
then splits off and is recycled.
– Occurs in the matrix of the mitochondria
– Compared with glycolysis, Kreb’s Cycle pays big energy
dividends to the cell
– This makes 1 ATP, 4 NADH and 1 FADH2, per
acetyl coA (double that for each glucose molecule)
– Releases CO2 as waste
– is aerobic (requires oxygen)
Kreb’s Cycle

Details of the citric acid cycle: Figure 6.9B:
– Step 1
 Acetyl coA is stripped via enzymes: coA is recycle and the
remaining acetyl (2-C) is combined with oxaloacetate
already present in the mitochondria forming citrate (6-C)
– Step 2 and 3
 Redox reactions take place stripping hydrogen atoms from
organic intermediates producing NADH molecules and
dispose of 2-C that came from oxaloacetate, which are
released as CO2.
 Substrate-level phos. of ADP occurs to form ATP.
 A 4-C molecule called succinate forms.
– Step 4 and 5
 Oxaloacetate gets regenerated from maltate, and FAD and
NAD+ are reduced to FADH2 and NADH, respectively.
 Oxaloacetate is ready for another turn of the cycle by
accepting another acetyl group
Kreb’s Cycle
Electron Transport Chain

Involves oxidative phosphorylation
– A clear illustration of structure fitting function:
the spatial arrangement of electron carriers
built into a membrane makes it possible for
the mitochondrion to use the chemical energy
released by redox reactions to create an H+
gradient and then use the energy stored in
the gradient to drive ATP synthesis

Chemiosmosis also occurs
– The potential energy of the concentration
gradient is used to make ATP.
Electron Transport Chain
Built into the inner membrane of the
mitochondrion, or in the cristae folds,
providing space for thousands of copies of
the electron transport chain and many
ATP synthase complexes
 With all these ATP-making “machines,” a
mitochondrion can produce many ATP
molecules simultaneously.

Electron Transport Chain

Figure 6.10:
– Path of electron flow from the shuttle molecules
NADH and FADH2 to O2, the final electron
acceptor.
– Each oxygen atom (1/2 O2) accepts two
electrons from the chain and picks up two
hydrogen ions from the surrounding solution to
form H2O, one of the final products of cellular
respiration.
– Most of the carrier molecules reside in the three
main protein complexes, while two mobile
carriers transport electrons between the

Electron Transport Chain
Figure 6.10 (continued):
– All of the carriers bind and release electrons in
redox reactions, passing electrons down the
“energy staircase.”
– Protein complexes shown in the diagram use
the energy released from the electron transfers
to actively transport H+ across the membrane,
from where they are less concentrated to where
they are more concentrated.
– Hydrogen ions are transported from the matrix
of the mitochondrion (its innermost
compartment) into the mitochondrion’s
intermembrane space.
Electron Transport Chain

Figure 6.10 (continued):
– The resulting H+ gradient stores potential energy,
similar to a dam storing energy by holding back
elevated water.
 Dams can be harnessed to generate electricity when the water
is allowed to rush downhill, turning giant wheels called
turbines.
 Similarly, ATP synthases built into the inner mitochondrial
membrane act like minature turbines. H+ can only cross
through ATP synthases bc they are not permeable to the
membrane.
 Hydrogen ions rush back “downhill” through an ATP synthase,
spinning a component of the complex, just as water turns the
turbine in a dam.
 Rotation activates catalytic sites in the synthase that attach
phosphate groups to ADP molecules to generate ATP.
Electron Transport Chain

Why is this process called oxidative
phosphorylation?
– The energy derived from the oxidationreduction reactions of the electron transport
chain that transfer electrons from organic
molecules to oxygen is used to phosphorylate
ADP.
– By chemosmosis, the exergonic reactions of
electron transport produce an H+ gradient
that drives the endergonic synthesis of ATP.
Cell Respiration Summary

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TOTAL= 38 ATP (theoretical)
Glycolysis
–
–
–
–
–
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Kreb’s Cycle (including pyruvate grooming)
–
–
–
–

Occurs in cytoplasm
2 ATP
2 NADH
2 H20 get released
2 pyruvate
2
8
2
6
ATP
NADH
FADH2
CO2 get released
Electron Transport Chain
– H20 gets released
– 10 NADH get converted to 3ATP= 30 ATP
– 2 FADH2 get converted to 2 ATP= 4 ATP
Poisons
Some poisons block the electron transport chain.
 Rotenone

– Often used to kill pest insects and fish.
– binds tightly with the electron carrier molecules in the
first protein complex, preventing electrons from
passing to the next carrier molecule.
– Literally starves an organism’s cells of energy bc it
blocks the ETC near its start thus preventing ATP
synthesis.
Poisons

Cyanide and Carbon Monoxide
– Bind with an electron carrier in the third
protein complex
– Block the passage of electrons to oxygen
– Similar to turning off a faucet; electrons cease
to flow through the “pipe”
– Result is the same as that or rotenone: no H+
gradient is generate and no ATP is made.
***refer to page 99 in your book for other
examples***
Fermentation-Anaerobic
Respiration
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Glycolysis is the metabolic pathway that generates ATP
during fermentation.
No O2 is required; it generates a net gain of 2 ATP while
oxidizing glucose to two molecules of pyruvate and
reducing NAD+ to NADH.
Significantly less ATP is generated, but it is enough to
keep your muscles contracting for a short while when
the need for ATP outpaces the delivery of O2 via the
blood stream
Many microorganisms supply all their energy needs with
the 2 ATP yield of glycolysis.
Fermentation-Anaerobic
Respiration
Strict Anaerobes
require anaerobic conditions and are
poisoned by oxygen
Facultative Anaerobes
can make ATP either by fermentation or
by oxidative phosphorylation, depending
on whether O2 is available.
Fermentation-Anaerobic
Respiration
Fermentation provides an anaerobic step
that recycles NADH back to NAD+;
essential to harvest food energy by
glycolysis.
 Two types of fermentation:

– Lactic acid
– Alcohol

Fermentation-Anaerobic
Respiration
Lactic acid fermentation
– Figure 6.13A
– NADH is oxidized to NAD+ as pyruvate is
reduced to lactate (the ionized form of lactic
acid)
– Lactate builds up in muscle cells during
strenuous exercise is carried in the blood to
the liver, where it is converted back to
pyruvate
– Dairy industry use this to with bacteria to
make cheese and yogurt
Fermentation-Anaerobic
Respiration

Alcohol fermentation
–
–
–
–
Figure 6.13A
Used in brewing, winemaking, and baking
Used by yeasts and bacteria (facultative anaerobes)
Recycle their NADH to NAD+ while converting
pyruvate to CO2 and ethanol (ethyl alcohol).
– CO2 provides bubbles in beer and champagne, and
bread dough to rise
– Ethanol is toxic to organisms that produce it; must
release it to their surroundings
Fuels for cell respiration
Free glucose molecules are not common in
our diet
 We obtain most of our calories as fats,
proteins, sucrose, and other disaccharide
sugars, and starch

Fuels for cell respiration

Carbohydrates (polysaccharides and
starch)
– Figure 6.14
– Enzymes in our digestive tract hydrolyze
starch to glucose; glycogen can be hydrolyzed
to glucose to serve as fuel between meals.
Fuels for cell respiration

Proteins:
– First must be digested to their constituent
amino acids
– Typically, cell will use most of the amino acids
to make its own proteins, but enzymes will
convert excess a.a. to intermediates of
glycolysis or the Kreb’s cycle, and their energy
is harvested by cell respiration.
– Amino groups unused are disposed in urine.
Fuels for cell respiration

Fats:
– Excellent cellular fuel bc they contain many hydrogen
atoms and thus many energy-rich electrons
– Cell first hydrolyzes fats to glycerol and fatty acids
– It converts glycerol to G3P; fatty acids are broken into
2-carbon fragments that enter the Kreb’s as acetyl coA
– A gram of fat yields more than twice as much ATP as a
gram of starch.
– Because so many calories are in each gram of fat, a
person must expend a large amount of energy to burn
fat stored in the body.
Fuels for cell respiration
Food is also used as the raw materials a
cell uses for biosyntheis, to make its own
molecules for repair and growth…not just
for ATP!
 To make cells, tissues, and organisms:

– Amino acids proteins
– Fatty acids and glycerols fats
– Sugars carbohydrates