Download aerobic respiration - juan-roldan

How cells Make ATP:
Energy Releasing
Pathways
Metabolism
• Metabolism has two complementary components:
• catabolism, which releases energy by splitting complex
molecules into smaller components
• anabolism, the synthesis of complex molecules from
simpler building blocks
• Most anabolic reactions are endergonic and require ATP or
some other energy source to drive them
Cellular Respiration
• Every organism extracts energy from food molecules that it
manufactures by photosynthesis or obtains from the
environment
• Exergonic metabolic pathways (cellular respiration and
fermentation) release free energy that is captured by the cell
• cellular respiration
• Catabolic processes that convert energy in the chemical
bonds of nutrients to chemical energy stored in ATP
• May be either aerobic or anaerobic
Aerobic Respiration
• Cells use aerobic respiration to obtain energy from glucose
C6H12O6 + 6 O2 + 6 H2O →
6 CO2 + 12 H2O + energy (chemical bonds of ATP)
• aerobic respiration
• Cellular respiration that requires molecular oxygen (O2)
• Nutrients are catabolized to carbon dioxide and water
Aerobic Respiration (cont.)
• Aerobic respiration is a redox reaction in which glucose
becomes oxidized and oxygen becomes reduced
• Aerobic respiration transfers electrons (associated with
hydrogen atoms in glucose) to oxygen in a series of steps that
control the amount of energy released
• Free energy of the electrons is coupled to ATP synthesis
• Aerobic respiration is an exergonic redox process in which
glucose becomes oxidized, oxygen becomes reduced, and
energy is captured to make ATP
The Four Stages of Aerobic Respiration
Summary of Aerobic Respiration
Table 8-1, p. 174
Reactions Involved
in Aerobic Respiration
• dehydrogenations
• Reactions in which two hydrogen atoms are removed from
the substrate and transferred to NAD+ or FAD
•
decarboxylations
• Reactions in which part of a carboxyl group (COOH) is
removed from the substrate as a molecule of CO2
• Other reactions
• Reactions in which molecules are rearranged so they can
undergo further dehydrogenations or decarboxylations
Introduction to Glycolysis
• Glycolysis
• Takes place in the cytosol
• Metabolizes the 6-carbon sugar glucose into two 3-carbon
molecules of pyruvate
• Does not require oxygen; proceeds under aerobic or
anaerobic conditions
• Net yield: 2 ATP molecules and 2 NADH molecules
• Two major phases:
• Endergonic reactions that require ATP (investment phase)
• Exergonic reactions that yield ATP and NADH (payoff
phase)
GLYCOLYSIS
Glucose
Energy investment
phase and splitting of
glucose Two ATPs
invested per glucose
3
steps
Fructose-1,6-bisphosphate
Glyceraldehyde
phosphate (G3P)
Glyceraldehyde phosphate
(G3P)
(G3P)
(G3P)
5
step
s
Energy capture
phase Four ATPs
and two NADH
produced per
glucose
Pyruvate Pyruvate
Net yield per glucose: Two
ATPs and two NADH
Fig. 8-3, p. 176
First Phase of Glycolysis
• Phosphate groups are transferred from ATP to glucose In two
separate phosphorylation reactions
• The phosphorylated sugar (fructose-1,6-bisphosphate) is
broken enzymatically into two three-carbon molecules,
yielding 2 glyceraldehyde-3-phosphate (G3P)
glucose + 2 ATP → 2 G3P + 2 ADP
Second Phase of Glycolysis
• G3P is converted to pyruvate
• G3P is oxidized by removal of 2 electrons (as hydrogen
atoms), which combine with NAD+
NAD+ + 2 H → NADH + H+
• ATP is formed by substrate-level phosphorylation
2 G3P + 2 NAD+ + 4 ADP → 2 pyruvate + 2 NADH + 4 ATP
Energy investment phase and splitting of
glucose Two ATPs invested per glucose
Glucose
Hexokinase
1
Glucose-6-phosphate
Phosphoglucoisomerase
Fig. 8-4a (1), p. 178
2
Fructose-6-phosphate
Phosphofructokinase
3
Fructose-1,6-bisphosphate
Aldolase
4
Isomerase
5
Dihydroxyacetone
phosphate
Glyceraldehyde3-phosphate (G3P)
Fig. 8-4a (2), p. 178
Two glyceraldehyde-3phosphate (G3P) from bottom of
previous page
Energy capture phase Four ATPs
and two NADH produced per
glucose
Glyceraldehyde-3-phosphate dehydrogenase
6
Two 1,3bisphosphoglycerate
Phosphoglycerokinase
7
Two 3phosphoglycerate
Phosphoglyceromutase
Fig. 8-4b (1), p. 179
8
Two 2-phosphoglycerate
Enolase
9
Two phosphoenolpyruvate
Pyruvate kinase
10
Two pyruvate
Fig. 8-4b (2), p. 179
Pyruvate is Converted to Acetyl CoA
• Pyruvate undergoes oxidative decarboxylation
• A carboxyl group is removed as CO2, which diffuses out of
the cell
• Occurs in mitochondria of eukaryotes
• The two-carbon fragment is oxidized (NAD+ accepts the
electrons), and is attached to coenzyme A, yielding acetyl
coenzyme A (acetyl CoA)
2 pyruvate + 2 NAD+ + 2 CoA →
2 acetyl CoA + 2 NADH + 2 CO2
Formation of Acetyl CoA
Overview of the Citric Acid Cycle
• The citric acid cycle is also known as the Krebs cycle
• Takes place in the matrix of the mitochondria
• A specific enzyme catalyzes each of the eight steps
• Begins when acetyl CoA transfers its two-carbon acetyl
group to the four-carbon acceptor compound
oxaloacetate, forming citrate, a six-carbon compound:
oxaloacetate + acetyl CoA → citrate + CoA
The Citric Acid Cycle (cont.)
• Citrate goes through a series of chemical transformations,
losing two carboxyl group as CO2
• One ATP is formed (per acetyl group) by substrate-level
phosphorylation – most of the oxidative energy (in electrons)
is transferred to NAD+, forming 3 NADH
• Electrons are also transferred to FAD, forming FADH2
Overview of the Citric Acid Cycle
Introduction to the
Electron Transport Chain
• All electrons removed from a glucose during glycolysis, acetyl
CoA formation, and the citric acid cycle are transferred as part
of hydrogen atoms to NADH and FADH2
• NADH and FADH2 enter the electron transport chain (ETC),
where electrons move from one acceptor to another
• Some electron energy is used to drive synthesis of ATP by
oxidative phosphorylation
Transfer of Electrons
• In eukaryotes, the ETC is a series of electron carriers
embedded in the inner mitochondrial membrane
• Electrons pass down the ETC in a series of redox reactions,
losing some of their energy at each step along the chain
Transfer of Electrons (cont.)
• Cytochrome c reduces O2, forming H2O
• Oxygen is the final electron acceptor in the ETC
• Lack of oxygen blocks the entire ETC – no additional ATP is
produced by oxidative phosphorylation
• Some poisons also inhibit normal activity of cytochromes
• Example: Cyanide binds to iron in cytochrome, blocking
ATP production
Overview of the
Electron Transport Chain
The Chemiosmotic Model
of ATP Synthesis
• 1961: Peter Mitchell proposed that electron transport and ATP
synthesis are coupled by a proton gradient across the inner
mitochondrial membrane in eukaryotes (chemiosmosis)
• Mitchell’s experiments used a bacterial model:
• Bacterial cells placed in an environment with a high
hydrogen ion (proton) concentration synthesized ATP
even if electron transport was not taking place
KEY EXPERIMENT:
Evidence for Chemiosmosis
Bacterial cytoplasm
(low acid)
Synthesized
Plasma
membrane
Acidic environment
Fig. 8-9, p. 183
The Proton Gradient
• As electrons pass down the ETC, energy is used to move
protons (H+) across the inner mitochondrial membrane into
the intermembrane space
• The intermembrane space has a higher concentration of
protons; the mitochondrial matrix has a lower concentration
• The resulting proton gradient is a form of potential energy that
provides energy for ATP synthesis
The
Proton
Gradient
Outer
mitochondrial
membrane
Cytosol
Inner mitochondrial
membrane
Intermembrane
space—low pH
Matrix—higher pH
Fig. 8-10, p. 184
Synthesis of ATP
• Protons diffuse from the intermembrane space (high
concentration) to the matrix (low concentration) through the
enzyme complex ATP synthase
• A central structure of ATP synthase rotates, catalyzing the
phosphorylation of ADP to form ATP
• Chemiosmosis allows exergonic redox reactions to drive the
endergonic reaction in which ATP is produced by oxidative
phosphorylation
Cytosol
Overview of the ETC
Outer mitochondrial
membrane
Intermembrane
space
Complex V:
ATP synthase
Inner
mitochondrial
membrane
Complex
III
Complex I
Complex
IV
Complex
II
Matrix of
mitochondrion
Fig. 8-11a, p. 185
ATP Production
• Aerobic respiration of one glucose molecule:
1. Glycolysis: glucose + 2 ATP → 2 pyruvates + 2 NADH + 4
ATPs (net profit of 2 ATPs)
2. Pyruvate conversion: 2 pyruvates → 2 acetyl CoA + 2
CO2 + 2 NADH
3. Citric acid cycle: 2 acetyl CoA → 4 CO2 + 6 NADH + 2
FADH2 + 2 ATPs
• Total = 4 ATP + 10 NADH + 2 FADH2
ATP Production (cont.)
• Oxidation of NADH in the electron transport chain yields
up to 3 ATPs per molecule (10 NADH X 3 = 30 ATPs)
• Oxidation of FADH2 yields 2 ATPs per molecule
(2 FADH2 X 2 = 4 ATPs)
ATP Production (cont.)
• Summing all the ATPs:
• 2 from glycolysis
• 2 from the citric acid cycle
• 32 to 34 from electron transport and
chemiosmosis
• Complete aerobic metabolism of one molecule of
glucose yields a maximum of 36 to 38 ATPs
Energy Yield from Oxidation of Glucose by
Aerobic Respiration
Cells Regulate Aerobic Respiration
• Glycolysis is partly controlled by feedback regulation of the
enzyme phosphofructokinase
• Phosphofructokinase has two allosteric sites:
• An inhibitor site that binds ATP (at very high ATP levels)
• An activator site to which AMP binds (when ATP is low)
KEY CONCEPTS 8.2
• Aerobic respiration consists of four stages: glycolysis,
formation of acetyl coenzyme A, the citric acid cycle, and the
electron transport chain and chemiosmosis
Animation: Recreating the reactions of
glycolysis
Nutrients Other Than Glucose
• Nutrients other than glucose are transformed into metabolic
intermediates that enter glycolysis or the citric acid cycle
• Amino acids:
• The amino group (NH2) is removed (deamination)
• The carbon chain is used in aerobic respiration
• Lipids:
• Glycerol is converted to a compound that enters glycolysis
• Fatty acids are converted by β-oxidation to acetyl CoA,
which enters the citric acid cycle
PROTEINS
Amino
acids
CARBOHYDRATES
FATS
Fatty
Glycolysis Glycerol
acids
Glucose
G3P
Pyruvate
Energy from
Proteins,
Carbohydrates,
and Fats
CO 2
Acetyl
coenzyme
A
Citric
acid
cycle
Electron
transport
and
chemiosmosis
End
products:
NH3
H2O
CO2
Fig. 8-13, p. 187
Anaerobic Respiration
• anaerobic respiration
• Does not use oxygen as the final electron acceptor
• Used by prokaryotes in anaerobic environments, such as
waterlogged soil, stagnant ponds, and animal intestines
• Electrons from glucose pass from NADH down an ETC
coupled to ATP synthesis by chemiosmosis
• End products of this of anaerobic respiration are CO2, one or
more reduced inorganic substances, and ATP
Fermentation
• fermentation
• An anaerobic pathway that does not involve an ETC
• Only two ATPs are formed per glucose (by substrate-level
phosphorylation during glycolysis)
• NADH molecules transfer H atoms to organic molecules,
regenerating NAD+ needed for glycolysis
• Fermentation is highly inefficient, because fuel is only partially
oxidized
Alcohol Fermentation
• Yeasts are facultative anaerobes that carry out aerobic
respiration when oxygen is available but switch to alcohol
fermentation when deprived of oxygen
• alcohol fermentation
• Enzymes decarboxylate pyruvate, forming acetaldehyde
• NADH produced during glycolysis transfers hydrogen
atoms to acetaldehyde, reducing it to ethyl alcohol
Lactate Fermentation
• Certain fungi bacteria perform lactate fermentation –
vertebrate muscle cells also produce lactate when oxygen is
depleted during exercise
• lactate (lactic acid) fermentation
• NADH produced during glycolysis transfers hydrogen
atoms to pyruvate, reducing it to lactate
Fermentation
Aerobic Respiration, Anaerobic Respiration,
and Fermentation
Table 8-2, p. 188