Download Chapter 25: Metabolism and Energetics

Chapter 25:
Metabolism
Introduction to Cellular Metabolism
Energy production
begins in cytosol
Formation of
Organic
Molecules
Energy is
captured to
produce ATP
Figure 25–1
Energy
• Large food molecules contain energy
– Energy in the form of chemical bonds
– Work required to liberate energy
• ATP: breaking the P bond provides
energy for cells
ATP ADP + P + free energy from food
Food Energy + ADP + P  ATP
(This ATP permits anabolism)
Energy
• Cells break down organic molecules to
obtain energy:
– used to generate ATP
• Most energy production takes place in
mitochondria
Essential Materials
• Oxygen
– absorbed at the lungs
• Water
• Nutrients: absorbed at digestive tract
– vitamins
– mineral ions
– organic substrates
Materials Transport
• Cardiovascular system:
– carries materials through body
• Materials diffuse:
– from bloodstream into cells
Metabolism
• Refers to all chemical reactions in an
organism
• Includes all chemical reactions within cells
• Provides energy to maintain homeostasis
and perform essential functions
Essential Metabolic Functions
• Metabolic turnover:
– periodic replacement of cell’s organic
components
• Growth and cell division
• Special processes:
– secretion
– contraction
– propagation of action potentials
The Nutrient Pool
• Contains all organic building blocks cell needs:
– to provide energy
– to create new cellular components
• Is source of substrates for catabolism and
anabolism
– Catabolism:
• Is the breakdown of organic substrates
• Releases energy used to synthesize high-energy
compounds (e.g., ATP)
– Anabolism:
• Is the synthesis of new organic molecules via forming
new chemical bonds
Nutrient Use in
Cellular Metabolism
Figure 25–2 (Navigator)
Organic Compounds
• Glycogen (carbohydrates) short carbon chains
– most abundant storage carbohydrate
– a branched chain of glucose molecules
• Triglycerides fatty acids and glycerol
– most abundant storage lipids
– primarily of fatty acids
• Proteins amino acids
– most abundant organic components in body
– perform many vital cellular functions
KEY CONCEPT
• There is an energy cost to staying alive
• Even at rest cells must spend ATP to:
– perform routine maintenance
– remove and replace structures and
components
• Cells spend additional energy for vital
functions:
– growth
– secretion
– contraction
Oxidation-Reduction Reactions (Redox Rxns)
• Oxidation = the removal of electrons (Or
addition of oxygen)
• Reduction = the addition of electrons
• These reactions are always coupled
– One molecule must be oxidized while another is
reduced
A-e’ + B  A + B-e’
• Oxidized molecule (A): Loses energy
• Reduced molecule (B): Gains energy
OIL
RIG
Oxidation-Reduction Reactions (Redox Rxns)
• Cells perform dehydrogenation reactions
– Hydrogen (1 proton + 1 electron) is exchanged
instead of a free electron, but this is still a redox
reaction
• Catabolism of large molecules result in
reduced carrier compounds
– e.i. ADP  ATP; NAD  NADH; FAD  FADH2
– These reduced compounds are later oxidized to
generate ATP
ATP Production
•
•
Requires the addition of a phosphate to ADP
Two Methods for ATP Productions
1. Substrate Level Phosphorylation
-
High energy phosphate is transferred directly from a
substrate to ADP forming ATP
2. Oxidative Phosphorylation
-
Electrons are transferred from an organic compound to
a cofactor carrier molecule (e.g. NAD+)
Electrons are passed through other carriers (the
electron transport chain) to a final acceptor (oxygen)
The passing of electrons releases energy that is
harvested to add a phosphate to ADP
- Process is called chemiosmosis.
What are the basic steps in
glycolysis, the TCA cycle,
and the electron transport
system?
Carbohydrate Catabolism
(Metabolism)
• Carbohydrates are the primary source of
cellular energy for most organisms
• Glucose is the most commonly used
carbohydrate and will always be used first
• Generates ATP and other high-energy
compounds by breaking down
carbohydrates:
glucose + oxygen  carbon dioxide + water
Carbohydrate Catabolism
(Metabolism)
Two methods for ATP productions via
catabolism of glucose
1. Cellular Respiration
-
-
Requires oxygen to serve as the final electron
acceptor in a series of redox reactions
- Generate ATP by oxidative phosphorylation
Most efficient method of ATP production
- 1 glucose generates 36 ATP
Involves reaction performed inside the mitochondria
Carbohydrate Catabolism
(Metabolism)
Two methods for ATP productions via
catabolism of glucose
2. Fermentation
-
-
Requires an organic molecule to serve as the final
electron acceptor
Can be done in the absence of oxygen
ATP is synthesized using substrate level
phosphorylation
- Less efficient, 1 glucose generates 2 ATP
In humans, results in lactic acid
Anaerobic Vs. Aerobic Respiration
Glycolysis
• Anaerobic reactions: Fermentation
– Do not require oxygen
– Example: Glycolysis
• Breaks down glucose in cytosol:
– into smaller molecules used by mitochondria
• Aerobic reactions: Cellular Respiration
– Occur in mitochondria:
• consume oxygen
• produce ATP
Aerobic Respiration of Glucose
C6H12O6 + 6O2  6 CO2 + 6H2O
Three Stages
1. Glycolysis
-
Oxidation of glucose to pyruvic acid
Some ATP and NADH produced
2. Citric Acid Cycle
-
Oxidation of acetyl to carbon dioxide
Some ATP, NADH and FADH produced
3. Electron Transport Chain
-
NADH and FADH2 are oxidized providing electrons for redox
reactions
-
-
coenzymes that function to transport electrons in the form of
hydrogen
Reduce oxygen to generate ATP
Majority of ATP is produced at this step
Nutrient Use in
Cellular Metabolism
Figure 25–2 (Navigator)
Glycolysis (Anaerobic Process)
• Does not require oxygen
• Occurs in cytoplasm
• 10 step metabolic pathway:
– Catabolizes and oxidizes one 6-carbon glucose molecule
into two 3-carbon pyruvic acid molecules
– Generates 2 ATP by substrate level phosphorylation
• Many cells can survive on glycolysis alone
– Not very efficient
– Generates lactic acid as a waste product
• Needs to be removed and processed to prevent
– Drastic alterations in pH
– Loss of homeostasis
Glycolysis Factors
•
•
•
•
•
Glucose molecules
Cytoplasmic enzymes
ATP and ADP
Inorganic phosphates
NAD (coenzyme)
Two Stages in Glycolysis
1. Preparatory Stage:
-
Enzyme phosphorylates last (sixth) carbon
atom of glucose molecule:
1. Glucose-6-phosphate is formed using 1 ATP
molecule
- traps glucose molecule within cell
2. Fructose 1,6-bisphosphate is formed using 1
ATP
– Therefore, two ATP molecules are used to
phosphorylate one 6-carbon glucose and
catabolize it into two 3-carbon molecules
Two Stages in Glycolysis
2. Energy Conservation Stage:
– the two 3-carbon molecules are oxidized to
generate two 3-carbon pyruvic acid
molecules
– Two NAD+ molecules are reduced to two
NADH molecules
– 4 ATP molecules are produced by substrate
level phosphorylation
– net gain 2 ATP per 1 glucose
Summary of Glycolysis
1 glucose + 2 NAD+ + 2 ADP + 2P 
2 pyruvic acid + 2 NADH + 2H+ + 2 ATP
Aerobic Reactions
• If oxygen supplies are adequate:
– mitochondria absorb and break down pyruvic
acid molecules
Mitochondrial Membranes
• Outer membrane:
– contains large-diameter pores
– permeable to ions and small organic
molecules (pyruvic acid)
• Inner membrane:
– contains carrier protein
– moves pyruvic acid into mitochondrial matrix
• Intermembrane space:
– separates outer and inner membranes
Mitochondrial ATP Production
• H atoms of pyruvic acid:
– are removed by coenzymes
– are primary source of energy gain
• C and O atoms:
– are removed and released as CO2
– process of decarboxylation
The TCA Cycle
PLAY
TCA Cycle
Figure 25–4a (Navigator)
Decarboxylation
• Preparation of the Citric Acid Cycle
• First step in aerobic process of glucose
metabolism (oxygen is necessary)
• 3 carbon pyruvic acid is decarboxylated
into carbon dioxide and a 2 carbon acetyl
• Acetyl is attached to coenzyme A (serves
as a carrier) and one NAD+ is reduced to
NADH
• Occurs in the matrix of the mitochondria
Summary of Decarboxylation
2 pyruvic acid + 2 NAD+ + 2 CoA 
2 Acetyl CoA + 2 CO2 + 2 NADH
Citric Acid Cycle
• a.k.a. Kreb’s Cycle or Tricarboxylic Acid Cycle
• Aerobic metabolism of glucose involves
– 8 enzymatic reactions occurring in the mitochondrial matrix
• Function to reduce the coenzyme NAD+ and FAD
• 2-carbon acetyl + 4-carbon and 2 CO2 molecules
• At the same time oxaloacetic acid 
6-carbon citric acid
• Oxidation and decarboxylation reactions:
– Catabolize the 6-carbon citric acid back into a 4-carbon
oxaloacetic acid
– 3 NAD+ and 1 FAD are reduced into 3 NADH and 1 FADH2
– 1 ATP is produced by substrate level phosphorylation
The TCA Cycle
Figure 25–4b
Citric Acid Cycle
• Remember:
– 1 glucose  2 pyruvic acid 
2 acetyl so this cycle will run twice
2Acetyl Co A + 6NAD+ + 2FAD + 2ADP +
2P + 4H20 
2CoA + 4CO2 + 6NADH + 4H+ +
2FADH2 + 2ATP
Oxidative Phosphorylation
Figure 25–5a (Navigator)
Oxidative Phosphorylation
Occurs on a membrane, the mitochondrial cristae,
to generate most of the ATP produced from glucose
Figure 25–5b
Oxidative Phosphorylation
• Is the most important mechanism for generation
of ATP within the mitochondria
– Produces more than 90% of ATP used by body
• Requires oxygen, electrons, and coenzymes:
– rate of ATP generation is limited by oxygen or
electrons
• Cells obtain oxygen by diffusion from
extracellular fluid
• Results in 2 H2 + O2 2 H2O
The Electron Transport
System (ETS)
• Key reaction in oxidative phosphorylation
• Is in inner mitochondrial membrane
• Coenzymes from the previous reactions
pass electrons (which transfer energy) to a
series of electron carrier molecules
– Molecules carry out redox reactions resulting
in the chemiosmotic generation of ATP
The Electron Transport
System (ETS)
•
Three classes of carrier molecules
1. FMN (flavin mononucleotide): protein +
flavin coenzyme
2. Coenzyme Q: nonprotein
3. Cytochromes: protein + an iron group
-
Most common
Events of the Electron Transport Chain
1.
NAD+ and FAD collected energy in the form of hydrogens
(electrons) from organic molecules during Glycolysis,
Decarboxylation, and the Citric Acid Cycle becoming that
reduced forms NADH and FADH2
NAD+ + FAD  NADH + FADH2
2.
NADH and FADH2 are oxidized and pass hydrogens
(electrons and protons) to the electron transport chain
consisting of flavoproteins, cytochromes, and coenzyme Q.
NADH + FADH2  NAD+ + FAD
As electrons are passed along the chain, protons are
pushed out through the membrane. This sets up a
concentration gradient with protons (+ charge) on the
outside and electrons (- charge) on the inside
Events of the Electron Transport Chain
3.
4.
5.
At the end of the chain the electrons are accepted by oxygen
creating an anion (O-) inside, which has a strong affinity for the
cations (H+) outside.
Chemiosmosis generates ATP:
- H+ from the outside moves toward O- on the inside through
special membrane channels that are coupled to ATP
synthase
- High-energy diffusion of H+ drives the reaction
ADP + P  ATP.
a. Energy from 1 NADH from glycolysis generate 2 ATP
b. Energy from 1 NADH from decarboxylation and the
citric acid cycle generate 3 ATP
c. Energy from 1 FADH2 generate 2 ATP for a total of
32 ATP
H+ combines with O- inside the mitochondria creating water
(H2O)
Oxidative Phosphorylation
Occurs on a membrane, the mitochondrial cristae,
to generate most of the ATP produced from glucose
Figure 25–5b
Summary of Electron Transport
2 NADH from glycolysis + 2 NADH from
decarboxylation + 6 NADH from Citric Acid
Cycle + 2 FADH2 from Citric Acid Cycle + 6 O2 +
32 ATP + 32 P 
12 H2O + 32 ATP + 10 NAD+ + 2 FAD
Final Summary of Aerobic
Respiration
C6H12O6 + 6 O2 + 36 ADP + 36 P 
6 CO2 + 6 H2O + 36 ATP
36 ATP:
2 from Glycolysis in cytoplasm
2 from Citric Acid Cycle by substrate level
phosphorylation in matrix of mitochondria
32 from Electron Transport by oxidative
phosphorylation in the cristae of the
mitochondria
Energy Yield of Aerobic Metabolism
Figure 25–6
Lipid Catabolism: Beta–Oxidation
Figure 25–8 (Navigator)
Lipolysis – Lipid Catabolism
• Hydrolyzes triglycerides (fat storage) 
glycerol and three fatty acids
• Glycerol:
– Glycerol  pyruvic acids in the cytoplasm
– Pyruvic acid catabolized through TCA in mitochondria
• Fatty Acids:
– Fatty acids are catabolized by beta-oxidation into acetyl-CoA
– In mitochondria to enter the TCA as two-carbon fragments
– For each two-carbon fragment of fatty acid produced by
beta-oxidation, the cell can generate 17 molecules of ATP
• This is 1.5 times the energy production as with
glucose
• Generates more energy but requires more oxygen
– Occurs much more slowly than equal carbohydrate
metabolism
Amino Acid Catabolism
Figure 25–10 (Navigator)
Protein and Amino Acid Catabolism
1. Protein  amino acids
2. Amino group (-NH2) is removed from amino acid
in process called deamination
– Requires vitamin B6
3. Amino group is removed with conjunction with a
hydrogen creating ammonia (NH3)
– Toxic
4. Liver converts the NH3  urea
– Harmless and excreted by the kidney
5. Remaining amino acid carbon chains are used at
various stages in the Citric Acid Cycle to generate
ATP
– Amount of ATP produced varies
Protein and Amino Acid Catabolism
• Not a Practical Source of Quick Energy
• Typically only used in starvation situations
• Harder to break apart than carbohydrates or
lipids
• Proteins are structural and functional parts of
every cell
– Thus tend to only be used when no other energy
source is available
• Amino acids are simply recycled by hydrolysis of
peptide bonds in one protein, to be reassembled
by dehydration synthesis into the next.
Nucleic Acid Catabolism
• DNA is never catabolized for energy
• RNA can be broken down into:
– Simple sugars
– Nitrogenous bases
• Sugars:
– Metabolized in glycolysis but only the pyrimidine bases
(uracil and cytosine) can be processed in the TCA cycle
• Purines (adenine and guanine) are deaminated and excreted as uric
acid making RNA metabolism very inefficient
• Typically nucleotides are simply recycled into new
nucleic acid molecules and are not used for energy
production
Pathways of Catabolism and
Anabolism
Figure 25–12
What is the primary role of the
TCA cycle in the production of
ATP?
A.
B.
C.
D.
break down glucose
create hydrogen gradient
phosphorylate ADP
transfer electrons from
substrates to coenzymes
How would a decrease in the level
of cytoplasmic NAD affect ATP
production in mitochondria?
A. ATP production would increase.
B. ATP production would decrease.
C. ATP production would fluctuate
randomly.
D. ATP is not produced in
mitochondria.
How would a diet that is deficient in
pyridoxine (vitamin B6) affect protein
metabolism?
A. It would interfere with protein
metabolism.
B. It would enhance protein
metabolism.
C. It would cause the use of different
coenzymes.
D. Pyridoxine is not involved in
protein metabolism.
Elevated levels of uric acid in the
blood can be an indicator of
increased metabolism of which
organic compound?
A.
B.
C.
D.
nucleic acids
proteins
carbohydrates
lipids
SUMMARY
•
•
•
•
•
•
•
•
•
Cellular metabolism
Catabolism and Anabolism
Carbohydrate metabolism
Glycolysis
Cellular Respiration
Mitochondrial ATP production
Lipid catabolism (Beta-oxidation)
Amino acid catabolism
Protein synthesis