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Energy Transformation: Cellular Respiration
Outline
Sources of cellular ATP
Turning chemical energy of covalent bonds between C-C into
energy for cellular work (ATP)
Importance of electrons and H atoms in oxidation-reduction
reactions in biological systems
Cellular mechanisms of ATP generation
The four major central metabolic pathways
Glycolysis- Fermentation
Transition step
Krebs Cycle
Electron Transport chain and oxidative phosphorylation
Poisons of cellular respiration and their mechanisms of action.
Use of biomolecules other than glucose as carbon skeletons &
energy sources
A cell has enough ATP to last for about three seconds
http://health.howstuffworks.com/sports-physiology3.htm
Necessary ATP is supplied to muscle cells by
1. Phosphagen system (8 to 10 seconds.)
2. Glycogen-lactic acid system (90 seconds)- glucose
3. Aerobic Respiration (unlimited)- glucose
Phosphagen system (muscle fibers)
*
• Creatine phosphate contains (a
high-energy phosphate
compound).
• Creatine kinase transfers the
phosphate to ADP to form ATP.
Creatine-Phosphate (tri)
Creatine
Kinase
Creatine-Phosphate + ADP
(Tri)
Creatine-Phosphate + ATP
(*) Arezki Azzi et al. Protein Sci 2004; 13: 575-585
(Di)
During cellular respiration the potential energy in the
chemical bonds holding C atoms in organic molecules
in turned into ATP.
In presence of oxygen
complete breakdown
ATP
C-C-C-C-C-C + O2……………… CO2 + H2O +
large amount
In absence of oxygen
incomplete breakdown
C-C-C-C-C-C ……………… 2 C-C-C + ATP
small amount
Cellular respiration: ATP is produced aerobically, in the
presence of oxygen
Fermentation: ATP is produced anaerobically, in the
absence of oxygen
Chemical bonds of organic molecules
Chemical bonds involve electrons of atoms
Electrons have energy
Chemical bonds are forms of potential energy
If a molecule loses an electron does it lose energy?
When a chemical bond is broken sometimes
electrons can be released
Oxidation-Reduction
• Oxidation: removal of electrons.
• Reduction: gain of electrons.
• Redox reaction: oxidation reaction paired with a
reduction reaction.
Electron Carriers in Biological Systems
 The electrons associated with hydrogen atoms. Biological oxidations
are often dehydrogenations
 Soluble electron carriers
NAD+, and FAD (ATP generation)
NADP+ (biosynthetic reactions)
Nicotinamide Adenine Dinucleotide (NAD+) a co-enzyme that acts as
an electron shuttle (Niacin)
2 e– + 2 H+
NAD+
2 e– + H+
NADH
H+
Dehydrogenase
+ 2[H]
(from food)
Nicotinamide
(oxidized form)
+ H+
Nicotinamide
(reduced form)
Flavin Adenine Dinucleotide (FAD+) is another co-enzyme that acts as
an electron shuttle (Vitamin B2)
In Metabolic pathways
Oxidation
Reduction
• Energy loss or gain
• Exergonic or endergonic
• Catabolic or anabolic
In cells, energy is harvested from an energy
source (organic molecule) by a series of coupled
oxidation/reduction reactions (redox) known as
the central metabolic pathways
becomes oxidized
becomes reduced
Dehydrogenase
Metabolic Pathways
Linear Pathways
Branched Pathways
Cyclic Pathways
An overview of the central metabolic pathways
•
•
•
•
Glycolysis
Transition step
Tricarboxylic acid cycle (TCA or Krebs cycle)
The Electron Transport Chain and oxidative
phosphorylation
Mechanisms of ATP Generation during
cellular respiration
1. Oxidative phosphorylation/ Chemiosmosis
•
•
•
•
ADP and inorganic PO-4
ATP synthase
Cellular respiration in mitochondria
Electron Transport Chain
Generation of ATP
2. Substrate-level phosphorylation
-Transfer of a high-energy
PO4- from an organic
molecule to ADP
- Different enzymes
- During glycolysis and the
TCA or Krebs cycle
Glycolysis
• Multi – step breakdown of glucose into
intermediates
• Turns one glucose (6-carbon) into two
pyruvate (3-carbon) molecules
• Generates a small amount of ATP (substratelevel phosphorylation)
• Generates reducing power - NADH + H+
Figure 9.18 Pyruvate as a key juncture in catabolism
Fermentation
An extension of glycolysis
Takes place in the absence of oxygen (anaerobic)
Regenerates NAD+ from NADH + H +
ATP generated by substrate - level phosphorylation
during glycolysis
• Does not use the Krebs cycle or ETC (no oxidative phosphorylation)
• A diversity of end products produced (i.e. lactic acid,
alcohols, etc.)
•
•
•
•
Figure 9.17a Fermentation
Figure 9.17b Fermentation
Figure 9.9 A closer look at glycolysis: energy investment phase
Figure 9.9 A closer look at glycolysis: energy payoff phase
The energy input and output of glycolysis
Energy investment phase
Glucose
Glycolysis
Citric
acid
cycle
2 ATP used
2 ADP + 2 P
Oxidative
phosphorylation
Energy payoff phase
ATP
ATP
ATP
4 ADP + 4 P
2 NAD+ + 4 e– + 4 H+
4 ATP
formed
2 NADH + 2 H+
2 Pyruvate + 2 H2O
Net
Glucose
4 ATP formed – 2 ATP used
2 NAD+ + 4 e– + 4 H+
2 Pyruvate + 2 H2O
2 ATP
2 NADH + 2 H+
Transition step
• Under aerobic conditions, the pyruvate produced
during glycolysis is directed to the mitochondrion.
• A multi-enzyme complex, spanning the 2
mitochondrial membranes, turns pyruvate (3-carbon)
into Acetyl-CoA (2-carbon) and releases one CO2
molecule
• Uses coenzyme A (Vit B derivative)
• Generates reducing power NADH + H+
Figure 9.10 Conversion of pyruvate to acetyl CoA, the junction between glycolysis and the Krebs cycle
Krebs or TCA Cycle
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•
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A cyclical pathway
Accepts – acetyl – CoA
Generates 2 – CO2 molecules (for every acetyl-CoA
Generates reducing power NADH + H+ and FADH2
Generates a small amount of ATP (substrate-level
phosphorylation)
A closer look at the Krebs cycle
Figure 9.12 A summary of the transition step and Krebs cycle
Figure 9.5 An introduction to electron transport chains
Electron Transport Chain and ATP
generation
Electron transport chain:
• membrane-embedded or
bound electron carriers
• Mostly proteins with nonprotein groups.
• NADH vs. FADH2
• No direct formation of ATP
• Pumps H+ into the intermembrane space
Oxidative Phosphorylation and ATP generation
ATP Generation :
• Membrane-embedded protein
complex , ATP synthase
• Proton-motive force
• Direct formation of ATP by
Chemiosmosis
Chemiosmosis: the coupling of the
transport of H+ across membrane
by facilitated diffusion with
chemical reaction producing ATP
ATP Synthase Gradient: The Movie
http://vcell.ndsu.edu/animations/at
pgradient/movie.htm
Chemiosmosis couples the electron transport chain to ATP synthesis
Certain poisons interrupt critical events in cellular respiration
• Block the movement of electrons
• Block the flow of H+ through ATP synthase
• Allow H+ to leak through the membrane
Cyanide,
carbon monoxide
Rotenone
H+
H+
H+
Oligomycin
H+
H+
+
H
H+ H+ H+
ATP
Synthase
DNP
FADH2
NADH
NAD+
H+
FAD
1 O2 + 2 H+
2
H+
H+
Electron Transport Chain
H2 O
ADP + P
ATP
Chemiosmosis
Energy yield: How many ATP molecules are generated during
cellular respiration of a single glucose molecule?
Sources of cellular glucose
• glucose from food in the intestine
• glycogen supplies in the muscles
• breakdown of the Liver's glycogen into
glucose
The Catabolism of various food molecules
Beta oxidation
http://www.brookscole.
com/chemistry_d/templ
ates/student_resources
/shared_resources/ani
mations/carnitine/carni
tine1.html
Simple lipids
• Triglycerides consist of glycerol and fatty acids
• Fatty acids broken down into
through beta
oxidation
Proteins
Protein
Extracellular proteases
Deamination, decarboxylation,
dehydrogenation
Amino acids
Organic acid
Krebs cycle
and glycolysis
Transition step
Feedback control of cellular respiration
Phosphofructokinase
Allosteric enzyme
Activity modulated by
inhibitors and activators
Adjustment of cellular
respiration in response to
demand
Food, such as
peanuts
Carbohydrates
Sugars
Fats
Proteins
Glycerol Fatty acids
Amino acids
Amino
groups
Pyruvate
Glucose G3P
GLYCOLYSIS
Acetyl
CoA
ATP
CITRIC
ACID
CYCLE
OXIDATIVE
PHOSPHORYLATION
(Electron Transport
and Chemiosmosis)
Food molecules provide raw materials for
biosynthesis consuming ATP
ATP needed to drive biosynthesis
ATP
CITRIC
ACID
CYCLE
GLUCOSE SYNTHESIS
Glucose
Pyruvate
G3P
Acetyl
CoA
Amino
groups
Amino acids
Proteins
Fatty
acids
Glycerol
Fats
Cells, tissues, organisms
Sugars
Carbohydrates