Cellular Respiration
Can utilize carbohydrates, lipids, and proteins
Converts energy in fuel molecules into ATP
Allows the cell to do work
2 types of phosphorylation
Substrate-Level
Oxidative
Stages of CR
Stage 1: Glycolysis → Cytoplasm
Glucose is partially broken down → small amount of energy is released
Note: fatty acids and amino acids may also be broken down by different pathways (we will not examine these)
Stage 2: Pyruvate Oxidation → Mitochondria
Pyruvate is produced from the breakdown of glucose in glycolysis → converted to acetyl-CoA and CO2
Stage 3: Citric Acid Cycle → Mitochondria
Acetyl-CoA from the end of step 2 is broken down
This releases: CO2, small amount of energy and electron carriers
Stage 4: Oxidative Phosphorylation → Mitochondria
All the electron carriers from stages 1-3 release their high-energy electrons to the ETC
This produces ATP
Overview of ATP Generation
ATP is generated using several mechanisms:
Substrate-level phosphorylation is the process by which ATP is synthesized by a hydrolysis reaction involving an enzyme/substrate complex
A small amount of ATP is generated
The energy is transferred to electron carriers → carry energy from one reaction to another
The electron carriers transport electrons to the respiratory electron transport chain → which transfers electrons along membrane-associated proteins to final acceptor
These proteins harness the energy released to produce ATP → process called Oxidative Phosphorylation
The majority of ATP is produced using oxidative phosphorylation
Oxidation Reactions in CR
NAD+ and FAD are important electron carriers in cellular respiration
In cellular respiration → the energy stored in glucose is harnessed in electron carriers as glucose is oxidized into CO2
In the breakdown of glucose, glucose is oxidized to CO2 and O2 is reduced to H2O
Remember → oxidation is the loss of electrons & reduction is the gain of electrons
Recall that the carbon atoms of glucose are bonded to other carbon atoms, hydrogen atoms, and oxygen atoms
“OIL RIG”, “LEO the lion says “GER””
Oxidation Is Loss of Electrons
Reduction Is Gain of Electrons
Loss Electrons is Oxidation
Gain of Electrons is Reduction
Reduction Reactions in CR
Oxygen is the final electron acceptor in cellular respiration
When O2 is reduced → forms H2O
The original electron donor in cellular respiration is glucose
But the electrons move from one molecule to the next during cellular respiration via reduction reactions
Electron Carriers
2 important electron carriers in cells are NAD+/NADH & FAD/FADH2
The oxidized forms of these carriers are NAD+ and FAD; the reduced forms are NADH and FADH2
Through glycolysis, pyruvate oxidation, and the citric acid cycle, the form of the electron carrier accepts electrons and becomes reduced
The reduced form of the electron carriers has high potential energy
This is used to synthesize ATP in the final stage of cellular respiration
Glycolysis
Background
Catabolic Pathway → Universal
The sum of ten chemical reactions that breakdown glucose
Goes from 6-carbon glucose → to 2,3-carbon pyruvates
Occurs in cytosol n presence or absence of O2
Considered in anaerobic process because no oxygen is consumed
Phases of Glycolysis
3 different phases
1. Preparatory Phase → where energy is consumed
2. Cleavage Phase → where glucose is split into 2
3. Payoff Phase → where ATP is one of the products
Glycolysis - Phase One
- Preparatory Phase
Remember: It takes energy to make energy
Events in Phase 1 Include:
Preparation of glucose for next 2 phases → addition of 2 phosphate groups, producing fructose 1,6-biphosphate
This process requires an input of energy in the form of 2 molecules of ATP
The phosphorylation of glucose traps the molecule inside the cell and destabilizes it so that it is ready for phase 2
Phase 1 are energy consuming reactions
Glycolysis - Phase Two
- Cleavage Phase
Events in Phase 2 include:
Cleavage of fructose 1,6-biphosphate into 2 molecules:
Glyceraldehyde 3-phosphate
Dihydroxyacetone Phosphate → converted into another molecule of glyceraldehyde 3-phosphate
Ends with 2 molecules of glyceraldehyde 3-phosphate
Glycolysis - Phase Three
- Payoff Phase
Events in phase 3 include:
2 molecules of pyruvate are formed
2 molecules of the electron carrier NADH are produced
4 molecules of ATP are produced
Phase 3 are energy producing reactions
At The End Of Glycolysis
After glycolysis → there is a net gain of two ATP
Phase 3 produced 4 ATP → but 2 were consumed in phase 1
Phase 3 also produces two NADH
These will be used in the last reaction of CR
Understand the Steps
Stages 2-4 of CR take place in mitochondria
Recall: Glycolysis is stage 1 and takes place in cytoplasm
Stage 2 is Pyruvate Oxidation
Stage 3 is the Citric Acid Cycle
Stage 4 is Oxidative Phosphorylation
2, 3, 4 in mitochondria
Pyruvate Oxidation: Where? Mitochondria basics
A mitochondria has an inner membrane and an outer membrane → this defines the 2 spaces
Space between 2 membranes → Inter Membrane Space
Space inside inner membrane → Mitochondrial Matrix
What is Pyruvate Oxidation?
When oxygen is present → pyruvate can be oxidized to produce carbon dioxide and NADH
Ultimately to acetyl-CoA
These reactions occur in the mitochondrial matrix
Pyruvate is converted to acetyl-CoA
Then it is further broken down in the citric acid cycle
The pyruvate is initially oxidized to form CO2 and an acetyl group
The acetyl group is then transferred to coenzyme A → carries the acetyl group to the citric acid cycle
These reactions are catalyzed by a group of enzymes called the Pyruvate Dehydrogenase Complex
One molecule of pyruvate produces?
one molecule of CO2
one molecule of NADH
one molecule of acetyl-CoA
At the end of glycolysis, there are two molecules of pyruvate, thus, at the end of the process, for each glucose molecule there are:
two molecules of acetyl-CoA
two molecules of NADH
two molecules of CO2
Citric Acid Cycle
Has additional names
Krebs Cycle and the TCA (Tricarboxylic acid) cycle
Cycle b/c the 1st reactant (oxaloacetate) in the process is also regenerated at the end
During this stage of cellular respiration → the fuel molecules are completely oxidized
The citric acid cycle completes the oxidation of glucose → CO2
Substrate-level phosphorylation is used to produce ATP
Additional energy management molecules → NADH & FADH2 are also produced
Citric Acid Cycle: Use of intermediates
Some organisms can use products from different steps in the citric acid cycle as intermediates in other metabolic pathways
E.g., ⍶-Ketoglutarate can be modified to form other amino acids and purine bases
Citric Acid Cycle: Carbon Dioxide Produced
Have you ever thought about why we exhale CO2?
The oxidation of acetyl-CoA produces the carbon dioxide we exhale
There is a transfer of the potential energy stored within acetyl-CoA to be transferred and stored in NADH and FADH2
Also, the production of GTP is catalyzed by substrate-level phosphorylation
Citric Acid Cycle: Fate of High Energy Molecules
The NADH and FADH2 that is produced transfers electrons to other carriers in the Electron Transport Chain → ETC
Recall that they are produced through redox reactions in the 1st 3 stages of CR
We will look at their fate in the ETC
Oxidative Phosphorylation
ETC is in the mitochondrial inner membrane → electrons enter & move from donors to acceptor
Until they reach the final electron acceptor → Oxygen
When oxygen accepts the electron → Reduced to H2O
