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Biology - Cell Metabolism


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What is cellular metabolism?
It is the sum of all chemical reactions that take place in the cell
What are these reactions generally categorized as?
Anabolic or catabolic
Which requires energy?
Anabolic requires energy
What does it involve?
It is the biosynthesis of complex organic compounds from simpler molecules
What is catabolism?
It releases energy as they break down complex organic compounds into smaller molecules
What are autotrophic organisms?
They convert sunlight into bond energy
What is an example?
Green plants
What type of bond energy is sunlight converted into?
It is converted into the bond energy in the bonds of organic compounds like glucose
When does this occur?
During the anabolic process of photosynthesis
Because of this, what do autotrophs not need?
They do not need an exogenous supply of organic compounds
What are heterotrophic organisms?
They obtain their energy catabolically, via the breakdown of organic nutrients that must be ingested
What is the net reaction of photosynthesis?
6CO2 + 6H20 + Energy à C6H12O6 + 6O2
Why do heterotrophic organisms metabolize glucose and other organic molecules?
They do it to release the stored bond energies
What is the net reaction of glucose catabolism?
C6H12O6 + 6O2 à 6CO2 + 6H20 + Energy
What is interesting about this reaction?
It’s basically the backward reaction of Photosynthesis!
What are some of the various molecular carriers used to shuttle energy between reactions?
ATP is one of them, along with the coenzymes NAD+, NADP+, and FAD
What is ATP?
Adenosine triphosphate is the cell’s main energy currency
What happens through its formation and degradation?
Cells have a quick way of releasing and storing energy
How is ATP synthesized?
It occurs during glucose catabolism
What is ATP composed of?
It has the nitrogenous base adenine, the sugar ribose, and three weakly linked phosphate groups
Where is the energy of ATP stored?
In the covalent bonds attaching the phosphate groups
What are they often referred to as?
High-energy bonds
What does hydrolysis of ATP to ADP and Pi release?
It releases stored bond energy that the cell can use in metabolic processes
How many kcal of energy are released per mole of ATP?
Approximately 7kcal of energy
What does this energy provide for?
Endergonic (endothermic) reactions such as muscle contraction, motility, and the active transport of substances across plasma membranes
What can ATP be hydrolyzed into?
AMP and PPi
What do ADP and Pi combine to form?
What is the purpose of this?
To regenerate its ATP supply
Does this process require energy?
Where does the necessary input of energy come from?
It comes from the degradation of glucose
What is a second mechanism by which the cell stores chemical energy?
It stores energy in the form of high potential electrons
What are electrons transferred as?
Hydride ions (H:-) or as pairs of hydrogen atoms
What happens during glucose oxidation?
Hydrogen atoms are removed
What happens to these hydrogen atoms?
They are accepted by NAD+, FAD, and NADP+
What do these molecules do?
They transport the high-energy electrons of the hydrogen atoms to a series of carrier molecules on the inner mitochondrial membrane
What are the carrier molecules known as?
The electron transport chain
What is oxidation?
It is the loss of an electron
What are NAD+, NADP+, and FAD referred to as?
Oxidizing agents
They are known as respiratory poisons
In the process what do they themselves undergo?
What does this mean?
It means it loses its CO2
What do NADH, NADPH, and FADH2 all behave as?
Reducing agents
What does NADH transfer its electrons to?
Another electron acceptor, thereby reducing it, and in the process NADH is oxidized back to NAD+
So, what do these coenzymes ultimately do?
They temporarily store and release energy in the form of electrons through their successive oxidations and reductions
What are the two stages of the degradative oxidation of glucose?
Glycolysis and cellular respiration
What is the first stage of glucose catabolism?
What is glycolysis?
A series of reactions that lead to the oxidative breakdown of glucose into two molecules of pyruvate
What else is produced besides pyruvate?
What is reduced?
NAD+ becomes NADH
Where do all of these reactions take place?
In the cytoplasm
What are they mediated by?
Specific enzymes
How many molecules of PGAL are formed per molecule of glucose?
2 molecules
From one molecule of glucose, how many molecules of pyruvate are obtained?
2 molecules
How many ATP are used?
How many ATP are generated?
4 ATP are generated
What is the net production of ATP?
2 ATP per glucose molecule
What is this type of phosphorylation called?
It is called substrate level phosphorylation
How many NADH is produced per PGAL?
What is the total production of NADH?
2 NADH molecules per glucose
What is the net reaction of glycolysis?
Glucose + 2ADP + 2Pi + 2NAD+ à 2 Pyruvate + 2ATP + 2NADH + 2H+ + 2H20
Does this occur in eukaryotes and prokaryotes or just eukaryotes?
It takes place in both kinds
Where can pyruvate degradation occur from this point?
It can occur to fermentation or cellular respiration in the mitochondria
What causes it to go the fermentation route?
If it is an anaerobic environment
What about toward the cellular respiration route in the mitochondria?
If there is oxygen the route will be this way
What is fermentation?
This is the reduction of pyruvate into ethanol or lactic acid
Why does this occur?
Because NAD+ must be regenerated for glycolysis to continue in the absence of O2
How many ATP does fermentation produce per glucose molecule?
It produces 2 ATP total
What is alcoholic fermentation?
It is a type that commonly occurs only in yeast and some bacteria
What occurs?
The pyruvate produced in glycolysis is decarboxylated to become acetaldehyde
Then what happens?
The acetaldehyde is reduced by the NADH generated in glycolysis to yield ethanol
What does this produce?
It produces NAD+ so that glycolysis can continue
What is lactic acid fermentation?
It occurs in certain fungi and bacteria
When does it occur in humans?
It occurs during strenuous activity
Why would it occur then?
It occurs then because during that time the oxygen supply to muscle cells lags behind the rate of glucose catabolism
What happens here?
The pyruvate generated is reduced to lactic acid
What is a similar thing that occurs between this an alcoholic fermentation?
They both include the regeneration of NAD+ when pyruvate is reduced
What may accumulate in humans during exercise as a result of this type of fermentation?
Lactic acid
What happens then?
The pH goes down in the blood, which causes muscle fatigue
What happens when the oxygen supply is replenished?
The lack acid is oxidized back to pyruvate and the cell enters cellular respiration
What is the amount of oxygen needed for this conversion to occur called?
Oxygen debt
What is cellular respiration?
It is the most efficient catabolic pathway used by organisms to harvest energy stored in glucose
How much energy does cellular respiration yield?
What type of process is it?
It is an aerobic process
What does oxygen act as?
It acts as the final acceptor of electrons that are passed from carrier to carrier during the final stage of glucose oxidation
Where does it occur?
It occurs in eukaryotic mitochondrion
What is it catalyzed by?
Reaction-specific enzymes
What are the three stages of cellular respiration?
Pyruvate decarboxylation, the citric acid cycle, and the electron transport chain
What happens during pyruvate decarboxylation?
The pyruvate formed during glycolysis is transported from the cytoplasm into the mitochondrial matrix where it is decarboxylated
What happens to the remaining acetyl group?
It is transferred to coenzyme A to form acetyl CoA
During this process, what happens to NAD+?
It is reduced to NADH
What is the citric acid cycle also known as?
It is known as the Krebs cycle or the tricarboxylic acid cycle
When does the cycle begin?
It begins when the two carbon acetyl group from acetyl CoA combined with oxaloacetate
What is oxaloacetate?
A four-carbon molecule
What does this form?
What happens during the series of reactions that follow?
2 CO2 are released, and oxaloacetate is regenerated for use in another turn of the cycle
How many ATP are produced per turn of the cycle?
1 ATP is produced
What type of phosphorylation occurs here?
Substrate level phosphorylation
What happens to NAD+ and FAD during the cycle?
Electrons are transferred to them, making NADH and FADH2
Where do these coenzymes transport the electrons to?
They transport the electrons to the electron transport chain, where more ATP is produced
How is it produced?
Through oxidative phosphorylation
Per molecule of glucose, how many pyruvates are decarboxylated and channeled into the citric acid cycle?
2 Pyruvates
What is the net reaction of the citric acid cycle per glucose molecule?
2 Acetyl CoA + 6NAD+ + 2FAD + 2GDP + 2Pi + 4H2O --> 4CO2 + 6NADH + 2FADH2 + 2ATP + 4H+ + 2CoA
What is the electron transport chain?
It is a complex carrier mechanism located on the inside of the inner mitochondrial membrane
What happens during oxidative phosphorylation?
ATP is produced when high energy potential electrons are transferred from NADH and FADH2 to oxygen by a series of carrier molecules located in the inner mitochondrial membrane
What happens as electrons are transferred from carrier to carrier?
Free energy is released
What is this new free energy used for?
It is used to form ATP
What are most of the molecules in the ETC chain called?
They are mostly cytochromes
What are those?
They are electron carriers that resemble hemoglobin in the structure of their active site
What does the functional unit contain?
A central iron atom, which is capable of undergoing a reversible redox reaction
What does that mean?
It means that is can alternatively be reduced and oxidized
What is FMN or flavin mononucleotide?
It is the first molecule of the ETC
What happens to it when it accepts electrons from NADH?
It is reduced
What happens to NADH?
It is reduced from NADH to NAD+
What is the last carrier of the ETC?
Cytochrome a3
What does it do?
It passes its electron to the final electron acceptor, O2
What else does the O2 pick up besides electrons?
It also picks up a pair of hydrogen ions from the surrounding medium
What happens if there is no oxygen?
The ETC becomes backlogged with electrons
What does this do?
NAD+ cannot be regenerated and glycolysis cannot continue unless lactic acid fermentation occurs
What does cyanide or dinitrophenol do?
They stop ATP synthesis
How do they work?
Cyanide blocks the transfer of electrons from cytochrome a3 to O2. Dinitrophenol uncouples the electron transport chain from the proton gradient established across the inner mitochondrial membrane
What are the three large protein complexes that electron carriers are classified as?
NADH dehydrogenase, the b-c1 complex, and cytochrome oxidase
An electron passing through the entire ETC supplies enough energy to generate how many ATP?
NADH delivers its electrons to NADH dehydrogenase complex, which produces how many ATP?
For each NADH, 3 ATP are produced
What does FADH2 do that is different?
It bypasses the NADH dehydrogenase complex and delivers its electrons directly to carrier Q
What is carrier Q?
It is called ubiquinone, which lays between the NADH dehydrogenase and b-c1 complexes
For each FADH2, how many energy drops and corresponding ATPs are produced?
2 energy drops, so 2 ATPs are produced
What does the operating mechanism in this type of ATP production involve?
The coupling of oxidation of NADH to the phosphorylation of ADP
What is the coupling agent for these two things?
The proton gradient across the inner mitochondrial membrane
What is it maintained by?
The electron transport chain
What happens as NADH passes its electrons to the ETC?
Free hydrogen ions are released and accumulate in the mitochondrial matrix
What does the ETC do to these ions?
It pumps them out of the matrix, across the inner mitochondrial membrane, and into the intermembrane space at each of the three protein complexes
What does this continuous translocation of H+ create?
A positively charged acidic environment in the intermembrane space
What does the electrochemical gradient generate?
A proton-motive force
What is that?
It is what drives the H+ back across the inner membrane and into the matrix
There is a problem, what is it?
To pass through the membrane which is impermeable to ions, the H+ must flow through specialized channels provided by enzyme complexes
What are the enzyme complexes called?
ATP synthetases
What happens as the H+ passes through the ATP synthetases?
Energy is released to allow for phosphorylation of ADP to ATP
What is the coupling of the oxidation of NADH with the phosphorylation of ADP called?
Oxidative phosphorylation
What happens when glucose supplies run low?
The body utilizes other energy sources
What are the sources used by the body?
It uses carbohydrates first, then fats, then proteins
How does it work?
These substances are first converted to either glucose or glucose intermediates, which can be then degraded in the glycolytic pathway and the TCA cycle
How does it work with carbohydrates?
Disaccharides are hydrolyzed into monosaccharides, most of which can be converted into glucose or glycolytic intermediates
What can glycogen in the liver be converted into?
When needed, it can be converted into glucose-6-phosphate, a glycolytic intermediate
What about fats?
Fat molecules are stored in adipose tissue in the form of triglyceride. When needed, they are hydrolyzed by lipases to fatty acids and glycerol, and are carried by the blood to other tissues for oxidation
What can glycerol be converted into?
PGAL, a glycolytic intermediate
What must first happen for the fatty acid?
It must be activated in the cytoplasm
What does this process require?
Once activated, what occurs?
The fatty acid is transported into the mitochondrion and taken through a series of beta-oxidation cycles
What do the cycles do?
They convert it into two-carbon fragments
What happens to them?
They are converted into acetyl CoA
What happens to the acetyl CoA?
It enters the Citric Acid Cycle
With each round of beta-oxidation of a saturated fatty acid, what is generated?
1 NADH and 1 FADH2
What yields the largest number of ATP per gram?
What does this imply?
It means they are extremely efficient energy storage molecules
How do proteins get converted?
The body degrades amino acids only when there is not enough carbohydrate available
How does this occur?
Most undergo a transamination reaction
What happens with one of those?
They lose an amino group to form an alpha-keto acid
What happens to the carbon atoms of most amino acids?
Most are converted into acetyl-CoA, pyruvate, or one of the intermediates of the citric acid cycle

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