Chapter 2 - Cellular Respiration Flashcards
Glucose
glucose contains about 2800 kJ/mol of free energy that is slowly released in small redox reactions
converting ADP to ATP temporarily stores 31 kJ/mol of this energy
the energy then gets used for
active transport
synthesis of macromolecules
mechanical work
Gibb’s free energy
predicts the amount of useful energy released by a reaction
positive change in G = non-spontaneous and endergonic
negative change in G = spontaneous and exergonic
Reaction coupling
in metabolism, non-spontaneous reactions happen routinely
to do this, spontaneous reactions are coupled with non-spontaneous ones
as long as the overall change in G is negative, both reactions will proceed
ATP hydrolysis releases just the right amount of energy to power metabolic processes
more = inefficient, less = ineffective
Glycolysis
occurs in the cytoplasm of all organisms and is oxygen independent
requires the input of 2 ATP, but produces 4 ATP (per glucose)
consists of ten reactions grouped into 4 stages
4 stages of glycolysis
mobilization
cleavage
oxidation
ATP generation
Mobilization
glucose (6 carbons) gains potential energy
it is phosphorylated twice with the input of 2 ATP
Cleavage
the product of mobilization (fructose -1,6 - biphosphate) is split in half
two molecules of PGAL are formed (3 carbons each)
Oxidation
a pair of high energy electrons (and a proton) are removed from PGAL
these are used to reduce NAD+ to NADH
ATP Generation
each molecule is then converted to pyruvate
in the process, 2 ATP are generated from each by substrate level phosphorylation
substrate level phosphorylation = coupled reaction where, in this case, a phosphate is added to ADP
Free energy of glycolysis
all exergonic reactions require activation energy, in this case 2 ATP are invested
the first several steps of glycolysis increase the free energy of glucose
the largest release of energy is the oxidation
there are two smaller releases of energy during the production of ATP
Problem with glycolysis
glycolysis depletes the cell’s NAD+ reserves
the cell then requires other metabolic processes to regenerate NAD+
aerobic conditions = oxidative respiration
anaerobic conditions = fermentation
Mitochondria
made up of outer membrane, inner membrane, intermembrane space, and mitochondrial matrix
the folds in the inner membrane are called “cristae”
Oxidative decarboxylation of pyruvate (step 1)
transition step between glycolysis and Krebs cycle
step 1 = pyruvate enters mitochondria
Oxidative decarboxylation of pyruvate (step 2)
in mitochondrial matrix, pyruvate molecules are decarboxylated to form CO2 and an acetyl group
the acetyl group is added to the carrier molecule coenzyme A, forming acetyl-CoA
this step also reduces a molecule of NAD+ to NADH (per pyruvate)
Oxidative decarboxylation of pyruvate (step 3)
acetyl-CoA molecule (2 carbons) binds to oxaloacetate (4 carbons) to make citrate (6 carbons), the first substrate of Krebs cycle
coenzyme A is then released to go fetch another acetyl group
Krebs cycle
for one glucose, the Krebs cycle is completed twice (once per pyruvate molecule)
at the end of the Krebs cycle, oxaloacetate is regenerated
the oxaloacetate combines with another acetyl-CoA and re-enters the cycle
Krebs cycle (stage A)
stage A (preparation) a rearrangement prepares citric acid (citrate) for energy extraction
Krebs cycle (stage B)
in the first few steps, two NAD+ are converted to NADH, and two CO2 are released (oxidative decarboxylation)
the CO2 are waste and exhaled through the lungs
one reaction directly makes GTP (functionally equivalent to ATP) by substrate level phosphorylation
then there are two more oxidations, one converting FAD to FADH2, and another converting NAD+ to NADH
Proton pumping
each NADH and FADH2 molecule contains a pair of high energy electrons
the NADH molecules diffuse to the inner mitochondrial membrane and are oxidized by the embedded protein NADH dehydrogenase
the electron pairs pass through a series of cytochromes (heme group attached to a protein), losing energy with each transfer
the cytochromes use the electron’s energy to move a pair of protons from the mitochondrial matrix to the intermembrane space
the final cytochrome in the chain reduces an oxygen to form water
Free energy of electron transport chain
each cytochrome is oxidized, then reduced
the free energy decreases throughout the ETC, leaving a pair of non useful electrons at the end of the chain that get converted to water
NADH pumps 3 protons, while FADH2 only pumps 2 as it does not activate the first cytochrome proton pump
this means that overall, NADH makes 3 ATP, while FADH2 only make 2 ATP
Chemiosmosis
the proton gradient generated by the electron transport chain is a concentration gradient and also a charge gradient, which both have potential energy
protons are allowed to pass back into the mitochondrial matrix through ATP synthase
passage of these protons back into the matrix powers ATP production (chemiosmotic phosphorylation)
Total ATP yield of aerobic metabolism
36 ATP per glucose
2 ATP from glycolysis
2 ATP from substrate level phosphorylation in Kreb
28 ATP from 10 NADH molecules (2 from glycolysis, 2 from oxidizing pyruvate, 6 from Krebs)
4 ATP from 2 FADH2 (Krebs cycle)
most NADH yield 3 ATp from chemiosmotic phosphorylation, however, the ones produced by glycolysis require transport into the matrix, costing 1 ATP
Fermentation
under anaerobic conditions, glycolysis is the only means of making ATP in cells
step 4 of glycolysis forms NADH from NAD+
this means that unless there is a source of NAD+, glycolysis will stop
during fermentation, NADH is used to reduce pyruvate to lactate in animal cells, or to ethanol and CO2 in yeast cells
both of these processes regenerate NAD+ to continue glycolysis
Lactate fermenation
lactic acid fermentation
glucose becomes 2 pyruvate
add 2 ATP, and 2 lactates are produced
the energy yield for fermentation is extremely low, and is not a sustainable strategy for obtaining energy in animals, however works for yeast
Ethanol fermentation
ethanol fermentation
glucose becomes 2 pyruvate
add 2 ATP, and 2 ethanols and 2 CO2 are produced
unfortunately, ethanol produced by fermentation kills yeast if it remains in the environment
this limits alcohol content of spirits to 10% unless they are distilled
Carbolizing triglycerides
triglycerides are hydrolyzed to fatty acids and glycerol
glycerol (3 carbons) is then converted to pyruvate (3 carbons)
fatty acids with an even number of carbons (most likely)
2-carbon fragments are snipped off the carboxyl end, forming acetyl-CoA
the pyruvate and acetyl-CoA then enter the Krebs cycle
fatty acids with an odd number of carbons
a 3 carbon fragment remains at the end, and is carboxylated and enters Krebs cycle as succinyl-CoA (4 carbons)
Protein
proteins are hydrolyzed to amino acids
amino acids have an amino group, which if released as NH3 would be toxic
they are therefore transported outside of your body
Amino group transport
as amino groups are snipped off, they combine with alpha-ketoglutarate (carrier from Krebs cycle) forming glutamic acid (MSG)
this is a transaminase reaction
this glutamic acid is carried to the liver by the blood
in the liver, amino groups are removed from glutamic acid and combined in urea
blood carries the urea from the liver to your kidneys
kidneys filter it from your blood and it leaves your body as urine
Carbon skeleton entry to respiration
after the amino groups are dealt with, the remaining carbon skeletons of amino acids can enter respiration pathways
where these carbon skeletons enter respiration pathways depends on the number of carbons they have
sometimes the carbon skeletons are carboxylated or decarboxylated if the number of carbons is wrong
Synthetic pathways
all the interconnected pathways form a metabolic pool
many reactions in protein and lipid metabolism are reversible, so you can interconvert sugars, proteins, and fats
however, you cannot make essential fatty acids or essential amino acids
molecules from the metabolic pool can be used either as energy sources or as substrates for synthesis of other macromolecules
if there is excess sugar, the body breaks it down to PGAL and acetyl-CoA and used to synthesize glycerol and fatty acids
