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ch4 - cellular respiration
cellular respiration​: metabolic reactions and processes that take place in the cells of organisms
to convert biochemical energy from nutrients into ATP and then release waste products
energy basics
two laws of thermodynamics: (how energy works)
- first law of thermodynamics​: the total amount of energy in the universe remains constant;
energy cannot be created/destroyed, only converted from one form to another, including:
- kinetic energy​: the energy of motion
- potential energy​: stored energy (ex: in a chemical bond)
- second law of thermodynamics​: when energy is converted from one form to another,
some is lost (unusable, usually in the form of heat); as additional energy conversions
occur, ​entropy​ increases (the disorder/randomness of energy that causes more of it to
be unusable)1
energy conversion is usually in the context of a system (ex: reaction, cell, organism, planet)
- closed system​: exchange of energy/matter with other systems is not considered
- open system​: exchange of energy/matter with other systems is considered
gibbs free energy​: energy in a system that is available for conversions (change in free energy
that occurs as the result of a conversion is represented by ∆G); not all of this energy is available
for chemical reactions (some will be lost as heat, as entropy increases)
- exergonic reaction​: when ∆G is negative for a reaction, there is a net release of energy;
ex: respiration (breakdown of glucose): ∆G = -686 kcal/mol
C​6​H12​O6​ + 6O​2​ = 6CO​2​ + 6HO​2 ​ (energy is produced)
​ ​
- endergonic reaction​: when ∆G is positive for a reaction, energy must be added to the
reaction for it to occur; ex: photosynthesis (making of glucose): ∆G = +686 kcal/mol
6CO​2​ + 6HO​2​ = C​6​H12​O6​ + 6O​2​ (energy comes from sunlight)
​ ​
2
activation energy​: required before most reactions occur to destabilize the reactants
most metabolic reactions are endergonic (positive ∆G) and require an input of energy (usually
from hydrolysis of ATP to ADP where ∆G = -7
...
substrate level phosphorylation​: when a phosphate group and its associated energy are
transferred to ADP to form ATP; the substrate molecule (a molecule with the phosphate
group) donates the high energy phosphate group
...
oxidative phosphorylation​: when a phosphate group is added to ADP to form ATP, but
energy for the bond does not accompany the phosphate group; instead, electrons give
up energy for generating ATP during each step of a process, where electrons are
transferred from one molecule (electron carrier) to another in a chain of reactions
cellular respiration​: the ATP-generating process that occurs in cells (energy extracted from
energy-rich glucose3):4 C​6​H12​O6​ + 6O​2​ = 6CO​2​ + 6H​2​O + energy
​ ​
aerobic respiration​: cellular respiration using oxygen (most cells use); 3 steps:
1
...
krebs cycle (citric acid cycle
3
...
alcohol fermentation
2
...
) glycolysis
glycolysis​: the decomposition (lysis) of glucose (glyco) to pyruvate​ (or ​pyruvic acid​); occurs in
​
the cytosol; steps (for each molecule of glucose):
1
...
2 NADH are produced (NADH, a ​coenzyme​, is an electron carrier when NAD​+​ combines
with two energy-rich electrons and H​+​, obtained from an intermediate molecule during
the breakdown of glucose; thus, NADH is an energy-rich molecule)
3
...
2 pyruvate are formed
summary: 1 glucose into 2 pyruvate, 2 NADH, and 2 ATP (made 4 but used 2)
3

other carbohydrates (starch, glycogen, sucrose) can by hydrolyzed to glucose in order to enter the
glycolytic pathway
body proteins, if necessary, can be hydrolyzed to amino acids (amino groups are stripped from the amino
acids and excreted as waste, the remainders are enzymatically converted to various substances that can
enter intermediate steps of glycolysis or the krebs cycle
glycerol and fatty acids are obtained from fats by hydrolysis or digestion (glycerol enters the glycolysis
pathway and fatty acids enter the krebs cycles (as acetyl CoA)
4
C6​H12​O6​ = glucose, sometimes transcribed as (CH2​O)n
​ ​ ​
​ ​

(aerobic respiration 2
...
pyruvate to acetyl CoA (before actual krebs cycle): pyruvate combines with coenzyme A,
or CoA, to produce acetyl CoA (1 NADH and 1 CO​2​ are also produced)
2
...
) oxidative phosphorylation
oxidative phosphorylation​: process of producing ATP from NADH and FADH​2
electrons from NADH and FADH​2 ​pass along an electron transport chain (ETC)​, which consists
​
of proteins that pass these electrons from one carrier protein7 to the next; along each step of the
chain, electrons give up energy (which is used to phosphorylate ADP to ATP;
NADH provides electrons that have enough energy to generate about 3 ATP (FADH​2 ​about 2);
the final electron acceptor of the electron transport chain is oxygen (the ½ O​2​ accepts the two
electrons and, together with 2 H​+​, forms water);
cytochrome c​: a carrier protein in the electron transport chain that is so ubiquitous among living
organisms that its ~100-amino-acid sequence is often used to compare species genetically
mitochondria
krebs cycle and oxidative phosphorylation occur in the mitochondria​; areas of mitochondrion:
​
1
...
intermembrane space​: narrow area between inner and outer membranes (H​+​ ions
(protons) accumulate here)
3
...
matrix​: the fluid material that fills the area inside the inner membrane (the krebs cycle
and conversion of pyruvate to acetyl CoA occur here)
chemiosmosis in mitochondria
chemiosmosis​: mechanism of ATP generation that occurs when energy is stored in the form of a
proton concentration gradient​ across a membrane; process during oxidative phosphorylation:
1
...
electrons are removed from NADH and FADH​2​ by protein complexes in the inner
membrane; the electrons move along the ETC from one protein complex to the next
5

the krebs cycle details what happens to 1 pyruvate, but glycolysis produces 2, so products would be
doubled
6
this CO2​ is that which animals exhale when they breathe
​
7
some carrier proteins, such as ​cytochromes​, include nonprotein parts containing iron
8
​ATP synthase​: another protein complex responsible for phosphorylation of ADP to form ATP

3
...
a pH and electrical gradient across the inner membrane is created; as H​+​ are
transferred, the concentration of H​+​ increases (pH decreases) in the intermembrane
space and decreases (pH increases) in the matrix; the concentration of H​+​ in the matrix
decreases further as electrons at the end of the ETC combine with H​+​ and oxygen to
form water (result: proton gradient, equivalent to a pH gradient, and an electric charge
/voltage gradient; these gradients are potential energy reserves)
5
...
therefore, NADH accumulates
...

in ​anaerobic respiration​, the objective of the two metabolic pathways alcohol fermentation​ and
​
lactic acid fermentation​ is to replenish NAD​+​ so glycolysis can proceed
...
pyruvate to acetaldehyde​: for each pyruvate, 1 CO​210 and 1 acetaldehyde are produced
2

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