Transcript Mitochondrial Respiration

Mitochondrial Respiration
Respiration
• Glycolysis
• Citric acid cycle/kreb’s cycle
Glycolysis -- partial
oxidation of a hexose
phosphate and triose
phosphates to produce an
organic acid: pyruvate
(occurs in the cytosol.
Note- pyruvate = pyruvic
acic)
Citric acid cycle
complete oxidation of
pyruvate to produce
CO2, H2O, reducing
power (NADH, FADH2)
and ATP
glycolysis
Citric acid
cycle =
TCA= Kreb’s
Cycle
No O2 required
O2 required
Anaerobic respiration,
or fermentation
Glycogen
Glucose-1-P
Glucose
Hexokinase or Glucokinase
Glucose-6-Pase
Glucose-6-P
Glucose + Pi
Glycolysis
Pathway
Pyruvate
Inhibition of the Glycolysis enzyme
Phosphofructokinase when [ATP] is high prevents
breakdown of glucose in a pathway whose main role
is to make ATP.
It is more useful to the cell to store glucose as
glycogen when ATP is plentiful.
Anaerobic
catabolism
Lactate Dehydrogenase
O
O
C
C
NADH + H+ NAD+
O
O
O
C
HC
OH
CH3
CH3
pyruvate
lactate
Lactate is also a significant energy source for neurons
in the brain.
Astrocytes, which surround and protect neurons in the
brain, ferment glucose to lactate and release it.
Lactate taken up by adjacent neurons is converted to
pyruvate that is oxidized via Krebs Cycle.
Anaerobic catabolism
Pyruvate
Decarboxylase
Alcohol
Dehydrogenase
CO2
NADH + H+ NAD+
O
O
C
C
O
CH3
pyruvate
H
O
C
CH3
acetaldehyde
H
H
C
OH
CH3
ethanol
Some anaerobic organisms metabolize
pyruvate to ethanol, which is excreted as a
waste product.
NADH is converted to NAD+ in the reaction
catalyzed by Alcohol Dehydrogenase.
There is evidence that glycolysis predates the
existence of O2 in the Earth’s atmosphere and
organelles in cells (it happens in the cytoplasm, not
in some specialized organelle) and it is a metabolic
pathway found in all living organisms.
Comparing energy yield:
Things I’d like you to know about the citric acid cycle
• Like the Calvin cycle, it is a cycle (the Calvin cycle
involves energy capture through incorporation of carbon
into small sugars, which are reduced by energy from
photosynthetic electron transport. The citric acid cycle
involves energy release through loss of carbon from small
organic acids which are oxidized, producing electrons to
be used in mitochondrial electron transport).
• The cycle is “flexible”. The organic acids are all involved
in a very large number of other biosynthetic pathways
• Most of the ATP production is through electron transport
in mitochondrial membranes (cristae)
• As in photosynthesis, regulation energy
production/consumption is critical
3C
This is all
occurring in the
matrix of the
mitochondrion
Lignin; alkaloids;
flavanoids
2C
4C
Fatty acids; lipids;
carotenoids;
abscisic acid
6C
4C
5C
N-assimilation,
amino acid
formation
(proteins),
chlorophylls
This complex is blocked
by cyanide
ATP
synthase
Most of the ATP produced in respiration comes from electrons of NADH
and FADH2 that enter a membrane-bound electron transport process,
producing a membrane potential, leading to oxidative phosphorylation
Mitochondrial electron transport
is controlled by both “supply”
(availability of carbohydrates and
organic acids) and demand
“demand”– (energy requirements
for growth, maintenance and
transport processes)
Demand regulation: when energy
demand for growth, maintenance
and transport processes is high,
ATP is rapidly consumed,
producing ADP, which increases
the rate of respiration)
An “alternate path” (aka, the cyanide resistant path) de-couples
respiratory electron transport from ATP production. This
pathway produces O2, but not ATP. It can serve as an “energy
overflow valve” when supply exceeds demand – but it results in a
net loss of energy from the plant. Is this a relic “error” or an
important physiological function?
An “alternative
oxidase” (AOX)
accepts electrons
coming from
complex II,
preventing them from
getting to complex III
Respiration and Plant Carbon
Balance
On a whole-plant basis, respiration
consumes from 30% to 70% of total fixed
carbon
Leaves account for about half of the total
(Is it possible to increase net growth by reducing
respiration rates?)
The amount of
photosynthate
consumed in
respiration varies
with tissue type and
with environmental
conditions.
When nutrients are
limiting, respiration
rates in roots
increase
dramatically.
Q10: the multiplicative
change in respiration
over a 10 degree C
change in
temperature
Mitochondrial
Respiration (like
photorespiration)
increases rapidly
with temperature.
Conifer roots appear to have relatively low capacity
to acclimate to low temperatures (Lambers et al.
1996)
In cold-hardened conifers, needles maintain low
respiration rates even during warm periods,
apparently maintaining higher concentrations of
sugars (the higher osmotic potential lowers the
freezing point and helps maintain turgor during
water stress)
Respiration is often subdivided into Growth,
Maintenance and Transport costs
Growth respiration: (a.k.a. “construction
respiration”) – a “fixed cost” that depends on the
tissues or biochemicals that are synthesized.
Maintenance respiration: The cost of maintaining
existing tissues and functions
Do high maintenance “costs” reduce growth of large trees?
Why high CO2 concentrations
reduce rates of mitochondrial
respiration?