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

Citric acid cycle = TCA= Kreb’s Cycle glycolysis Citric acid cycle = TCA= Kreb’s Cycle

Anaerobic respiration, or fermentation No O2 required Anaerobic respiration, or fermentation O2 required

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 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 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 Lignin; alkaloids; flavanoids This is all occurring in the matrix of the mitochondrion 2C Fatty acids; lipids; carotenoids; abscisic acid 4C 6C N-assimilation, amino acid formation (proteins), chlorophylls 4C 5C

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?