Respiration: converting

reduced carbon to energy

Biol 3470Plant Physiol Biotechnol

Lecture 10Thurs. 9 Feb. 2006

Chapter 7From Rost et al., “Plant biology,” 2nd edn.

Cellular respiration oxidizes fixed carbon to CO2

• Photosynthesis has fixed (reduced) atmospheric carbon into carbohydrates (photoassimilate)

• Respiration mobilizes the energy in these macromolecules using oxidative enzyme reactions via– Glycolysis (6C 3C)– The citric acid cycle or CAC (a/k/a the TCA cycle) (using the 3C

molecules to generate reducing power [electrons] for…)– Oxidative phosphorylation: making ATP

• Photoassimilate also represents a source of C skeletons for anabolism

• When we discuss respiration, we will concentrate on plant respiration and cover:– Interesting differences between plant and animal respiration– Influence of environmental factors on respiration– The relationship of respiration to plant productivity (yield)

Compare and contrast respiration and photosynthesis

• Identical and reciprocal substrates and products but– Occur in different cellular compartments– Use different enzymes

• The two processes are extensively linked

CAC + oxidative P’n (respiration)

Dependent on photosynthesis!

Glyco-lysis

Gluco-neogenesis

Fig. 7.1

Glyco-lysis

Gluco-neogen-esis

Note that plastids have a parallel set of carbohydrate metabolizing enzymes!

To fuel respiration, starch must be moved into the cytosol

• In most species, starch is the main “fuel” for respiration

• Starch is degraded where it is synthesized: in __________

• But glycolysis takes place (mostly) in cytosol – need to export starch hydrolysis products

into cytosolThis occurs by 2 possible routes:• As Glucose by a glucose transporter• As triose-P by the triose-P/Pi antiporter

already discussed• Sucrose breakdown in cytosol occurs via

– Sucrose synthase • Recall that the enzyme’s high G for

sucrose synthesis actually favors its breakdown (+14kJ/mol)

– Invertase: alkaline (cytosol), acid (cell walls, vacuoles) Figure 7.3

glucose transporter

Glc-1-P

Plastidic glycolysis

Favored energetically!

amylase

Starch phosphorylase

Cytosolic

Triose-P

trans-porter

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The net products of the oxidation of hexoses are water, CO2 and ATP

• We will not rehash the basics of respiration in any detail– See section 7.1 for summary, sections 7.3 to 7.7 to review in detail

• Know that entropy greatly favors hexose oxidation over its synthesis

• Plants may get from hexose to CO2 the same way (using the same enzymes) as animals

• However, plants need to be more metabolically flexible than animals because of their sessile nature

• Adaptation to their environment means coping with abiotic (“non-living”) stresses

• In plants, this often means using different enzymes to accomplish the same metabolic goals easily performed in homeothermic animals

• The activity of regulatory enzymes in glycolysis is often limited under nutrient stress– e.g., Pi stress: occurs often in nature!– Complete plant fertilizer is “NPK”

• These “bypass” or alternate enzymes allow glycolysis to progress even in the absence of Pi

Examples:• PFP to bypass PFK• nonphosphorylating NADP-G3PDH to

bypass Pi requiring equilibrium enzymes• PEP carboxylase as part of a bypass of

pyruvate kinase

Theodorou and Plaxton 1993

Plants possess “bypass” enzymes for regulatory steps of glycolysis

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2

31

2

3

The activity of alternative oxidase generates heat instead of ATP

Figure 7.12

• The terminal electron acceptor in mitochondrial electron transport is normally c_______ C oxidase (a/k/a Complex IV)

• Its activity is inhibited by CN ( ______ ), CO, N3

- in animals and plants

Normally donates its e- to complexes II III IV

• These are effective poisons in animals• However, plants possess a CN- resistant oxidase called

the alternative oxidase• Accepts electrons from UQ pool (bypasses complexes II-

IV) and transfers electrons directly to oxygen• Bypasses stepwise electron reduction of energy allowing

H+ extrusion and ATP synthesis!

The activity of the alternative oxidase (AOX) may be used to generate heat

• The activity of the AOX severely limits ATP synthesis

• only allows 2 H+ to exit the mitrochondria’s matrix to generate 0 or 1 ATP

• Compared to ~9 H+ 3 ATP in oxidative phosphorylation

So why use it?• Reason 1: It may raise the temperature of plant

tissues up to 10°C above ambient • This thermogenesis volatilizes amines that

attract pollinators (skunk cabbage and other fly-pollinated species of the Arum family)

• Reason 2: Energy overflow hypothesis AOX engages only after photosynthesis derived substrates saturate the oxidative E.T.C.

• This allows the plant to– burn off excess fixed C that interferes with

source-sink relationships and inhibits photoassimilate translocation

– prevent overreduction (lots of e-) of mt E.T.C. which causes production of ROS (e.g. superoxide)

Skunk cabbage.From Rost et al., “Plant biology,” 2nd edn.

Oilseeds are able to convert stored oil to carbohydrate

• Many seeds store a significant portion of photoassimilate as oil, not carbohydrate

• This oil is mobilized as an energy source upon germination– e.g., canola (45% oil by dry weight versus

maize 5%)• Oil – not water soluble, not transportable• Most plants convert oil droplets (triglycerides)

sucrose to mobilize its energy• Animals cannot interconvert lipids and

carbohydrates!• Again, this gives plants metabolic flexibility in

allocating carbon between lipids and carbohydrates– Seeds can be smaller because lipids store more energy

per gram!

cbc.ca

Mobilizing the energy in stored oil involves the glyoxylate cycle and gluconeogenesis

• Triglyceride conversion to sucrose involves 3 organelles + cytosol

• Fatty acids are removed from triglyceride by lipase

• FA imported into glyoxysome – specialized plant organelle

• Cleaved at every 2nd C to generate acetyl CoA via ß-oxidation

• Glyoxylate cycle take home messages:– Borrowing oxaloacetate from the

mitrochondrion allows citrate synthesis from fatty acids

– It’s a cycle! Regeneration of OAA in mt keeps acetyl CoA incorporation high

– The products of the cycle enter gluconeogenesis to generate sucrose in the __________

– Glycerol from triglyceride also enters gluconeogenesis for sucrose biosynthesis

– NADH enters oxidative phosphorylation

Figure 7.13

a/k/a _____________

Respiration rate varies depending on the plant tissue

• Respiration rate varies with plant age– Related to metabolic demand– High during active growth– Falls during maturation of

tissues– Difficult to relate to individual

pathway flux in mature plants – varies with nutrient status, organ maturation state, temperature

Figure 7.15

• Senescing tissues increase CO2 evolution (e.g., in many ripening fruits)– This is known as the respiratory climacteric– Accompanied by “uncoupling” of oxidative

phosphorylation from ATP synthesis

• Respiration accounts for 30-60% of photoassimilate lost as CO2 30-70% of this is from roots!

http://tfphotos.ifas.ufl.edu/022401.HTM

Environmental effects also change the respiration rate

• Light– Rate in light not always = dark rate– Difficult to measure respiration in light during photosynthesis due to O2

evolution, CO2 recycling – higher in shade-intolerant vs tolerant species (low irradiance low

respiration)• Due to/cause of lower growth rate in shade-grown plants?

• Temperature– Respiration rises exponentially from 5 30°C, doubles with every

10°C rise– This parameter can be defined as the temperature coefficient Q10 (=2)– Optimum temperature for respiration varies depending on environment

• 10°C for arctic species, 30°C for tropical species• Likely due to other factors limiting respiration (ATP availability, different

temperature optima for photosynthetic enzymes)

• Oxygen– Cytochrome C (terminal electron acceptor) has high affinity for O2 – Normally its high concentration is not limiting except in bulky tissues

(fruit, roots) – air spaces circulate gases, flooding causes anoxia