RESPIRATION IN PLANTS

• Biological process whereby the energy stored in carbohydrates from Photosynthesis is released in a step-wise, controlled manner.
• Energy released is coupled to the synthesis of ATP.
• ATP is essential for plant cell maintenance, growth and development

Carbohydrate Conversion

Starch glucose.

​

Sucrose + water glucose + fructose.

Equation for Aerobic Respiration

C₆H₁₂O₆ + 6O₂ + 6H₂O

(glucose)

6CO₂ + 12H₂O + energy (ATP)

1 mole glucose

36 ATP

3 Stages of Respiration

• Glycolysis.
• TCA Cycle.
• Electron Transport Chain.

GLYCOLYSIS

• Occurs in all living organisms.
• Only stage which can occur without oxygen
• Oldest stage of respiration.
• Operated for billions of years in anaerobic organisms.
• Converts glucose to 2 pyruvates in cytosol.
• with O₂ goes on to TCA cycle
• without O₂ pyruvate is converted to lactate or ethanol. (fermentation)
• Yields 2ATP/mole glucose in the absence of O₂

Glycolysis

Glucose (6C)

Ethanol

Lactate

TCA Cycle

CO₂

+O₂

2 Pyruvate (3C)

-O₂ -O₂

IN CYTOSOL

IN CYTOSOL

IN CYTOSOL

IN MITOCHONDRIA

TCA CYCLE

Electron Transport System

NADH and FADH₂

_e-

_e-

4e^- + 4H⁺ + O₂ 2H₂O

cyt. oxidase

_H+

_H+

ATP

Chemi osmotic model

_H+

_H+

_H+

_H+

_H+

_H+

_H+

_H+ _H+

_H+ _H+ _H+

_H+ _H+

_H+

Ion concentration difference represents a source of free energy

Chemiosmotic model

H H

+ +

_H+

_H+

_H+

_H+

_H+

_H+

_H+

_H+ _H+ _H+

_H+ _H+

_H+

The energy represented by the H⁺ gradient is converted to a chemical form (ATP) via the ATP synthase

3 Stages of Respiration

• Glycolysis:
• cytoplasm
• with or without oxygen present
• breaks glucose (6C) into 2 pyruvates (3C)
• TCA Cycle:
• mitochondrial matrix
• only if oxygen present
• converts pyruvate via acetyl CoA into CO₂; generates NADH and FADH₂
• Electron Transport Chain:
• mitochondrial membranes = cristae
• transfers electrons from NADH and FADH₂ to reduce O to Water and generate ATP

Mitochondria

• Spherical to oval
• about 1 micron diameter
• # mito./cell increases with demand for

respiration; 300-1000/root tip cell

• Double-membrane bound organelle.
• outer smooth
• inner folds forming cristae
• controls movement in/out
• site of electron transportm
• Matrix
• soluble phase

^ site of TCA cycle; DNA, RNA, ribosomes

matrix

cristae

Alternate Fates of Glucose C

• Not all C respired to CO₂
• Intermediates of respiration branch off:
• amino acids.
• pentoses for cell wall structure.
• Nucleotides.
• porphyrin biosynthesis.
• fatty acid synthesis.
• lignin precursors.
• precursors for carotenoid synthesis, hormones

Factors Affecting Resp. Rate

• [Substrate]
• [ATP]
• [Oxygen]
• Temperature
• Plant type
• Plant organ
• Plant age

Factors: Substrate Availability

• Resp. higher right after sundown compared to right before sunrise due to [S]
• Shaded leaves respire slower than lighted leaves
• Starvation of plant tissue results in utilization of proteins
• High [ATP] in cell and get negative feedback on resp.

Factors: [Oxygen]

• No effect until [O₂] < 1%
•  Cyt oxidase not sensitive to O₂ until 0.05%
• O₂ diffuses in water 10,000 X slower than in air
• Some plants have intercellular air system, e.g., aerenchyma in shoots and roots (rice)
• Very low levels of O₂ see accelerated breakdown of sugars to ethanol and CO₂ evolved = Pasteur Effect

Factors: Temperature

• Q₁₀ for respiration is 2.0 - 2.5 between 5 and 25C.
• Q₁₀ = rate of process at one temperature divided by the rate at 10C lower temp.

^ Decreases with most plant tissues at 30-35C

^ O₂ being used so fast, it can’t diffuse fast enough into tissues.

• Tropical regions - 70-80% PS C lost to resp. due to high night temperatures and resp. rates

Factors: Plant Type/Organ/Age

• Resp. rate tends to increase with age of plant.
• Young trees lose about 1/3 daily PS C to resp. and doubles with older trees as ratio of PS/Non-PS tissue decreases.
• Greater metabolic activity = greater resp. rates
• Root tips, dev. buds and meristematic regions in general have higher respiration rates.
• In veg. tissues, resp. decreases from the tip to the mature regions.
• Seeds - low resp. rates, dormant, desiccation results in slowdown of respiration

Factors: Plant Type/Organ/Age (cont.)

• Ripening Fruit.
• Resp. high when young cells are dividing and growing.
• Climacteric Fruit. (apples, tomatoes)
• Sharp increase in rate immediately before fruit ripening

= climacteric rise in respiration.

• Coincides with full ripeness and flavor and preceded by huge increase in ethylene production.
• This leads to senescence and decrease in respiration.
• Non-climacteric Fruit.
• Citrus, cherries, grapes, pineapple, strawberries
• Insensitive to ethylene.

Controlled Atmosphere Storage

• Lower O₂ (2% - 3%) & raise CO₂ (5% - 10%)
• slows down resp.
• No ethylene.
• high CO₂ also inhibits ethylene synthesis.
• Temps. typically about -1 to -0.5C
• Pick apples in Sept./Oct. when green and immature and store in CA.
• expose to normal air with ethylene when ready to sell fresh apples in March.h

Cyanide Resistant Respiration

• Aerobic resp. (cyt oxidase) in plants and animals inhibited by CN^- and N₃^- (azide).
• bind to Fe in enzyme and halts e^- transport.
• Animals: CN causes resp. to decrease fast, virtually irreversible and fatal.
• Plants: display a 10-25% CN-resistant resp. and alternate pathway for electron flow.
• electron flow branches off to alternate oxidase
• less ATP produced.

Cyanide Resistant Respiration (cont.)

• Metabolic Role?
• No clear role
• Operates when cyt oxidase poisoned
• Energy overflow hypothesis.
• overflow for electrons when resp. rate exceeds demand for ATP; high with high carbo. levels
• Skunk cabbage, Voodoo lily, Stinking lily: CN- res. pathway causes temp. of spadix to increase 10-20C.
• volatilization of odiferous cmpds which attract pollinators.