This chapter explores plant respiration, detailing processes like glycolysis, fermentation, and aerobic respiration, emphasizing energy release from substrates and the roles of ATP in cellular processes.
Plants require Oxygen (O2) for respiration and release Carbon Dioxide (CO2).
They utilize stomata and lenticels for gas exchange, which allows each plant part to manage its own gas exchange efficiently.
The diffusion distance for gases in plants is short, facilitating easier exchange of gases at the cell surface.
12.2 Glycolysis
Glycolysis refers to the breakdown of glucose into pyruvic acid and occurs in the cytoplasm.
It is an anaerobic process and is critical for all organisms, yielding energy from glucose in the form of ATP.
Glucose originates from sucrose (photosynthesis product) and is phosphorylated by enzymes into glucose-6-phosphate.
The pathway involves a series of enzymatic reactions, resulting in 2 molecules of pyruvate, 2 NADH, and a net gain of 2 ATP molecules following a series of steps.
12.3 Fermentation
Fermentation allows energy extraction without oxygen, producing either lactic acid or alcohol (ethanol) as byproducts.
It yields little energy (2 ATP) compared to aerobic processes, and can be hazardous due to the accumulation of acids or alcohol.
Aerobic respiration occurs in the presence of O2 and fully oxidizes pyruvic acid into CO2 and H2O, generating more ATP compared to fermentation.
Key Processes:
Krebs Cycle: Converts acetyl CoA into CO2 and supports electron carriers (NADH, FADH2) production.
Electron Transport System (ETS): Utilizes electrons from NADH and FADH2 to generate ATP through oxidative phosphorylation.
12.5 The Respiratory Balance Sheet
The theoretical yield for complete aerobic oxidation of glucose can net up to 38 ATP molecules, based on efficient energy transfer through glycolysis, Krebs cycle, and ETS.
Assumptions made in this calculation must account for ideal conditions which rarely occur in living systems where pathways are non-linear.
12.6 Amphibolic Pathway
The respiratory pathway is classified as amphibolic because it facilitates both catabolic (breaking down molecules for energy) and anabolic (using respiratory intermediates to synthesize molecules) processes.
Substrates like fats and proteins enter the respiration pathway through various stages after initial breakdown.
12.7 Respiratory Quotient (RQ)
RQ is the ratio of CO2 produced to O2 consumed during respiration; it varies with the type of substrate utilized:
Carbohydrates: RQ = 1.0
Fats: RQ < 1
Proteins: RQ ≈ 0.9
Summary
Plants respire primarily through glycolysis, followed by fermentation or aerobic respiration depending on oxygen availability, facilitating energy production necessary for cellular processes.
The pathways of respiration are interconnected and complex, allowing for energy extraction through catabolic and anabolic processes, reflected in various respiratory quotients based on the substrate utilized.
Key terms/Concepts
Plants breathe through stomata and lenticels, exchanging gases by diffusion.
Glycolysis is the initial step in cellular respiration, occurring in the cytoplasm, where glucose is split into pyruvate.
Fermentation occurs under anaerobic conditions, producing lactic acid or ethanol with low energy yield.
Aerobic respiration fully oxidizes pyruvate in the presence of oxygen, utilizing the Krebs Cycle and ETS to synthesize ATP.
The net gain of ATP from one glucose molecule can theoretically reach 38 ATP in aerobic conditions.
The respiratory pathway is amphibolic, involved in both breaking down and synthesizing biomolecules.
The Respiratory Quotient indicates substrate type, varying for carbohydrates (RQ=1), fats (RQ<1), and proteins (RQ≈0.9).
ATP acts as the energy currency, synthesized primarily during oxidative phosphorylation in aerobic respiration.