Respiration in Plants

The One Thing

Plant respiration is a stepwise, enzyme-controlled oxidation of organic molecules that releases energy gradually and stores it mainly in ATP. With oxygen, glucose is metabolised through glycolysis, the link reaction, the Krebs cycle, and the electron transport system; without sufficient oxygen, glycolysis continues through fermentation but yields much less ATP.

Reactions, Processes and Experiments

Key Terms

• Cellular respiration: The oxidation or breakdown of food molecules inside cells to release energy in the form of ATP.
• Glycolysis: The ten-step breakdown of one glucose molecule into two molecules of pyruvate in the cytoplasm; it does not directly require oxygen.
• Glucose: A six-carbon carbohydrate that commonly serves as the starting substrate for cellular respiration.
• Pyruvate: A three-carbon compound formed at the end of glycolysis; it enters mitochondria during aerobic respiration.
• Krebs cycle: A cyclic series of reactions in the mitochondrial matrix that oxidises acetyl-CoA and produces carbon dioxide, NADH, FADH2, and ATP or GTP.
• Acetyl-CoA: A two-carbon compound formed from pyruvate during the link reaction and used in the Krebs cycle.
• Link reaction: The conversion of pyruvate into acetyl-CoA, releasing carbon dioxide and producing NADH.
• Electron transport system: A series of electron carriers on the inner mitochondrial membrane that transfers electrons and helps produce ATP.
• Oxidative phosphorylation: ATP formation using the energy released as electrons pass through the electron transport system and drive proton movement.
• Chemiosmosis: The movement of hydrogen ions through ATP synthase down their electrochemical gradient, producing ATP.
• NADH and FADH2: Reduced electron carriers that transport high-energy electrons and hydrogen to the electron transport system.
• Substrate-level phosphorylation: Direct formation of ATP when a phosphate group is transferred from a phosphorylated substrate to ADP.
• Fermentation: Anaerobic breakdown of pyruvate that regenerates NAD+ so glycolysis can continue when oxygen is insufficient.
• Respiratory quotient: The ratio of carbon dioxide produced to oxygen consumed during respiration, written as RQ = CO2 evolved / O2 consumed.
• Amphibolic pathway: A metabolic pathway that functions in both breakdown and synthesis; respiration provides intermediates for many biosynthetic processes.
• ATP: The principal energy-storage molecule produced by respiration and used to power growth, transport, synthesis, movement, and other cellular activities.
• Aerobic respiration: Respiration that uses oxygen and includes glycolysis, the link reaction, the Krebs cycle, and the electron transport system.
• Anaerobic respiration: Respiration without sufficient oxygen; in plant tissues it produces ethanol and carbon dioxide and yields only 2 ATP per glucose.

Easily Confused

• Glycolysis and the Krebs cycle: Glycolysis occurs in the cytoplasm and produces pyruvate; the Krebs cycle occurs mainly in the mitochondrial matrix and begins with acetyl-CoA.
• Aerobic and anaerobic respiration: Aerobic respiration uses oxygen and has a high ATP yield; anaerobic respiration produces ethanol and carbon dioxide in plant tissues and yields only 2 ATP per glucose.
• Link reaction and Krebs cycle: The link reaction converts pyruvate into acetyl-CoA; the Krebs cycle oxidises acetyl-CoA and regenerates oxaloacetate.
• Electron transport system and oxidative phosphorylation: The electron transport system transfers electrons and establishes a proton gradient; oxidative phosphorylation uses the resulting energy to form ATP.
• Oxidative phosphorylation and substrate-level phosphorylation: Oxidative phosphorylation depends on electron transfer and proton movement; substrate-level phosphorylation transfers phosphate directly from a phosphorylated substrate to ADP.
• NADH and FADH2 versus ATP: NADH and FADH2 carry high-energy electrons and hydrogen to the electron transport system; ATP directly supplies usable energy for cellular work.
• Respiratory quotient for carbohydrates and fats: Carbohydrates have an RQ of approximately 1, whereas fats generally have an RQ less than 1.
• Respiration and photosynthesis: Photosynthesis stores solar energy in organic molecules; respiration releases that stored chemical energy for cellular work.

What Gets Asked

• Describe the stages and locations of aerobic respiration. Marks depend on identifying glycolysis in the cytoplasm, the link reaction and Krebs cycle in relation to the mitochondrial matrix, and the electron transport system on the inner mitochondrial membrane.
• Calculate or state respiratory yields. Common errors include omitting that glycolysis has a net yield of 2 ATP and 2 NADH, or failing to double the Krebs-cycle products because one glucose produces two acetyl-CoA molecules.
• Explain the role of oxygen. Oxygen is the final electron acceptor and combines with electrons and hydrogen ions to form water; it is not used directly as the substrate for glycolysis.
• Compare aerobic and anaerobic respiration in plants. The required anaerobic products are ethanol and carbon dioxide, and the energy yield is only 2 ATP per glucose.
• Interpret respiratory quotient values. Carbohydrates give approximately RQ = 1, fats generally give values below 1, and organic acids may give values above 1.
• Explain why respiration is amphibolic. Respiration supplies carbon skeletons and intermediate compounds for synthesising amino acids, fatty acids, and other biomolecules, rather than functioning only as an energy-producing process.