Plant Physiology - Respiration Notes

Respiratory Gas Exchange

• Overview of respiration in plants, including key processes and locations within the cell.

Respiratory Substances

• Briefly touches on the various substrates that can be used for respiration.

Nature of Biological Oxidation

• Discusses the oxidation processes involved in respiration.

Outline of Respiratory Metabolism

• Provides an outline of the metabolic pathways involved in respiration, including glycolysis, the citric acid cycle, the pentose phosphate pathway, and oxidative phosphorylation.

Mitochondria: Power House of Cells

• Mitochondria are the cellular sites of respiration.
• They are separated from the cytosol by a double membrane.
• Mitochondria can vary in shape from spherical to tubular, featuring a smooth outer membrane and a highly convoluted inner membrane.
• Infoldings of the inner membrane are called cristae.
• The compartment enclosed by the inner membrane is known as the mitochondrial matrix.
• The mitochondrial matrix contains the enzymes of the Krebs cycle.

Respiration: What is it?

• Releases the energy stored in carbon compounds in a controlled manner for cellular use with or without O_2.
• Aerobic respiration is the biological process by which reduced organic compounds are mobilized and oxidized in a controlled manner.
• This control is achieved by a series of step-by-step reactions which can be grouped to major processes such as:
  • Glycolysis
  • The citric acid cycle
  • The reactions of the pentose phosphate pathway
  • Oxidative phosphorylation

Glycolysis

• Derived from the Greek words glykos, meaning "sugar," and lysis, meaning "splitting."
• It is an anaerobic series of reactions carried out by a group of soluble enzymes located in both the cytosol and the plastid.
• Glycolysis converts carbohydrates into pyruvate, producing NADH and ATP.
• When molecular oxygen is unavailable—for example, in plant roots in flooded soils—glycolysis can be the main source of energy for cells.
• Glycolysis occurs in all living organisms (prokaryotes and eukaryotes).
• The fate of pyruvate depends on oxygen availability.
• When oxygen is present, pyruvate is oxidized to acetyl-CoA, which enters the Krebs cycle.
• Without oxygen, pyruvate is reduced in order to oxidize NADH back to NAD^+.
• Photosynthetic products can also directly enter the glycolytic pathway as triose phosphate.
• For more details, refer to page no. 228 of Chapter 11 of Taiz and Zeiger’s book.

Pentose Phosphate Pathway

• The glycolytic pathway is not the only route available for the oxidation of sugars in plant cells.
• The pentose phosphate pathway also oxidizes sugars into simple sugars.
• It is located both in the cytosol and the plastid.
• Six-carbon glucose-6-phosphate is initially oxidized to the five-carbon ribulose-5-phosphate.
• Carbon is lost as CO_2, and reducing power is conserved in NADPH molecules.
• Ribulose-5-phosphate is converted into three- to seven-carbon sugars.

Respiratory Substrates

• Starch
• Sucrose or other sugars
• Organic acids
• Fats
• Proteins

Respiratory Quotient (RQ)

• RQ indicates an approximation of the type of substrate that is being oxidized.
• RQ = \frac{Amount \ of \ CO2 \ released}{Amount \ of \ O2 \ utilized}
  • Carbohydrates: RQ = 1 (e.g., Leaves, cereal grains, germinating seeds)
  • Proteins: RQ = 0.9 (e.g., Seeds containing proteins)
  • Lipids: RQ = 0.7 (e.g., Seeds with fat and oil-containing plants)

Fate of Pyruvate

• Fate of pyruvate with O_2.

Alternative Glycolytic Pathways

• Gluconeogenesis: metabolizing PEP

Metabolizing PEP - Applications in Agriculture

• Some plants can operate the glycolytic pathway in the opposite direction to synthesize sugar from organic acids, known as gluconeogenesis.
• Castor bean and sunflower, which store a significant quantity of their carbon reserves in the form of oils (triacylglycerols), are converted to sugars to support seedling growth.
• Synthesis of sucrose from photosynthetic triose phosphate is typical and common in many plants.