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1.1 Discuss the significance of both substrate level and oxidative phosphorylation. (10)
Substrate Level Phosphorylation:
• Definition: Substrate-level phosphorylation is a metabolic process where a
phosphate group is directly transferred from a high-energy substrate molecule to
ADP, forming ATP.
• Occurrence: This process occurs during glycolysis in the cytoplasm and in the Krebs
cycle within the mitochondrial matrix.
• Mechanism: Enzymes such as kinases facilitate the transfer of the phosphate group.
For example, in glycolysis, phosphoenolpyruvate (PEP) transfers a phosphate to ADP
to form ATP.
• ATP Yield: It generates a relatively small amount of ATP; during glycolysis, 2 ATP
molecules are produced per glucose molecule, and during the Krebs cycle, 1 ATP (or
GTP) per cycle.
• Significance:
• Anaerobic Conditions: It is crucial in anaerobic conditions where oxygen is
not available, allowing cells to produce ATP and survive temporarily.
• Rapid Energy Supply: Provides a quick supply of ATP for immediate energy
needs, especially important in cells that rapidly expend energy, such as
muscle cells during intense activity.
• Metabolic Flexibility: Allows organisms to maintain ATP production under
varying environmental conditions.
Oxidative Phosphorylation:
, • Definition: Oxidative phosphorylation is the production of ATP using energy derived
from the redox reactions of the electron transport chain (ETC), coupled with
chemiosmosis.
• Location: This process takes place in the inner mitochondrial membrane.
• Mechanism:
• Electron Transport Chain (ETC): Electrons are transferred through a series of
protein complexes (I to IV) in the inner mitochondrial membrane, ultimately
reducing oxygen to water.
• Proton Gradient: The energy released from electron transfers is used to pump
protons (H+) from the mitochondrial matrix to the intermembrane space,
creating a proton gradient.
• Chemiosmosis: Protons flow back into the matrix through ATP synthase,
driving the synthesis of ATP from ADP and inorganic phosphate (Pi).
• ATP Yield: Oxidative phosphorylation is highly efficient, producing about 34 ATP
molecules per glucose molecule, significantly more than substrate-level
phosphorylation.
• Significance:
• Main ATP Source: It is the primary source of ATP in aerobic organisms,
providing the bulk of cellular energy.
• Efficient Energy Conversion: Converts the energy stored in NADH and FADH2
into ATP efficiently, supporting various energy-demanding cellular processes.
• Regulation: The process is tightly regulated by the availability of ADP, oxygen,
and the mitochondrial membrane potential, allowing the cell to adapt to
changing energy demands.
Comparison:
, • Energy Yield: Oxidative phosphorylation yields significantly more ATP compared to
substrate-level phosphorylation.
• Dependence on Oxygen: Oxidative phosphorylation requires oxygen, whereas
substrate-level phosphorylation can occur without it.
• Location: Substrate-level phosphorylation occurs in the cytoplasm and
mitochondrial matrix, while oxidative phosphorylation occurs in the inner
mitochondrial membrane.
• Role in Metabolism: Both processes are essential for energy production; substrate-
level phosphorylation provides quick, immediate ATP, while oxidative
phosphorylation supplies sustained energy for prolonged activities.
1.2 Describe the role of metabolic channelling and compartmentalisation in regulation
of metabolism. (10)
Metabolic Channelling:
• Definition: Metabolic channelling refers to the direct transfer of intermediates
between enzymes within a multi-enzyme complex or metabolic pathway without
diffusing into the surrounding solution.
• Mechanisms:
• Multi-enzyme Complexes: Enzymes involved in sequential steps of a
pathway are physically associated, facilitating the direct transfer of
intermediates. For example, in the pyruvate dehydrogenase complex,
intermediates are passed directly from one enzyme to the next.
• Substrate Channeling: This minimizes the transit time of intermediates,
reducing the risk of their degradation or diversion into competing pathways.
• Significance:
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