Enzymes and Energy Diagrams

Understanding Energy Diagrams in Biochemical Reactions

Energy diagrams are graphical representations used to illustrate the energy changes during a chemical reaction. They help visualize the concepts of activation energy and free energy change (ΔG), and the effect of enzymes on these parameters.

• Activation Energy (Ea): The minimum energy required for reactants to undergo a chemical reaction. It is measured from the energy level of the reactants up to the peak of the energy diagram.

• ΔG (Free Energy Change): The difference in energy between reactants and products. It determines whether a reaction is spontaneous (negative ΔG, exergonic) or not.

• Enzyme Function: Enzymes are biological catalysts that lower the activation energy, making reactions proceed faster. They do not change ΔG.

• Energy Diagram Axes: Y-axis represents free energy; X-axis represents the progress of the reaction.

• With vs. Without Enzyme: The catalyzed reaction has a lower peak (activation energy) but the same reactant and product energy levels (ΔG unchanged).

Example: If a reaction has ΔG = −20 kJ/mol, adding an enzyme does not change ΔG; it remains −20 kJ/mol.

Key Equations:

Additional info: Enzymes increase reaction rates by stabilizing the transition state, not by altering the thermodynamic properties of the reaction.

Order of Diffusion Through the Membrane

Membrane Permeability and Molecular Transport

The plasma membrane is selectively permeable, allowing certain molecules to diffuse more readily than others. The hydrophobic (nonpolar) interior favors the passage of small nonpolar molecules, while charged ions require transport proteins.

• Small Nonpolar Molecules: (e.g., O2, CO2) diffuse fastest due to their size and nonpolarity.

• Large Nonpolar Molecules: (e.g., steroid hormones, lipids) are hydrophobic but diffuse slower because of their size.

• Small Polar Molecules: (e.g., water) can diffuse, but less efficiently than nonpolar molecules.

• Large Polar Molecules: (e.g., glucose) diffuse slowly due to both size and polarity.

• Small Ions: (e.g., H+, K+) cannot diffuse without a transport protein due to their charge.

Example: Glucose diffuses slower than water because it is both large and polar, while water is small and polar.

Additional info: Transport proteins such as channels and carriers facilitate the movement of ions and large polar molecules across the membrane.

Counting Carbons in Cellular Respiration and the Calvin Cycle

Tracking Carbon Atoms Through Metabolic Pathways

Understanding how carbon atoms are transferred and released during cellular respiration and photosynthesis is essential for grasping metabolic cycles.

• Glucose: 6 carbons

• G3P (Glyceraldehyde-3-phosphate): 3 carbons

• Pyruvate: 3 carbons

• Acetyl CoA: 2 carbons

• CO2: 1 carbon

• 1 glucose (6C) is split into 2 G3P (3C each).

• Each G3P is converted to pyruvate (3C), then to acetyl CoA (2C) with 1C released as CO2.

• Acetyl CoA enters the citric acid cycle, releasing 2 more CO2 per G3P.

• Thus, 1 G3P yields 3 CO2; 1 glucose yields 6 CO2.

Calvin Cycle: Each turn fixes 1 CO2. Three turns are needed to produce 1 net G3P. To make 1 glucose (2 G3P), 6 turns and 6 CO2 are required.

Example: The first CO2 is released during pyruvate oxidation; the other two during the citric acid cycle.

Additional info: The Calvin cycle regenerates RuBP (ribulose bisphosphate) to continue fixing carbon.

Receptor-Mediated Endocytosis and Exocytosis

Mechanisms of Vesicular Transport Across the Plasma Membrane

Cells use vesicular transport to move large molecules and particles across the plasma membrane. Endocytosis brings substances into the cell, while exocytosis exports them.

• Receptor-Mediated Endocytosis:

  • Receptors on the plasma membrane bind specific ligands.

  • Once enough ligands are bound, the membrane invaginates and forms a vesicle.

  • The receptors and ligands are now inside the vesicle.

• Exocytosis:

  • Proteins are synthesized on the rough ER, packaged into vesicles, and sent to the Golgi apparatus.

  • Vesicles exit the Golgi (trans face) and fuse with the plasma membrane.

  • The contents of the vesicle are released outside the cell.

Key Concept: In endocytosis, what was outside the cell becomes inside the vesicle; in exocytosis, what was inside the vesicle becomes outside the cell.

Example: For a secreted protein: rough ER → Golgi cis face → Golgi trans face → plasma membrane.

Additional info: Vesicular transport is essential for nutrient uptake, neurotransmitter release, and hormone secretion.