Enzymes and Reaction Energy

Enzyme Function and Activation Energy

Enzymes are biological catalysts that accelerate chemical reactions by lowering the activation energy required for the reaction to proceed. However, they do not alter the overall free energy change of the reaction.

• Activation Energy (Ea): The minimum energy required to initiate a chemical reaction.

• Free Energy Change (ΔG): The difference in energy between reactants and products; determines whether a reaction is spontaneous.

• Enzyme Effect: Enzymes lower activation energy but do not change ΔG.

Equation:

Example: The hydrolysis of ATP is catalyzed by enzymes, making the reaction proceed faster, but the amount of energy released (ΔG) remains unchanged.

Diffusion Through Membranes

Types of Molecules and Diffusion Rates

Diffusion is the passive movement of molecules across cell membranes. The rate of diffusion depends on the size and polarity of the molecules.

• Small Nonpolar Molecules: Diffuse fastest (e.g., O2, CO2).

• Large Nonpolar Molecules: Diffuse slower than small nonpolar molecules.

• Small Polar Molecules: Diffuse slower than nonpolar molecules (e.g., H2O).

• Large Polar Molecules: Diffuse slowest among polar molecules.

• Small Ions: (e.g., H+, K+) require transport proteins and diffuse slowest overall.

Example: Oxygen diffuses rapidly across the plasma membrane, while sodium ions require channel proteins.

Carbon Flow in Respiration and Photosynthesis

Tracking Carbon Atoms in Metabolic Pathways

Carbon atoms are transferred and transformed during cellular respiration and photosynthesis. Understanding their flow helps clarify how energy and matter are cycled in cells.

• Glucose: Contains six carbon atoms.

• G3P (Glyceraldehyde-3-phosphate): Contains three carbon atoms; one glucose yields two G3P molecules.

• Respiration: Each G3P's three carbons are released as three CO2 molecules during pyruvate oxidation and the citric acid cycle; one glucose yields six CO2 molecules.

• Photosynthesis (Calvin Cycle): The cycle must occur three times to produce one G3P; carbon dioxide is fixed, and remaining molecules regenerate the cycle's starting compound.

Equation:

Example: In the Calvin cycle, three CO2 molecules are required to synthesize one G3P.

Endocytosis and Exocytosis

Cellular Transport Mechanisms

Cells use endocytosis and exocytosis to transport large molecules and particles across the plasma membrane. These processes are essential for nutrient uptake, waste removal, and cell signaling.

• Receptor-Mediated Endocytosis: Ligands bind to specific receptors on the cell surface; the membrane folds inward, forming a vesicle that contains the receptors and ligands.

• Exocytosis: Vesicle contents are released outside the cell when the vesicle fuses with the plasma membrane.

• Secreted Proteins: Synthesized at the rough endoplasmic reticulum, processed in the Golgi apparatus (from cis to trans), and packaged into vesicles for exocytosis.

Example: Insulin is secreted by pancreatic cells via exocytosis after being processed through the ER and Golgi.

Summary Table: Diffusion Rates of Molecules

Additional info: Academic context was added to clarify the mechanisms and significance of each process, including definitions, examples, and equations.