Bioenergetics: The Flow of Energy in the Cell

Types of Cellular Work

Cells require a constant supply of energy to perform various forms of work essential for life. These include:

• Chemical Work: Synthesis of macromolecules and other cellular components.

• Mechanical Work: Movement of organelles, muscle contraction, and cell motility.

• Transport Work: Pumping substances across membranes against concentration gradients.

Example: ATP hydrolysis provides energy for active transport of ions across the plasma membrane.

Energy Flow in the Biosphere

Energy flows linearly through the biosphere, primarily from sunlight to chemical energy via photosynthesis, and is coupled to the cyclic flow of organic molecules through metabolic pathways.

• Linear Energy Flow: Energy enters as light and leaves as heat.

• Cyclic Molecule Flow: Organic molecules are recycled through processes like glycolysis and the citric acid cycle.

Bioenergetics and Thermodynamics

Bioenergetics applies thermodynamic principles to biological systems, focusing on energy conservation and transformation.

• System + Surroundings = Universe: In thermodynamics, the universe is divided into the system (the part under study) and its surroundings.

• Open System: Cells are open systems, meaning they exchange energy and matter with their surroundings.

Additional info: Open systems can maintain non-equilibrium states necessary for life.

Calorie vs. Calorie

The calorie is a unit of energy. One calorie is the amount of energy required to raise the temperature of 1 gram of water by 1°C. On food labels, "Calorie" (with a capital C) refers to a kilocalorie (1,000 calories).

• 1 Calorie (food) = 1 kilocalorie = 1,000 calories

Direction of Chemical Reactions

The direction of chemical reactions is governed by two key thermodynamic principles:

• Second Law of Thermodynamics: The entropy of the universe must increase.

• Free Energy: The free energy of the system must decrease for a spontaneous reaction.

Equation:

Where is the change in free energy, is the change in enthalpy, is temperature, and is the change in entropy.

Free Energy Changes and Equilibrium

Free energy changes () are related to equilibrium constants for biochemical reactions. They indicate whether a reaction is at equilibrium and, if not, the direction it will proceed to reach equilibrium.

• At equilibrium:

• Spontaneous reaction:

• Non-spontaneous reaction:

Equation:

Where is the standard free energy change, is the gas constant, and is temperature.

Standard Free Energy Change Values

Tables of standard free energy change values () are available for common biochemical reactions. These values help predict reaction spontaneity under standard conditions.

Meanings and Differences: and

: Actual free energy change under cellular conditions. : Standard free energy change at pH 7, 25°C, 1 atm, and 1 M concentrations.

Exergonic vs. Endergonic Reactions

• Exergonic: Reactions with ; release energy; spontaneous.

• Endergonic: Reactions with ; require energy input; non-spontaneous.

Example: ATP hydrolysis is exergonic; glucose phosphorylation is endergonic.

Key Equations (5-17, 5-19, 5-21, 5-22)

• Equation 5-17:

• Equation 5-19:

• Equation 5-21:

• Equation 5-22: at equilibrium

Steady State in Cellular Reactions

In cells, reactions often operate in a steady state, where concentrations of reactants and products remain constant over time, but the system is not at equilibrium. This allows cells to maintain metabolic flux and perform work.

Driving Reactions with Positive

Reactions with positive values (non-spontaneous) can be made to proceed by:

• Altering Reactant/Product Ratios: Increasing reactant concentration or decreasing product concentration.

• Coupling Reactions: Linking an endergonic reaction to an exergonic one (e.g., ATP hydrolysis) to drive the overall process.

Example: Glucose phosphorylation is coupled to ATP hydrolysis to make the overall reaction exergonic.