Bioenergetics: The Flow of Energy in the Cell (Chapter 5) – Study Notes

Bioenergetics: The Flow of Energy in the Cell

Types of Biological Work

Cells perform various types of biological work, all of which require energy. Energy is defined as the capacity to do work, causing specific physical or chemical changes.

• Biosynthetic work: Synthesis of cellular components (e.g., photosynthesis in plants).

• Mechanical work: Movement of cells or cellular structures (e.g., muscle contraction, movement of cilia).

• Concentration work: Accumulation of molecules in a cell or organelle against a concentration gradient.

• Electrical work: Movement of ions across membranes, generating electrical potentials (e.g., nerve impulses).

• Heat production: Generation of heat to maintain body temperature.

• Light production: Bioluminescence, as seen in some fungi and marine organisms.

Example: Active transport of ions, muscle contraction, and synthesis of macromolecules are all forms of biological work.

Introduction to Metabolism

Cells are analogous to chemical factories, carrying out millions of chemical reactions per second. Metabolism is the sum total of all chemical reactions occurring in a cell.

• Catabolic pathways: Breakdown of molecules to release energy.

• Anabolic pathways: Synthesis of complex molecules from simpler ones, requiring energy input.

Metabolism Includes Catabolic and Anabolic Pathways

Metabolic pathways are divided into two main types:

• Catabolic pathways: Degradative, energy-yielding processes (e.g., glycolysis, cellular respiration).

• Anabolic pathways: Biosynthetic, energy-requiring processes (e.g., protein synthesis, DNA replication).

These pathways are governed by the First Law of Thermodynamics (energy cannot be created or destroyed).

Source of Energy	Source of Carbon
Phototrophs – Light feeders	Autotrophs – Self-feeders (CO2)
Chemotrophs – Chemical feeders	Heterotrophs – Other feeders

Additional info: Photoautotrophs use light and CO2; chemoheterotrophs use chemicals and organic carbon.

Cells Use Energy to Create and Maintain Order

According to the Second Law of Thermodynamics, the entropy (disorder) of the universe (or a closed system) will increase over time. Cells maintain order by using energy, but the total entropy of the system (cell + environment) still increases.

• State of disorder is more energetically favorable.

• Chemical reactions inside cells must increase the total entropy of the entire system.

Change in Free Energy Determines Whether a Chemical Reaction Occurs Spontaneously

The spontaneity of a chemical reaction is determined by the change in free energy (ΔG):

• ΔG < 0: Reaction is spontaneous (exergonic).

• ΔG > 0: Reaction is non-spontaneous (endergonic; requires energy input).

• ΔG = 0: Reaction is at equilibrium.

The useful energy in a system is called free energy (G).

Equation:

$\Delta G = G_{\text{products}} - G_{\text{reactants}}$

$\Delta G = \Delta H - T\Delta S$

• ΔH: Change in enthalpy (heat content)

• T: Temperature in Kelvin

• ΔS: Change in entropy (disorder)

Equilibrium and the Equilibrium Constant (Keq)

At equilibrium, there is no net change in the concentrations of reactants or products:

A + B ⇌ C + D

The equilibrium constant (Keq) is the ratio of product concentrations to reactant concentrations at equilibrium:

$K_{eq} = \frac{[C]_{eq}[D]_{eq}}{[A]_{eq}[B]_{eq}}$

• If the concentration ratio is less than Keq, the reaction proceeds to the right (toward products).

• If the concentration ratio is greater than Keq, the reaction proceeds to the left (toward reactants).

Factors Affecting Free Energy Change (ΔG)

ΔG is affected by:

• Concentration of reactants and products

• Temperature

• pH

• Pressure

To compare ΔG values, standard conditions are used:

• Temperature: 25°C (298 K)

• Pressure: 1 atm

• Concentration: 1 M for all reactants and products

• pH: 7

This is called the standard free energy change (ΔG°').

Calculating ΔG

For the reaction A ⇌ B:

$G'_B = G_B^\circ' + RT \ln[B]$

$G'_A = G_A^\circ' + RT \ln[A]$

$\Delta G' = G'_B - G'_A$

$\Delta G' = (G_B^\circ' - G_A^\circ') + (RT \ln[B] - RT \ln[A])$

$\Delta G' = \Delta G^\circ' + RT \ln \frac{[B]}{[A]}$

At equilibrium:

$\Delta G' = 0 = \Delta G^\circ' + RT \ln K_{eq}$

$\Delta G^\circ' = -RT \ln K_{eq}$

• R = gas constant (1.987 cal/mol·K)

• T = temperature in Kelvin

Reactions in Cells

Many cellular reactions are endergonic (ΔG°' > 0). Cells drive these reactions by:

1. Regulating concentrations of products and reactants (shifting equilibrium).

2. Input of energy (e.g., coupling to ATP hydrolysis).

Example: Synthesis of macromolecules (proteins, nucleic acids) is endergonic and requires energy input.