Energy Transformations, Membrane Structure, and Cell Biology Study Notes

Energy Transformations

Thermodynamics in Biology

Thermodynamics describes how energy is transferred and transformed in biological systems. Understanding these laws is essential for explaining cellular processes and metabolism.

• First Law of Thermodynamics: Energy cannot be created or destroyed; only transformed. Example: Glucose oxidation releases energy, which is captured as ATP and heat.

• Second Law of Thermodynamics: Energy transformations increase entropy (disorder) in the universe. Example: Heat released from metabolism increases disorder in surroundings.

• Why life requires constant energy: Living systems maintain order (low entropy) by continuously consuming energy to drive cellular work.

ΔG Practice – Conceptual

Gibbs free energy (ΔG) determines whether a reaction is spontaneous. Entropy (ΔS) and enthalpy (ΔH) are key factors.

• If ΔG < 0: The reaction is exergonic and spontaneous, releasing free energy.

• If ΔH = +300 kJ and TΔS = +350 kJ: $\Delta G = \Delta H - T\Delta S$ $\Delta G = +300\ \text{kJ} - (+350\ \text{kJ}) = -50\ \text{kJ}$ The reaction is spontaneous and entropy-driven.

Coupled Reactions

Cells often couple energetically unfavorable reactions to favorable ones, such as ATP hydrolysis, to drive essential processes.

• ATP hydrolysis drives endergonic reactions: Phosphorylation of molecules increases their potential energy. Example: Na+/K+ pump uses ATP to move ions against gradients.

• Proton pump and cotransport: ATP powers proton export; H+ gradient forms; H+ re-entry drives symport of molecules like sucrose.

Application

Enzyme function is sensitive to temperature. Denaturation affects reaction rates and efficiency.

• When an enzyme stops functioning above 37°C: Enzyme denaturation reduces reaction efficiency and spontaneity.

Membrane Structure and Function

Labeling Challenge

Biological membranes are complex structures that regulate transport and communication.

• Phospholipids: Form amphipathic bilayer that controls selective permeability.

• Cholesterol: Buffers membrane fluidity—prevents rigidity in cold and melting in heat.

• Integral vs. Peripheral Proteins: Integral proteins span the bilayer for transport; peripheral proteins attach loosely for signaling or structure.

• Carbohydrate chains: Enable cell recognition and communication (glycolipids, glycoproteins).

Transport Scenarios

Cells use various transport mechanisms to move substances across membranes.

Scenario	Transport Type	Justification
a. O2 diffuses into a cell	Simple diffusion	Nonpolar, moves with gradient
b. Na+ moves against gradient using ATP	Primary active transport	Requires ATP via Na+/K+ pump
c. Glucose enters via channel	Facilitated diffusion	Uses carrier/channel protein
d. Water moves into hypotonic cell	Osmosis	Water follows concentration gradient

Predict the Outcome

• RBC in 10% NaCl: Water exits—cell shrinks (crenation).

Higher-Order Connection

• Na+/K+ Pump: Pumps 3 Na+ out and 2 K+ in per ATP. Maintains resting potential, nerve impulse conduction, and osmotic balance.

Cell Structure

Surface Area : Volume Ratio

Cell size affects efficiency of diffusion and nutrient exchange. Smaller cells have higher surface area to volume (SA:V) ratios.

• Why cells are small: High SA:V ratios allow faster diffusion and nutrient exchange.

• Example:

  • 2 cm cube — SA:V = 3:1

  • 4 cm cube — SA:V = 1.5:1

  • Smaller cell = more efficient.

Organelle ID & Function

Cell organelles perform specialized functions essential for life.

Organelle	Function
Ribosome	Protein synthesis
Rough ER	Protein modification and transport
Smooth ER	Lipid synthesis, detoxification
Golgi Apparatus	Protein packaging and secretion
Mitochondrion	Energy conversion (ATP synthesis)

Endosymbiotic Theory

Summary

The endosymbiotic theory explains the origin of mitochondria and chloroplasts as formerly free-living bacteria engulfed by ancestral eukaryotic cells.

• Evidence:

  • Double membranes

  • Circular DNA

  • Bacterial-type ribosomes

  • Independent replication

Final Mini Quiz

1. What happens to ΔG when entropy increases? ΔG becomes more negative; reaction becomes more spontaneous.

2. Why do phospholipids form bilayers spontaneously in water? Hydrophobic tails aggregate away from water; increases water's entropy.

3. Two features mitochondria and chloroplasts share indicating prokaryotic origin: Double membranes and circular DNA (also bacterial ribosomes and autonomous division).