Bond Energy & Cell Membrane Essentials

Bond Potential Energy

• Potential Energy (P.E.) vs. Internuclear Distance curve:
  • \text{Positive P.E.}>0 ⇒ repulsion; \text{Negative P.E.}<0 ⇒ attraction.
  • Bonded state = energy minimum (point 3); unbound state = plateau at long distance (point 1).
• Energy required to break a bond:
  • $\Delta E = E<em>{\text{unbonded}} - E</em>{\text{bonded}}$ (always positive).
• Typical single-bond data (approx.):
  • $\text{C–C}:\,-347\,\text{kJ mol}^{-1},\;154\,\text{pm}$
  • $\text{C–O}:\,-358\,\text{kJ mol}^{-1},\;143\,\text{pm}$
  • $\text{O–O}:\,-204\,\text{kJ mol}^{-1},\;148\,\text{pm}$

Cell Types & Organelle Abundance

• Phagocytes: many lysosomes (digest engulfed material).
• B cells: extensive rough ER & Golgi (antibody secretion).
• Red blood cells: virtually no mitochondria & no nucleus (maximize $\text{O}_2$ transport).

Scientific Models

• Built from observation → hypothesis → experimental test → refinement.
• Fluid-mosaic model: dynamic bilayer of phospholipids, proteins, carbohydrates; proteins can laterally diffuse (shown by cell-fusion fluorescence experiment).
• Models useful for visualizing complexity but can oversimplify ("cartoon" cells, pipe/cell analogies).

Membrane Structure & Components

• Phospholipid: glycerol + two fatty acids + phosphate head (amphipathic).
• Other lipids: triglycerides (storage), cholesterol (rigidity buffer), glycolipids (cell ID).
• Protein associations: integral (transmembrane), lipid-anchored, peripheral.

Factors Affecting Membrane Fluidity

• Temperature ↑ ⇒ fluidity ↑ (until disordered).
• Fatty-acid tail length: shorter tails ↑ fluidity.
• Unsaturation (double bonds): more kinks ↑ fluidity.
• Cholesterol: buffers; at low T prevents packing (↑ fluidity), at high T restrains motion (↓ fluidity).
• Adaptive response to cold: insert shorter & unsaturated tails.
  • Example: algae & bacteria add double bonds at low T.
  • Warming oceans → fewer double bonds (e.g., DHA levels may drop), risking excess fluidity.

Selective Permeability & Diffusion

• Pure lipid bilayer permeability trend:
  \text{Hydrophobic} > \text{Small polar} > \text{Large polar} > \text{Ions}
• Hydrophobic molecules & gases (e.g.
  $\text{O}<em>2$) diffuse freely down concentration gradient $\Delta C = C</em>1 - C_2$.
• Glucose & ions require transport proteins; $\text{Na}^+$ least permeable.
• Diffusion rate ∝ concentration gradient magnitude.

Osmosis & Tonicity

• Water crosses membrane via diffusion; moves toward higher solute (lower water) concentration.
• Terms: hypertonic (higher solute), hypotonic (lower solute), isotonic (equal).

Gecko Adhesion Case Study

• Hypotheses: (1) hydrogen bonding, (2) van der Waals.
• Observations: geckos climb dry, polar glass, and non-polar plastic.
  • Plastic surface (hydrophobic) is inconsistent with hydrogen-bonding hypothesis ⇒ supports van der Waals as dominant force.

Nature of Science & Equity

• Scientific knowledge is observational, tentative, creative, and culturally influenced.
• Climate change & membrane fluidity illustrate societal impact; DHA shortages may unequally affect low-income populations.
• Emphasis on diversity, positionality, and avoiding "helicopter science" to improve inclusivity.