Biological Systems and Environmental Factors

Thermal Noise and Biological Systems

• Developed to counteract thermal noise in sensory systems.
• Certain types of noise can enhance the sensitivity of a system (stochastic resonance).
• Biological mechanisms may involve amplification cascades;
  • Small energy amounts can induce significant biological control changes.
  • Interaction doesn't need specific receptors, can occur at critical points in reaction networks.

Example of Biological Amplification

• Ionizing radiation can induce mutations;
  • Even one DNA alteration can have dramatic effects.
• Initially observed in radiation biology, but applies to various physical influences.
• Organism evolution adapts to physical environmental parameters:
  • Development of double-stranded DNA protects against ionizing radiation-related breaks.
  • Numerous repair mechanisms in place for DNA damage.

Environmental Influences

• Contemporary issues stem from man-made environmental changes;
  • Focus on electromagnetic influences from power lines and radio frequencies.

Temperature Effects

• Introduction of the Arrhenius equation (Sect. 2.1.5):
  • Describes how temperature affects chemical reactions.
  • Reaction rates will increase with temperature due to overcoming activation energies.

Steady-State Systems and Temperature

• Changes in metabolite concentrations depend on the temperature's effect on opposing fluxes.
• Increased temperature may lead to:
  • Decrease in steady-state concentration if decomposition flux is affected.
  • Complexity arises as temperature changes impact overall activity.

Biological Mechanisms to Manage Temperature

• Protective mechanisms against extreme temperature effects exist.
• Some organisms maintain constant internal temperatures irrespective of the environment:
  • Need effective thermoreceptors for regulation.

Thermoreception Systems

• Homeothermic animals (e.g., birds, mammals) exhibit specific thermoregulation mechanisms.
  • Infrared receptors help some animals like snakes for hunting.
  • Detect temperature changes via thermosensitive membranes (e.g., pit organs in snakes).

Thresholds of Temperature Perception

• Sensors in snakes activate at temperature shifts of $0.003–0.01 \, K$;
• Human skin receptors trigger at $0.06 \, K$.

Thermotaxis Behavior

• Thermotaxis: directed movement along a thermal gradient,
  • Observed across various organisms including bacteria and C. elegans.

Molecular Insights into Thermoreception

• Two types of thermosensitive molecules identified:
  • RNA molecules (riboswitches):
  • Trigger enzyme activation for phospholipid synthesis, heat-shock protein expression in response to temperature changes.
  • Membrane proteins (TRPV channels):
  • Cation channels involved in thermoreception;
  • Structurally change around specific temperature ranges for activation.

Functionality of TRP Channels

• Example from TRPV4 channels shows drastic changes in permeability at $24 \, °C$:
  • Q10 parameter
  • Below $24 \, °C$: Q10 = 1.6;
  • Above $24 \, °C$: Q10 = 19.2.

Non-Arrhenius Behavior

• Some reactions show 'non-Arrhenius' properties;
  • Activities may increase in one temperature range while decreasing in another due to opposing receptor actions.

Thermoregulation Control

• Thermoreceptors influence a steady state of heat production vs. dissipation in organisms.
• Various heat transport methods between organism and environment:
  • Conduction, convection, radiation, evaporation, each influenced by temperature gradients and flow rates.

Heat Transport Calculations

• Fourier’s law for thermal conductivity: $JQc = -l \frac{dT}{dx}$
  • $l$ = thermal conductivity, measured in $\text{J m}^{-1} \text{s}^{-1} \text{K}^{-1}$

Temperature Gradient Impact

• Heat conduction is defined under steady state conditions; for time-dependent situations, $\frac{\partial T}{\partial t} = l \rho C \nabla^2 T$
  • Includes specific heat capacity $C$.

Convection

• Important for heat transport at physiological temperatures, relying on blood circulation and environmental airflow.

Thermal Radiation

• Governed by the Stefan–Boltzmann Law:
  $JQR = s T^4$, where $s = 5.67 \times 10^{-8} \text{W m}^{-2} \text{K}^{-4}$
• Evaporative cooling mechanism for animals in air, dependent on ambient conditions.

Bioheat Equation for Body Temperature Control

• Developed by Harry H. Pennes in 1948: $C \frac{\partial T}{\partial t} = l \rho \nabla^2 T + SAR + F - B(T - T_B)$
  • Represents internal versus external heat dynamics and metabolic heat production.