Biology Chapter Homeostasis and Feedback Mechanisms

Homeostasis

• Homeostasis is a steady state, maintained through energy expenditure.
• Equilibrium is a state that does not require energy; homeostasis regulates a parameter without being in equilibrium.
• A steady state occurs when a vital parameter is well-regulated, staying at a constant value because actions that lower the parameter are matched by actions that raise it.

Negative Feedback

• Negative feedback is the most common feedback mechanism in homeostasis.
• It reverses any deviation from the stable point.
• If a factor becomes excessive or deficient, a negative feedback system will reduce the factor to its mean value.

Positive Feedback

• Positive feedback loops maintain or accelerate a change to a particular stimulus.
• The effect of each feedback cycle perpetuates the stimulus.
• Positive feedback is less common than negative feedback and can lead to instability or death.

Electrolyte Content

• Electrolyte content in bodily fluids varies between interstitial fluid and intracellular fluid.
• Sodium is highest in the plasma at 140 mEq/L.
• Potassium is highest in the intracellular fluid at 160 mEq/L.
• Chloride is highest in interstitial fluid at 117 mEq/L.

Osmotic Pressure and Reflection Coefficient

• Osmolarity is the concentration of solutes in a solution.
• Iso-osmotic solutions have the same solute concentration.
• Isotonic solutions have a reflection coefficient of zero and equal solute concentrations.
• Osmotic pressure is determined by the concentration of impermeable solutes.

Tonicity of Solutions

• The three types are isotonic (no change), hypotonic (cell swells), and hypertonic (cell shrinks).
• Tonicity depends on the concentrations of impermeable solutes.
• Some solutes can still permeate the cell membrane.

Transport Function of the Plasma Membrane

• Simple diffusion occurs through the phospholipid layer, following the electrochemical gradient (e.g. O₂ , CO₂, glycerol).
• Facilitated diffusion occurs through protein channels or pores, also following the electrochemical gradient, but with a protein carrier. (e.g., Na⁺, K⁺, H₂O).
• Primary active transport goes against the electrochemical gradient and uses ATP directly (e.g., Na⁺-K⁺ ATPase, Ca²⁺ pump).
• Secondary active transport also goes against the electrochemical gradient, using gradients from primary active transport (e.g. Na⁺-glucose symport).
• Bulk transport moves large substances across membranes (e.g., endocytosis and exocytosis).

Fick's Law of Diffusion

• Fick's law relates diffusive flux to the concentration gradient.
• Permeability coefficient (P), partition coefficient (K), diffusion coefficient (D), thickness of the membrane (Δx), and cross-sectional area (A) all affect the rate of diffusion.

Saturation in Carrier-Mediated Transport

• Transport maximum (Tm) is the upper limit of carrier-mediated transport.
• This limit is reached when all transport proteins are occupied, and occurs as binding sites for solute become scarce.

Stereospecificity in Carrier-Mediated Transport

• Carrier proteins are stereospecific, meaning they only bind to specific isomers of solutes.
• The transporter recognizes and transports a closely related isomer but not the opposite.
• D-glucose is used in the body; L-glucose isn't.

Competition in Carrier-Mediated Transport

• Some molecules compete for the transport capacity of carrier proteins.
• Related solutes can occupy binding sites making transport of another solute difficult.

Ion Channel Characteristics

• Conductance (g) of a channel depends on its probability of being open; greater probability = greater conductance = faster diffusion.
• Voltage-gated channels respond to changes in membrane potential.
• Ligand-gated channels respond to changes in ligands such as hormones or neurotransmitters.
• Second-messenger-gated channels respond to changes in signaling molecules.
• Mechanically-gated channels respond to changes in membrane tension.

Mechanisms Responsible for the Resting Membrane Potential

• Chemical gradients from active transport pumps create concentration differences.
• Selective membrane permeability allows specific ions to cross (e.g., potassium is selectively permeable).
• Electrical gradients oppose chemical gradients as K⁺ leaks outside the cell, establishing a negative charge intracellularly.

Equilibrium Potential

• There is an equilibrium potential when the electrical and chemical forces are balanced for each specific ion.
• This is described by the Nernst equation.
• The membrane potential moves toward the equilibrium potential of the most permeable ion.

Driving Force for Diffusion

• Driving force affects the diffusion of ions across membranes.
• The driving force is determined by calculating the difference between the membrane potential (RMP) and the equilibrium potential for each ion.

Characteristics of Action Potentials

• Stereotypical size and shape; every action potential is identical for that cell type.
• Propagation; action potential is propagated down, maintaining its size and shape.
• All-or-none response; only a stimulus above a threshold triggers an action potential.

Refractory Periods

• During the absolute refractory period, another action potential can't be triggered. Inactivation gates of Na⁺ channels do not allow this.
• The relative refractory period begins after the absolute refractory period and a larger stimulus can trigger an action potential due to higher K⁺ conductance.

Propagation of Action Potentials

• Action potentials are propagated along a nerve or muscle fiber through spread of local currents.
• The initial segment of the axon initiates action potentials.

Factors Affecting Conduction Velocity in Nerves

• Factors affecting conduction velocity include myelination, axon diameter, and temperature.

Receptors, Signaling Pathways and Messengers

• Different types of receptors exist affecting signaling pathways.
• Types include ligand-gated ion channels, G protein-coupled receptors (GPCRs), enzyme-linked receptors, and nuclear receptors.

Good Study Habits

• Consistent scheduling, active note-taking, study breaks, organized workspace, prioritized tasks, distraction-free environment, peer study groups, and periodic review sessions contribute to effective studying.