5C · Module 1 Review: Homeostasis and Cellular Function

Practice this module: MCQ · Flashcards · Problems, Module 1

Feedback¶

Negative feedback reverses a departure: sensor → afferent → integrating center with set point → error signal → efferent → effector.
Positive feedback amplifies a departure and needs an outside cutoff. The full course list: childbirth, milk ejection, ovulation, defecation, clotting.
Homeostasis is dynamic. A cake raising blood glucose and the body bringing it back is homeostasis in action.

Four membrane purposes: barrier, sensitivity, regulated exchange, structural support. ATP synthesis is not one.
About 50% lipid and 50% protein. Phosphate heads hydrophilic, fatty acid tails hydrophobic.
Nonpolar (even sharing): crosses unaided. Polar (uneven sharing, no net charge) and ionic (net charge): hydrophilic, need a protein.
Ligand: any chemical that binds a receptor. Four receptor types; only the intracellular one is outside the membrane.
Body water 42 L in a 70 kg person; intracellular 28 L holds the most.

Transporter	What moves	Energy
Channel	One ion, one direction	Its own gradient
Carrier	One polar molecule	Its own gradient
Cotransporter	Two or more, same direction	Usually the Na⁺ gradient
Countertransporter (exchanger)	Two, opposite directions	Usually the Na⁺ gradient
Pump (ATPase)	Up the gradient	ATP

Outside: Na⁺ 135–145, Cl⁻ 95–105, HCO₃⁻ 22–28, Ca²⁺. Inside: K⁺ 120–140, Mg²⁺, proteins, phosphate.
Diffusion rate rises with gradient, area, temperature, permeability; falls with thickness, molecular size, medium density.

Osmolality = molality × particles. NaCl n = 2, glucose n = 1. Range 280–310, target about 290 (the quiz answer is “about 300”).
Water moves toward the higher osmolality, only when the solute cannot cross.
Dilute bath → swelling, hemolysis. Concentrated bath → crenation.
π = nCRTφ converts mOsm to mmHg so osmotic and hydrostatic pressures can be compared.