Chapters 5-6: Cellular Biology - Homeostasis

osmosis

• Water moves across a selectively permeable membrane in response to a solute gradient.

• Happens via aquaporins (water channels).

How are body fluids compartmentalized?

• Intracellular fluid (ICF) and extracellular fluid (ECF).

• Water distribution varies by age and sex.

Osmotic equilibrium

water concentration equalized.

Chemical disequilibrium

• different solute concentrations

Electrical disequilibrium

• charge differences (more + outside, more - inside).

Isosmotic

Same osmotic pressure

Hyperosmotic

Higher osmotic pressure.

Hypoosmotic

Lower osmotic pressure.

molarity vs osmolarity

• Molarity = moles/L

• Osmolarity = number of particles/L (important for dissociation, e.g., NaCl → Na⁺ + Cl⁻)

tonicity

Tonicity considers solute concentration and membrane permeability

• Isotonic: No net water movement.

• Hypertonic: Water moves out → cell shrinks.

• Hypotonic: Water moves in → cell swells.

types of transport across membranes

• Bulk flow (fluids via pressure gradients)

• Diffusion (passive transport)

• Protein-mediated transport (channels, carriers)

• Vesicular transport (endo-, exocytosis)

properties that define diffusion

• Passive process (high → low concentration)

• Fast over short distances

• Increases with temperature

• Decreases with large molecular weight

• Movement based on electrochemical gradients for ions

types of membrane transport proteins

• Channel proteins: Water and ion channels (open, gated)

• Carrier proteins: Uniport, symport, antiport mechanisms

carrier-mediated transport

• Facilitated diffusion (no energy, down gradient)

• Active transport (requires ATP, against gradient)

  • Primary active transport: Direct ATP use (e.g., Na⁺/K⁺ pump)

  • Secondary active transport: Uses energy from another gradient

transporter saturation

• Transport rate plateaus when all carrier proteins are occupied (transport maximum).

• Saturation and competition affect transport efficiency.

resting membrane potential (RMP)

• Inside of the cell is negative relative to outside.

• Caused mainly by K⁺ leaving the cell.

• Electrical and concentration gradients create an electrochemical gradient.

Depolarization

Membrane potential becomes less negative.

Repolarization

Return to resting state.

Hyperpolarization

Membrane potential becomes more negative.

G-protein coupled receptors (GPCRs)

GPCR activates G-protein → G-protein activates amplifier enzymes:

• Adenylyl cyclase → cAMP → Protein kinase A → cellular responses

• Phospholipase C (PLC) → DAG + IP₃ → PKC activation and Ca²⁺ release

vesicular transport

• Phagocytosis: Cell engulfs large particles (e.g., bacteria).

• Endocytosis: Formation of vesicles (receptor-mediated or pinocytosis).

• Exocytosis: Vesicle contents released outside the cell.

insulin secretion

integrates multiple membrane processes

Glucose enters β-cell → metabolism ↑ ATP → K⁺ channels close → membrane depolarization → Ca²⁺ channels open → insulin exocytosis.