Homeostasis, Membrane Transport, and Cellular Communication: Study Notes for Anatomy & Physiology

Homeostasis

Definition and Importance

Homeostasis refers to the maintenance of a stable internal environment within the body, despite changes in external conditions. It is essential for proper cellular function and overall health.

• Extracellular Fluid (ECF): The fluid outside cells, including interstitial fluid (between cells) and plasma (in blood vessels).

• Intracellular Fluid (ICF): The fluid within cells.

• Chemical Composition: ECF and ICF differ in their concentrations of ions and molecules, which is crucial for cellular processes.

Equilibrium

• Osmotic Equilibrium: Water moves freely between compartments to balance solute concentrations.

• Chemical Disequilibrium: Solute concentrations differ between ECF and ICF.

• Electrical Disequilibrium: There is a difference in charge across the cell membrane.

Transport Processes

Overview

Transport processes move substances across cell membranes, either passively or actively, and are vital for maintaining homeostasis.

Types of Solutes

• Penetrating vs. Non-penetrating Solutes: Penetrating solutes can cross the membrane, while non-penetrating solutes cannot.

Osmosis

• Movement of water across a semipermeable membrane from low to high solute concentration.

Tonicity

• Isotonic: Solutions with equal osmolarity.

• Hypertonic: Solution with higher osmolarity; water leaves the cell, cell shrinks.

• Hypotonic: Solution with lower osmolarity; water enters the cell, cell swells.

Osmolarity and Tonicity of Saline

• Important for IV solutions to avoid damaging cells.

Gradients

• Chemical Gradient: Difference in solute concentration.

• Electrical Gradient: Difference in charge.

• Electrochemical Gradient: Combined effect of chemical and electrical gradients.

Diffusion

• Passive movement of molecules from high to low concentration.

Protein-Mediated Transport

• Carrier Proteins: Transport specific molecules.

• Water Channels: Allow water movement.

• Facilitated Diffusion: Passive transport via carrier proteins.

Active Transport

• Requires energy (ATP).

• Primary: Direct use of ATP.

• Secondary: Uses gradient created by primary transport.

• Sodium-Potassium Pump: Maintains gradients by pumping Na+ out and K+ in.

Vesicular Transport

• Phagocytosis

• Endocytosis

• Exocytosis

Epithelial Transport

• Paracellular: Between cells.

• Transcellular: Through cells.

• Transcytosis: Combination of endo- and exocytosis.

Energy Requirements

• Passive transport does not require energy; active transport and vesicular transport do.

The Resting Membrane Potential

Definition

The resting membrane potential is the electrical charge difference across the cell membrane when the cell is at rest.

• Chemical Disequilibrium: Difference in ion concentrations between ICF and ECF.

• Leak Channels: Allow ions to move down their gradients, contributing to membrane potential.

• Membrane Potential: Typically -70 mV in neurons. Generated by Na+/K+ pump and leak channels.

Equation:

$E_K = \frac{RT}{zF} \ln \left( \frac{[K^+]_{out}}{[K^+]_{in}} \right)$

(Nernst equation for potassium)

Cellular Communication

Types of Cellular Communication

Cells communicate to coordinate functions and respond to changes. Communication can be local or long-distance.

• Local Communication:

  • Contact-dependent: Cell-to-cell

  • Paracrine: Nearby cells

• Long-distance Communication:

  • Endocrine: Hormones (circulatory system)

  • Neuronal: Neurotransmitters (nervous system)

  • Cytokines: Regulatory proteins for immune responses

Signal Pathways

Signal pathways involve the transmission of information from the cell surface to intracellular targets, resulting in a cellular response.

• Signaling Molecules:

  • Lipophilic: Can cross membranes

  • Lipophobic: Cannot cross membranes

• Membrane Receptor Proteins:

  • G Protein-Coupled Receptors (GPCR):

    • 7-transmembrane domain

    • ATP converted to cAMP

    • cAMP activates PKA

    • PKA phosphorylates other proteins

  • Receptor-Enzymes: Protein kinases (e.g., tyrosine kinase)

• Intracellular Signal Molecules:

  • Second messengers (e.g., calcium ions, cAMP)

• Target Proteins: Proteins that carry out the response.

• Response: The final effect of the signaling pathway.

Signal Transduction

Signal transduction is the process by which a cell converts an extracellular signal into a functional response.

• Amplification: One signal molecule can activate many downstream molecules.

• Signaling Cascades: Series of steps leading to a response.

Novel Signal Molecules

• Some signaling molecules are unique and play specialized roles in communication.

• Examples: Gases (e.g., nitric oxide), lipids, calcium.

Modulation of Signal Pathways

• Cells can regulate their sensitivity to signals by upregulating or downregulating receptors.

Homeostatic Reflex Pathways

Homeostatic reflex pathways maintain internal stability through feedback mechanisms.

• Cannon's Postulates: Principles describing homeostatic regulation.

• Tonic Control: Ongoing regulation by a single system.

• Antagonistic Control: Opposing effects by different systems.

• Homeostatic Control Systems: Integrate signals to maintain balance.

Agonists and Antagonists

• Agonists: Molecules that activate receptors.

• Antagonists: Molecules that block receptors.

Review Table: Types of Membrane Transport

Type	Energy Required?	Direction	Example
Simple Diffusion	No	High to Low	O2 across membrane
Facilitated Diffusion	No	High to Low	Glucose via GLUT transporter
Active Transport	Yes	Low to High	Na+/K+ pump
Osmosis	No	Water: Low to High solute	Water via aquaporins
Vesicular Transport	Yes	Varies	Endocytosis, exocytosis

Additional Review Questions

• Does homeostasis mean that your body is in equilibrium?

• How do substances get moved against their concentration gradient?

• Do exocytosis and endocytosis require an input of cellular energy?

• What is responsible for the resting membrane potential?

• What makes up the ECF?

• Explain what it means for an enzyme to be saturated.

• What are the 5 steps of a general signaling pathway?

• Give some examples of signaling molecules, receptor subtypes, and second messengers.

• What are some novel signaling molecules?

• What does it mean for a receptor to be up- or down-regulated?

• What are Cannon's postulates?