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

Homeostasis

Definition and Components

Homeostasis refers to the maintenance of a stable internal environment within the body, despite changes in external conditions. It is essential for the proper functioning of cells and organs.

• Extracellular fluid (ECF): The fluid outside cells, including plasma and interstitial fluid.

• Interstitial fluid: The fluid between cells within tissues.

• Plasma: The liquid component of blood.

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

• Osmotic equilibrium: A state where water concentration is equal across membranes.

• Chemical disequilibrium: Unequal distribution of solutes (e.g., ions) across membranes.

• Electrical disequilibrium: Difference in charge across the cell membrane.

Example: The body maintains blood glucose levels within a narrow range through hormonal regulation.

Transport Processes

Types of Solutes and Osmosis

Transport processes regulate the movement of substances across cell membranes, crucial for cellular function.

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

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

• Isosmotic: Solutions with equal osmolarity.

• Hyperosmotic: Solution with higher osmolarity than another.

• Hyposmotic: Solution with lower osmolarity than another.

• Tonicity: The effect of a solution on cell volume (isotonic, hypertonic, hypotonic).

• Isotonic: No net movement of water; cell volume remains unchanged.

• Hypertonic: Water moves out of the cell; cell shrinks.

• Hypotonic: Water moves into the cell; cell swells.

Example: IV saline solutions are designed to be isotonic to prevent cell damage.

Osmolarity and Tonicity of Saline as an IV Solution

• Osmolarity: Total concentration of solute particles per liter ($osmolarity = \frac{moles\ of\ solute}{liters\ of\ solution}$).

• Tonicity: Depends on the concentration of non-penetrating solutes.

Chemical Gradient

• Chemical Gradient: Difference in concentration of a substance across a membrane.

Types of Membrane Transport

• Diffusion: Passive movement of molecules from high to low concentration.

• Protein-Mediated Transport: Transport involving membrane proteins.

• Carrier Proteins: Bind and transport specific molecules across membranes.

• Water Channels (Aquaporins): Facilitate rapid water movement.

• Ion Channels: Allow ions to pass through membranes.

• Open Channels: Always open for passage.

• Gated Channels: Open or close in response to stimuli.

• Transporter Specificity, Competition, Saturation: Transporters are specific for substrates, can be competed for, and have a maximum rate (saturation).

• Facilitated Diffusion: Passive transport via carrier proteins.

• Active Transport: Movement against the gradient, requires energy.

• Primary Active Transport: Direct use of ATP (e.g., sodium-potassium pump).

• Secondary Active Transport: Uses energy from another gradient.

• Sodium Potassium Pump: Maintains gradients by pumping 3 Na+ out and 2 K+ in per ATP ($3Na^+_{out} : 2K^+_{in}$).

• Vesicular Transport: Movement via vesicles (phagocytosis, endocytosis, exocytosis).

• Phagocytosis: Cell engulfs large particles.

• Endocytosis: Cell takes in substances via vesicles.

• Exocytosis: Cell expels substances via vesicles.

• Epithelial Transport: Movement across epithelial layers.

• Paracellular: Between cells.

• Transcellular: Through cells.

The Resting Membrane Potential

Definition and Mechanisms

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

• Chemical disequilibrium between ICF and ECF: Different ion concentrations inside and outside the cell.

• Leak channels: Allow passive movement of ions, contributing to membrane potential.

• Membrane potential: Voltage across the cell membrane, typically -70 mV in neurons.

• Responsible factors: Na+, K+ gradients, and selective permeability.

Equation:

$V_m = \frac{RT}{F} \ln \frac{[K^+]_{out}}{[K^+]_{in}}$ (Nernst equation for potassium)

Cell Communication

Types and Mechanisms

Cells communicate to coordinate functions via chemical and electrical signals.

• Local Communication: Includes gap junctions and contact-dependent signaling.

• Gap junctions: Direct cytoplasmic connections between cells.

• Contact dependent: Requires membrane-bound molecules.

• Paracrine & Autocrine signaling: Paracrine acts on nearby cells; autocrine acts on the same cell.

• Long distance: Hormones (endocrine), neurocrines (nervous system), cytokines (immune system).

Signal Pathways and Molecules

• Signaling molecules: Lipophilic (can cross membranes) and lipophobic (cannot cross membranes).

• G protein coupled receptors (GPCR): Membrane receptors that activate intracellular pathways.

• 3 part transducer receptor: Receptor, G protein, effector enzyme.

• ATP converted to cAMP: Adenylyl cyclase converts ATP to cyclic AMP.

• cAMP activates PKA: cAMP binds to and activates protein kinase A.

• PKA phosphorylates other proteins: Alters their activity.

• Receptor-enzymes: Receptors with intrinsic enzyme activity (e.g., tyrosine kinases).

• Protein kinases: Enzymes that add phosphate groups to proteins.

• Intracellular signal molecules: Second messengers like calcium ions and cAMP.

• Target proteins: Proteins affected by signaling pathways.

• Response: Cellular change due to signaling.

• Signal Transduction: Conversion of extracellular signal to intracellular response.

• Amplification: One signal molecule leads to many responses.

• Signaling cascades: Series of reactions amplifying the signal.

• Novel Signal Molecules: Gases (NO), lipids, calcium.

Second Messengers Table

Second Messenger	Source	Main Effect
cAMP	ATP via adenylyl cyclase	Activates PKA, regulates metabolism
Ca2+	ER, extracellular influx	Muscle contraction, secretion
NO (Nitric Oxide)	Enzymatic synthesis	Vasodilation
Lipids	Membrane phospholipids	Inflammation, signaling

Regulation and Control

Up and Down Regulation

• Up regulation: Increase in receptor number/sensitivity.

• Down regulation: Decrease in receptor number/sensitivity.

• Agonists: Molecules that activate receptors.

• Antagonists: Molecules that block receptors.

Homeostatic Reflex Pathways

• Cannon's Postulates: Principles describing homeostatic regulation (e.g., nervous system role, tonic/antagonistic control).

• Tonic Control: Ongoing activity regulates physiological parameters.

• Antagonistic Control: Opposing effects (e.g., sympathetic vs. parasympathetic nervous system).

• Homeostatic Control Systems: Feedback mechanisms maintaining stability.

Additional Review Questions

• Does homeostasis mean that your body is in equilibrium? No, it means dynamic stability, not equilibrium.

• How do substances get moved against their concentration gradient? Via active transport, requiring energy.

• Do exocytosis and endocytosis require an input of cellular energy? Yes, they require ATP.

• What is responsible for the resting membrane potential? Ion gradients and selective permeability.

• What makes up the ECF? Plasma and interstitial fluid.

• Explain what it means for an enzyme to be saturated. All active sites are occupied; maximum rate achieved.

• What are the 5 steps of a general signaling pathway? Signal molecule, receptor, intracellular signal, target protein, response.

• Give some examples of signaling molecules, receptor subtypes, and second messengers. Hormones, neurotransmitters, GPCRs, cAMP, Ca2+.

• What are some novel signaling molecules? Gases (NO), lipids.

• What does it mean for a receptor to be up or down regulated? Change in receptor number/sensitivity.

• What are Cannon's postulates? Principles of homeostatic regulation.

Additional info: Some definitions and examples were expanded for clarity and completeness.