Anatomy & Physiology: Cellular Physiology, Membrane Transport, and Endocrine System Study Guide

Cellular Physiology and Homeostasis

Homeostasis and Body Fluid Compartments

Homeostasis refers to the maintenance of a stable internal environment within the body. The body contains several fluid compartments, each with distinct chemical and osmotic properties.

• Intracellular Fluid (ICF): The fluid within cells, rich in potassium and proteins.

• Extracellular Fluid (ECF): The fluid outside cells, including plasma and interstitial fluid, high in sodium and chloride.

• Interstitial Fluid: The fluid between cells in tissues.

• Plasma: The liquid component of blood.

Example: The difference in sodium concentration between ICF and ECF is essential for nerve impulse transmission.

Cellular Organelles and Their Functions

Cells contain specialized structures called organelles, each with unique functions.

• Nucleus: Contains genetic material and controls cellular activities.

• Mitochondria: Site of ATP production through cellular respiration.

• Lysosomes: Contain digestive enzymes for breaking down waste.

• Endoplasmic Reticulum (ER): Involved in protein and lipid synthesis.

Additional info: The plasma membrane separates the ICF from the ECF and regulates transport.

Membrane Transport Mechanisms

Types of Membrane Transport

Cells exchange substances with their environment through various transport mechanisms.

• Passive Transport: Movement of substances down their concentration gradient without energy input (e.g., diffusion, osmosis).

• Active Transport: Movement of substances against their concentration gradient, requiring energy (e.g., sodium-potassium pump).

Example: Glucose uptake in the intestine involves both facilitated diffusion and active transport.

Facilitated Diffusion vs. Simple Diffusion

Both are passive transport processes, but they differ in mechanism.

• Simple Diffusion: Direct movement of molecules through the lipid bilayer.

• Facilitated Diffusion: Movement of molecules via specific membrane proteins (channels or carriers).

Additional info: Facilitated diffusion is essential for polar molecules that cannot cross the lipid bilayer directly.

Concentration Gradients and Determinants

A concentration gradient is the difference in the concentration of a substance between two regions.

• Determinants: Membrane permeability, molecule size, and temperature.

• Formula: Fick's Law of Diffusion: $J = -D \frac{dC}{dx}$

Example: Oxygen diffuses from alveoli into blood due to a concentration gradient.

Transport Proteins: Uniport, Symport, Antiport

Transport proteins facilitate movement of molecules across membranes.

• Uniport: Transports a single type of molecule in one direction.

• Symport: Transports two different molecules in the same direction.

• Antiport: Transports two different molecules in opposite directions.

Example: The sodium-glucose symporter in intestinal cells.

Cell Surface Receptors

Receptors are proteins that bind signaling molecules (ligands) and initiate cellular responses.

• Types: Ion channel-linked, G protein-coupled, and enzyme-linked receptors.

• Function: Convert extracellular signals into intracellular actions.

Example: Insulin receptor triggers glucose uptake in muscle cells.

Signal Transduction and Cellular Communication

Agonists and Antagonists

Agonists and antagonists are molecules that interact with receptors to modulate their activity.

• Agonist: Binds to a receptor and activates it, mimicking the natural ligand.

• Antagonist: Binds to a receptor but blocks its activation.

Example: Beta-blockers are antagonists of beta-adrenergic receptors.

Negative and Positive Feedback

Feedback mechanisms regulate physiological processes.

• Negative Feedback: Reduces the output or activity to maintain homeostasis.

• Positive Feedback: Enhances the output, leading to amplification of the process.

Example: Regulation of blood glucose by insulin (negative feedback).

Endocrine System and Hormonal Regulation

Major Endocrine Glands and Hormones

The endocrine system consists of glands that secrete hormones to regulate body functions.

• Pituitary Gland: Master gland controlling other endocrine glands.

• Thyroid Gland: Regulates metabolism via thyroid hormones (T3, T4).

• Adrenal Glands: Produce cortisol, aldosterone, and adrenaline.

• Pancreas: Regulates blood glucose via insulin and glucagon.

Additional info: Disorders of these glands can lead to conditions such as hypothyroidism, Cushing's syndrome, and diabetes.

Thyroid Hormone Pathways

Thyroid hormones are regulated by the hypothalamic-pituitary-thyroid axis.

• TRH (Thyrotropin-releasing hormone): Released from hypothalamus.

• TSH (Thyroid-stimulating hormone): Released from anterior pituitary.

• T3 and T4: Released from thyroid gland, regulate metabolism.

Formula: $\text{TRH} \rightarrow \text{TSH} \rightarrow \text{T3/T4}$

Example: Low T3/T4 levels stimulate increased TRH and TSH secretion.

Growth Hormone Regulation

Growth hormone (GH) is produced by the anterior pituitary and regulated by hypothalamic hormones.

• GHRH (Growth hormone-releasing hormone): Stimulates GH release.

• Somatostatin: Inhibits GH release.

Example: GH promotes growth and metabolism; somatostatin prevents excessive GH secretion.

Effects of Insulin-like Growth Factors (IGFs)

IGFs are hormones that mediate many actions of GH, especially growth and development.

• IGF-1: Stimulates cell growth and division.

• Regulation: GH increases IGF-1 production in the liver.

Example: IGF-1 levels are used clinically to assess GH activity.

Hormonal Disorders and Symptoms

Imbalances in hormone levels can lead to various disorders.

• Hypothyroidism: Low thyroid hormone levels; symptoms include fatigue, weight gain, and cold intolerance.

• Hyperthyroidism: High thyroid hormone levels; symptoms include weight loss, heat intolerance, and anxiety.

• Cushing's Syndrome: Excess cortisol; symptoms include obesity, hypertension, and skin changes.

Additional info: Hormonal feedback loops are essential for maintaining endocrine balance.

Table: Comparison of Membrane Transport Types

Transport Type	Energy Required	Direction	Example
Simple Diffusion	No	Down gradient	Oxygen across alveolar membrane
Facilitated Diffusion	No	Down gradient	Glucose via GLUT transporter
Active Transport	Yes	Against gradient	Sodium-potassium pump
Symport	Varies	Same direction (2 molecules)	Sodium-glucose symporter
Antiport	Varies	Opposite directions	Sodium-calcium exchanger