ANP Study Guide: Structure-Function, Cell Membranes, Gradients, Metabolism, Communication, and Homeostasis

Structure-Function & Levels of Organization

Definitions and Relationships

• Structure: The physical composition and organization of a biological entity (e.g., cell shape, tissue arrangement, protein design).

• Function: The specific job or role performed by a structure (e.g., pumping blood, absorbing nutrients).

• Mechanism: The process by which a function is carried out (e.g., ion movement through channels to generate nerve impulses).

Example: The biconcave shape of red blood cells increases surface area and flexibility, enabling efficient oxygen exchange and passage through capillaries.

Compartmentation

• Compartmentation refers to the separation of physiological functions into specialized spaces (e.g., organelles, organs, blood vs. tissues).

• This allows for distinct chemical environments and regulation of processes.

Primary Tissue Types and Functions

• Connective Tissue: Supports, connects, protects, and stores materials (e.g., bone, blood, adipose).

• Nervous Tissue: Enables rapid communication and coordination (e.g., neurons, glia).

• Epithelial Tissue: Forms selective boundaries, covers surfaces, lines organs, involved in secretion, absorption, and protection.

• Muscle Tissue: Produces force and movement (skeletal, cardiac, smooth).

Muscle Tissue Types

• Skeletal Muscle: Long, striated fibers; voluntary movement; posture and breathing.

• Cardiac Muscle: Branched, interconnected cells; involuntary; pumps blood with synchronized contractions.

• Smooth Muscle: Non-striated, sheet-like arrangement; involuntary; moves substances through organs, controls tube diameter.

Nervous Tissue Structure

• Neurons have dendrites (receive signals) and axons (send signals long distances).

• Myelin sheaths insulate axons, increasing signal speed.

• Complex networks enable integration and rapid coordination.

Epithelial Tissue Functions

• Barrier: Tightly packed cells prevent unwanted passage.

• Exchange: Thin layers allow rapid diffusion.

• Secretion: Glandular cells produce hormones, enzymes, mucus.

• Absorption: Microvilli increase surface area for uptake.

• Protection: Multiple layers resist friction and pathogens.

Connective Tissue and Extracellular Matrix (ECM)

• ECM composition (collagen, elastin, minerals, fluid) determines tissue properties (strength, flexibility, hardness, transport).

Tissue	ECM Structure	Function
Tendon	Dense parallel collagen	Strong muscle-bone attachment
Cartilage	Firm, flexible matrix	Cushions joints
Bone	Mineralized, rigid	Support, protection, calcium storage
Blood	Fluid (plasma)	Transport of gases, nutrients, wastes
Dermis	Collagen, elastin	Skin strength, elasticity
Adipose	Fat-filled cells	Energy storage, insulation

Effects of Skin Damage

• Loss of barrier, fluid balance, temperature regulation, sensation, nutrient exchange, and healing capacity.

Cell Membrane Structure and Transport

Membrane Permeability

• Phospholipid bilayer: hydrophilic heads (outward), hydrophobic tails (inward).

• Small, nonpolar molecules cross easily; large, polar, or charged molecules require proteins.

Membrane Fluidity

• Unsaturated fatty acids increase fluidity and permeability.

• Temperature: higher = more fluid; lower = less fluid.

• Cholesterol buffers fluidity: restrains at high temp, maintains at low temp.

Transport Mechanisms

• Simple Diffusion: Direct movement down gradient, no protein, not saturable.

• Facilitated Diffusion: Uses membrane proteins, saturable, plateaus at high substrate.

• Channels: Fast, low saturability, often gated (e.g., voltage-gated Na+ channel).

• Carriers: Slower, high saturability, not gated (e.g., GLUT4 glucose transporter).

Transporter Specificity and Selectivity

• Specificity: Ability to recognize a particular substrate.

• Selectivity: Preference for one substance over others (e.g., GLUT1 for glucose).

Types of Transporters

• Uniporter: One solute, one direction.

• Symporter: Two solutes, same direction (e.g., Na+-glucose cotransporter).

• Antiporter: Two solutes, opposite directions.

Active and Passive Transport

• Simple Diffusion: Down gradient, no energy.

• Facilitated Diffusion: Down gradient, protein-mediated.

• Secondary Active Transport: Uses energy from another ion's gradient.

• Primary Active Transport: Direct ATP use (e.g., Na+/K+-ATPase).

Endocytosis and Exocytosis

• Endocytosis: Uptake into cell via membrane invagination (e.g., phagocytosis).

• Exocytosis: Release from cell via vesicle fusion (e.g., neurotransmitter release).

Resting Membrane Potential

• Set by ion gradients and relative permeabilities (mainly K+ leak channels).

• Increased Na+ permeability: depolarization (less negative).

• Increased K+ permeability: hyperpolarization (more negative).

Key Equation:

$E_{ion} = \frac{RT}{zF} \ln \left( \frac{[ion]_{out}}{[ion]_{in}} \right)$

(Nernst equation for equilibrium potential)

Action Potential Sequence (Neuromuscular Junction)

1. AP arrives at motor neuron terminal.

2. Voltage-gated Ca2+ channels open; Ca2+ enters.

3. Vesicles release ACh; ACh binds muscle receptors.

4. Ligand-gated channels open; Na+ influx depolarizes membrane.

5. Threshold reached; voltage-gated Na+ channels open (AP upstroke).

6. Na+ channels inactivate; K+ channels open (repolarization).

7. ACh is degraded by acetylcholinesterase.

Flow Down Gradients

Flow, Gradient, and Resistance

• Flow: Movement of a substance (e.g., blood, ions).

• Gradient: Driving force (pressure, concentration, electrical).

• Resistance: Opposition to flow.

Key Equation:

$\text{Flow} = \frac{\text{Gradient}}{\text{Resistance}}$

Types of Gradients

• Pressure Gradient: Drives blood flow.

• Concentration Gradient: Drives diffusion (e.g., O2 from alveoli to blood).

• Electrical Gradient: Drives ion movement (e.g., Na+ into cells).

• Electrochemical Gradient: Combination of concentration and electrical forces.

Factors Affecting Resistance

• Vessel radius (most significant), length, and fluid viscosity.

• Small changes in radius cause large changes in resistance (Poiseuille's Law).

Key Equation:

$R \propto \frac{1}{r^4}$

(Resistance inversely proportional to the fourth power of radius)

Capillary Exchange Forces

• Hydrostatic Pressure: Pushes fluid out of capillaries.

• Osmotic (Oncotic) Pressure: Pulls fluid into capillaries (mainly due to plasma proteins like albumin).

Pathological Changes

• Hyponatremia: Water moves into cells, causing swelling.

• Liver disease: Low albumin reduces oncotic pressure, leading to edema.

• Pulmonary edema: Increases resistance to O2 diffusion.

Energy: Metabolic Pathways, Bioenergetics, Enzymes

Types of Cellular Work

• Chemical: Synthesis of molecules (e.g., proteins).

• Mechanical: Movement (e.g., muscle contraction).

• Transport: Moving substances across membranes.

Enzyme Function and Regulation

• Activation Energy: Minimum energy required to start a reaction; enzymes lower this barrier.

• Factors affecting rate: substrate/enzyme concentration, temperature, pH, cofactors, inhibitors.

• Regulation types:

  • Allosteric: Binding at regulatory site changes activity.

  • Competitive Inhibition: Competes for active site.

  • Noncompetitive Inhibition: Reduces activity regardless of substrate.

  • Covalent Modification: e.g., phosphorylation.

  • Feedback Inhibition: End product inhibits pathway.

  • Enzyme Amount: Synthesis/degradation changes capacity.

Enzyme Specificity and Affinity

• Specificity ensures correct reactions; lack of specificity can cause harmful byproducts.

• High affinity: efficient at low substrate; low affinity: requires higher substrate.

Cofactors and Coenzymes

• Cofactors: Inorganic helpers (e.g., Mg2+, Zn2+).

• Coenzymes: Organic helpers (e.g., NAD, FAD).

• Essential for electron transfer and metabolic reactions.

ATP Resynthesis and Energy Systems

• ATP levels remain stable due to rapid resynthesis (phosphocreatine, glycolysis, oxidative phosphorylation).

• Phosphocreatine donates phosphate to ADP to buffer ATP drops.

• During intense exercise, glycolysis is faster than oxidative phosphorylation but less efficient.

• Lactate production regenerates NAD+ for glycolysis.

• Glucose oxidation is favored at high intensity/low O2 because it yields more ATP per O2 molecule.

Cell–Cell Communication

Signal Life Cycle

1. Synthesis of signal molecule

2. Release from signaling cell

3. Transport to target

4. Binding to receptor

5. Signal transduction

6. Cellular response

7. Termination of signal

Direct vs Indirect Communication

• Direct (Gap Junctions): Fast, local, synchronized (ions, small molecules pass).

• Indirect (Chemical Messengers): Slower, more flexible, allows amplification and long-distance signaling.

Messenger Types

• Water-soluble: Stored in vesicles, released by exocytosis (e.g., peptide hormones).

• Lipid-soluble: Synthesized on demand, diffuse out, require carrier proteins in blood (e.g., steroid hormones).

Signaling Modes

• Synaptic: Across synapse to nearby cell.

• Paracrine: Local diffusion to nearby cells.

• Autocrine: Acts on same cell.

• Endocrine: Via bloodstream to distant cells.

• Neuroendocrine: Neurons release hormones into blood.

Receptor Properties

• Response depends on receptor presence, number, and affinity.

• Competitive antagonists shift dose-response right; noncompetitive reduce maximal response.

• Second messengers (e.g., cAMP, Ca2+) amplify signals.

Homeostasis and Regulation

Internal Environment and Regulated Variables

• Extracellular fluid (ECF) is the internal environment; homeostasis maintains its stability.

• Only critical variables (e.g., temperature, blood glucose, pressure) are tightly regulated.

Homeostatic Control Components

• Receptors/Sensors: Detect changes (mechanical, chemical, thermal, osmotic, nociceptive).

• Comparator/Control Center: Compares variable to set-point, generates error signal.

• Effectors: Carry out corrective actions (e.g., sweat glands, heart).

• Response: Actual physiological change (e.g., sweating, vasoconstriction).

Feedback Loops

• Negative Feedback: Opposes deviation, stabilizes variable (e.g., insulin lowers blood glucose after a meal).

• Positive Feedback: Amplifies change, requires endpoint (e.g., childbirth, blood clotting).

Sensor Properties

• Gain: Sensitivity of response.

• Threshold: Minimum stimulus for activation.

• Saturation: Maximum response capacity.

• Adaptation: Decreased response to constant stimulus.

Hierarchical Prioritization

• Body may sacrifice less critical variables to preserve vital ones (e.g., maintain brain perfusion during hemorrhage).

Examples of Homeostatic Failure

• Type 1 diabetes: Effector (insulin secretion) impaired.

• Type 2 diabetes: Insulin present, but signaling/glucose uptake reduced (insulin resistance).

• Heat stroke: Regulatory mechanisms overwhelmed, core temperature rises uncontrollably.

Summary Table: Feedback Types

Feedback Type	Effect	Example
Negative	Stabilizes variable	Blood glucose regulation
Positive	Amplifies change	Childbirth, action potential

Additional info: Some explanations and context have been expanded for clarity and completeness, including the Nernst equation, Poiseuille's Law, and examples of homeostatic failure.