Costanzo | Physiology | Chapter 1 | Cellular Physiology | Study Guide
This study guide provides a structured review of the fundamental principles of cellular physiology, including body fluid composition, membrane transport mechanisms, excitable cell electrical properties, and the mechanics of muscle contraction.
Chapter 1: Cellular Physiology Notes
I. Volume and Composition of Body Fluids
Total Body Water (TBW): Water accounts for 50% to 70% of total body weight. It is inversely correlated with body fat; thus, females typically have lower TBW percentages than males due to higher adipose tissue.
Body Fluid Compartments: TBW is distributed into two main areas: Intracellular Fluid (ICF), which contains two-thirds of TBW, and Extracellular Fluid (ECF), which contains one-third.
ECF Subcompartments: The ECF is further divided into interstitial fluid (the larger portion bathing the cells) and plasma (the fluid circulating in blood vessels). These are separated by the capillary wall, which is virtually impermeable to large proteins, meaning interstitial fluid is an ultrafiltrate of plasma and contains almost no protein.
Compositional Differences:
The major cation in ECF is sodium (Na+), with chloride (Cl−) and bicarbonate (HCO3−) as balancing anions.
The major cations in ICF are potassium (K+) and magnesium (Mg2+), balanced by proteins and organic phosphates.
Osmolarity is equal in both the ICF and ECF (approximately 290 mOsm/L) because water flows freely across cell membranes to dissipate any transient differences.
II. Characteristics of Cell Membranes
Lipid Component: Membranes consist of phospholipids, cholesterol, and glycolipids. Phospholipids are amphipathic, featuring hydrophilic glycerol "heads" and hydrophobic fatty acid "tails" that form a lipid bilayer.
Protein Component:
Integral Proteins: These are embedded in the membrane via hydrophobic interactions and often span the entire bilayer (transmembrane proteins) to act as channels, transporters, or receptors.
Peripheral Proteins: These are loosely attached to one side of the membrane.
III. Transport Across Cell Membranes
Downhill Transport (Passive): Moves substances down an electrochemical gradient and requires no metabolic energy.
Simple Diffusion: The only transport not involving a protein carrier. It depends on concentration gradients, partition coefficients (solubility in oil), and membrane thickness.
Facilitated Diffusion: Uses a membrane carrier but is still passive (e.g., GLUT4 glucose transport).
Uphill Transport (Active): Moves substances against a gradient and requires energy.
Primary Active Transport: Uses ATP directly. The most critical example is the Na+-K+ ATPase (pump), which pumps three Na+ ions out and two K+ ions in, maintaining the essential concentration gradients across the cell.
Secondary Active Transport: Uses the Na+ gradient established by primary active transport as an indirect energy source.
Cotransport (Symport): All solutes move in the same direction (e.g., Na+-glucose in the small intestine).
Countertransport (Antiport): Solutes move in opposite directions (e.g., Ca2+-Na+ exchange in muscle cells).
Carrier Features: All carrier-mediated transport exhibits saturation (transport maximum or Tm), stereospecificity (recognizing specific isomers), and competition between chemically related solutes.
IV. Diffusion and Equilibrium Potentials
Diffusion Potential: A potential difference generated when a charged ion diffuses down its concentration gradient. It only occurs if the membrane is permeable to that specific ion.
Equilibrium Potential: The potential difference that exactly balances the tendency of an ion to diffuse down its concentration gradient. It is calculated using the Nernst Equation.
Typical Values: ENa+=+65 mV, ECa2+=+120 mV, EK+=−85 mV, and ECl−=−90 mV.
V. Resting Membrane Potential
Excitable cells have a resting potential of -70 to -80 mV.
It is determined by the ions with the highest permeabilities at rest, primarily K+ and Cl−.
The Na+-K+ ATPase indirectly contributes by maintaining the K+ concentration gradient required for the K+ diffusion potential.
VI. Action Potentials
Terminology: Depolarization makes the potential less negative; hyperpolarization makes it more negative. Inward current (e.g., Na+ entry) depolarizes the cell, while outward current (e.g., K+ exit) hyperpolarizes it.
Ionic Basis:
Resting State: Conductance to K+ is high.
Upstroke: Depolarization to threshold opens Na+ activation gates, causing a rapid increase in Na+ conductance and an inward Na+ current.
Repolarization: Na+ inactivation gates close and K+ conductance increases, leading to an outward K+ current.
Hyperpolarizing Afterpotential (Undershoot): K+ conductance is briefly higher than at rest, driving the potential closer to the K+ equilibrium potential.
Refractory Periods: The absolute refractory period occurs when Na+ inactivation gates are closed and no stimulus can trigger a new AP. The relative refractory period requires a stronger-than-normal stimulus because K+ conductance is high.
Propagation: APs spread via local currents. Conduction velocity is increased by increasing fiber diameter (reducing internal resistance) or by myelination, which allows for saltatory conduction at the nodes of Ranvier.
VII. Synaptic and Neuromuscular Transmission
Neuromuscular Junction: An AP in the motoneuron opens Ca2+ channels, causing Ca2+ entry and the quantal release of Acetylcholine (ACh). ACh binds to nicotinic receptors on the motor end plate, opening Na+ and K+ channels to produce a depolarization called the End Plate Potential (EPP).
Synaptic Arrangements: Can be one-to-one, one-to-many, or many-to-one (common in the CNS, where inputs must summate to reach threshold).
Summation: Spatial summation involves simultaneous inputs at different locations; temporal summation involves successive inputs in rapid succession.
VIII. Muscle Physiology
Skeletal Muscle: Ca2+ is released from the sarcoplasmic reticulum (SR) and binds to troponin C. This causes a conformational change that moves tropomyosin, allowing actin and myosin to interact for cross-bridge cycling.
Smooth Muscle: Lacks striations because it is not organized into sarcomeres. Contraction is triggered by Ca2+ entering the cell or being released from the SR via IP3-gated channels. Ca2+ binds to calmodulin, which activates myosin-light-chain kinase to phosphorylate myosin and initiate tension
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Part I: Short-Answer Quiz
Instructions: Answer the following questions using two to three sentences based on the provided text.
How does the proportion of body fat affect total body water?
What are the primary differences between the composition of extracellular fluid (ECF) and intracellular fluid (ICF)?
Define the three characteristics shared by all forms of carrier-mediated transport.
How does the Na+-K+ ATPase maintain the electrochemical gradients across a cell membrane?
What is the functional difference between an isosmotic solution and an isotonic solution?
Explain the role of the activation and inactivation gates on the nerve Na+ channel during the upstroke of an action potential.
What is the "all-or-none" response in the context of action potentials?
How do inhibitory postsynaptic potentials (IPSPs) affect the likelihood of a postsynaptic cell firing an action potential?
Describe the role of the transverse (T) tubules in skeletal muscle excitation-contraction coupling.
What is the difference between multiunit and single-unit smooth muscle (implied by the description of multiunit)?
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Part II: Quiz Answer Key
Body Fat and Water: Total body water correlates inversely with body fat, meaning it is a higher percentage of body weight when fat is low and a lower percentage when fat is high. Because females generally have a higher percentage of adipose tissue than males, they tend to have a lower percentage of total body water.
ECF vs. ICF Composition: The major cation in ECF is sodium (Na+), with chloride (Cl−) and bicarbonate (HCO3−) serving as balancing anions, whereas the major cations in ICF are potassium (K+) and magnesium (Mg2+). Additionally, ICF is more acidic than ECF and contains a much lower concentration of ionized calcium (Ca2+).
Carrier-Mediated Characteristics: All carrier-mediated transport processes exhibit saturation, where the rate levels off at a transport maximum (Tm); stereospecificity, where the carrier recognizes specific isomers like D-glucose; and competition, where chemically related solutes compete for binding sites.
Na+-K+ ATPase Function: This primary active transport mechanism pumps three Na+ ions out of the cell for every two K+ ions pumped in, using ATP as a direct energy source. By transporting both ions against their electrochemical gradients, it maintains low intracellular Na+ and high intracellular K+ concentrations.
Isosmotic vs. Isotonic: Isosmotic refers to two solutions having the same calculated osmolarity regardless of whether the solutes can cross the membrane. Isotonic refers to solutions having the same effective osmotic pressure, meaning no net water flow occurs because the solutes are restricted by the membrane (reflection coefficient = 1).
Na+ Channel Gates: During the upstroke, depolarization causes the activation gate to open quickly while the inactivation gate is still open, allowing Na+ to flow into the cell. The upstroke ends when the slower inactivation gate finally closes in response to the same depolarization, terminating the Na+ current.
All-or-None Response: This principle states that an action potential either occurs fully or not at all; if a cell is depolarized to threshold, a stereotypical action potential is inevitable. If the stimulus fails to reach the threshold potential, no action potential is generated.
IPSPs and Inhibition: IPSPs hyperpolarize the postsynaptic cell by opening Cl− or K+ channels, which drives the membrane potential further away from the threshold. This makes it more difficult for excitatory inputs to trigger an action potential.
T Tubule Role: T tubules are invaginations of the sarcolemmal membrane that carry depolarization from action potentials on the muscle surface deep into the fiber. This depolarization triggers a conformational change in dihydropyridine receptors, which then opens Ca2+-release channels in the sarcoplasmic reticulum.
Smooth Muscle Types: Multiunit smooth muscle consists of separate fibers that behave as independent motor units with little coupling between cells, whereas the text implies other types (single-unit) may involve coordinated activity via gap junctions as seen in other tissues. Multiunit fibers are typically regulated by dense autonomic innervation.
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Part III: Essay Format Questions
Instructions: Use the provided source material to develop detailed responses for the following prompts.
Transport Mechanisms: Compare and contrast simple diffusion, facilitated diffusion, primary active transport, and secondary active transport. Include examples of specific solutes and the energy requirements for each.
The Action Potential: Detail the ionic basis of the nerve action potential. Discuss the specific conductance changes for sodium and potassium and how these changes correlate with the phases of depolarization, repolarization, and hyperpolarization.
Neuromuscular Transmission: Trace the sequence of events at the neuromuscular junction from the arrival of an action potential at the motoneuron terminal to the initiation of an action potential in the muscle fiber.
Muscle Mechanics: Explain the length-tension and force-velocity relationships in skeletal muscle. Discuss how the overlap of thick and thin filaments determines the active tension a muscle can generate.
Clinical Physiology: Analyze how disorders such as Myasthenia Gravis and Multiple Sclerosis disrupt normal physiologic signaling. Contrast the site of failure in each disease (synaptic receptor vs. axonal conduction).
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Part IV: Glossary of Key Terms
Term
Definition
Amphipathic
Describing a molecule, such as a phospholipid, that possesses both hydrophilic (water-soluble) and hydrophobic (water-insoluble) properties.
Cotransport (Symport)
A form of secondary active transport where all solutes are moved in the same direction across the cell membrane, utilizing the Na+ gradient.
Countertransport (Antiport)
A form of secondary active transport where solutes move in opposite directions across the membrane (e.g., Na+-Ca2+ exchange).
Diffusion Potential
The potential difference generated across a membrane when an ion diffuses down its concentration gradient.
Electrogenic
A process that creates a charge separation and a potential difference across a membrane, such as the Na+-K+ ATPase.
End Plate Potential (EPP)
A local depolarization of the motor end plate caused by the binding of acetylcholine to nicotinic receptors.
Equilibrium Potential
The diffusion potential that exactly balances and opposes the tendency for an ion to diffuse down its concentration gradient.
Glycosides (Cardiac)
A class of drugs (e.g., digoxin, ouabain) that inhibit the Na+-K+ ATPase by binding to the extracellular side of the enzyme.
Length Constant (\lambda)
The distance from a current injection site where the change in membrane potential has fallen by 63%; it indicates how far a current will spread.
Osmolarity
The concentration of osmotically active particles in a solution, expressed as osmoles per liter (Osm/L).
Reflection Coefficient (\sigma)
A dimensionless number between 0 and 1 that describes the ease with which a solute crosses a membrane; 1 indicates an impermeable solute.
Refractory Period
A period following an action potential during which a new normal action potential cannot be elicited (Absolute) or requires a higher stimulus (Relative).
Saltatory Conduction
The rapid propagation of action potentials in myelinated nerves where the impulse "jumps" from one node of Ranvier to the next.
Sarcomere
The basic contractile unit of striated muscle, delineated by Z disks and containing interdigitating thick and thin filaments.
Tetanus
A sustained muscle contraction resulting from repeated stimulation where the sarcoplasmic reticulum cannot reaccumulate Ca2+ quickly enough.
Time Constant (\tau)
The time required for the membrane potential to change to 63% of its final value following current injection; it reflects how quickly a membrane depolarizes.
