Cell Membrane and Ion Channels Quiz

Homeostasis

• Homeostasis is a steady state, requiring energy.
• Equilibrium is a state without energy consumption. When a vital parameter, like blood glucose, is well-regulated, it isn't in equilibrium.
• A steady state maintains a vital parameter at a constant value, achieved by the body or cell carefully balancing actions that raise and lower the parameter.

Negative Feedback

• Negative feedback is the most common feedback mechanism.
• It reverses any deviation from a stable point.
• If a factor becomes excessive or deficient, a control system initiates negative feedback to return the factor to a mean value, maintaining homeostasis.
• Negative feedback involves a series of changes.

Positive Feedback

• Positive feedback loops maintain and possibly accelerate the direction of a stimulus.
• Positive feedback exaggerates deviations from a stable point.
• Each cycle of positive feedback leads to more of the same variable, potentially causing instability and death.
• Positive feedback is less common than negative feedback.

Electrolyte Content of Body Fluids

• Data shows the electrolyte content of plasma, interstitial fluid, and intracellular fluid.
• Includes cation concentrations (Sodium, Potassium, Calcium, Magnesium) and anion concentrations (Chloride, Bicarbonate, Sulfate, Phosphate, Protein).
• Review the electrolyte levels in the body's fluids.

Osmotic Pressure and Reflection Coefficient

• Review the relationship between osmotic pressure and reflection coefficient.
• Isotonic solutions have the same solute concentration in compartments, but may not be truly isotonic if substances have reflection coefficients of 0.
• Urea is freely permeable and quickly reaches equilibrium between compartments (making it an "ineffective osmole").
• Osmotic pressure is determined by the concentration of impermeable solutes, influencing water movement.

Tonicity of Solutions

• The terms isotonic, hypotonic, and hypertonic describe solutions based on whether they cause changes in cell volume.
• Tonicity primarily depends on the concentration of impermeable solutes.
• Some solutes can permeate the cell membrane.

Effects of Volume Changes on Osmolarity of Body Fluids

• Table 1 shows how changes in volume affect body osmolarity during various hydrational conditions (Loss of Isotonic fluid, Loss of hypotonic fluid, Gain of isotonic fluid, Gain of hypotonic fluid, Gain of hypertonic fluid).
• Different scenarios (e.g., hemorrhage, diarrhea, dehydration) are considered.

Membrane Transport Mechanisms

• Simple diffusion across a phospholipid bilayer.
• Facilitated diffusion via protein channels/pores; carrier proteins needed.
• Primary active transport uses ATP against the electrochemical gradient.
• Secondary active transport uses electrochemical gradients created by primary active transport.
• Bulk transport involves large particles or large volumes of material (Endocytosis and Exocytosis).
• Different examples of transport mechanisms are provided.

Fick's Law of Diffusion

• Fick's Law describes the relationship between the diffusive flux and the gradient of concentration across a membrane.
• Factors affecting diffusion rate are permeability coefficient (P), partition coefficient (K), diffusion coefficient (D), cross-sectional area (A) and the thickness of membrane (Δx).

Saturation in Carrier Mediated Transport

• Carrier proteins have limited binding sites; transport rate increases with concentration until all sites are occupied (Transport maximum -Tm).
• At low concentrations, many binding sites are available, and transport increases steeply with solute concentrations as binding sites are available, while at high concentrations, binding sites become scarce slowing the rate of transport.

Stereospecificity in Carrier-Mediated Transport

• Binding sites on carrier proteins are stereospecific, recognizing and transporting specific isomers.
• The transporter for glucose (in the renal proximal tubule) recognizes and transports D-glucose, but not L-glucose.

Competition in Carrier-Mediated Transport

• Related substances can compete for the same binding sites.
• An example is D-Galactose which inhibits glucose transport by occupying binding sites.

Ion Channel Characteristics

• Conductance (g) is the probability a channel is open
• Ion channel gates are controlled by sensors. Types include voltage-gated, ligand-gated channels, second messenger-gated, and mechanically-gated channels.

Mechanisms Responsible for the Resting Membrane Potential

• Chemical gradients from active transport pumps (e.g. Na+/K+ ATPase pump).
• Selective membrane permeability with higher permeability to potassium ions.
• Electrical gradients.
• Electrochemical equilibrium.

Equilibrium Potential

• The membrane potential at which the diffusive force and electrical force are balanced for an ion.
• This potential is significant because the overall current flow is directly proportional to the net force and conductance of the membrane for that ion.

Driving Force for Diffusion: Equilibrium Potential vs. RMP

• The driving force for diffusion is the difference between the equilibrium potential of an ion and the resting membrane potential (RMP).
• Depending on the direction of the driving force, an ion will either enter or exit the cell.

Characteristics of Action Potentials

• Action potentials have stereotypical size and shape for a given cell type.
• They are propagated down the entire length of an axon without a reduction in size or shape, due to regeneration.
• Action potentials are all-or-none, triggered only by a threshold stimulus.

Action Potential Sequence of Events

• Depolarization (opening voltage-gated Na+ channels).
• Repolarization (voltage-gated Na+ channels close, and voltage-gated K+ channels open).
• Hyperpolarization (voltage-gated K+ channels remain open after the potential reaches resting level).

Refractory Periods

• Absolute refractory period: no new action potential can be generated. The Na+ channel inactivation gates must be open again.
• Relative refractory period: a new action potential CAN be generated but a greater than normal depolarizing current will be needed. The voltage-gated K+ channels remain open, which creates more resistance to depolarization.

Propagation of Action Potentials

• Propagation occurs by the spread of local currents from active regions to inactive regions.
• Action potentials are initiated in the initial segment of the axon, then propagate down the axon.

Factors Affecting Conduction Velocity in Nerves

• Myelination
• Axon diameter
• Temperature

Receptors, Signaling Pathways, and Messengers

• Different receptors (ligand-gated ion channels, G protein-coupled receptors, enzyme-linked receptors, nuclear receptors) along with their associated signal transduction pathways and secondary messengers are identified.
• Examples of ligands are listed for each receptor type

Good Study Habits

• Good study habits are consistent and effective practices used for retention and understanding of information.
• Examples of good study habits include a consistent schedule, active note-taking, study breaks, organized workspace, prioritizing tasks, distraction-free environment, peer study groups, and periodic review sessions.

Test your knowledge on cell membrane functions, ion channels, and transport proteins. This quiz covers various aspects of membrane potentials, ion conductance, and the effects of different forces on ion movement. Challenge yourself with these questions related to membrane biology!