Cell Membrane Transport Quiz

Homeostasis

• Homeostasis is a steady state, not equilibrium
• Homeostasis requires energy
• Equilibrium does not require energy and is a state where there is no change in a vital parameter
• A steady state is when a vital parameter is well-regulated
• The body maintains a constant value for a parameter adjusting actions up or down as needed
• Negative feedback returns a vital parameter to its stable point
• Negative feedback maintains homeostasis
• When a parameter deviates from its stable point, negative feedback is activated
• Negative feedback leads to a series of changes
• A stimulus is recognized by a receptor
• The control center compares input to the reference value (set point)
• Effectors make adjustments
• Positive feedback loops amplify a stimulus and accelerate it
• A positive feedback loop can lead to instability and death

Electrolyte Content of Body Fluids

• Electrolytes are measured in mEq/L (milliequivalents per liter)
• Cations and anions are listed in a table displaying the concentration of various electrolytes in plasma, interstitial fluid, and intracellular fluid
• Examples of cations are sodium, potassium, calcium, and magnesium
• Examples of anions are chloride, bicarbonate, sulfate, phosphate, and protein

Osmotic Pressure and Reflection Coefficient

• Osmolarity is the concentration of solute particles within a solution and used to determine osmotic pressure
• Solutes with a reflection coefficient of 0 (σ = 0) move freely across a membrane
• Determining which solute is permeable is crucial when measuring osmotic pressure
• Isotonic solutions are always iso-osmotic, but iso-osmotic solutions may not be isotonic; if the reflection coefficient is not zero
• The concentration of impermeable solute determines osmotic pressure and water movement

Tonicity of Solutions

• Isotonic solutions do not change cell volume
• Hypotonic solutions cause cells to swell
• Hypertonic solutions cause cells to shrink

Transport Function of the Plasma Membrane

• Simple diffusion follows the electrochemical gradient
• Facilitated diffusion follows the electrochemical gradient with a protein carrier
• Primary active transport goes against the electrochemical gradient using ATP directly
• Secondary active transport uses the electrochemical gradient produced by a primary active transport system

Fick's Law of Diffusion

• Fick's Law relates the diffusive flux to the concentration gradient across a membrane
• Rate of diffusion is proportional to the permeability coefficient times the concentration difference divided by the thickness of the membrane
• Temperature positively correlates with the rate of diffusion

Saturation in Carrier-Mediated Transport

• Membrane proteins transport solute molecules
• Carrier-mediated transport has a maximum transport rate
• At low concentrations, transport increases proportionally to the concentration of solute until all the transport proteins are occupied.
• Transport maximum (Tm) is when all transport sites are occupied.
• The rate of transport becomes constant no matter how high the solute concentration increases

Stereospecificity in Carrier-Mediated Transport

• Binding sites are highly specific
• Stereospecificity means the transported molecule must be the correct isomer
• The transporter for glucose is highly specific to naturally-occurring D-glucose and doesn't transport its mirror image L-glucose

Competition in Carrier-Mediated Transport

• Similar solutes can compete for binding sites
• A closely related solute might occupy transport sites reducing the availability of sites to transport the primary solute

Ion Channel Characteristics

• Conductance (g) is the probability the channel is open, increasing the rate of diffusion
• Channels are controlled by different sensors such as: Voltage, Ligand, Second-messenger, Mechanically-gated
• Different types of sensors elicit different responses
• Ion movement through the channels depends on the relative concentration of ions and the voltage gradient across the membrane

Mechanisms Responsible for the Resting Membrane Potential

• Concentration gradients (chemical gradient) are across the membrane due to active transport (pumps)
• Selective membrane permeability selectively allows potassium (K+) to move across the membrane.
• Electrical gradients are generated by K+ leak which creates a negative intracellular charge

Equilibrium Potential

• The electrical potential difference that exactly balances an ion's concentration gradient across a membrane
• At equilibrium potential, there is no net flow

Driving Force for Diffusion: Equilibrium Potential vs. RMP

• The driving force for an ion is the difference between the membrane potential (RMP) and the equilibrium potential for the ion
• Forces can be positive or negative

Characteristics of Action Potentials

• Stereotypical in size and shape for a given cell type
• Propagate along a membrane, non-decremental
• All-or-none response

Action Potential Sequence

• Depolarization: Na+ channels open and Na+ enters
• Repolarization: Na+ channels close and K+ channels open, K+ leaves
• Hyperpolarization: K+ channels are slow to close and K+ continues to leave.

Refractory Periods

• Absolute refractory period: No action potential can be generated no matter the stimulus
• Relative refractory period: An action potential can be generated but requires a stronger stimulus than usual

Propagation of Action Potentials

• Action potentials propagate along a membrane
• Spread of local currents to adjacent, inactive regions
• Initiation of Aps in the initial segment of axon

Factors Affecting Conduction Velocity in Nerves

• Factors affecting conduction velocities include myelination, axon diameter, temperature.

Receptors, Signaling Pathways, and Messengers

• Different types of receptors and how they signal inside a cell.
• Ligand-gated ion channels, G protein-coupled receptors, enzyme-linked receptors, and nuclear receptors are listed.
• The signaling pathway for each receptor and the 2nd messengers are listed

Good Study Habits

• Effective study habits include a consistent schedule, active note-taking, study breaks, an organized workspace, prioritized tasks, a distraction-free environment, peer study groups, and periodic review sessions.