Cell Membrane and Transport Quiz

Cell Membrane

• Plasma membrane surrounds all cells.
• Functions: structural barrier, controls substance passage, maintains internal/external composition, contributes to cell fluidity, enables cell communication, and allows response to stimuli.
• Two main components: phospholipid bilayer and proteins.

Membrane Transport

• Diffusion: Movement of molecules from high to low concentration (e.g., oxygen, carbon dioxide, fat-soluble substances).
• Protein-mediated transport:
  • Channel proteins: act as selective pores for specific molecules.
  • Carrier proteins: bind to molecules and facilitate their movement across the membrane.
• Endocytosis: engulfing substances from the extracellular environment.
  • Phagocytosis: engulfing solid particles.
  • Pinocytosis: engulfing liquids.
• Exocytosis: releasing substances from within the cell to the extracellular environment.

Membrane Potential

• Membrane potential results from the separation of charges across the cell membrane.
• All cells have a membrane potential, which is the electrical difference across the membrane.
• Excitable cells (nerve and muscle) actively change their membrane potential, which is the basis for their electrical excitability.
• Membrane potential is also used to transport substances.

Resting Membrane Potential (RMP)

• RMP is the potential difference maintained across the cell membrane when the cell is at rest.
• RMP is usually negative inside the cell compared to the outside.
• Neurons typically have an RMP of approximately -70 mV.
• RMP is a result of two factors:
  • Unequal distribution of ions: K+ ions are higher inside the cell, Na+ ions are higher outside the cell, and large protein anions are only found inside the cell.
  • Selective permeability of the plasma membrane: the membrane is more permeable to K+ ions than Na+ ions.

Na+/K+ ATPase (Pump)

• This protein plays a crucial role in establishing and maintaining the concentration gradients of Na+ and K+ across the cell membrane.
• It actively transports 3 Na+ ions out of the cell for every 2 K+ ions transported into the cell.
• This process requires energy (ATP) and contributes to the negative charge inside the cell.

Factors contributing to RMP

• The Na+/K+ ATPase creates a small potential difference by transporting more positive charges out than in.
• The key factor contributing to the RMP is the diffusion of K+ ions out of the cell down their concentration gradient. This movement leaves the inside of the cell more negative compared to the outside.

Membrane Potential Changes

• Depolarization: The membrane potential becomes more positive.
• Hyperpolarization: The membrane potential becomes more negative.
• Repolarization: The membrane potential returns to its resting state.

Na+/K+ Pump and Transport Processes

• The Na+/K+ pump provides the driving force for various transport processes across the membrane.
• Cells utilize the Na+ concentration gradient created by the pump to drive other transport mechanisms.
• Cotransporters: move substances in the same direction as Na+ (e.g., glucose, amino acids).
• Exchangers: move substances in the opposite direction of Na+ (e.g., Ca2+, H+).

Classes of Ion Channels

• Leak channels: constantly open and allow ions to flow passively down their concentration gradients (e.g., K+ leak channels).
• Voltage-gated channels: open and close in response to changes in membrane potential (e.g., voltage-gated sodium channels).
• Ligand-gated channels: open and close in response to the binding of specific molecules (ligands) (e.g., acetylcholine receptor).

Drug Modification of Ion Channels

• Local anesthetics: block voltage-gated Na+ channels, leading to reduced nerve impulse transmission (e.g., Xylocaine/Lidocaine).
• Ca2+-channel blockers: inhibit Ca2+ channels, affecting smooth muscle contraction and blood vessel constriction (e.g., nifedipine).

Membrane Permeability

• Lipid soluble and small uncharged molecules (e.g. oxygen) freely diffuse across cell membranes
• Movement across the membrane is driven by the concentration gradient
• Charged ions and water-soluble molecules cannot freely move across the hydrophobic interior of the cell membrane
• Cell membranes are selectively permeable
• The cell membrane is permeable to certain substances and impermeable to others

Membrane Potential

• Opposite charges attract and the separation of these creates a membrane potential
• There is a difference in concentration of positive and negative ions across the cell membrane
• All cells have a membrane potential
• Excitation cells actively induce changes in the membrane potential
• The membrane potential is the basis for the electrical excitability of nerve and muscle cells
• Membrane potential can be harnessed for transporting substances

Absence of a Membrane Potential

• No charge difference across the membrane, resulting in no membrane potential

Presence of a Membrane Potential

• Separated charges are responsible for the presence of a membrane potential

Resting Membrane Potential (RMP)

• The cell membrane is more negative inside than outside
• The RMP of a neuron is approximately -70mV
• The unequal distribution of potassium, sodium, and protein anions (A-) between the inside and outside of the cell contributes to the RMP
• The permeability of the cell membrane to potassium plays a major role
• The cell membrane contains many leak channels, that allow potassium to diffuse out of the cell down its concentration gradient

Concentration of Ions

• The intracellular concentration of potassium is significantly higher than the extracellular concentration
• Conversely, the extracellular concentration of sodium is significantly higher than the intracellular concentration
• The concentration of protein anions (A-) is only found intracellularly

Sodium-Potassium ATPase (Na-K Pump)

• The Na-K pump is a membrane-spanning enzyme that establishes the concentration gradients of Na+ and K+
• The pump transports three sodium ions out of the cell and two potassium ions into the cell
• The pump uses ATP as an energy source
• The pump actively transports approximately 200 million ions per second

Contribution to RMP

• The Na-K pump generates a small potential due to unequal transport of positive ions across the membrane
• The inside of the cell becomes slightly negatively charged relative to the outside because of the 3Na+ out for 2K+ in ratio
• Diffusion of potassium out of the cell down its concentration gradient contributes significantly to the RMP
• Potassium leaks out of the cell down its concentration gradient, further increasing the negativity inside the cell

Additional Contributors to Negative Membrane Potential

• Phosphatidylinositol (PIP2) and Phosphatidylserine (PS) contribute to the negative inner membrane potential
• These negatively charged lipids provide a negative charge to the inner part of the plasma membrane

Nernst Equilibrium

• The chemical gradient acts as the driving force for diffusion of potassium out of the cell
• The chemical gradient is balanced by the electrical gradient which draws potassium back into the cell
• At equilibrium, the electrical force balances the chemical force, resulting in no net transport of potassium

Nernst Equation

• The Nernst equation is used to calculate the equilibrium potential for an ion
• The equilibrium potential is the membrane potential at which the electrical driving force for the ion movement is equal and opposite to the chemical driving force

Membrane Potential Changes

• Depolarization: The membrane potential becomes more positive
• Hyperpolarization: The membrane potential becomes more negative
• Repolarization: The membrane potential returns to its resting state

NA-K Pump and Transport Processes

• The cell utilizes the chemical gradient of sodium to transport substances into the cell
• Cotransporters are involved in this process

Cell Membrane

• Plasma Membrane: 3-10 nm thick membrane made of fluid lipid-protein bilayer, enclosing all cells
• Functions:
  • Structural barrier
  • Controls substance passage, maintaining cell's internal environment
  • Maintains cell fluidity
  • Facilitates communication between cells
  • Enables response to external stimuli
• Membrane Proteins:
  • Integral/transmembrane: Span the entire membrane
  • Extrinsic/peripheral: Attached to the membrane surface

Transport Across Membranes

• Diffusion: Movement of substances across membrane driven by concentration gradient, applies to lipid-soluble and small uncharged molecules (e.g., O2, CO2)
• Protein-mediated transport: Facilitated by channel or carrier proteins for charged (ions) or water-soluble molecules
  • Channels: Provide specific pathways for substances to move through the membrane
  • Carriers: Bind to the transported substance, change conformation, and release it on the other side of the membrane
• Endocytosis: Process of bringing substances into the cell by engulfing them in membrane-bound vesicles:
  • Phagocytosis: Ingestion of large particles, e.g., bacteria
  • Pinocytosis: Ingestion of fluid and small molecules
• Exocytosis: Process of releasing substances from the cell by fusing vesicles with the plasma membrane

Movement Across Plasma Membrane

• Lipid-soluble and small uncharged molecules: Freely diffusible across the membrane, driven by concentration gradient
• Charged (ions) / water-soluble molecules: Movement is restricted due to the hydrophobic interior of the membrane
• Selectively permeable membrane: Allows for the control of ion movement across the membrane

Membrane Potential

• Separation of charges: Gives rise to "membrane potential" due to attraction between opposite charges (cations (+) and anions (-)) across the membrane.
• Role in all cells: Cells possess a membrane potential, representing an electrical difference across the membrane
• Excitable cells: Nerve and muscle cells actively change their membrane potential, forming the basis for their electrical excitability
• Transport: Used for active transport of substances
• Mechanism: The membrane acts as a barrier, establishing concentration gradients of key ions across the membrane. The magnitude of the potential depends on the degree of charge separation

Absence of Membrane Potential

• Electrically neutral: When there is no charge difference across the membrane, despite the membrane barrier, there is no membrane potential

Presence of Membrane Potential

• Separated charges: A charge difference across the membrane, due to the presence of separated charges (cations (+) and anions (-))
• Remainder of fluid: The remainder of the intracellular and extracellular fluids remain electrically neutral

Resting Membrane Potential (RMP)

• Negative inside: The inside of the cell membrane is more negative than the outside.
• Neuron: Typically -70mV
• Factors contributing to RMP:
  • Unequal distribution of ions: Differing concentrations of K+, Na+, and large protein anions (A-) inside and outside the cell
  • Selective permeability: The cell membrane is more permeable to K+ than other ions, allowing K+ to leak out of the cell down its concentration gradient

Distribution of ions responsible for RMP

• High concentration of K+ inside the cell: 150 mM
• High concentration of Na+ outside the cell: 150 mM
• High concentration of A- inside the cell: 65 mM

Na+/K+ ATPase (Pump)

• Role: Maintains concentration gradients of K+ and Na+ across the membrane
• Mechanism: Transports 3 Na+ ions out of the cell and 2 K+ ions into the cell, using energy from ATP (ATP Hydrolysis, creating a "pump")
• Importance:
  • Establishes concentration gradients for Na+ and K+
  • Contributes to the negative charge inside the cell

Resting Membrane Potential - Factors Contributing to it

• Na+/K+ ATPase pump: Transports ions unequally, resulting in slight negative charge inside the cell due to 3Na+ out for 2K+ in
• Diffusion of K+: Most significant factor; the diffusion of K+ out of the cell down its concentration gradient established by the pump makes the inside more negative

PM is "Leaky" to K+

• Ion movement: K+ is constantly moving across the membrane, but cannot fully leak out because of the Na+/K+ ATPase pump constantly working to maintain ion gradients

Membrane Potential Changes

• Depolarization: Increase in positivity of the membrane potential (becomes less negative)
• Hyperpolarization: Increase in negativity of the membrane potential (becomes more negative)
• Repolarization: Return of the membrane potential to its resting state following depolarization

Na+-K+ Pump is the Major Driving Force for Transport

• Coupled Transport: Utilizes the concentration gradient of Na+ (high outside the cell) to drive the transport of other substances into the cell
• Cotransporters: Proteins that simultaneously move two or more substances in the same direction across the membrane
• Example: The sodium-glucose cotransporter uses the energy from the movement of Na+ down its concentration gradient to bring glucose into the cell.

Test your understanding of the cell membrane's structure and functions, including membrane transport mechanisms such as diffusion, endocytosis, and exocytosis. Dive into concepts like membrane potential and how cells interact with their environment.