Cell Membrane and Permeability Quiz

Cell Membrane

• The cell membrane is not a simple container; it plays a crucial role in cell function.
• It is composed of a phospholipid bilayer.
• It allows lipid-soluble molecules and gases (like oxygen, carbon dioxide, ethanol, and urea) to diffuse easily.
• Water-soluble molecules require assistance to cross the membrane.
• The membrane is impermeable to organic anions, such as proteins.
• Permeability depends on the size, lipid solubility, and charge of the molecule.

Membrane Permeability

• If a substance can cross the membrane, the membrane is permeable to that substance.
• Gases can diffuse freely across the membrane.
• Polar molecules and ions need the help of proteins (channels or carriers) to cross.

Simple Diffusion

• Small, lipid-soluble molecules and gases move across the membrane directly through the phospholipid bilayer or through pores.
• Movement occurs down the concentration gradient.
• The rate of diffusion is proportional to the concentration gradient.
• It's a passive process, requiring no energy input from ATP.

Facilitated Diffusion

• Facilitated diffusion relies on carrier proteins to help molecules cross the membrane.
• This is used for polar molecules like sugars and amino acids.
• Movement occurs down the concentration gradient.
• The energy comes from the concentration gradient of the solute.
• It's a passive process, requiring no energy input from ATP.

Active Transport

• Active transport moves molecules against their concentration gradient, requiring energy expenditure.
• A specific protein carrier binds to the substance, changes conformation, and transports it across the membrane.
• It uses energy from ATP hydrolysis, typically via ATPases like the Na+/K+ pump.

Secondary Active Transport

• This type of transport moves molecules against their concentration gradient without directly using ATP hydrolysis.
• It harnesses the kinetic energy of one substance moving down its concentration gradient to drive the transport of another against its gradient.
• It relies on the energy built up by primary active transport, like the Na+/K+ pump.
• Sequential binding of substances and ions to the transporter protein induces conformational changes.

Channels

• Channels are formed by membrane-spanning proteins creating pores through the membrane.
• These pores are composed of multiple protein subunits, allowing specific ions to diffuse.
• The 'pore loops' inside the channel act as selectivity filters, allowing only specific molecules to pass based on their size and charge.

Gated Channels

• Most channels are not continuously open.
• They possess a 'gate', which can open and close under specific conditions.
• These gates are controlled by different factors:
  • Ligand-gated channels are controlled by the binding of a chemical agent.
  • Voltage-gated channels are controlled by the voltage difference across the membrane.

Ligand Gated Channels

• These receptors are part of the body's chemical signaling system.
• Binding of a ligand to its receptor often triggers events at the membrane, such as enzyme activation.

Voltage Gated Channels

• These channels are sensitive to the potential difference across the membrane.
• Changes in voltage, such as depolarization, cause conformational changes in the channel subunits, leading to the formation of a diffusion pore.
• The voltage-sensing mechanism is located in the S4 segment of the protein's fourth transmembrane domain.

Endo/Exocytosis

• Endocytosis is the 'pinching in' of the membrane to form a vesicle, typically through receptor-mediated processes to capture proteins from outside the cell.
• Exocytosis is the process of partially or completely fusing vesicles with the cell membrane, transporting molecules from the inside to the outside.
• Exocytosis can be divided into two types:
  • Kiss and Run: Rapid exocytosis.
  • Full Fusion: Complete exocytosis.

Exocytosis 1 (Kiss and Run)

• Secretory vesicles dock and fuse with the plasma membrane at specific locations called 'fusion pores'.
• The vesicle can connect and disconnect several times before releasing its contents.
• Only a portion of the contents is released in each 'Kiss', allowing for repeated releases.
• This type of exocytosis is used for low-rate signaling.

Exocytosis 2 (Full Fusion)

• This involves complete fusion of the vesicle with the membrane, releasing all contents at once.
• It's necessary for delivery of membrane proteins and high-level signaling.
• It must be balanced by endocytosis to maintain stable membrane surface area.

Membrane Potential

• All cells in the body generate a Membrane Potential (MP).
• To create MP, two conditions are needed:
  • Concentration Gradient: An enzyme ion pump (acting as an ATPase) actively transports specific ions across the membrane to create a concentration gradient.
  • Semi-permeable Membrane: Allows one ion species to diffuse across the membrane more than others, creating an electrical gradient.

Resting Membrane Potential

• This potential is established by the balance of ion movement across the membrane.
• The flow of potassium (K+) ions out of the cell, driven by its concentration gradient, creates a negative charge inside the cell.
• This negative charge opposes further K+ efflux, until an equilibrium is reached.

Equilibrium Potential

• At equilibrium, the electrical force opposing ion movement is balanced by the chemical force driving it.
• The membrane potential at equilibrium is determined by the concentration gradient of the ion.
• The Nernst equation can be used to calculate the equilibrium potential.

Nernst Equation

• The Nernst equation describes the balance between the chemical forces and electrical forces acting on ions.
• It provides the potential difference between the inside and outside of the membrane at equilibrium.
• It's accurate only if one ion species is diffusing across the membrane.

K+ Equilibrium Potential

• The equilibrium potential for potassium (EK+) can be calculated using the Nernst equation.
• In a typical neuron, it is around -90 mV.
• This is the potential if only K+ ions were involved, but the resting membrane potential is usually around -70 to -80 mV.
• This difference is due to the presence of other ions like sodium (Na+).

Test your knowledge on the structure and function of the cell membrane. This quiz covers key concepts such as membrane permeability, diffusion processes, and the types of molecules that can traverse the membrane. Enhance your understanding of cell biology fundamentals with this interactive quiz.