Chapter 13 Biochem.... Membrane Channels and Pumps Notes
Membrane Transport Overview
Three Classes of Membrane Transporter Proteins:
Pumps: Active transport mechanisms that require energy.
Carriers: Transport molecules across the membrane without direct energy use.
Channels: Facilitated diffusion for ions and small molecules.
Active vs. Passive Transport
Active Transport: Movement against a concentration gradient, requiring energy (e.g., ATP).
Positive change in Gibbs free energy (ΔG > 0).
Passive Transport: Movement down a concentration gradient without energy input.
Negative change in Gibbs free energy (ΔG < 0).
Examples: Simple diffusion (lipophilic molecules), facilitated diffusion (polar molecules).
Free Energy Change for Charged Species
Electrochemical Potential:
Defined as the sum of concentration and electric terms.
Free energy change for moving a charged species:
where:
= charge of the species
= voltage across the membrane
= Faraday's constant (96.5 kJ V⁻¹ mol⁻¹)
Na⁺–K⁺ ATPase
Function: Maintains K⁺ and Na⁺ gradients across the plasma membrane.
Mechanism: Catalyzes the hydrolysis of ATP to transport 3 Na⁺ out and 2 K⁺ in per cycle.
Requires magnesium (Mg²⁺).
Standard Gibbs energy change: .
P-Type ATPases
Characteristics: Form a phosphorylated aspartate intermediate.
Examples: Na⁺–K⁺ ATPase, SERCA (pumps calcium), H⁺–K⁺ ATPase (regulates stomach pH).
Multidrug Resistance (MDR) Proteins
Function: ATP-dependent transporters that expel toxins and drugs from cells.
Structure: Contain multiple membrane-spanning and ATP-binding domains.
Secondary Active Transport: Lactose Permease
Definition: Coupled transport that uses the gradient of one ion to drive the transport of another.
Types:
Symporters: Transport substrates in the same direction.
Antiporters: Transport substrates in opposite directions.
Uniporters: Transport one substrate based on concentration.
Ion Channels
Function: Facilitate passive transport of ions rapidly across membranes.
Type: Voltage-gated (Na⁺ and K⁺ channels) and ligand-gated (e.g., acetylcholine receptor).
Action Potentials
Mechanism: Mediated by transient changes in membrane permeability to Na⁺ and K⁺.
Depolarization occurs when Na⁺ flows into the cell, while repolarization happens as K⁺ flows out.
Phases:
Threshold depolarization (e.g., -40 mV leads to action potentials).
Refractory period: Prevents back-propagation of impulses.
Nernst Equation
Equilibrium potential for a given ion can be calculated as:
Where and are the intracellular and extracellular concentrations, respectively.
Gap Junctions and Aquaporins
Gap Junctions: Facilitate intercellular communication, allow small molecules and ions to pass between cells.
Composed of connexins.
Aquaporins: Water channels facilitating rapid water transport across membranes, prevent proton conduction.
Acetylcholine Receptor
Role: Ligand-gated ion channel critical for muscle contraction and nerve impulse transmission.
Structure includes four types of subunits forming a pentameric complex, which opens upon ligand binding.
Summary of Ion Channel Activity
Ion channels change their conformation based on voltage (voltage-gated) or ligand binding (ligand-gated).
The Charge Movement: - Membrane potentials oscillate during action potentials based on Na⁺ and K⁺ flow as influenced by channel states (open, closed, inactivated).
REVIEW GUIDE
Membrane Transport Overview
Three Classes of Membrane Transporter Proteins:
Pumps: Active transport mechanisms that require energy to move substances from low to high concentration.
Carriers: Facilitate movement of molecules across the membrane without direct energy use, taking advantage of high to low concentration gradients.
Channels: Facilitate ion or water transport, moving substances from high to low concentration.
Active vs. Passive Transport
Active Transport: Movement against a concentration gradient, requiring energy (e.g., ATP).
Positive change in Gibbs free energy (ΔG > 0).
Passive Transport: Movement down a concentration gradient without energy input.
Negative change in Gibbs free energy (ΔG < 0).
Examples: Simple diffusion (lipophilic molecules), facilitated diffusion (polar molecules).
Free Energy Change for Charged Species
Electrochemical Potential: Defined as the sum of concentration and electric terms.
Free energy change for moving a charged species: where:
= charge of the species
= voltage across the membrane
= Faraday's constant (96.5 kJ V⁻¹ mol⁻¹)
Na⁺–K⁺ ATPase
Function: Maintains K⁺ and Na⁺ gradients across the plasma membrane.
Mechanism: Catalyzes the hydrolysis of ATP to transport 3 Na⁺ out and 2 K⁺ in per cycle, requiring magnesium (Mg²⁺).
Standard Gibbs energy change: .
Multidrug Resistance (MDR) Proteins
Function: ATP-dependent transporters that expel toxins and drugs, allowing cancer cells to remove toxic drugs that enter the cell.
Structure: Contain multiple membrane-spanning and ATP-binding domains.
Secondary Active Transport: Lactose Permease
Definition: Coupled transport that uses the gradient of one ion (usually proton) to drive the transport of another substrate.
Types:
Symporters: Transport substrates in the same direction.
Antiporters: Transport substrates in opposite directions.
Uniporters: Transport one substrate based on its concentration.
Ion Channels
Function: Facilitate passive transport of ions rapidly across membranes.
Mechanism of Action Potentials:
Depolarization occurs when Na⁺ flows into the cell; repolarization happens as K⁺ flows out.
Channels are voltage-gated (Na⁺ and K⁺ channels) and open in response to changes in membrane potential.
The ball and chain mechanism explains why channels remain inactive for a period of time (refractory period) after activation.
Nernst Equation
The equilibrium potential for a given ion can be calculated as:
Where and are the intracellular and extracellular concentrations, respectively.
Gap Junctions and Aquaporins
Gap Junctions: Facilitate intercellular communication, allowing small molecules and ions to pass between cells.
Composed of connexins.
Aquaporins: Water channels that facilitate rapid transport of water across membranes while preventing the conduction of protons and other charged ions.
Their structure allows for selective permeability, ensuring that only water pass through rapidly while
Yes, the notes discuss voltage in several sections:
Electrochemical Potential: The free energy change for moving a charged species includes the voltage across the membrane, defined by the equation: ΔG = RT ln(c2/c1) + ZFΔVΔV represents the voltage.
Nernst Equation: The equilibrium potential for a given ion is calculated as: V{eq} = -\frac{RT}{zF} ln\frac{[X]{in}}{[X]_{out}}, highlighting the role of voltage in determining the movement of ions across the membrane.
Action Potentials: The mechanism of action potentials is mediated by transient changes in membrane permeability to Na⁺ and K⁺, which involves changes in voltage across the membrane during depolarization and repolarization phases.
C-type lectins are a type of protein that binds carbohydrates (sugars) in a calcium-dependent manner. They are called "C-type" because their carbohydrate-binding activity requires calcium ions (Ca²⁺) for function.
Key Features:
Carbohydrate Binding: C-type lectins specifically recognize and bind to certain sugar molecules, which can play important roles in cell-cell interactions, immune response, and pathogen recognition.
Calcium Dependency: The presence of calcium ions is essential for the structure and function of C-type lectins. Without calcium, they cannot effectively bind to their carbohydrate targets.
Functions: These lectins are involved in various biological processes such as cell signaling, immune defense against infections, and developing a response to pathogens like viruses and bacteria. Some examples of C-type lectins include the mannose receptor and DC-SIGN, which are important in the immune system.
In summary, C-type lectins are important proteins that help cells recognize and interact with carbohydrates, playing a crucial role in many physiological functions.