Chapter 11 Flashcards
Define active vs passive membrane transport
Passive: diffusion of a solute down its concentration gradient
Active: transport against a concentration gradient and/or electrical potential
How is a transmembrane electrical gradient created?
membrane potential, Vm, is created when ions of opposite charge are separated by a permeable membrane
The electrical gradient + the chemical gradient= ______
electrochemical gradient
What are the four types of ion channel?
- Ligand-gated
- Mechanically- Gated
- Always Open
- Voltage Gated
What are the Structural features of the voltage gated Na+ channels in neurons
Subunit α is essential
-Contains 4 homologous domains, each containing 6 transmembrane helices
- Segments between helices 5 and 6 create selectivity filter
- Segment connecting domains 3 and 4 is the inactivation gate
- Helix 4 is involved in the voltage-sensing mechanism
What is an example of a ligand-gated ion channel
The acetylcholine receptor
Important to note that acetylcholine is an extracellular ligand.
Both the acetylcholine receptor and the voltage gated Na+ channel relate to _____
Ca2+ transport and ultimately muscle contraction
Describe if active transport requires energy
Process is thermodynamically unfavorable (endergonic)
Requires either direct or indirect coupling to an exergonic process (i.e. ATP hydrolysis)
What is an ABC Transporter
(ATP-binding cassette transporter) a type of membrane transport protein that uses energy from ATP hydrolysis to move molecules across cellular membranes.
ATP binds, is hydrolyzed, and this induces a conformational change in the transporter that facilitates the movement of the substrate from one side of the membrane to the other.
Give the important info for ABC Transporters
Also ATP-driven; ABC= ATP-binding cassette
48 human genes encode ABC transporters
Some are really specific for substrates, others more “promiscuous”
Common therapeutic target because of drug transport (export) ability
- MDR1 causes resistance to
antitumor drugs, pumping out
chemotherapeutics like
doxorubicin and vinblastine;
MDR1 overexpression in liver,
kidney, and colon is associated
with treatment failure
- ABCs in pathogenic microbes
contribute to antibiotic resistance
BCRP (breast cancer resistance
protein) is an ABC overexpressed
in breast cancer cells which
confers resistance to anticancer
drugs
Define Uniport
A transporter system that carries only one substrate
Define symport and antiport
Transporter systems that carry two substrates
Sym-same direction
Anti-opposite directions
Describe Glucose Transporter – GLUT1
Passive transporter
Transfers glucose from blood plasma via facilitated diffusion
Integral membrane protein of erythrocytes
12 hydrophobic segments
Assembly provides hydrophilic channel
Define the GLUT1 Mechanism
Two major conformations
T1: Glucose binds to outside & induces a conformational shift
T2: Resulting conformation for glucose release inside cell
Diffusion is favored down the concentration gradient
[Glucose] ≈ 5 mM in plasma
Internalized glucose is metabolized instantly to maintain concentration gradient
Define Kt
Kt is a term used to describe the affinity of a transporter for its substrate. It is analogous to Km (Michaelis constant) in enzyme kinetics.
A lower Kt value indicates higher affinity for a substrate
Describe the diabetes defect
Diabetes is a metabolic disorder primarily characterized by impaired glucose regulation, leading to elevated blood glucose levels (hyperglycemia).
In Type 1 diabetes, the pancreas does not produce insulin, so the body cannot signal cells to take up glucose, leading to a glucose transport deficiency.
In Type 2 diabetes, the insulin receptors or the insulin signaling pathway become resistant, meaning that even though insulin is present, glucose uptake into tissues especially muscle and fat is impaired.
Describe F-type ATPases
AKA ATP synthases, are responsible for the synthesis of ATP in mitochondria, chloroplasts, and some bacteria.
F-type ATPases use the proton gradient (H⁺ gradient) across the membrane to drive the synthesis of ATP.
Protons flow down their concentration gradient (from the intermembrane space to the matrix in mitochondria, or from the thylakoid space to the stroma in chloroplasts) through the F₀ part of the enzyme, causing it to rotate.
This rotation is transmitted to the F₁ complex, which catalyzes the synthesis of ATP from ADP and inorganic phosphate (Pi).
Describe V-Type ATPases
Responsible for pumping protons (H⁺) into various intracellular organelles, including lysosomes, endosomes, and vacuoles
V-type ATPases use the energy from ATP hydrolysis to pump protons against their concentration gradient into intracellular compartments.
Define ligand gated channel
A type of ion channel that opens or closes in response to the binding of a specific ligand (a molecule, typically a neurotransmitter or other signaling molecule) to a receptor on the channel
Define voltage gated ion channel
A type of ion channel that opens or closes in response to changes in the membrane potential (voltage) across the cell membrane
Describe the steps of myosin/actin binding
- Resting State
- Myosin heads are “cocked” and attached to ADP and an inorganic phosphate (Pi), but they are not yet bound to actin.
- Tropomyosin is blocking the active sites on actin, preventing myosin from binding to actin. This is controlled by troponin, which binds calcium ions when released from the sarcoplasmic reticulum (SR). - Calcium Ion Release
- When an action potential reaches the muscle, calcium ions are released from the sarcoplasmic reticulum into the cytoplasm.
-Calcium binds to troponin, causing a conformational change in the troponin-tropomyosin complex. - Exposure of Binding Sites
- The conformational change in the troponin-tropomyosin complex shifts tropomyosin away from the myosin-binding sites on the actin filament.
This exposes the binding sites on actin, allowing myosin heads to bind to actin. - Cross-Bridge Formation
- The myosin heads, which have ADP and Pi bound, now attach to the exposed binding sites on actin. This forms the cross-bridge between myosin and actin. - Power Stroke
-Once the myosin head binds to actin, the release of Pi from myosin causes a conformational change in the myosin head, pulling the actin filament toward the center of the sarcomere. This is the power stroke.
-This action generates force, causing the sliding of the actin filament relative to the myosin filament, which leads to muscle contraction. - ADP Release
-After the power stroke, ADP is released from the myosin head. - ATP Binding and Myosin Head Detachment
-A new molecule of ATP binds to the myosin head, causing the myosin head to detach from the actin filament.
-The ATP is hydrolyzed into ADP and Pi, which re-cocks the myosin head, readying it for another cycle of binding and pulling.
Explain sodium potassium pump
The pump actively transports three sodium ions (Na⁺) out of the cell and two potassium ions (K⁺) into the cell, against their concentration gradients.
This ion gradient is essential for processes like nerve impulse transmission, muscle contraction, and osmoregulation.
The sodium-potassium pump helps maintain cell volume by preventing excessive water intake (since sodium attracts water), supports the electrical excitability of neurons and muscle cells, and is involved in the absorption of nutrients (such as glucose) in the intestines and kidneys.
Explain the steps of an action potential
- Resting Potential: Neuron is at rest, with a negative charge inside the cell.
- typically around -70 mV
- Depolarization: Stimulus causes sodium channels to open, and sodium ions rush in.
- around -55 mV
- Rising Phase: The inside of the neuron becomes more positive.
-+30 to +40 mV - Repolarization: Potassium channels open, and potassium ions leave, restoring a negative charge.
- Hyperpolarization: Membrane potential becomes more negative than the resting potential.
-80 to -90 mV - Restoration to Resting Potential: Sodium-potassium pump restores ion balance.
Explain the steps of muscle contraction
- Action Potential in Motor Neuron: An action potential is sent to the muscle fiber.
- Neuromuscular Junction: Acetylcholine is released, causing muscle membrane depolarization.
- Action Potential in Muscle Fiber: The action potential travels down T-tubules and activates the sarcoplasmic reticulum.
- Release of Calcium: Calcium is released from the sarcoplasmic reticulum into the cytoplasm.
- Troponin-Ca²⁺ Binding: Calcium binds to troponin, shifting tropomyosin to expose actin binding sites.
- Cross-Bridge Formation: Myosin heads bind to actin, forming cross-bridges.
- Power Stroke: Myosin heads pull on actin, causing muscle contraction.
ATP Binding and Cross-Bridge - Detachment: ATP binds to myosin, releasing actin.
- Relaxation: Calcium is pumped back into the sarcoplasmic reticulum, and the muscle relaxes.
