Transport Mechanisms in Cells

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Questions and Answers

What does the Hill coefficient indicate regarding ligand binding?

The degree of saturation of a binding site

The rate at which ligands dissociate from receptors

The amount of ligand present in solution

The cooperativity of binding between multiple ligands (correct)

How does Gibb's free energy relate to the dissociation constant (Kd) of a binding interaction?

Lower Kd indicates higher free energy

Higher Kd indicates more stable binding

Gibb's free energy is directly proportional to Kd

Lower Kd indicates more favorable free energy (correct)

What mechanism is described by ligand-gated ion channels and how do they function?

They depend on voltage changes to open and close

They open in response to the binding of specific ligands (correct)

They allow ions to pass when they are selectively permeable

They remain constantly open regardless of ligand presence

What is a characteristic of multipart ligands with regard to their affinity for targets?

They can create stronger binding interactions as a whole Signup and view all the answers

What role do non-covalent forces play in intermolecular interactions?

They facilitate transient interactions between molecules Signup and view all the answers

What is the primary difference between passive and active transport?

Passive transport occurs along concentration gradients, while active transport moves against them. Signup and view all the answers

Which of the following statements about membrane permeability is accurate?

Small, non-polar molecules can cross lipid bilayers more easily than larger, polar ones. Signup and view all the answers

What role do aquaporins play in membrane transport?

They are involved in the passive transport of water. Signup and view all the answers

Which substance would require a transporter protein to move across a lipid bilayer?

Glucose Signup and view all the answers

What is the significance of the dissociation constant (Kd) in relation to ligand binding?

A lower Kd value signifies a higher affinity between ligand and target. Signup and view all the answers

Which statement about cooperative binding is true?

Cooperative binding refers to the binding of multiple ligands that enhance the overall binding strength. Signup and view all the answers

Which of the following molecules would cross a lipid bilayer most efficiently?

CO2 Signup and view all the answers

What distinguishes active transport from passive transport?

Active transport can move substances against their concentration gradient. Signup and view all the answers

What is the primary mechanism of transport for carrier molecules?

They create a hydrophobic shield around polar molecules. Signup and view all the answers

Which characteristic is NOT true about pores/channels in facilitated transport?

They undergo conformational changes during transport. Signup and view all the answers

What distinguishes transporters from channels in facilitated transport?

Transporters undergo conformational changes. Signup and view all the answers

What is the unique feature of the Na+/K+-ATPase pump?

It uses ATP to maintain ion gradients. Signup and view all the answers

Which of the following ion concentration levels is NOT characteristic of the inside of a cell?

Ca++ concentration of 1-2 mM. Signup and view all the answers

What happens during the uniport of glucose by GLUT transporters?

The transporter exists in two distinct conformations. Signup and view all the answers

Which statement about facilitated transport is true?

Pores and channels can be gated for regulation. Signup and view all the answers

What type of transport does the Na+/K+-ATPase represent?

Active transport requiring ATP. Signup and view all the answers

What does a small dissociation constant (Kd) indicate about the affinity of two molecules?

High affinity interaction Signup and view all the answers

At what concentration of ligand (L) does the dissociation constant (Kd) represent?

The concentration at which 50% of protein (P) is bound Signup and view all the answers

Which expression correctly relates the fraction bound (Y) to the ligand concentration [L] and the dissociation constant (Kd)?

Y = [L] / Kd Signup and view all the answers

What is the significance of the Hill coefficient in binding interactions?

It indicates cooperativity in ligand binding Signup and view all the answers

In the binding equation P + L ⇄ PL, what does 'P' represent?

Protein Signup and view all the answers

How can the affinity of multipart ligands compare to their individual components?

They can show substantially greater affinity Signup and view all the answers

What does the term 'fraction bound' (Y) indicate in the context of molecular binding?

The ratio of bound protein to total protein Signup and view all the answers

What is the primary role of the constants Kon and Koff in the binding equation?

They denote the rates of ligand-protein interactions Signup and view all the answers

Study Notes

Transport around the Cell

There are two types of transport across a membrane:
- Passive transport: Does not require energy input.
- Active transport: Requires energy input.

Membrane Permeability

Lipid bilayers: Are not permeable to polar molecules or ions, which require protein-mediated transport.
Small, non-polar molecules (like CO2): Can cross the lipid bilayer more efficiently than larger, polar molecules.
Water: Can cross lipid bilayers to some extent, but aquaporins (water channel proteins) facilitate water movement.
Glucose and ions (MgSO4): Require transporter proteins facilitate their movement across membranes.

Methods of Facilitated Transport

Carrier molecules: Form a hydrophobic shield around polar molecules, often ions.
Pores/Channels: Transport can be facilitated by pores/channels, which are non-stoichiometric, fast, selective, and may be gated.
Transporters: Conformational changes facilitate transport, and can be either passive or active (pumps).

Transport Protein Action

Transporter proteins: Can move multiple ligands through conformational changes.

Uniport of Glucose

Glucose transporters (GLUT): Bind glucose and facilitate its transport across membranes.

Effect of Na+/K+-ATPase on Cellular Ion Concentrations

Na+/K+-ATPase: Is an ATP-dependent ion pump that maintains Na+ and K+ gradients across cell membranes.
Importance of Na+/K+-ATPase: Uses one-third of total energy at rest.

Regulated Transport

Ion channels: Can be gated (regulated) by voltage or small organic molecules.
Ligand binding: Can open ion channels, allowing ions to cross the membrane.

The Nicotinic Acetylcholine Receptor

Ligand-gated channel: The nicotinic acetylcholine receptor is an ion channel regulated by acetylcholine binding.
Conformation change: Acetylcholine binding changes the receptor's conformation from closed to open, allowing ions to pass through.

Non-covalent Forces and Intermolecular Interactions

Non-covalent forces: Drive intermolecular interactions, such as heme binding to O2, antibody binding, and transcription factor binding to DNA.

Quantitative Treatment of 1:1 Binding

Dissociation constant (Kd): A measure of affinity between two molecules.
Small Kd: Indicates high affinity.
Kd: Can be determined by measuring rates or concentrations.
Fractional saturation (Y): Represents the fraction of protein bound to ligand.

Classic Binding Curve

Kd: Is equal to the concentration of ligand at which 50% of the protein is bound.
Key point on the binding curve: When 50% of the protein is bound, [L] = Kd.

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Description

Explore the different transport mechanisms that cells utilize to move substances across their membranes. This quiz covers concepts such as passive and active transport, membrane permeability, and facilitated transport methods. Test your knowledge on how cells manage the movement of essential molecules!