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What determines the diffusion rate of a molecule across a protein-free lipid bilayer?
What is the primary function of ATP-driven pumps?
Why are protein-free lipid bilayers essentially impermeable to charged molecules?
What is the characteristic of P-type pumps?
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What is a characteristic of transporter proteins?
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What is the primary function of channel proteins?
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What is the function of V-type proton pumps?
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What is the advantage of ion channels over transporters?
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What is the main difference between transporter and channel proteins?
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Which of the following molecules would be most likely to diffuse across a protein-free lipid bilayer?
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What is the primary function of ion channels?
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What is the characteristic of aquaporins?
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What is the role of conformational changes in transporter proteins?
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Why are protein-free lipid bilayers important for understanding membrane transport?
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What is the difference between ATP-driven pumps and ion channels?
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What is the primary function of ABC transporters?
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What happens during facilitated diffusion?
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What is the main difference between passive and active transport?
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What is the role of a conformational change in a transporter?
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What is the purpose of ATP-driven pumps?
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What is the characteristic of uniporters?
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What problem do water channels in the kidney and exocrine cells need to solve?
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What is the difference between symporters and coupled transporters?
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What is a characteristic of ion channels that distinguishes them from aqueous pores?
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How can active transport be driven?
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What is the function of gated channels in ion channels?
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What is the role of a transporter in active transport?
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What type of gated channel is opened by a mechanical stress?
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What is the main contributor to the membrane potential in animal cells?
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What is the result of a difference in electrical charge on the two sides of a membrane?
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What is the purpose of ion channels?
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What is an example of a mechanically gated channel?
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Study Notes
ATP-Driven Pumps
- ATP-driven pumps are also called transport ATPases, which hydrolyze ATP to ADP and phosphate and use the energy released to pump ions or other solutes across a membrane.
- There are four types of ATP-driven pumps: P-type, ABC transporters, V-type, and F-type ATPases.
P-Type Pumps
- P-type pumps are structurally and functionally related to multipass transmembrane proteins.
- They are called "P-type" because they phosphorylate themselves during the pumping cycle.
- This class includes many of the ion pumps that set up and maintain gradients of Na+, K+, H+, and Ca2+ across cell membranes.
ABC Transporters
- ABC transporters primarily pump small organic molecules across cell membranes.
V-Type Pumps
- V-type pumps are turbine-like protein machines constructed from multiple different subunits.
- The V-type proton pump transfers H+ into organelles, such as lysosomes, synaptic vesicles, and plant or yeast vacuoles to acidify the interior of these organelles.
F-Type ATPases
- F-type ATPases are found in mitochondria and chloroplasts.
Ion Channels
- Most channels in the plasma membrane of animal and plant cells that connect the cytosol to the cell exterior have narrow, highly selective pores that can open and close rapidly.
- Ion channels are concerned specifically with inorganic ion transport.
- They have an advantage over transporters, as they can pass up to 100 million ions through one open channel each second – a rate 10^5 times greater than even the fastest transporter.
- Ion channels cannot be coupled to an energy source to perform active transport, so the conductance they mediate is always passive (downhill).
Aquaporins
- Aquaporins (water channels) allow water to move more rapidly.
Protein-Free Lipid Bilayers
- Protein-free lipid bilayers are impermeable to charged molecules (ions), regardless of size, due to their charge and high degree of hydration.
- The smaller the molecule and the less strongly it associates with water, the more rapidly it diffuses across the bilayer.
Transport Proteins
- There are two main classes of membrane transport proteins: transporters and channels.
- Transporters alternate between two conformations, allowing the solute-binding site to be sequentially accessible on one side of the bilayer and then on the other.
- Channels form continuous pores across the bilayer, allowing specific solutes to pass through at a much faster rate.
Active Transport
- Active transport is mediated by transporters coupled to an energy source.
- It involves movement of the solute against its concentration or electrochemical gradient and requires an input of metabolic energy.
Conformational Change in Transporters
- A conformational change in a transporter mediates the passive movement of a solute.
- The transporter has three conformational states: outward-open, occluded, and inward-open states.
Three Ways of Driving Active Transport
- Coupled transporters harness the energy stored in concentration gradients to couple the uphill transport of one solute to the downhill transport of another.
- ATP-driven pumps couple uphill transport to the hydrolysis of ATP.
Uniporters, Symporters, and Coupled Transport
- Uniporters facilitate the passive movement of a single solute from one side of the membrane to the other.
- Symporters (co-transporters) involve the transfer of one solute strictly dependent on the transport of a second solute in the same direction.
- Coupled transport involves the intimately coupled transfer of two solutes in the same direction.
Ion Channels
- Ion channels are ion-selective and fluctuate between open and closed states.
- They have two important properties: ion selectivity, permitting some inorganic ions to pass but not others, and gating, which allows them to open briefly and then close again.
- Voltage-gated channels are gated by a change in the voltage across the membrane.
- Mechanically gated channels are gated by a mechanical stress.
- Ligand-gated channels are gated by the binding of a ligand.
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Description
This quiz covers ATP-driven pumps, also known as transport ATPases, which use ATP hydrolysis to pump ions and solutes across cell membranes. It focuses on P-type pumps, including ion pumps responsible for setting up and maintaining ionic gradients.
