ATP-Driven Pumps in Cellular Transport

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.

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.