20 Questions
What is the primary difference between passive and active transport?
Energy requirement
Which type of transporter facilitates the passive movement of a single solute?
Uniporter
What drives the transport of solutes through symporters?
Transport of a second solute in the same direction
How do ATP-driven pumps transport solutes?
By coupling uphill transport to the hydrolysis of ATP
What is the primary function of ATP-driven pumps?
To pump ions or other solutes across a membrane
What is the role of conformational changes in mediated transport?
To facilitate the passive movement of solutes
Which type of ATP-driven pumps phosphorylate themselves during the pumping cycle?
P-type pumps
What is the primary function of V-type pumps?
To transfer H+ into organelles
What is the advantage of ion channels over transporters?
Ion channels can pass up to 100 million ions through one open channel each second
What is the primary function of aquaporins?
To allow water to move more rapidly across the cell membrane
What is the main factor that determines the diffusion rate of a molecule across a protein-free lipid bilayer?
Both size and hydrophobicity of the molecule
Why are protein-free lipid bilayers impermeable to charged molecules?
Due to the charge and high degree of hydration of the molecules
What is the primary function of a transporter protein?
To bind to specific solutes and undergo conformational changes
What is the main difference between transporter and channel proteins?
Transporters bind to specific solutes, while channel proteins do not
Which of the following molecules is most likely to diffuse rapidly across a protein-free lipid bilayer?
A small, hydrophobic molecule
What is the main difference between ion channels and aqueous pores?
Ion channels are selective, whereas aqueous pores are non-selective.
What is the purpose of narrow pores in ion channels?
To force permeating ions into intimate contact with the walls of the channel.
What is the main mechanism by which mechanically gated channels are activated?
A mechanical stress.
What is the main factor that contributes to the membrane potential in animal cells?
K+ leak channels and the K+ gradient across the plasma membrane.
What is the function of gated channels in ion channels?
To open briefly and then close again, allowing for regulated ion flow.
Study Notes
ATP-driven Pumps
- ATP-driven pumps, also known as transport ATPases, hydrolyze ATP to ADP and phosphate, using the energy released to pump ions or other solutes across a membrane.
- There are three types of ATP-driven pumps: P-type, ABC transporters, and V-type pumps.
P-type Pumps
- Structurally and functionally related to multipass transmembrane proteins.
- Phosphorylate themselves during the pumping cycle.
- Responsible for setting up and maintaining gradients of Na+, K+, H+, and Ca2+ across cell membranes.
ABC Transporters
- Primarily pump small organic molecules across cell membranes.
V-type Pumps
- Turbine-like protein machines constructed from multiple different subunits.
- Transfer H+ into organelles, such as lysosomes, synaptic vesicles, and plant or yeast vacuoles, to acidify the interior of these organelles.
Channels and Electrical Properties of Membranes
- Ion channels are membrane proteins that form narrow, highly selective pores, allowing specific inorganic ions to diffuse rapidly down their electrochemical gradients.
- Ion channels are classified into voltage-gated, mechanically gated, and ligand-gated channels.
Ion Selectivity and Gating
- Ion channels are ion-selective, permitting some inorganic ions to pass while blocking others.
- Ion channels fluctuate between open and closed states, controlled by gating mechanisms such as voltage, mechanical stress, or ligand binding.
Aquaporins and Water Transport
- Aquaporins are water channels that allow water to move rapidly across the membrane.
- Found in cells that require high water transport rates, such as epithelial cells in the kidney or exocrine cells.
Membrane Transport Proteins
- Two main classes of membrane transport proteins: transporters and channels.
- Transporters bind specific solutes and undergo conformational changes to transport solutes across the membrane.
- Channels form continuous pores that allow specific solutes to pass through the membrane.
Active Transport and Energy Sources
- Active transport involves the movement of solutes against their concentration or electrochemical gradient.
- Requires an input of metabolic energy, which can be coupled to the hydrolysis of ATP or the energy stored in concentration gradients.
Uniporters, Symporters, and Coupled Transport
- Uniporters facilitate the passive movement of a single solute across the membrane.
- Symporters (co-transporters) transfer one solute strictly dependent on the transport of another solute.
- Coupled transport involves the intimately coupled transfer of two solutes in the same direction.
This quiz covers ATP-driven pumps, also known as transport ATPases, which use ATP to pump ions and solutes across cell membranes, including P-type pumps that phosphorylate themselves during the pumping cycle.
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