Pores and Transport Channels S1M1

• Lipid-based hormones (aldosterone),
• O2,
• CO2,

• Diffusion (passive)
• Carrier-mediated Transport
• Vesicular Transport

• Diffusion through plasma membrane (simple diffusion)
• Diffusion through channels/pores

• Facilitated diffusion (Passive)
• Active Transport
• - Primary active transport
• - Secondary active transport

Transporting molecules from a region of high concentration to a region of low concentraction.

• O2
• H2O (to an extent)
• CO2
• steroid hormones
• thyroid hormone

• Porins
• Connexins
• Aquaporins

• Always open on both sides (no gate)
• Substances move through a pore “downhill” or down the concentration gradient
• Subunits undergo no conformational changes
• Do not exhibit saturation

Jx = PxA (C1-C2)

• Jx (Rate of Transport)
• A (Surface area)
• w (Membrane thickness)
• C1 (area of High conc.)
• C2 (area of Low conc.)

• 1. Not always open
• 2. Contain a gate
• 3. Gate must receive a signal to open (stretch, Δ voltage, ligand binding)
• 4. “Downhill” movement
• 5. Do not exhibit saturation
• 6. Most are selective for the molecules they transport

The Kidneys

• Change in voltage
• Binding of a chemical messenger (ligand)
• Mechanical forces (stretch)

• Specificity
• Saturation limited
• Competition (different substances that can be carried by the same transport compete slowing it down)

• 1. Not always open
• 2. Transported molecules bind to the carrier & subunits undergo a conformational change
• 3. Responsible for facilitated diffusion; move substances “downhill” without ATP
• 4. Responsible for secondary active transport; moves substances “downhill” and/or “uphill”
• 5. Transporters can be saturated.

• Transport against a gradient
• Requires energy (directly or indirectly)

• 1. Solutes are transported against their concentration gradient.
• 2. Energy required to transport molecules “uphill” is derived from the breakdown of ATP or GTP.

• 1. Not always open
• 2. Solute binds to pump, then pump undergoes a conformational change; No true “gate”
• 3. Responsible for primary active transport, requires ATP to move a substance “uphill”.
• 4. Pumps can be saturated
• 5. Highly selective for the molecule they transport

• Sodium Potassium ATPase
• 3 Sodium from inside of cell to the outside
• 2 Potassium from the outside in
• *Requires one ATP molecule
• In cardiac and smooth muscles

• Does not use ATP directly but instead takes advantage of a previously established concentration gradient
• Na/Ca transport (3Na in 2Ca out) uses gradient of high Na outside the cell built by Na/K+ ATPase

Energy from one gradient drives transport of another molecule against its gradient

• e.g. as Na+ moves
• into the cell, molecule X is pumped out.

• Sodium Glucose transporter in kidney
• or
• Sodium-Calcium exchanger in cardiac muscle