Transporters

Introduction and Passive Transport

(Simple) Diffusion

• Diffusion always occurs from high to low concentration regions (driving force), also known as the concentration gradient.
• When the concentrations are equal throughout, this system is said to be at equilibrium.
• In order for diffusion to occur across a membrane, the membrane must be permeable to the molecule.

Diffusion of Non-Electrolytes

Facilitated Diffusion

• No energy required just like simple diffusion but instead uses the help of a membrane protein.
• Down the concentration gradient (i.e. molecules go from areas of high to low concentration).
• Two types of membrane proteins that help in facilitated diffusion: Channel Proteins and Carrier Proteins
• Channel proteins
• Specific for a certain molecule;
• Can be open all the time or need a trigger ("gated"). Example: channels for Na+ or Cl- ions.
• Carrier proteins
• Not just a hollow channel: when its specific molecule binds, it changes shape (conformation) and enables the passage of the molecule inside. Example: glucose transporters.

Active Transport

• Can transport solutes against its concentration gradient.
• Therefore, requires the input of energy.
• When there's no energy available, this cannot occur.

Primary Active Transport

• ATP is used as the source of energy.
• The transporter/carrier itself is an ATPase enzyme.
• Carrier takes a phosphate from ATP, changing its conformation. Examples: Na+/K+ ATPase, Ca2+ ATPase, H+ ATPase, H+/K+ ATPase

Secondary Active Transport

• Also known as co-transport. Energy used is from the movement of one ion down its electrochemical gradient, while another ion moves up its gradient.
• When both solutes move in the same direction, the carrier molecule is called a symporter (hitching a ride). Example: Na+/glucose cotransporter, Na+/amino acid cotransporter.
• When solutes move in opposite directions, the carrier molecule used is called an antiporter (club is at capacity). Example: Na+/H+ exchanger, Cl-/HCO3-exchanger, Na+/Ca2+ exchanger.

Summary

• Both require input of energy.
• Both move solutes against their electrochemical gradients
• Primary active transport uses ATPases.
• Secondary active transport uses another molecule that moves down its gradient.

Ion Channels

• Very selective for their respective ions
• Move ions at very high rates (108 s-1)
• They are gated, meaning they have an open and closed state

Potassium Ion Channels

• Play a key role in electrical impulse formation in neurons
• But has many other roles
• Made up pf 4 identical subunits (predominantly alpha-helices)
• Each subunit has 5 key amino aids lining the active site (TVGYG)
• The small size of the opening forces these residues to replace the water that surrounds the ion extracellularly
• Also is size specific for K+
• Channel can be opened or closed via a conformational change in a conserved glycine residue