Transporters

Simple Diffusion (Passive Transport)

Small, nonpolar molecules

Facilitated Diffusion (Passive Transport)

Large molecules/ions not able to bypass the hydrophobic core

Primary Active Transport

Large molecules/ions moving against the electrochemical gradient

Membrane Pumps (Ion Transporters)

• actively move ions against the concentration gradient

• create ion concentration gradients

Membrane Channel (Ion Channel)

• allow ions to diffuse down the concentration gradient

• cause selective permeability to certain ions

Passive Transport

• down the electrochemical gradient

• No energy expenditure

• molecules are obeying the law of entropy

  • “Go with the flow”

Facilitated Diffusion

• molecules also move down the electrochemical gradient

• Unable to pass through the membrane

• Passive: ΔG transport is negative

• Very specific: D-glucose, not L-glucose

• Competitive: galactose competes with glucose

• Saturable: transport limited

• Example: Glucose transport into red blood cell

• Down a concentration gradient: glucose that enters the cell is used up almost immediately and so the [glucose] inside is lower than [glucose] in the bloodstream

Channels

• ions can diffuse without ever interacting with the membranes’ hydrophobic core

• Still, no energy expenditure

• Generally allow transmembrane movement of ions at rates that are orders of magnitude greater than those typical of transporters, approaching the limit of unhindered diffusion

• Typically show some specificity for which ion is preferentially transported

• Not saturable with the ion substrate (there is no substrate concentration above which further increases will not produce a greater rate of transport)

• Always passive (i.e. ions always move down the electrochemical gradient)

Active Transport

• Molecules are moved against the electrochemical gradient (from low to high potential)

• Unlike channels, pumps utilize energy (i.e., ATP, light, etc)

• Essentially, pumps transform one form of energy into another

  • ATPases: pumps using ATP

  • ATP is hydrolyzed

  • Conformational change

  • Ion/molecule can move

Secondary Transporters

pumps that use ion A’s gradient to move ion B

For uncharged molecule:

ΔG = RTln(𝐶2/𝐶1) where C1 is the concentration in the area the molecule is coming from C2 is concentration in the area it is moving to

• If ΔG > 0, the molecule will not move spontaneously, and energy expenditure is necessary – Active transport

• If ΔG < 0, the molecule will move spontaneously, and energy will be released

  – Passive transport

For charged molecules:

ΔG = RTln(𝐶2/𝐶1) + ZFΔψ where C1 is the concentration in the area the molecule is coming from, C2 is concentration in the area it is moving to, Z is the charge on the ion, F is Faraday’s constant (96,485 J/Vmol) and 𝝙ѱ is the transmembrane electrical potential

• If ΔG > 0, the molecule will not move spontaneously, and energy expenditure is necessary - Active transport

• If ΔG < 0, the molecule will move spontaneously, and energy will be released

  – Passive transport

Transporters

• Bind their substrates with high specificity,

• Catalyze transport at rates well below the limits of free diffusion (slow), and

• Are saturable in the same sense as are enzymes (there is a substrate concentration above which further increases will not produce a greater rate of transport)

• Can be active or passive

Gated Ion Channels

• Ion channels that respond to the binding of specific cellular event (ligand binding, voltage, stress)

• Acetylcholine receptors found on postsynaptic cells

  • They respond to the binding of acetylcholine, a neurotransmitter

How gated ion channels work:

1. Action potential arrives at the terminal end of the pre-synaptic nerve cell, stimulating the release of vesicles containing acetylcholine

2. Acetylcholine diffuses across the synaptic cleft and binds onto target acetylcholine receptors

3. Upon binding of acetylcholine, a conformational change is induced on the channels. They open, exposing small and polar residues, which allows the ions to pass through

Ligand-Gated Ion Channel

• The acetylcholine receptor is a heteropentamer that consists of four types of subunits – 2α,β,γ,δ chains

• Integral membrane protein

• Acetylcholine binds each of the two α subunits

• Each polypeptide chain (subunit) crosses the plasma membrane four times, each as an α-helix.

• Helices are named M1, M2, M3, M4

• The M2 helix has a polar side and a nonpolar side, i.e., it is amphipathic

• The M2 helix from each subunit lines the channel

• Channel opens very rapidly when acetylcholine binds to the α-subunits.

• Binding causes a conformational change in the receptor which causes the helices of the α-subunits to twist

How can the ligand-gated ion channel be specific?

• There are acidic residues near the end of each helix so anions cannot pass

• Size of channel opening does not allow larger ions to pass

Ion Channels

• Acts as a controlled “pore”. When the channel is open, there is very rapid, fairly specific movement of a large number of ions down an electrochemical gradient (107 to 108 ions/second, close to the theoretical maximum for unrestricted diffusion)

• Pores can be ligand, voltage or stress-gated

• Transport by channels is passive

• Transport by channels is not saturable

• A channel typically opens in a fraction of a millisecond and may remain open for only a few milliseconds

GLUT1

• Glucose transporter responsible for generating the basal rate of glucose uptake

• High affinity for glucose molecules

• Glucose transporter in red blood cell makes 12 α-helical passes across the membrane

• 9 helices have 3 or more polar residues

• A cluster of amphipathic helices are arranged so their polar sides face each other to form a hydrophilic pore through which glucose can pass

• Hydrophobic faces interact with the surrounding membrane lipids

How facilitated diffusion works:

1. Glucose in blood plasma binds to transporter and lowers the activation energy for

2. a conformational change from the T1 conformation to T2 conformation - effecting the transmembrane passage of the glucose

3. Glucose is released into the cytoplasm and

4. the transporter returns to the T1 conformation

Primary Active Transport

• Electrical signalling in neurons

• ATP hydrolysis is coupled to pumping:

  • Na + out of cells

  • K + into cells

• Why active?

  • Na + is way more concentrated outside the cells (Uphill battle)

  • Cells spend 30-70% of the ATP they produce

Co-transporters (Secondary Active Transporters)

• All three classes could be active (energy-requiring) or passive (energy-independent)

• Energy required can come from co-transport of an ion moving down its electrochemical gradient

Secondary Active Transport

Transport of a solute against its electrochemical gradient is coupled to transport of an ion down its electrochemical gradient so the overall free energy change is negative (ATP hydrolysis is required to maintain the ion gradient).

• Example: glucose transport in intestine

• Glucose transport in intestine requires a different transporter and different mechanism of transport than glucose transport in red blood cells

Sodium-glucose Symporter

• Two sodium ions and one glucose molecule bind to the symporter on the apical membrane. No transport unless all are bound

• Conformational change in symporter; sodium and glucose released inside cell

• Relies on the sodium gradient produced by the Na⁺ K⁺ ATPase

Types of Transport

1. Simple Diffusion: if the membrane is permeable enough to the solute (O2, etc.)

2. Membrane proteins: required for many solutes since membrane not permeable enough – channels or transporters (aka carriers)

3. Membrane vesicles are used to transport large molecules (e.g., proteins) or large amounts of smaller solutes (e.g., neurotransmitters)

Endocytosis (Membrane Vesicles)

uptake into cell

Exocytosis (Membrane Vesicles)

excretion out of cell