Chapter 8: Transport Across Membranes: Overcoming the Permeability Barrier

Simple Diffusion

Direct unaided movement dictated by differences in concentration on the two sides of the membrane

Transport Proteins

Assist most molecules across membranes. They are also integral membrane proteins with great specificity for substances they transport

Facilitated Diffusion(Passive Transport)

Moves solutes from regions of higher concentration to regions of lower concentration

Passive Transport

Movement that requires no energy

Active Transport

Transport moves solutes against the concentration gradient or from low concentration to high concentration

(Requires energy; from ATP or the simultaneous transport of another solute down an energy gradient)

The Movement of a Solute Across a Membrane Is Determined by Its Concentration Gradient

The movement of a molecule that has no net charge is determined by its concentration gradient

Simple diffusion and facilitated diffusion involve exergonic movement “down” the concentration gradient

Simple diffusion and facilitated diffusion involve exergonic movement “down” the concentration gradient (negative ΔG)

Active transport involves endergonic movement “up” the concentration gradient (positive ΔG)

Active transport involves endergonic movement “up” the concentration gradient (positive ΔG)

Electrochemical Potential

The combination of an ions concentration gradient & charge gradient across the membrane

Membrane potential

The active transport of ions across a membrane creates a charge gradient across the membrane

Active Transport of Ions

Most cells have a high concentration of negatively charged solutes inside the cell.

Favors the inward movement of Na⁺ and outward movement of Cl^-

Diffusion(Equillibrium)

Always tends to create equal concentrations

Factors Affecting Diffusion

size, polarity, and charge

Solute Polarity

Lipid bilayers are more permeable to nonpolar substances than to polar ones

Nonpolar substances dissolve readily into the hydrophobic region of the bilayer

Solute Charge—Relevance to Cell Function

Every cell must maintain an electrochemical potential across its plasma membrane

This potential is a gradient of either sodium ions in animal cells.

Membranes must still be able to allow ions to cross the bilayer in a controlled manner

Facillitated Diffusion

If the process of moving substances requires no energy

Carrier Proteins

Bind solute molecules on one side of a membrane, undergo a conformation change, and release the solute on the other side of the membrane

Channel Proteins

Form channels through the membrane to provide a passage route for solutes

Alternating Conformation Model(analagous to enzymes)

carrier protein alternates between two conformational states(increase rate of Diffusion)

Carrier Protein 1st State

The solute-binding site of the protein is accessible on one side of the membrane

Carrier Protein 2nd State

The protein shifts to the alternate conformation, with the solute-binding site on the other side of the membrane, triggering solute release

How are carrier Proteins analagous to enzymes?

Carrier proteins are analogous to enzymes(Similar to enzymes that they have an active site and 2 confirmations)(however they do not catalyze a reaction)

Uniport

a carrier protein that transports a single solute across the membrane

Coupled Transport

Two solutes are transported simultaneously, and their transport is coupled

Symport

If two solutes are moved in the same direction

Antiport

If the solutes are moved in opposite directions

Why is glucose phosphorylized when it enters the cell?

The immediate phosphorylation of glucose upon entry into the cell keeps the internal concentration of glucose low.

Once phosphorylated, glucose cannot bind the carrier protein any longer and is effectively locked into the cell

What enzyme phosphorylates glucose upon entry to the cell?

Hexokinase

Gated Channels

Ion channels the open and close in response to stimulus

Voltagge Gated Channels

Respond to changes in membrane potential

Ligand Gated Channels

Are triggered by the binding of certain substances to the channel proteins

Mechanosensitve Channels

Respond to mechanical forces acting on the membrane

Active Transport

Is used to move solutes up a concentration gradient using energy

Active Transport performs 3 Cellular Functions

1.Uptake of Essential Nutrients

2.Removal of Wastes

3.Maintenance of Nonequillibrium concentration of certain ions

Direct Active Transport

movement of solute molecules to one side of the membrane is coupled directly to the use of ATP.

Indirect active transport

depends on the simultaneous transport of two solutes

Favorable movement of one solute down drives the unfavorable movement of the other up