Chapter 8 - Transport Across Membranes

Homeostasis

Simple Diffusion

Gases, nonpolar molecules, small polar molecules (water, glycerol, ethanol)

Facilitated Diffusion/Passive Transport

Can become saturated at high solute concentrations because there are a limited number of transport proteins

Active Transport

Transport against concentration gradient (Low concentration to high concentration); requires ATP

Bulk Transport

Movement of large molecules. Endocytosis (into cell) & Exocytosis (Out of cell)

Rates of Solutes Crossing Lipid Bilayer

• Small, nonpolar molecules - Fast; readily diffuse

• Uncharged polar molecules - Fast; readily diffuse if small enough

• Ions - NONE; needs transport proteins/channels

Exergonic Vs. Endergonic

Simple & facilitated diffusion (release of energy)

Active Transport (input of energy)

Electrochemical Potential

the combined effect of an ion’s concentration gradient

Membrane Potential (Vm)

charge gradient across a membrane

Most cells have an excess of ____ charged solutes ___ the cell.

negatively; inside. This charge favors the inward movement of cations (Na+) and the outward movement of anions (Cl-)

Thermodynamic equilibrium

No further net movement occurs because the free energy of the system is at a minimum

Osmosis

movement of water

Water will move towards

the region of higher solute concentration. For most cells, water tends to move inward.

Osmolarity

total solute concentrations inside versus outside of the cell

Hypertonic

solute concentration is higher outside the cell

Hypotonic

solute concentration is lower outside the cell

Isotonic

same solute concentration inside and outside the cell

Plant Cells & Turgor Pressure

Cell walls keep a cell from swelling and bursting in a

hypotonic solution. Turgor pressure is the pressure that pushes plasma membrane against cell wall. Plasma membrane pulls away

from the cell wall by a process called plasmolysis

Cells without cell walls

pump out inorganic ions, reducing the intracellular

osmolarity

Liposomes

small vesicles forming a closed, spherical lipid bilayer lacking proteins

Factors affecting diffusion

size, polarity (separation of charge on a molecule), and charge

If the molecular weight of a molecule is less than ___ it can diffuse across the membrane.

100

Estrogen and testosterone (large nonpolar molecules) cross the membrane ___.

easily.

Partition coefficient

Polarity of a solute can be measured by the ratio of its

solubility in an organic solvent to its solubility in water. The more nonpolar (hydrophobic) a substance is, the higher the partition coefficient

Shell of hydration

For these substances to move into a membrane, the water

molecules must be removed, a process that requires

energy.

Kinetics of Simple Diffusion and Facilitated Diffusion

Facilitated diffusion can become saturated, causing the rate to plateau.

Carrier Proteins

(transporters or permeases) 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 hydrophilic channels through the membrane

to provide a passage route for solutes

Cellular Pores

Pores are formed by transmembrane proteins called

porins that allow passage of solutes up to a certain

molecular weight to pass (600)

Ion channels

The movement of solutes through ion channels is much

faster than transport by carrier proteins. This is likely because conformation changes are not required

Alternating Conformation Model

Competitive Inhibition of Carrier Proteins

Competitive inhibition of carrier proteins can occur in the

presence of molecules or ions that are structurally related

to the correct substrate (Example: Glucose Transport Proteins > mannose and galactose)

Uniport

A carrier protein that transports a single solute

Symport

If the two solutes are moved across a membrane in the same direction

Antiport/countertransport

solutes are moved in opposite directions

Channel

forms a pore across the bilayer through which specific

inorganic ions or, in some cases, polar organic molecules can diffuse (can either be open or closed)

Transporter

undergoes a series of conformational changes to

transfer small solutes across the lipid bilayer (are selective)

Passive Transporters Vs. Pumps

• Passive = Movement Down Conc. Gradient; NO ENERGY

• Pump = Requires ATP; Against Conc. Gradient

Na+/K+ Pump

Ca2+ Pump

Keeps the cytosolic Ca2+ concentration low

Glucose/Na+ Symport