Membrane Transport: Passive Transport

Unaided by cellular transport proteins

• Simple diffusion

Aided by cellular transport proteins

• Facilitated diffusion

• Active transport

Conditions that differ on two sides of the membrane

• Solute concentrations

• Ion concentrations

Diffusion

Spontaneous movement of solutes and ions to achieve equilibrium. Area of high concentration to area of low concentration (down gradient).

Gradients govern movement of molecules (Laws)

• No Net Charge: governed by concentration gradient

• Ions: governed by sum of electrical and chemical forces

Simple Diffusion (Solutes transported)

Small polar, small non-polar, large nonpolar lipids and steroids

Facilitated Diffusion (Solutes Transported)

Small polar, large polar, ions

Active Transport (Solutes Transported)

Large polar, ions

Biological membrane permeable to what

small non-polar (uncharged) molecules

Osmosis

Movement of water across a membrane to the side with the higher solute concentration.

Tonicity of water and cells

• Hypertonic: More (membrane impermeable) solutes than in cells

• Hypotonic: Fewer solutes than in cells

• Isotonic: equal amounts of solutes to cells (Saline for intravenous hydration)

Simple Diffusion (Basic)

• No cellular proteins necessary

• Mostly limited to small nonpolar molecules (Eg: O2 and CO2)

Facilitated Diffusion (Transport proteins)

• Carrier proteins

• Channels

NO energy required

Similarities (Carriers and channels for facilitated diffusion)

Both use transmembrane proteins

Carrier Proteins

• Bind extracellular solutes; change chape to bring solute into the cell

• Highly specific; relatively slow transport

Channel proteins

• Create hydrophilic channels in the membrane

• Variable specificity; usually very rapid transport

Glucose Transporter

Changes shape (conformation) when bound by glucose (T1 and T2 conformations). This increases diffusion rate into the cell by 500000 folds

Is a uniporter (meaning that is can transport only one solute at a time)

Chloride carbonate exchanger

Red blood cells convert waste CO2 to HCO3-

• As HCO3- concentration increases, it is transported out

• Coupling with Cl- uptake prevents net charge imbalance

• Antiporter of chloride and carbonate (1:1 ratio) (transport stops it either ions is absent)

Channel proteins

Transmembrane proteins that create pores in the membrane. Allows specific solutes to cross the membrane (often gated so movement can be regulated)

Ex: porins

Channel proteins structure

• Transmembrane segments form a beta-barrel.

• Pore lined with polar amino acids (hydrophilic)

• Exterior of pore contains nonpolar amino acids (hydrophobic)

Aquaporins (transmembrane water channels)(function)

Allow rapid passage of water through membranes of specialized cells (kidney cells)

• Water passage is regulated

Aquaporins (structure)

Integral membrane protein creates a channel between outside and inside the cell.

• The channels allow water to pass, one H2O at a time.

• Homotetramer

Homotetramer

Four identical monomers form channels.