Cell Structure and Function Chapter 8

3 Transport Mechanisms

• Simple Diffusion

• Facilitated Diffusion

• Active Transport

Transport Proteins

• Assist most solutes across membranes

• Some move solutes to regions of lower concentration (facilitated diffusion) which uses no energy.

Active Transport

• Transport proteins move solutes against the concentration gradient

• Requires energy

  • Hydrolysis of ATP

  • Simultaneous transport of another solute down an energy gradient (diffusion down one gradient drives diffusion up another gradient)

Concentration gradient

• Determines the movement of a molecule that has no net charge

• Passive Transport - exergonic movement “down”

  • Negative free energy

• Active Transport - endergonic movement “up”

  • positive free energy

Electrochemical potential

• Determines movement of ion

  • Concentration gradient and charge gradient across membrane

Charged Gradient/Membrane Potential (Vm)

• Active Transport of ions across a membrane creates it

• It is a charge separation across a membrane (i.e. voltage)

• Membrane voltage can be used for nerve impulse conduction and driving transport of solutes

  • Excess of negatively charged solutes inside the cell

Simple Diffusion

• the unassisted net movement of a solute from high to lower concentration

• Only Possible for:

  • Gases

  • nonpolar molecules

  • small polar molecules (water, ethanol)

• Diffusion is always movement toward equilibrium

  • Free energy minimized

Osmosis

• Water moves towards the region of higher solute concentration

• For most cells water tends to move inward

Osmolarity

• Total solute concentrations inside versus outside the cell

• Hypertonic - solute concentration is higher outside the cell (concentrated)

• Hypotonic - solute concentration is lower outside the cell (diluted)

• Isotonic Solution - Same solute concentration inside and outside the cell

Cell Response

• Shrink in hypertonic

• Swell/burst in hypotonic 

• Cell walls prevent swelling and bursting and instead become very firm from turgor pressure

• Cells without cell walls pump out inorganic ions to reduce intracellular osmolarity

Solute Size

• lipid bilayers are more permeable to small molecules

• without a transporter however they most more slowly

• molecular weight less than 100 Da it can diffuse across the membrane.

Solute Polarity

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

• Dissolve readily into the hydrophobic region of the bilayer

• Even large nonpolar molecules cross easily

Solute Charge

• Charged Solutes do not spontaneously cross the bilayer

• Cells use this property to create electrochemical gradients

• IT IS NECESSARY FOR PROPER FUNCTION

  • ATP generation, signaling etc. 

• Could be gradient of either sodium ions of protons and are established by active transporter

Diffusion Kinetics

Rate of simple diffusion is directly proportional to the concentration gradient

Facilitated Diffusion

• Molecules move down their concentration gradient (from high to low) across a cell membrane with the help of membrane proteins

• Limited # of transport proteins so saturation occurs

Types of Transporters

Carrier Proteins - bind solute molecules on onside 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

Alternating Conformation Model

• a Carrier protein is allosteric protein and alternates between 2 conformation states

  • 1 - Solute binding site is accessible on one side of membrane

  • 2 - Shifts to the alternate conformation, with the solute binding site on the other side of the membrane, triggering solute release. 

Specificity of Carrier Proteins

• Carrier proteins are analogous to enzymes

• High specificity

Uniport

When a carrier protein (uniporter) transports a single solute across the membrane

Coupled Transport

• 2 solutes transported simultaneously and their transport is coupled

Symport

2 solutes moves across a membrane in the same direction

Antiport

2 solutes moves in opposite directions

Glucose Transporter

• Uniport carrier

• Glucose is transported inward by a glucose transported (GLUT; GLUT1 in erythrocytes)

• Integral membrane Protein with 12 transmembrane segments which form a cavity with hydrophilic side chains

Process of GLUT1

1. Glucose collides with and binds to GLUT1 in T1 conformation

2. GLUT1 shifts to T2 conformation

3. Conformation change releases glucose

4. GLUT1 returns to original T1 conformation

Can be reversed.

Glucose concentration kept low inside most animal cells. 

Anion Exchange Protein 

• Chloride bicarbonate exchanger 

• Facilities reciprocal exchange of Cl- and HCO3- ions only

• Strict 1:1 ratio and can only occur if both anions are present

“Ping Pong” Mechanism

• 2 conformation states

  • 1st - Protein binds a chloride ion on one side of the membrane which causes a change to the second state

  • 2nd - chloride moved across the membrane and released 

• This release causes the protein to bind bicarbonate cause a shift back to the first conformation

• Bicarbonate moves out of the cell allowing the carrier to bind chloride again.

Significance of Anion Exchange Protein 

• Waste co2 diffuses into erythrocytes where it is converted into HCO3-

• Moves out to prevent net charge imbalance

• In lungs this is reversed

Channel Proteins

Form hydrophilic transmembrane channels that allow specific solutes to cross the membrane directly

Types of Channels

• Ion

• porins

• aquaporins

Ion Channels

• tiny pores lined with hydrophilic atoms

• remarkably selective

Gated Channels

• Most ion channels are gated

  • Voltage Gated Channels - open and close in response to changes in membrane potential

  • Ligand Gated Channels -triggered by the binding of certain substances to the channel protein

  • Mechanosensitive channels - responds to mechanic forces on membrane

Functions of Ion Channels

• cellular communication

  • eg. muscle contraction and electrical signaling of nerve cells

• Maintain salt balance in cells and airways linking the lungs

Porins

• Pores on outer membranes of bacteria, mitochondria, and chloroplasts

• Larger and less specific

• Formed by multipass transmembrane proteins aka Porins

Structure of Porins

• Beta Barrel has a water filled pore at its center

• Polar side chains line the inside of the pore, allowing passage of many hydrophilic solutes

• The outside of the barrel contains many nonpolar side chains that interact with the hydrophobic interior of the membrane

Aquaporins (AQP)

Water channel proteins found in cell membranes that selectively facilitate the rapid passage of water, and sometimes small solutes like glycerol or urea, across the membrane.

• erythrocytes and kidney cells

• root cells and vacuolar membranes

AQP structure

• Tetrameric integral membrane proteins

• 4 central channels

• Just large enough for water molecules to pass through one at a time