BIOL 3450 Chapter Notes - Chapter 11: Membrane Transport Protein, Lipid Bilayer, Electrochemical Gradient
16 January 2019
BIOL 3450
Chapter 11 – Transmembrane Transport of Ions and Small Molecules
o Organelle membranes separate cytosol from interior of organelle; form permeability
barriers
o Cellular membranes (plasma and organelle membranes) consist of bilayer of
phospholipids in which other lipids and specific types of proteins are embedded
▪ Membrane transport proteins ~ integral membrane roteins wit multiple
transmembrane domains embedded in cellular membranes
11.1 Overview of Transmembrane Transport
Only Gases and Small Uncharged Molecules Cross Membranes by Simple Diffusion
o Phospholipid bilayer
▪ Dense hydrophobic core; largely impermeable to water soluble molecules and
ions → only gases and small uncharged polar molecules (urea and ethanol) can
readily move across artificial membrane composed of pure phospholipid or of
phospholipid and cholesterol
• Simple diffusion ~ net movement of molecule across membrane down its
concentration gradient at rate proportional to gradient and permeability
of membrane
• Movement from high concentration to low; movement is spontaneous,
has positive change of entropy vale and negative change in free energy
▪ Membrane potential ~ electric potential (voltage) across membrane
▪ Electrochemical gradient ~ determines energetically favorable direction of
movement of charged molecule across membrane
Three Main Classes of Membrane Proteins Transport Molecules and Ions Across Cellular
Membranes
o All transport proteins are transmembrane proteins with multiple membrane spanning
segments tha are normally alpha helices
▪ Channels ~ transport water, specific ions, or hydrophilic small molecules across
membranes down concentration or electric potential gradients
• Facilitated transport
• Form hydrophobic tubes or passages across membrane in which ions
move simultaneously, single file, very quickly
• Most are gatd
▪ Transporters ~ move wide variety of ions and moelcules across cellular
membranes but at slower rate than channels
• Uniporters ~ transport single typeof molecule down concentration
gradient
• Antiporters ~ couple movement of one type of ion or molecule against
concentration gradient with movement of one or more different ions in
different directions
• Symporters ~ couple movement in same direction
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▪ ATP powered pumps ~ ATPases that use energy of ATP hydrolysis to move ions
or small molecules across membrane against chemical concentration gradient,
electric potential, or both
• Active transport ~ requires energy
Key Concepts – Overview of Transmembrane Transport
o Cellular membranes regulate traffic of molecules and ions into and out of cells and
organelles
▪ Rate of simple diffusion of substance across membrane is proportional to its
concentration gradient and hydrophobicity
o With exception of gases and small uncharged, water soluble molecules, most molecules
cannot diffuse across pure phospholipid bilayer at rates sufficient to meet cellular needs
o Membrane transport proteins provide hydrophilic passageway for molecules and ions to
travel through hydrophobic interior of membrane
o Three classes of transmembrane proteins mediate transport of ions, sugars, amino acids,
and other metabolites across cellular membranes: channels, transporters, and ATP
powered pumps
o Channels form hydrophobic tube through which water or ions move down concentration
gradient, process known as facilitated transport
o Transporters fall into three groups
▪ Uniporters – transport molecule down concentration gradient (facilitated
transport)
▪ Symporters – couple movement of substrate against its concentration gradient to
movement of second substrate down concentration gradient; secondary active
transport
▪ Antiporters
o ATP powered pumps couple movement of substrate against its concentration gradient to
ATP hydrolysis, process known as active transport
o Conformational shapes are essential to function of all membrane transport proteins
▪ Speed of transport depends on number of substrate molecules or ions that can
pass through protein at once
11.2 Facilitated Transport of Glucose and Water
Uniport Transport is Faster and More Specific than Simple Diffusion
o Uniport ~ protein mediated transport of single type of molecule down concentration
gradient across cellular membrane
▪ Rate of substrate movement is higher than simple diffusion
▪ Since transported molecule never enters hydrophobic core of phospholipid layer,
partition coefficient K is irrelevant
▪ Transport occurs via limited nmber of uniporter molecules → there is Vmax
which depends on nu,ber of uniporters in membrane
• Achieved when concentration gradient across membrane is very large
and each uniporter is working at max rate
▪ Transport is reversible and direction will change if concentration gradient
direction changes
▪ Transport is specific → each uniporter transports only single type of molecule or
single group of loosely related moelcules
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• Measure of affinity of transporter for its substrate is Michaelis constant,
Km = ½ Vmax
Low Km of GLUT1 Uniporter Enables it to Transport Glucose into Most Mammalian Cells
Human genome Encodes Family of Sugar Transporting GLUT Proteins
o Genome encodes at least 14 highly homologous GLUT proteins that contain 12
membrane spanning alpha helices → evolved from ancestral transport protein
Transport Proteins Can BE Studied Using Artificial Membranes and Recombinant Cells
o 1) Specific transport protein is extracted from membrane with detergent and purified
Osmotic Pressure Causes Water to Move Across Membranes
o Aquaporins ~ family of membrane proteins that allow water and few other small
uncharged molecules to cross cellular membranes efficiently
o Osmosis ~ water spontaneously moves downhill across semi permeable membrane from
solution of lower solute concentration to higher solute concentration
▪ Osmotic pressure ~ hydrostatic pressure required to stop net flow of water across
membrane separating solutions of different water concentrations
o Hypotonic ~ concentration of non membrane penetrating solutes is lower than in cytosol;
animal cells swell owing to osmotic flow inward
o Hypertonic ~ one in which concentration of non membrane penetrating solutes is higher
than in cytosol; animal cells shrink as cytosolic water leaves cell by osmotic flow
o Isotonic ~ solute concentration similar to cell cytosol
o Contractile vacuole ~ permits protozoans to avoid osmotic lysis; takes up water from
cytosol and periodically discharges its contents through fusion with plasma membrane
Aquaporins Increase Water Permeability of Cellular Membranes
Key Concepts – Facilitated Transport of Glucose and Water
o Protein catalyzed transport of biological solutes across membrane occurs much faster
than simple diffusion, exhibits Vmax when limited number of transporter moelcules are
saturated with substrate, and is highly specific for substrate
o Uniport proteins are thought to shuttle between two conformational states
▪ One in which substrate binding site faces outward
▪ One in which binding sites face inward
o All members of GLUT protein family transport sugars and have similar structures
▪ Differences in Km values , expression in different cell types, and substrate
specificities are important for proper sugar metabolism in body
o Two common experimental systems for studying functions of transport proteins are
liposomes containing purified transport protein and cells transfected with gene encoding
particular transport protein
o Most cellular membranes are semipermeable, more permeable to water than to ions or
most other solutes
▪ Water moves by osmosis across membranes from solution of lower solute
concentration to one of higher solute concentration
o Rigid cell wall surrounding plant cells prevents their swelling and leads to generation of
turgor pressure in response to osmostic influx of water
o Aquaporins are water channel proteins that specifically increase permeability of cellular
membranes to water
o Aquaporin 2 in plasma membrane of certain kidney cells is essential for resorption of
water from urine being formed
▪ Absence of aquaporin 2 leads ti medical condition diabetes insipidus
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Document Summary
Three main classes of membrane proteins transport molecules and ions across cellular movement of charged molecule across membrane. Atp hydrolysis, process known as active transport: conformational shapes are essential to function of all membrane transport proteins, speed of transport depends on number of substrate molecules or ions that can pass through protein at once. Low km of glut1 uniporter enables it to transport glucose into most mammalian cells. Human genome encodes family of sugar transporting glut proteins: genome encodes at least 14 highly homologous glut proteins that contain 12 membrane spanning alpha helices evolved from ancestral transport protein. Transport proteins can be studied using artificial membranes and recombinant cells: 1) specific transport protein is extracted from membrane with detergent and purified. Isotonic ~ solute concentration similar to cell cytosol: contractile vacuole ~ permits protozoans to avoid osmotic lysis; takes up water from cytosol and periodically discharges its contents through fusion with plasma membrane. Aquaporins increase water permeability of cellular membranes.
