ACTIVE TRANSPORT AND GLUCOSE TRANSPORT

(Chapter 14 and 15, pp 140-143 and pp 146-151)

Overview• Active transport is the movement of molecules

across a cell membrane in the direction against their concentration gradient, i.e. moving from a low concentration to a high concentration.

• The membrane proteins which help in the transportation posses ATPase activity (enzyme which breaks ATP for energy which is used in transportation) and are called as transporters.

• If the process directly uses chemical energy, such as adenosine triphosphate(ATP), it is termed as primary active transport.

• Secondary active transport involves the use of concentration gradient, generated by ATP-powered pumps to power transport of molecules and ions against their own concentration gradient (indirect use of energy).

• Examples of active transport include the uptake of glucose in the intestines in humans.

Primary Active Transport• Four classes of transport proteins function as pumps (powered by

ATP) to bring about transportation against their concentration gradients.

• All the transport pumps have ATP binding sites on the cytosolic side of the membrane.

• ATP breakdown is coupled to the transport of molecules by these pumps.

• The classes differ in the types of ions/molecules they transport.

CLASS SUBSTRATES TRANSPORTED

P Ions(H+, Na+, K+, Ca+)

F H+ only

V H+ only

ABC Ions, Drugs

Primary Active Transport:P-Class Pumps

• They are named as P-class pumps because of the phosphorylation (adding a phosphate) of one of the subunits of the transport protein that occurs during the process.

• Transported molecules are moved through the phosphorylated subunit of the transport pump.

• For example sodium-potassium ATPase (P class pump) that is present on the plasma membrane of all cells.

• It maintains extracellular sodium concentrations and intracellular potassium concentrations.• Three sodium ions are pumped out of the cell and two potassium ions are pumped in for each ATP breakdown by the sodium-potassium ATPase.

Primary Active Transport:F- and V-Class Pumps

• Both F-class and V-class pumps transport protons.

• V-class pumps present on membranes of lysosomes maintain the low pH of lysosomes by pumping protons inside by using ATP.

• In mitochondria, F-pumps known as ATP synthase allows protons to be pumped from the mitochondrial matrix to the intermembrane space .

• This creates an proton gradient across the inner mitochondrial membrane.

• Proton gradient is used by ATP synthase (enzyme at inner mitochondrial membrane) to bring about synthesis of ATP from ADP + Pi.

Primary Active Transport :ABC-Class Pumps

• They have two cytosolic ATP-binding domains and two transmembrane domains that forms the passage for transported molecules.

• ABC pumps bind and hydrolyze ATP, causing conformational changes in the membrane-spanning domains, which results in transportation of molecules from one side of the membrane to the other.

• About forty eight human ABC transporters have been characterized.

• Cells can become resistant to several drugs as these cells can no longer respond to drug-multidrug resistance (MDR) and ABC transporters are often associated with it.

Secondary Active Transport

• Ion concentration gradient generated by primary active transport can power the movement of substrates against their gradient.

• Transport proteins in secondary active transport do not posses ATPase activity, infact they depend indirectly on ATP hydrolysis done during primary active transport.

• Cotransport can allow substrates to cross the membrane in the same direction or can allow for entry of one substrate into the cell and exit of another outside the cell-counter transport.

Secondary Active Transport-Symporter• They are secondary active transporters

which move 2 molecules in the same direction across plasma membrane.

• Both the molecules will enter or both will exit a cell by symport i.e. one is transported along its concentration gradient (i.e. from high concentration to low concentration) and another is actively transported against its gradient.

• For example sodium-dependent amino acid transport proteins also function in symport. The sodium ion travels downhill (i.e. from high concentration to low concentration) and energize the transport of amino acid across membrane.

• Antiporters transport two molecules in opposite directions across plasma membrane.

• Movement of one substrate into a cell is coupled to the movement of another substrate out of the cell i.e. one along its gradient and the other moved against its gradient.

• For example, sodium-calcium antitransporter in cardiac muscle cells.

• Sodium ions moved into the cell along its gradient (i.e. from high concentration to low concentration) and the transport of calcuim ions occurs out of the cell against its gradient.

Secondary Active Transport- Antiporter

Glucose Transport• Glucose is essential for life.• It is major source of energy for cells

and its transportation into cells is crucial for survival of both the cell and the individual.

• Most cells use facilitated transport or passive transport for uptake of glucoseby uniport.

• Different members of glucose transporters family are expressed in most cell types.

Facilitated Glucose Transport• Family of glucose transporters (GLUTs) function in facilitated transport of

glucose include at least 14 members.

• GLUTs 1 to 5 are the most commonly expressed.

SiteFunctionTransporterGLUT-1 in RBCs and blood brain barrier.GLUT-3 in neurons.GLUT-4 in muscle and adipose tissues.

Involved in glucose uptake from blood

GLUT-1,GLUT-3,GLUT-4

Liver, kidney and β cells of pancreas.

Transport glucose in these cells when blood glucose is high and out of cells when blood glucose is low.

GLUT - 2

Small intestine and testes. Primary transporter of fructose

GLUT - 5

Mechanism• GLUT family members transport glucose

from an area of higher concentration to an area of lower concentration.

• The sugar molecule binds to the GLUT protein.

• Then the sugar-GLUT complex is reversed so that the sugar is released to the other side of the membrane i.e. inside the cell.

• After the sugar has dissociated, the GLUT changes orientation and is ready for another cycle of transport.

Active Transport Of Glucose§ The location in the body where glucose is

found at a higher concentration inside cells than out and glucose must be transported against its gradient. For example to enter those cells that are the kidney and epithelial cells of small intestine.

§ In the cell membrane, there is a carrier protein called sodium dependant glucose transporter (SGLT-1) which transport glucose to inside the cell using energy.

§ The transporter has 2 separate sites, one for sodium and the other for glucose. The sodium is transported from high to low concentration and at the same time causes transport of glucose against its concentration gradient.

§ The sodium is send outside the cell by sodium pump which needs ATP as a source of energy. The reaction is catalyzed by an enzyme called "Adenosine triphosphatase (ATPase)".

§ Active transport is much more faster than facilitated (passive) transport.

SUMMARY• Active transport involves movement of

ions or molecules against their concentration gradient in an energy-dependent process.

• There is primary active transport requiring direct hydrolysis of ATP by the transporter.

• Four classes of primary active transporters are: P-class transporters move ions against the gradient.

• F- and V-class pumping proton against their gradient and ABC-class transporters move drugs, ions against their gradient.

• Ion gradient established by primary active transport can drive the transport of ions and small molecules in secondary active transport (indirect use of energy).

• Symporters that move substrates in the same direction.

• Antiporters move one substrate into the cell and another substrate out of the cell.

SUMMARY• Most cells use facilitated transport of glucose

mediated by GLUT proteins.

• In most cases, glucose move from higher concentration outside the cell toward the lower concentration inside the cell.

• Family of glucose transporters (GLUTs) function in facilitated transport of glucose include at least 14 members, 11of which are involved in glucose transport.

• GLUTs 1 to 5 are the most commonly expressed.

• Secondary active transport of glucose occurs via symport with sodium using SGLT proteins, in the kidney and intestinal cells against its concentration gradient.