ACTIVE TRANSPORT

ACTIVE TRANSPORT

Active transport is the movement of substances against the chemical or electrical or electrochemical gradient. It is like swimming against the water tide in a river. It is also called uphill transport. Active transport requires energy, which is obtained mainly by breakdown of high energy compounds like adenosine triphosphate (ATP).

Active Transport vs Facilitated Diffusion

Active transport mechanism is different from facilitated diffusion by two ways:

1. Carrier protein of active transport needs energy, whereas the carrier protein of facilitated diffusion

does not need energy

2. In active transport, the substances are transported against the concentration or electrical or electrochemical gradient. In facilitated diffusion, the substances are transported along the concentration or electrical or electrochemical gradient.

CARRIER PROTEINS OF ACTIVE TRANSPORT

Carrier proteins involved in active transport are of two types:

1. Uniport

2. Symport or antiport.

1. Uniport

Carrier protein that carries only one substance in a single direction is called uniport. It is also known as

uniport pump.

2. Symport or Antiport

Symport or antiport is the carrier protein that transports two substances at a time.

Carrier protein that transports two different substances in the same direction is called symport

or symport pump. Carrier protein that transports two different substances in opposite directions is called

antiport or antiport pump.

MECHANISM OF ACTIVE TRANSPORT

When a substance to be transported across the cell membrane comes near the cell, it combines with

the carrier protein of the cell membrane and forms substance-protein complex. This complex moves

towards the inner surface of the cell membrane. Now, the substance is released from the carrier proteins. The same carrier protein moves back to the outer surface of the cell membrane to transport another molecule of the substance.

SUBSTANCES TRANSPORTED BY ACTIVE TRANSPORT

Substances, which are transported actively, are in ionic form and non-ionic form. Substances in ionic form are sodium, potassium, calcium, hydrogen, chloride and iodide. Substances in non-ionic form are glucose, amino acids and urea.

TYPES OF ACTIVE TRANSPORT

Active transport is of two types:

1. Primary active transport

2. Secondary active transport.

PRIMARY ACTIVE TRANSPORT

Primary active transport is the type of transport mechanism in which the energy is liberated directly from

the breakdown of ATP. By this method, the substances like sodium, potassium, calcium, hydrogen and chloride are transported across the cell membrane.

Primary Active Transport of Sodium and Potassium: Sodium-Potassium Pump

Sodium and potassium ions are transported across the cell membrane by means of a common carrier protein called sodium-potassium (Na+-K+) pump. It is also called Na+-K+ ATPase pump or Na+-K+ ATPase. This pump transports sodium from inside to outside the cell and potassium from outside to inside the cell.This pump is present in all the cells of the body. Na+-K+ pump is responsible for the distribution of

sodium and potassium ions across the cell membrane and the development of resting membrane potential.

Structure of Na+-K+ pump

Carrier protein that constitutes Na+-K+ pump is made up of two protein subunit molecules, an α-subunit with a molecular weight of 100,000 and a β-subunit with a molecular weight of 55,000. Transport of Na+ and K+ occurs only by α-subunit. The β-subunit is a glycoprotein the function of which is not clear.

α-subunit of the Na+-K+ pump has got six sites:

i. Three receptor sites for sodium ions on the inner (towards cytoplasm) surface of the protein

molecule

ii. Two receptor sites for potassium ions on the outer (towards ECF) surface of the protein

molecule

iii. One site for enzyme adenosine triphosphatase (ATPase), which is near the sites for sodium.

Mechanism of action of Na+-K+ pump

Three sodium ions from the cell get attached to the receptor sites of sodium ions on the inner surface of the carrier protein. Two potassium ions outside the cell bind to the receptor sites of potassium ions located on the outer surface of the carrier protein . Binding of sodium and potassium ions to carrier

protein activates occurs only by α-subunit. The β-subunit is a glycoprotein the function of which is not clear.

α-subunit of the Na+-K+ pump has got six sites:

i. Three receptor sites for sodium ions on the inner (towards cytoplasm) surface of the protein

molecule

ii. Two receptor sites for potassium ions on the outer (towards ECF) surface of the protein

molecule

iii. One site for enzyme adenosine triphosphatase (ATPase), which is near the sites for sodium.

Mechanism of action of Na+-K+ pump

Three sodium ions from the cell get attached to the receptor sites of sodium ions on the inner surface of the carrier protein. Two potassium ions outside the cell bind to the receptor sites of potassium ions located on the outer surface of the carrier protein . Binding of sodium and potassium ions to carrier

protein activates the enzyme ATPase. ATPase causes breakdown of ATP into adenosine diphosphate (ADP) with the release of one high energy phosphate. Now, the energy liberated causes some sort of conformational change in the molecule of the carrier protein. Because of this, the outer surface of the molecule (with potassium ions) now faces the inner side of the cell. And, the inner surface of the protein molecule (with sodium ions) faces the outer side of the cell. Now, dissociation and release of the ions take place so that the sodium ions are released outside the cell (ECF) and the potassium ions are released inside the cell (ICF). Exact mechanisms involved in the dissociation and release of ions are not yet known.

Electrogenic activity of Na+-K+ pump

Na+-K+ pump moves three sodium ions outside the cell and two potassium ions inside cell. Thus, when the

pump works once, there is a net loss of one positively charged ion from the cell. Continuous activity of the

sodium-potassium pumps causes reduction in the number of positively charged ions inside the cell leading to increase in the negativity inside the cell. This is called the electrogenic activity of Na+-K+ pump.