Active Absorption of Ions

The active transport of ions into the cells occurs at rates or in quantities which are against the laws of diffusion and electrochemical equilibrium. Such a transport uses energy derived from metabolism of the cell.

Certain experiments suggest the presence of active transport in cells. In one experiment, accumulation of K+ ions was found to be proportional to temperature and oxygen, both affecting the metabolism. Active uptake to ions is proportional to rate of respiration also. In some experiments, the value of concentration of ions inside and outside the membrane are determined by methods of microchemistry. If these values do not correspond with the Nernst equation, then active transport is said to have occurred, because Nernet equation shows the relationship between electric potential and chemical-activity of the ions in passive absorption.

The major hypotheses that explain the mechanism of active transport of ions are:

• Carrier Concept – transport by a carrier protein.
• Cytochrome Pump – transport by electrochemical gradient generated by electron transport.
• ATPase Mediated Transport – by generating proton gradient.

Carrier Concept

Carrier Concept

According to this hypothesis the carrier protein picks up an ion from one side of the membrane and discharges it on the other side. The picking up and discharge of the ion requires energy. Energy is obtained by hydrolysis of ATP. ATP changes into ADP and energy released is used to change the conformation of the carrier which may be ATPase itself, so that the ion is picked up on one side of the membrane and released on the other. After discharge of an ion, carrier protein is reprimed to pick up an other ion.

The carrier protein may carry one ion inwards and may exchange it with another ion at the inner surface of membrane, so that the other ion is carried by the same carrier outwards.

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