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I know that RBCs use glycolysis of glucose to lactate to produce most of their energy, but, if they are just carrier cells, where do they use the energy?

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Erythrocytes are far more than inert "bags" of oxygen. They are complex, highly specialized cells and, like all cells, they require a number of basic processes to be maintained to work properly.

A good example (out of many) of why red blood cells need energy would be the active transport of ions across the membrane, a mechanism important for any type of human cell.

Quoting an article by Knight & Welt (J General Physio, 1974):

The human red blood cell maintains a high intracellular potassium (K) and low sodium (Na) apparently by balancing passive movements of the ions with their active transport in the opposite direction. Thus the cell extrudes Na and accumulates K in a linked process (...). The component of Na-K transport (...) is dependent on adenosine triphosphate (ATP) in the cell.

A visual explanation of how export of sodium and import of potassium are linked to the consumption of ATP:

enter image description here

This means that to maintain correct gradients across the membrane (important to maintain osmotic level, and to provide a potential energy exploited in other import-export processes) the cell must continuously spend molecular energy.

Producing ATP in red blood cell is also important to maintain a pool of phosphate-donors for phosphorylation signals, involved in cell signaling for example.

Interestingly, it was also shown that releasing ATP is also a way for erythrocytes to communicate with their environment, possibly to stimulate microvessel dilatation and thereby allowing proper circulation of oxygen near the destination site (see Ellsworth et al, Acta Physiol 2016 for a detailed review).

All these processes require ATP, and therefore a continuous metabolic activity.

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