BaSIC STRuCTuRE aNd fuNCTION Of CEllS
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erythrocytes lack mitochondria altogether. Cells with few mitochondria ent and its electronic charge, and the potential difference across the
generally rely largely on glycolysis for their energy supplies. These membrane. These factors combine to produce an electrochemical gradi-
include some very active cells, e.g. fast twitch skeletal muscle fibres, ent, which governs ion flux. Channel proteins are utilized most effec-
which are able to work rapidly but for only a limited duration. Mito- tively by the excitable plasma membranes of nerve cells, where the
chondria appear in the light microscope as long, thin structures in the resting membrane potential can change transiently from about −80 mV
cytoplasm of most cells, particularly those with a high metabolic rate, (negative inside the cell) to +40 mV (positive inside the cell) when
e.g. secretory cells in exocrine glands. In living cells, mitochondria stimulated by a neurotransmitter (as a result of the opening and sub-