About lactic acid and lactate
Lactic acid is a weak acid, which means that it only partially dissociates in water. Lactic acid dissociates in water resulting in ion lactate and H+. This is a reversible reaction and the equilibrium is represented below.
CH3CH(OH)CO2H 
H+ + CH3CH(OH)CO2-Ka= 1.38 x 10-4
Depending on the environmental pH, weak acids such as lactic acid are either present as the acid in its undissociated form at low pH or as the ion salt at higher pH. The pH at which 50% of the acid is dissociated is called the pKa, which for lactic acid is 3.86.
Under physiological circumstances the pH is generally higher than the pKa, so the majority of lactic acid in the body will be dissociated and present as lactate. In the undissociated (unionized) form the substrates are able to pass through the lipid membranes, unlike the dissociated (ionized) form which cannot.