Fig. 4. (A) Solubilised apical plasma membrane protein fraction (sAPM) dose response of the fast-linear sperm subpopulation in the presence (filled circles) and absence (open squares) of bicarbonate/CO₂. The data summarise group sperm frequencies (%), calculated for each individual boar (N=6) and treatment (mean % ± s.e.m.). Data representing bicarbonate/CO₂-treated spermatozoa are from treatments 7 min after bicarbonate/CO₂ addition. Data from the `bicarbonate-absent' groups are from the NaCl control samples, measured at 0 and 17 min. (B) Within the fast-linear sperm subpopulation, the mean (± s.e.m.) LIN (linearity) values increased in relation to the log_{(base 2)} sAPM concentration.