A discontinuous sucrose density gradient was used to separate membrane fractions from a homogenate of maize root tips. Endoplasmic reticulum-, Golgi apparatus-, plasma membrane- and mitochondria-rich fractions were identified by their enzymic characteristics and by their appearance under the electron microscope. Maize roots were incubated in vivo with D-[U-14C]glucose, [Me-14C]choline chloride and diazotized [U-3H]sulphanilic acid. The pattern of incorporation of radioactivity into the various membrane fractions was investigated. Analyses of the polypeptide chains of the membrane fractions by SDS-polyacrylamide gel electrophoresis showed that the mitochondria-rich fraction had a different pattern of polypeptides from that of the other membrane fractions. The results are discussed in relation to the hypothesis of endomembrane flow and differentiation.

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