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+# Preparation of organelles from castor bean endosperm
+
+The isolation of castor bean endosperm was performed according to Cooper & Beevers (1969) and Beevers & Breidenbach (1974) {Cooper:1969tz} {Beevers:1974ug}. The protocols were modified by using the grinding buffer as described by Reumann et al. (2007) {Reumann:2007wi}. All steps were carried out on ice in a cold room (4°C) unless indicated otherwise. 30 g of endosperm tissue from 5-day old dark-grown Ricinus seedlings was harvested by removing the yellow cotyledons using the blunt side of a scalpel blade. The resulting endosperm was chopped using an onion chopper in 60 mL grinding buffer (170 mM Tricine pH X (KOH), 1 M Sucrose, 1% (w/v) BSA, 10 mM KCl, 1 mM MgCl2, 2 mM EDTA, 0,5 % (w/v) PVP-40, and 5 mM DTT). The suspension was further homogenized using mortar and pestle. The homogenate was filtered through four layers cheesecloth. The crude extract was centrifuged at 1,200 g for 10 minutes to remove cell debris. The supernatant was carefully decanted into a new flasks (approx. 40 mL). To separate the organelles, 6 mL of the obtained extract was loaded onto the top of a discontinuous sucrose gradient prepared in 20 mM Tricine-KOH (pH 7.5) and 1 mM EDTA. The density gradient consists of the following sucrose steps (from top to bottom): 5 ml 30% (w/w) sucrose, 3 ml 44% (w/w) sucrose, 5 ml 48% (w/w) sucrose, 5 ml 49% (w/w) sucrose, 1 ml 50% (w/w) sucrose, 2 ml 54% (w/w) sucrose, and 2 ml 60% (w/w) sucrose. The organelles were separated by ultracentrifugation at 105,026 g using a swing-out rotor for 3 hours. Four visible bands at the interface 30% − 44% (w/w) sucrose solution (fraction 1), 44% − 48% (w/w) sucrose solution (fraction 2), 48% − 49% (w/w) sucrose solution (fraction 3), and 50% − 54% (w/w) sucrose solution (fraction 4) was carefully collected, pooled and stored at -80°C for further experiments.