That would be mitochondria.
The answer is proteins.
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membranes that's wrong its cell cement
Endoplasmic Reticulum
Cell nuclei are membranous organelles. Their membranes are similar to regular cellular membranes.
Golgi Complex
mitochondria, nucleus, chloroplast
The rough endoplasmic reticulum synthesizes membranes.
The cell membrane is a lipid bilayer while the organelle membranes are usually a single layer of lipids. The nucleus, mitochondria and chloroplasts are the exception and have a lipid bilayer. AS you see, it depends on which organelle. The nucleus, mitochondria and chloroplasts have a bilayer (2). The others have one.
There are none. Organelles are found IN cells. They are made up of membranes and proteins. An organelle to a cell is like an organ to a human.
all cell organelle are membrane bounded especially in eukaryotes