Define an organelle

An organelle is a specialized intracellular structure with a characteristic shape and present to perform specific functions in cellular growth, maintenance, and reproduction.

 

Which organelles are surrounded by a membrane and which are not?

The centrosome, cilia, flagella, ribosomes, and proteasomes are organelles not surrounded by a membrane. The rough endoplasmic reticulum, smooth endoplasmic reticulum, Golgi complex, lysosomes, peroxisomes, mitochondria and nucleus are organelles that are surrounded by a membrane.