Cell Coverings

• A cell membrane surrounds every cell. The membranes of most cells consist of fatty substances called phospholipids plus structural proteins inserted here and there. Plant cells have an additional covering --- a rigid structure called a cell wall located just outside the cell membrane. The cell wall consists mainly of cellulose.
  
  

Nucleus

• A typical cell contains a single nucleus, but some have more than one or none at all. A perforated structure called the nuclear envelope surrounds the nucleus and sets it apart from the rest of the cell. The nucleus contains a denser area called the nucleolus and chromosomes, the genetic material that directs cell activity and determines inheritance.
  
  

Cytoplasm with Organelles

• A viscous liquid called cytoplasm fills the area between the cell membrane and the nucleus. This cytoplasm contains various membrane-bound structures called organelles, such as ribosomes, which assist in the production of proteins, and mitochondria, where energy production takes place. Plant cells have structures that are lacking in animal cells, such as chloroplasts, where photosynthesis takes place.
  
  

Bacterial Cells

• Bacterial cells have neither a nucleus nor organelles, and their cell membranes and cell walls have a different composition from the generalized cell.