Biology

• All living organisms are made of one or more cells. Figure 1 shows human cheek cells as seen through a light microscope. They have been stained with a dye to make their structure more visible.
• Common features of cells 

  Animal cells and plant cells have these features in common: 

  • a cell membrane
  • cytoplasm
  • DNA.
• cytoplasm

  • description: jelly-like substance containing the other structures inside the cell
  • function: the site of most chemical reactions in the cell
• nucleus

  • description:
  • an approximately spherical structure that occupies about 10% of the cell’s volume
  • located towards the centre of animal cells and towards the edge of plant cells
• A photomicrograph of human cheek cells, as seen through a light microscope. The nucleus of each cell is visible as a dark dot in the middle of the cell.

  
• The DNA in animal cells and plant cells is found in a structure called the nucleus. This is contained in the cytoplasm of the cell. The cytoplasm and nucleus can be seen using a light microscope (Figure 2). However, the cell membrane cannot be seen. It is an extremely thin layer, only about 10 nm (1 × 10–8 m) thick.
• function of the nucleus:

  • contains the cell’s DNA (genetic code)
  • controls the activities of the cell
  • controls cell division