STPM Biology Structure of Cell and Organelle (Part 4)

Nucleus

Distribution:

Found in all cells except:

Structure of nucleus

• Prokaryotic cells (bacteria and blue-green algae)
• Red blood cells of mammals
• Matured sieve tube elements (part of the floem)

Structure:

• Shape: Usually spherical or oval.
• Size: Nucleus is the largest cell organelle.
• Nucleus is surrounded by a double membrane. This membrane is perforated by nuclear pores. These pores control exchange of materials between the nucleus and cytoplasm.
• The matrix of the nucleus is called nucleoplasm. Nucleoplasm contains chromatin, ions, proteins, nucleotides, enzymes and ribosomes. Chromosomes visible during cell division. Chromosome are made of chromatin and histone proteins.
• Nucleus also contains one or more nucleoli.

Functions of nucleus:

• Nucleus is the control center for most of the activities in cells.
• Serve as a container for chromosomes.
• Nucleolus manufactures ribosomes.
• Produces DNA and RNA.