Ultrastructure and Function of Organelles

Ultrastructure and Function of Organelles

Ultrastructure of Organelles


  • The nucleus is the control centre of a cell and contains genetic material in the form of DNA.
  • It is surrounded by a double membrane known as the nuclear envelope, which controls the entry and exit of substances.
  • Within the nucleus are chromosomes, long molecules of DNA wrapped around proteins, which carry the genes.
  • The nucleus also contains a structure called the nucleolus, where ribosome assembly begins.


  • Mitochondria are oval-shaped organelles that are the site of aerobic respiration, where energy is produced.
  • They contain a double membrane; the inner membrane is highly folded into structures called cristae to increase its surface area for respiration reactions.

Endoplasmic Reticulum

  • The endoplasmic reticulum (ER) is a network of tubules used for protein and lipid synthesis.
  • Rough ER has ribosomes on its surface and is involved in the synthesis and transport of proteins, while smooth ER does not have ribosomes and synthesises lipids.


  • Ribosomes are tiny, spherical structures where protein synthesis occurs.
  • They can be found free-floating in the cytoplasm or attached to the rough ER.

Golgi Apparatus

  • The Golgi apparatus modifies, sorts, and packages proteins and lipids for transport out of the cell.
  • It is a series of flattened, membrane-bound sacs called cisternae.


  • Lysosomes are small, round organelles filled with enzymes.
  • They play a crucial role in cellular digestion and the breakdown of waste materials.

Chloroplasts (plant cells)

  • Chloroplasts are found in the cells of green plants and are responsible for photosynthesis, the process by which light energy is converted into chemical energy.
  • Like mitochondria, they are surrounded by a double membrane and have an inner membrane system called thylakoids.

Cell Wall (plant cells)

  • The cell wall is a rigid layer surrounding the plant cell, providing strength and support.
  • It is principally made up of cellulose, a complex carbohydrate.

Functions of Organelles

Understanding the different structures found within the eukaryotic cell is essential. Each organelle has a specific function crucial to the survival of the cell:

  • Nucleus controls the cell activity and houses DNA.
  • Mitochondria provide the cell with energy through respiration.
  • Endoplasmic reticulum synthesises proteins and lipids.
  • Ribosomes are the site for protein synthesis.
  • Golgi apparatus modifies, sorts, and packages cellular products for transport.
  • Lysosomes perform cellular digestion and waste removal.
  • Chloroplasts (plant cells) carry out photosynthesis to create food for the plant cell.
  • Cell wall (plant cells) provides structural support to the plant cell.

Each organelle has a unique structure which is optimised for its specific function, an important concept to understand in cell biology.