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Important Parts of a Cell

Endoplasmic reticulum (ER)

  1. The ER is a system of membranous tubules and sacs
  2. The primary function of the ER is to act as an internal transport system, allowing molecules to move from one part of the cell to another
  3. The quantity of ER inside a cell fluctuates, depending on the cell’s activity. Cells with a lot include secretory cells and liver cells
  4. The rough ER is studded with 80s ribosomes and is the site of protein synthesis. It is an extension of the outer membrane of the nuclear envelope, so allowing mRNA to be transported swiftly to the 80s ribosomes, where they are translated in protein synthesis
  5. The smooth ER is where polypeptides are converted into functional proteins and where proteins are prepared for secretion. It is also the site of lipid and steroid synthesis, and is associated with the Golgi apparatus. Smooth ER has no 80s ribosomes and is also involved in the regulation of calcium levels in muscle cells, and the breakdown of toxins by liver cells
  6. Both types of ER transport materials throughout the cell.

Golgi apparatus

  1. The Golgi apparatus is the processing, packaging and secreting organelle of the cell, so it is much more common in glandular cells.
  2. The Golgi apparatus is a system of membranes, made of flattened sac-like structures called cisternae.
  3. It works closely with the smooth er, to modify proteins for export by the cell.


  1. Lysosomes are small spherical organelles that enclose hydrolytic enzymes within a single membrane
  2. Lysosomes are the site of protein digestion – thus allowing enzymes to be re-cycled when they are no longer required. They are also the site of food digestion in the cell, and of bacterial digestion in phagocytes
  3. Lysosomes are formed from pieces of the Golgi apparatus that break off
  4. Lysosomes are common in the cells of Animals, Protoctista and even Fungi, but rare in plants.


  1. Just as your body depends on your skeleton to maintain its shape and size, so a cell needs structures to maintain its shape and size
  2. In animal cells, which have no cell wall, an internal framework called the cytoskeleton maintains the shape of the cell, and helps the cell to move
  3. The cytoskeleton consists of two structures: a) microfilaments (contractile). They are made of actin, and are common in motile cells. b) microtubules (rigid, hollow tubes – made of tubulin).
  4. Microtubules have three functions:
    1. To maintain the shape of the cell
    2. To serve as tracks for organelles to move along within the cell
    3. They form the centriole