Why does the Rough Endoplasmic Reticulum look ‘rough’ under a microscope?

Correct — A, Due to the presence of ribosomes on its surface. The endoplasmic reticulum is a network of membrane sheets and tubes running through the cytoplasm and continuous with the outer membrane of the nuclear envelope. Where its cytoplasmic face carries ribosomes, the membrane appears studded with dark granules in an electron micrograph, and that granular look is the whole of the name — rough endoplasmic reticulum, sometimes called granular ER. Where the ribosomes are absent the membrane looks smooth, and it is called smooth ER. The appearance follows the function. A ribosome that begins making a protein carrying a signal sequence is drawn to the ER membrane and finishes the job by threading the growing chain into the ER's interior, so proteins destined for secretion, for insertion into membranes or for delivery to lysosomes are made on rough ER and folded and modified inside it before moving on to the Golgi apparatus. Smooth ER, having no ribosomes, does the jobs that need no protein synthesis: making lipids and steroids, storing calcium, and detoxifying drugs in liver cells. Cells that export protein heavily — pancreatic acinar cells, antibody-secreting plasma cells — are packed with rough ER, which is what you would expect if the ribosomes are the point.

• (b)Due to the absence of ribosomes on its surface — That describes the smooth endoplasmic reticulum, which is smooth precisely because it has no ribosomes. This option states the correct fact about the wrong half of the organelle.
• (c)Due to the presence of lysosomes on its surface — Lysosomes are separate membrane-bound sacs of digestive enzymes floating in the cytoplasm, and they are far larger than the granules seen on the ER. They are not attached to its surface, though the enzymes inside them are made on rough ER.
• (d)Due to the presence of mitochondria on its surface — Mitochondria are large double-membraned organelles with their own DNA. They lie close to the ER at contact sites where lipids and calcium are exchanged, but they are never stuck onto it as surface granules.

The endoplasmic reticulum is the largest membrane system in a eukaryotic cell and comes in two forms that are continuous with each other. Rough ER is a stack of flattened sheets whose outer face is covered in ribosomes; it is the entry point of the secretory pathway, where proteins meant for export or for membranes are synthesised, folded, given their first sugar chains and checked for correct shape before being sent on to the Golgi apparatus. Smooth ER is a network of tubules with no ribosomes, and it handles lipid and steroid synthesis, calcium storage and, in liver cells, the detoxification of drugs. Ribosomes themselves are not membrane-bound; each is a two-subunit particle of RNA and protein, and the same ribosome can work free in the cytosol or attached to the ER, depending on the protein it happens to be making.

The item is really testing whether a name has been understood or only memorised, and the four options are built so that the memoriser can still fail. Options (a) and (b) offer the same fact with opposite signs, which is the classic device for catching a candidate who remembers 'ribosomes and ER go together' without remembering which of the two forms is which. The reliable check is that a granular appearance requires something granular to be present, so an absence cannot make anything look rough. Options (c) and (d) offer organelles that are simply too large: lysosomes and mitochondria are structures on the scale of a bacterium, not surface granules, and neither attaches to the ER membrane. It is worth adding that the smooth and rough forms are not separate organelles but two regions of one continuous membrane system, which is why a cell can shift the balance between them when its work changes.

• Rough endoplasmic reticulum carries ribosomes on its cytoplasmic surface, which is what gives it a granular appearance under an electron microscope.
• It is also called granular ER; the ribosome-free tubular form is smooth or agranular ER.
• The ER is continuous with the outer membrane of the nuclear envelope.
• Proteins destined for secretion, for membranes or for lysosomes are made on rough ER and pass from there to the Golgi apparatus.
• Smooth ER makes lipids and steroids, stores calcium and detoxifies drugs in liver cells.
• Ribosomes are made of RNA and protein, have two subunits and are not membrane-bound; the same ribosome can work free in the cytosol or attached to the ER.
• Cells that secrete protein heavily, such as pancreatic acinar cells and plasma cells, are rich in rough ER.

• Swapping rough and smooth. An absence of ribosomes cannot make a membrane look granular.
• Attaching lysosomes or mitochondria to the ER surface; both are far too large to be the granules seen in a micrograph.
• Assuming ribosomes exist only on the ER. Free ribosomes in the cytosol make the proteins the cell keeps for itself.

As a why-is-it-called-rough item like this one, as an organelle-to-function match, or through a comparison of rough and smooth ER.

• practice — not a real PYQ

  Proteins that a cell is going to secrete are synthesised chiefly on which one of the following?

  • (a)Free ribosomes in the cytosol
  • (b)Ribosomes attached to the rough endoplasmic reticulum
  • (c)The inner membrane of mitochondria
  • (d)The lysosomal membrane

  Answer(b) Ribosomes attached to the rough endoplasmic reticulum — the growing chain is threaded into the ER interior and passes on to the Golgi apparatus for despatch.

• practice — not a real PYQ

  Which one of the following is a function of the smooth endoplasmic reticulum?

  • (a)Synthesis of secretory proteins
  • (b)Synthesis of lipids and steroids
  • (c)Production of ATP by oxidative phosphorylation
  • (d)Digestion of worn-out organelles

  Answer(b) Synthesis of lipids and steroids — smooth ER also stores calcium and, in liver cells, detoxifies drugs. Protein synthesis needs ribosomes, which it does not have.