AS Biology 9700: Rough vs Smooth Endoplasmic Reticulum and Lysosome Formation

Electron micrographs of a liver cell and a pancreatic acinar cell are compared. The liver cell contains extensive regions of tubular membrane that are not studded with ribosomes, together with many mitochondria. The pancreatic acinar cell, which secretes digestive enzymes, contains extensive stacks of flattened membrane that are studded with ribosomes, feeding into a prominent Golgi body. The Golgi body in the pancreatic cell also buds off small enzyme-filled vesicles that do not leave the cell.

(a) Name the type of endoplasmic reticulum found extensively in the liver cell, and describe one function of this type of ER that fits the liver's known role in detoxifying substances such as alcohol. [2]

(b) Name the type of endoplasmic reticulum found extensively in the pancreatic acinar cell, and describe how its appearance in an electron micrograph differs from that of the type of ER named in (a). [2]

(c) State how the small enzyme-filled vesicles described above are formed, and name the type of organelle they become. [2]

Part (a): Smooth ER in the liver cell

The liver cell’s extensive tubular membrane, lacking attached ribosomes, is smooth endoplasmic reticulum (smooth ER). Smooth ER synthesises lipids and steroids (for example, components of cell membranes and steroid hormones). In liver cells specifically, smooth ER also contains enzymes that carry out detoxification: they chemically modify harmful substances such as alcohol and other drugs, converting them into more soluble forms that can be safely excreted from the body. Liver cells contain unusually large amounts of smooth ER because of this ongoing detoxification role.

Part (b): Rough ER in the pancreatic acinar cell

The pancreatic acinar cell’s extensive stacks of flattened membrane, studded with ribosomes and feeding into the Golgi body, are rough endoplasmic reticulum (rough ER). In an electron micrograph, rough ER appears studded and granular, because ribosomes are attached across its outer (cytoplasmic) surface. Smooth ER, in contrast, has no ribosomes attached to it and so appears as smooth, unstudded tubular membrane. This difference reflects their functions: rough ER’s ribosomes synthesise proteins (such as digestive enzymes) directly into the ER, while smooth ER, with no ribosomes, synthesises lipids instead.

Part (c): Formation of lysosomes

The small enzyme-filled vesicles bud off, or pinch off, from the Golgi body. Each vesicle is enclosed by a section of Golgi membrane and contains a concentrated mixture of hydrolytic (digestive) enzymes that the Golgi body has processed and packaged. Once released from the Golgi body, these vesicles are called lysosomes. Because they do not leave the cell, they remain available for intracellular digestion, such as breaking down worn-out organelles or material engulfed by the cell.

Final answers

(a) Smooth ER; synthesises lipids/steroids and, in the liver, detoxifies substances such as alcohol.
(b) Rough ER; appears studded/granular due to attached ribosomes, unlike the smooth, ribosome-free appearance of smooth ER.
(c) The vesicles bud off from the Golgi body, becoming lysosomes.