Endomembrane system: ER and Golgi apparatus

The endomembrane system groups organelles whose functions are coordinated: endoplasmic reticulum, Golgi complex, lysosomes and vacuoles. Mitochondria, chloroplasts and peroxisomes are NOT part of it because their functions are not coordinated with these.

The ER is a network of tiny tubular structures in the cytoplasm that splits it into a luminal (inside ER) and an extra-luminal (cytoplasm) compartment.

ER carrying ribosomes on its surface is rough ER (RER); ER without ribosomes is smooth ER (SER). RER is abundant in cells busy making and secreting proteins and is continuous with the outer nuclear membrane.

Lipids are synthesised mainly in the SER, which in animal cells also makes the lipid-like steroidal hormones.

Camillo Golgi first saw the Golgi apparatus in 1898 as densely stained reticular structures near the nucleus.

It is a stack of flat, disc-shaped sacs called cisternae, about 0.5–1.0 μm in diameter, arranged parallel to one another in stacks, with cisternae varying in number.

The stack has two faces: the convex cis (forming) face and the concave trans (maturing) face. They are entirely different but interconnected.

Materials packed in ER vesicles fuse with the cis face and travel towards the trans face, which explains why the Golgi lies close to the ER.

The Golgi packages materials for intracellular targets or for secretion. Proteins made on ribosomes of the ER are modified in its cisternae before release from the trans face, and it is an important site for forming glycoproteins and glycolipids.