Membrane transport of macromolecules Membrane transport of macromolecules

Membrane transport of macromolecules

Membrane transport of macromolecules • Macromolecules, large liquid droplets and particular matter cannot be transported through the unbroken plasma membrane. • They are transported through the membrane bound vesicles and is called vesicular transport. • 2 types – Exocytosis – Endocytosis

Exocytosis • It involves expulsion of a substance which gets enclosed in membrane bound vesicle called exocytic vesicle. • It fuses with the plasma membrane and releases its contents outside the cell. • Synthetic and secretory materials are passed out of the cell in this way.

Endocytosis • It is the uptake of macromolecule from the extracellular space into the cell across the plasma membrane by the formation of an intracellular vesicle pinching off from the plasma membrane.

• Endocytosis can be divided into 3 types based on the size of ingested macromolecule and whether specific cell surface receptors are involved. • Phagocytosis • Pinocytosis • Receptor mediated endocytosis

Phagocytosis • It is the bulk transport of solid food materials into the cell. • It involves folding of the plasma membrane around the substance forming an intracellular vesicle called phagosome. • In protozoa, phagocytosis is a form of feeding, where the ingested material is broken down in the lysosomes and utilized as food. • Macrophages and neutrophils (WBC) use phagocytoss to protect the organism against infection by ingesting invading microorganisms.

Pinocytosis • It is the process of intake of liquid food material into the cell. • During the process the cell membrane folds to form small vesicles called pinocytic vesicles or pinosomes in which small drops of fluid materials are taken into the cell.

Receptor mediated endocytosis • It is the selective uptake of extracellular macromolecules (such as cholestrol) through their binding specific cell surface receptors. • The receptor – macromolecule complex then accumulates in clathrin coated pits and endocytosed through clathrin coated vesicle.

Clathrin coated pits and vesicles • They are involved in both the exocytosis of proteins from golgi apparatus and endocytosis of material at the plasma membrane. • Pits are the invaginations of the plasma membrane that are coated on their cytosolic surface with a densely packed material made up predominantly of the protein clathrin.