Membrane Transport for Macromolecules

Mechanical transport of substance like macromolecules, large liquid droplets and particular matter cannot occur through the unbroken plasma membrane. They are transported through the membrane bound vesicles and is called  vesicular transport. Exocytosis and endocytosis are the two forms of vesicular transport.

Exocytosis:

It involves the 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. 

Exocytosis

Endocytosis:

It is the uptake  of macromolecules from the  extracellular space into the cell across the plasma membrane via the formation of an intracellular vesicle pinching off from the plasma membrane.

Endocytosis

Endocytosis can be divided into three distinct types depending on the size of the ingested macromolecule and whether specific cell surface receptors are involved. These three processes are phagocytosis, pinocytosis and receptor mediated endocytosis.

Phagocytosis:

It is the bulk transport of solid food materials into cell. It involves folding of the plasma membrane around the substance forming an intracellular vesicle called phagosome.

Phagocytosis

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 phagocytosis to protect the organism against infection by ingesting invading microorganisms.

Pinocytosis:

It is the process of intake of fluid 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.

Pinocytosis

Receptor mediated endocytosis: 

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

Receptor mediated endocytosis

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.

Clathrin coated pits and vesicles

Pits are invaginations of the plasma membrane that are coated on their cytosolic surface  with a densely packed material made up predominantly of the protein clathrin.