Question

What is bulk transport?

Answer 1

Hint: As the name suggests, the transport of bulky/large molecules in and out of cells is termed as bulk transport. It is an active type of transport, meaning energy produced by cellular respiration is utilised.

Complete answer:
Molecules of large sizes, including macromolecules like proteins and polysaccharides or parts of cells or whole cells are required for many purposes in a cell, mainly food. Due to their size, transport of these molecules in and out of cells cannot be spontaneous. This transport demands energy expenditure in the form of ATP.
Bulk transport is of two types, depending on the direction of transport:
-Endocytosis: The movement of molecules from the outer environment to a cell's cytoplasm. This motion is facilitated by the formation of a pocket or invagination on the surface of the cell. Molecules to be transported, move into these pockets. This is then followed by the closing of invagination as the membranes fuse from over the molecule, looking like cliffs on either side of a valley fusing together. This forms an intracellular vesicle made from the cell membrane, containing the molecule.
Endocytosis can further be classified in three categories; phagocytosis or “cell eating”, the intake of solid molecules by active transport and can be seen in neutrophils as they engulf microbes entering the human body; pinocytosis or “cell drinking”, the intake of extracellular fluids, mainly for the consumption of water; and receptor-mediated endocytosis, the intake of specific substrates corresponding to the receptor proteins present on plasma membrane of the cell.
-Exocytosis: This process is the absolute opposite of endocytosis where molecules (waste material) from inside the cell are transported out to the extracellular fluid. A vacuole is formed within the cell containing the waste, whose membrane fuses with the cell’s plasma membrane. The plasma membrane opens on the exterior, releasing the waste material at the expense of energy.

Note:
Contents of a vesicle in phagocytosis and pinocytosis are broken down with the help of enzymes and absorbed according to the needs. The remaining debris and excreted material of the cell is removed by the same but reversed process, exocytosis.