Endocytosis and exocytosis

What is bulk transport, and why is it needed?

Bulk transport moves large quantities or very large materials (e.g. proteins, polysaccharides, bacteria) into or out of cells when they are too big to pass through carrier or channel proteins.

It involves the formation or fusion of vesicles with the plasma membrane and requires energy from ATP.

What are the two main types of bulk transport?

• Endocytosis: Bulk transport into cells.

• Exocytosis: Bulk transport out of cells.

Both are active processes, as they require ATP for membrane movement and vesicle transport via the cytoskeleton.

Describe the process of endocytosis.

The cell surrounds the substance with a section of its plasma membrane, the membrane then pinches off to form a vesicle inside the cell containing the ingested substance

This allows bulk materials to enter the cell.

What are the two forms of endocytosis, and how do they differ?

• Phagocytosis: Uptake of solid material (e.g. bacteria, cell debris). The vesicle formed is called a phagocytic vacuole. Carried out by phagocytes (e.g. neutrophils, macrophages).

• Pinocytosis: Uptake of liquids or small particles in solution. If vesicles are very small, the process is called micropinocytosis.

Both require ATP for vesicle formation and movement within the cell.

Give an example of phagocytosis in the body.

White blood cells (phagocytes) engulf bacteria or damaged cells to form a phagocytic vacuole, which then fuses with a lysosome. The lysosomal enzymes digest the ingested material.

Describe the process of exocytosis.

• Substances (e.g. enzymes, hormones, cell wall components) are packaged into secretory vesicles formed by the Golgi apparatus.

• These vesicles move along the cytoskeleton to the plasma membrane.

• They fuse with the membrane, releasing contents outside the cell.

• This process requires ATP for vesicle movement and membrane fusion.

Give an example of exocytosis in animal and plant cells.

• Animal cells: Secretion of digestive enzymes from pancreatic cells.

• Plant cells: Export of cell wall materials (e.g. cellulose) during cell wall formation.

Why does bulk transport require energy (ATP)?

• ATP provides energy for cytoskeletal movement of vesicles.

• It also powers membrane fusion and fission events during endocytosis and exocytosis.

• The process involves significant membrane rearrangement, which is energetically demanding.

How are vesicles involved in bulk transport formed and moved?

• Endocytosis: Vesicles form from the cell surface membrane.

• Exocytosis: Vesicles form from the Golgi apparatus.