Some molecules and particles are far too large to pass through channel or carrier proteins. Cells move them in bulk inside membrane-bound vesicles. This is possible because the phospholipid bilayer is fluid, so membranes can fold, pinch off and fuse. Both processes need energy from ATP.
1. The cell surface membrane folds inwards around the material.
2. The membrane surrounds it completely and fuses, pinching off a vesicle inside the cytoplasm.
There are two main types:
• phagocytosis ("cell eating"): uptake of solid particles, for example a phagocyte engulfing a bacterium. The vesicle may fuse with a lysosome, whose enzymes digest the contents;
• pinocytosis ("cell drinking"): uptake of liquid containing dissolved substances in small vesicles.Exocytosis releases material from a cell:
1. Proteins made on the rough endoplasmic reticulum are modified and packaged in the Golgi apparatus.
2. Vesicles containing the product bud off the Golgi apparatus and move to the cell surface membrane along the cytoskeleton.
3. The vesicle membrane fuses with the cell surface membrane and the contents are released outside the cell.Examples of exocytosis:
• beta cells in the pancreas releasing insulin;
• pancreatic cells secreting digestive enzymes;
• neurones releasing neurotransmitter at a synapse;
• plant cells exporting materials to build the cell wall.In exocytosis the vesicle membrane becomes part of the cell surface membrane, while endocytosis removes membrane, so the two processes together keep the cell surface area roughly constant.
1. The cell surface membrane folds inwards around the material.
2. The membrane surrounds it completely and fuses, pinching off a vesicle inside the cytoplasm.
There are two main types:
• phagocytosis ("cell eating"): uptake of solid particles, for example a phagocyte engulfing a bacterium. The vesicle may fuse with a lysosome, whose enzymes digest the contents;
• pinocytosis ("cell drinking"): uptake of liquid containing dissolved substances in small vesicles.Exocytosis releases material from a cell:
1. Proteins made on the rough endoplasmic reticulum are modified and packaged in the Golgi apparatus.
2. Vesicles containing the product bud off the Golgi apparatus and move to the cell surface membrane along the cytoskeleton.
3. The vesicle membrane fuses with the cell surface membrane and the contents are released outside the cell.Examples of exocytosis:
• beta cells in the pancreas releasing insulin;
• pancreatic cells secreting digestive enzymes;
• neurones releasing neurotransmitter at a synapse;
• plant cells exporting materials to build the cell wall.In exocytosis the vesicle membrane becomes part of the cell surface membrane, while endocytosis removes membrane, so the two processes together keep the cell surface area roughly constant.