Cell structure and function (cell membrane) - SDL Wksheet A

Endocytosis vs Exocytosis (opposite processes)
http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120068/bio02.swf::Endocytosis%20and%20Exocytosis

http://www.youtube.com/watch?v=K7yku3sa4Y8&list=FL-cFTJ8NxBjonnjuwSQgSsw&index=52&feature=plpp_video

Endocytosis
- Definition: Cellular uptake of biological molecules (eg. proteins) and particulate matter via formation of new vesicles (called endosomes in general) from the plasma membrane.

- Importance: This is required as most substances required by the cells cannot pass through the plasma membrane.

- Types:

(i) Phagocytosis ("cellular eating") - process of engulfing solid particles (eg. bacteria, food) by the cell membrane to form a phagocytic vacuole/ phagosome by phagocytic cells (eg. white blood cells). The resulting phagosome subsequently fuses with primary lysosomes to form secondary lysosomes. Hydrolases present in lysosomes hydrolyse the solid particles and the nutrients released will then diffuse out into the cytosol for use in other metabolic processes.

(ii) Pinocytosis ("cellular drinking") - non-specific process where a cell creates a pinocytic vacuole around a droplet of extracellular fluid and brings it into the cell. The resulting pinocytic vesicle/ pinocytic vacuole subsequently fuses with primary lysosomes to form secondary lysosomes. Hydrolases present in lysosomes hydrolyse the particles and the nutrients released will then diffuse out into the cytosol for use in other metabolic processes.

(iii) Receptor-mediated endocytosis - Specific binding of extracellular substances (eg. LDL) to protein receptors (eg. LDL receptor) on the cell surface membrane, triggering the formation of a vesicle, and the subsequent internalising of the substances.