Biology 重要concept 一問

Secretion 姐係分泌一d野...
e.g. vagina 會 secret 一d acidic ge mucus 去prevent bacterial infection
sperms 會 secret enzyme to digest 個 egg 的protective membrane去入去.
excretion就係完份全唔同ge野...
係 removal of metabolic waste
urea ge 排出...
CO2 from respiration of body cells...再由 ventilation排出體外咁..


仲有唔好 confuse左 excretion 同 egestion..,.
egestion 係 指 faeces...姐係 我地digest 唔到ge野...好似cellulose...or 係cast cells咁...
excretion唔包呢d!!!

Excretion is the process of eliminating waste products of metabolism and other non-useful materials.[1] It is an essential process in all forms of life.

In single-celled organisms, waste products are discharged directly through the surface of the cell. Multicellular organisms utilize more complex excretory methods. Higher plants eliminate gases through the stomata, or pores, on the surface of leaves. Animals have special excretory organs.


Secretion is the process of segregating, elaborating, and releasing chemicals from a cell, or a secreted chemical substance or amount of substance.

Eukaryotic cells have a highly evolved process of secretion. Proteins targeted for the outside are synthesized by ribosomes docked to the rough endoplasmic reticulum. As they are synthesized, these proteins translocate into the ER lumen, where they are glycosylated and where molecular chaperones aid protein folding. Misfolded proteins are usually identified here and retrotranslocated by ER-associated degradation to the cytosol, where they are degraded by a proteasome. The vesicles containing the properly-folded proteins then enter the Golgi apparatus.

In the Golgi apparatus, the glycosylation of the proteins is modified and further posttranslational modifications, including cleavage and functionalization, may occur. The proteins are then moved into secretory vesicles which travel along the cytoskeleton to the edge of the cell. More modification can occur in the secretory vesicles (for example insulin is cleaved from proinsulin in the secretory vesicles).

Eventually, the vesicle fuses with the cell membrane in a process called exocytosis, dumping its contents out of the cell's environment.

Strict biochemical control is maintained over this sequence by usage of a pH gradient: the pH of the cytosol is 7.4, the ER's pH is 7.0, and the cis-golgi has a pH of 6.5. Secretory vesicles have pHs ranging between 5.0 and 6.0; some secretory vesicles evolve into lysosomes, which have a pH of 4.8.