Cell membrane question

(2)n(3)

it's hard to explain.

protein is present because of active transport(one of the simplest reason), it acts as a carrier, to transport things from outside the cell into the cell.

for the reason of presence of lipid...i really don't know how to explain. hope somebody can help uuu..

sorry

It is indeed made up of mainly lipids and protein.

Lipid is an amphiphilic molecule, which means it consists of both polar and non-polar groups. Therefore, it has both hydrophobic and hydrophilic properties. Since the biological fluid is aqueous, the hydrophobic tails of lipid molecules would come together and form a lipid bilayer (which is shown in the above answer). The lipid bilayer, when it's in 3D, usually forms a vesicle, which gives the cell its boundary. One correction to the above answer, the hydrophilic sections (the heads) are used to prevent hydrophobic materials from going into the cell, whereas the hydrophobic tails are used to prevent hydrophilic materials from going in (e.g. water, Na+). (this is the opposit of what is stated above)

For transport of water, ions and other materials in and out of the cell, channels and transporters are used, which are essentially protein molecules. The cell membrane also contains various receptors and the cytoskeleton, which are mainly protein as well.

cell membrane is formed by lipid(2) and protein(3).
lipid combine with phosphate 成了phospholipid再排成以下圖像

oooooooooooooooo <--lipid
^^^^^^^^^^^^^^^^^^^^^^ <--carbon chain
vvvvvvvvvvvvvvvvvvvvvv <---carbon chain
oooooooooooooooo <--lipid

lipid既用途係prevent polar molecules such as water inter the cell，carbon chain就用黎prevent organic matter entry the cell such as fat. Protein就會藏係呢層layer度，as a channel 用molecules movement. 仲有可形成glycoprotein 用黎做receptor, 辨識cell外既物質。

所以cell membrane有控制 molecules movement via the cell.

(2) & (3)
From Wikpedia :
细胞膜，又称质膜，为细胞中最重要的分隔细胞内和细胞外不同介质和组分的界面。质膜普遍认为由脂双层分子(lipid )作为基本单位重复而成，其上镶嵌有各种类型的膜蛋白(protein)以及与膜蛋白结合的糖和糖脂。

质膜是细胞与周围环境和细胞与细胞间进行物质交换和信息传递的重要通道。质膜通过其上的孔隙和跨膜蛋白的某些性质，达到有选择性的，可调控的物质运输作用。