## Cell membrane or plasma membrane is the outer most boundary of the cell. In most plant cells, however, it is covered by cell wall. Cell membrane is chemically made up of proteins and lipids. The proteins are 60 – 80% and lipids are 20 – 40%. There is also small amount of carbohydrates present in plasma membrane.

<center>![Cell-membrane-detailed-diagram.png](https://steemitimages.com/DQmbsJ3AnAZTCKz2xrNn1r5DK88HcxrRfm9g6vJyrEWi3XV/Cell-membrane-detailed-diagram.png)</center> [image source](https://biologydictionary.net/plasma-membrane/)

## Structure of Cell membrane:

The structural composition of plasma membrane was described by many biologists. 

#### Unit Membrane:

Earlier, it was suggested that cell membrane is made up of two layers of lipids which are sandwiched between inner and outer layers of proteins. This basic structure is known as unit membrane and is found in all cellular organisms.

#### Fluid Mosaic Model:

<center>![1200-86007545-cell-membrane.jpg](https://steemitimages.com/DQmbGEkStfxvPg3uQFvmbmWtHij1hifCgP61nRVe3YewPU1/1200-86007545-cell-membrane.jpg)</center> [image source](https://biologywise.com/cell-membrane-structure-function)

According to this proposal, the proteins layers are not continuous rather they are embedded in lipid layers in a mosaic way. This model is most accepted model. 

In cell membrane, the movement of material takes place by active and passive transports through charged poles. In passive transport, many substances which are not required, may enter the cell by passive transport. Some of them move from area of low concentration to area of high concentration by active transport. This difficult movement of materials requires energy which is provided by ATP.

## Functions of Cell membrane:

One of the important functions of the cell membrane is the transport of materials. It allows only selective substances to pass through it because it provides barrier between cell contents and the environment. Due to this function, it is known as differentially permeable membrane. The lipid substances cross it more easily than other substances. That is why, it regulates the flow of materials and ions to maintain definite gradient.

Many small water, gas and glucose molecules are neutral; so they can easily cross the membrane, while ions are charged particles, so they have some difficulty in crossing the membrane.