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Cells are the smallest unit of a living organism. They can be considered as the basic building block of all living beings. A cell consists of a core nucleus and cytoplasm. The nucleus directs the activities performed by the cell. It is separated from the cytoplasm by a nuclear membrane.

Cytoplasm and the cell membrane
The cytoplasm is a gel like substance that supports the cellular molecules. It plays an important role in the functioning of the cell as many cellular processes occur in the cytoplasm. It is surrounded by the cell membrane.

The cytoplasm can be distinguished as two parts called as the endoplasm and the ectoplasm. The central area that contains the organelles is the endoplasm where as the ectoplasm is more gel like and is near the periphery of the cell.

Cell Membrane definition
The Cell membrane is defined as a thin, semi permeable membrane that separates the contents of the cell from the outside environment. This is the simplest cell membrane definition.

Fluid mosaic model
The cell membrane is composed of two layers of lipid molecules. Phospholipids are the most prevalent type of lipid found in the membrane. Each lipid molecule has a water loving polar end and a water hating nonpolar end. Proteins are scattered around this lipid layer. This structure is commonly referred to as the fluid mosaic model.

Carbohydrates present in the cell membrane help the cells to recognise each other.

Did you know: The molecules that make up a cell membrane are called glycerophospholipids. These molecules are composed of glycerol, a phosphate group, and two fatty acid chains.

Functions of the cell membrane
The main function of the cell membrane is to provide protection to the cells. Since it is semi permeable, it allows only certain substances to enter and leave the cells while keeping other unwanted substances out. It also acts as a cell wall, thus providing shape and support to the cell.

Cell Membrane | Image source: wikipedia.org

Transport mechanisms
There are a number of transport mechanisms that are used by different cell membranes for filtering substances that come inside or go outside the cell. These methods for transport of molecules and ions include:

Osmosis and diffusion - This is a passive transport process wherein some substances such as Carbon dioxide and oxygen can selectively diffused across the membrane. This is due to different concentrations of the substances on either side of the cell membrane that facilitates an osmotic flow of the substance much like the flow of water.

Endocytosis - Endocytosis is an active transport process. In this process the molecules are engulfed by the cell creating a deformation where the molecules are captured. This is sometimes also referred to as cell eating or cell drinking depending on the substances being captured.

Exocytosis - Exocytosis is also a active transport process. In this process, the substance is moved from the interior of the cell to the surface of the cell and then extruded out into the surrounding environment. This process is used to remove waste from the cell and also for secreting out enzymes and hormones across the cell membrane.

Passive diffusion through protein channels - Some molecules can also diffuse through protein channels. This is a passive process. The protein channels typically allow only select substances to diffuse.

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