# The Cell Membrane

The cell membrane is the outer layer of the cell. The primary function of the cell membrane is to regulate what substances enter and leave the cell. The cell membrane is selectively permeable, or "semi-permeable", meaning that the membrane only allows certain substances to enter or leave the cell. Scientists use what is called the Fluid Mosaic Model to describe the cell membrane's structure and function.

### Parts & their functions

This complex "biological membrane" contains several parts and each of those parts have a specific function:
* Phospholipid Bilayer, or otherwise known as the "Lipid Bilayer", is one of the largest parts of the cell membrane. It contains two layers of phospholipids (phosphate heads + lipid tails) and allows small molecules such as water, oxygen and carbon dioxide to enter or leave the cell.
* Proteins can be found within or on the surface of the cell membrane and are able to do many vital functions: Proteins transport large molecules in and out of cells (transportation), regulate chemical reactions that happen around the cell membrane (enzyme activity), send messages to other cells (messengers), recognize cell invaders/harmful cells (cell recognition), and proteins reinforce the structure of the cell membrane (support).
* Carbohydrate Chains, or glycolipids/glycoproteins, are chains found on the surface of the cell membrane that are made of carbohydrates + lipid (or) protein. The primary function of these chains is to recognize harmful cells (cell-cell recognition).
* Cholesterol Chains help with the structure of the cell membrane and its ability to control the passage of different molecules such as sodium ions (Na+).

### Transportation across the cell membrane

The cell membrane needs to move water, oxygen, carbon dioxide, glucose and ions across the cell membrane. This is to help organisms maintain homeostasis (maintaining the organism's temperature, water levels, and glucose levels) and get rid of waste. The cell membrane helps the organism do these vital actions. Our cell membrane move these molecules in [either?] two ways:

#### Passive Transport

Passive Transport is moving molecules in or out of the cell without using energy (population moving, not individual molecules). Molecules, in a passive transport, usually move across a membrane from where there are MORE molecules to where there are LESS molecules (High concentration to low concentration). The molecules are moved down the concentration gradient. Equilibrium has been reached when the concentration of the molecule that is moving is equal on both sides of a membrane. Types of passive transport include: diffusion, osmosis and faciliated diffusion.

##### Facilitated Diffusion

Facilitated Diffusion is when molecules that are too large to fit through the phospholipid bilayer, so a "channel protein" is used to help them move from where there are more molecules to where there are fewer molecules$^{[DIFFUSION]}$. Molecules that use this process to move in and out of the cell membrane include glucose, water, ions and amino acids/lipids.

#### Active Transport

Active transport is moving molecules in or out of the cell WITH energy! Molecules usually move across a membrane from where there are LESS molecules to where there are MORE molecules (Low concentration → High concentration). Molecules, in an active transport, move AGAINST the concentration gradient. Types of active transports include exocytosis and endocytosis.
Endocytosis is taking items into the cell by membrane folding (bulk transport). This is a type of active transport, therefore it requires energy. White blood cells are a great example of this form of active transport. When the white blood cells engulf invaders to your body, this is known as exocytosis. Exosytosis is taking items out of the cell by membrane folding. This is, again, a type of active transport, and therefore, it requires energy. Paramecium expelling water through the contractile vacuole (in a cell) is an example of exosytosis. Active transport will help move large molecules, such as glucose and proteins, across the cell membrane.

##### Endocytosis

In endocytosis, the cell takes in molecules by forming new vesicles from the plasma membrane,
* Phagocyotisis, only in animals, is where the cell catches a particle by forwarding pseudopodia to it and packaging it in a sac called a food vacuole. The particle will be digested when the food vacuole is fused with a lysosome.
* Pinocytosis: A cell "gulps" droplets of extracellular fluid into tiny vesicles formed by the infoldings of the plasma membrane.
* Receptor-mediated endocytosis: Specialized, specific type of pinocytosis that allows the cell to take in large amounts of substances. On the membrane are receptor sites, which embed itself to specific solutes. The receptor proteins then cluster together. After the ingested material is free from the vesicle, the receptors are recycled.