Movement through the cell 7-3

Cell Membrane From our vocabulary we should already know that the cell membrane has two jobs Regulates what enters and leaves the cell Supports and provides protection for the cell

If we look at the microscopic parts of a cell membrane we can see that there is a double-layer sheet called the lipid bilayer This lipid-bilayer is a tough flexible structure that forms a barrier around the cytoplasm of a cell

Lipid-Bilayer A lipid-bilayer of a cell contains proteins and carbohydrates as well as lipids For this reason it is sometimes referred to as a “molecular mosaic”

Structure of the lipid-bilayer

Diffusion Every living cell has a liquid interior (cytoplasm) and is usually surrounded by liquid The cytoplasm is a solution of many different substances that are dissolved in water

It is the job of the cell membrane to control traffic in and out of a cell One of the ways this is done is by concentration When a molecule moves from an area of high concentration to low it is called diffusion

Osmosis Osmosis is a specific type of diffusion Osmosis occurs in selectively permeable membranes This means that only certain molecules are allowed to pass through

Osmotic Pressure Osmosis works on the principle of osmotic pressure Water will move from an area of high concentration and pressure to areas of low concentration and pressure

Osmotic pressure Osmotic pressure can cause problems however In a hypotonic environment a cell absorbs the water from the environment and the cell will swell and burst

In a hypertonic environment the water will migrate out of a cell and the cell will collapse

Facilitated diffusion Facilitated diffusion occurs when a molecule can not directly diffuse through the lipid-bilayer however it will still go through This is accomplished by protein channels in the cell membrane that are large enough for those molecules to pass though

Active Transport Often times a molecule will need to move across a concentration gradient in the opposite direction When this occurs energy is needed to create this movement and we call it active transport