Unit 4: Cells 4.4 Homeostasis: Passive Transport

Passive Transport Overview Passive transport requires the cell to use no energy! There are three types: Diffusion Osmosis Facilitated Diffusion

Diffusion Substances will move from areas of high concentration to areas of low concentration. They will move until they are equal, called equilibrium. Molecules that are very small or non-polar can move across the membrane through diffusion. http://highered.mheducation.com/sites/0072495855/student_view0/chapter2/animation__how_diffusion_works.html

Facilitated Diffusion Ions and polar molecules have to use a transport protein to get across the cell membrane. Each transport protein moves only a specific substance. This allows our membrane to control what enters and exits the cell; called selectively permeable. Through the middle of the transport protein is an ion channel. Some channels are always open, some open and close based on stimuli from the environment.

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Steps of Facilitated Diffusion Substances will use transport proteins to move down their concentration gradients. The transport protein binds to a specific molecule on one side of the cell membrane. The shape of the transport protein changes and exposes the molecule to the other side of the cell. The molecule is released and the transport protein returns to its original shape.

Osmosis A type of facilitated diffusion The movement of water down its concentration gradient How is the concentration gradient made? Each side of the membrane will have a different number of dissolved ions and polar molecules that will attract and bond to water molecules. The water molecules that are not bonded are called “free” water molecules. The number of free water molecules on each side of the membrane will create the concentration gradient.

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Concentration Gradient of Water = Bonded Water Molecules

Look at Free Water Molecules ONLY: Which side has more free water molecules?

3 Ways a Water Molecule Can Move https://www.youtube.com/watch?v=IaZ8MtF3C6M 3 Ways a Water Molecule Can Move Out of the cell: causes cell to shrink A solution that causes a cell to shrink is called a hypertonic solution. Into the cell: causes the cell to swell A solution that causes a cell to swell is called a hypotonic solution. No net movement: the water moves equally in and out of the cell. A solution that causes no change in cell size is called an isotonic solution.