Active and Passive Transport !!!

The Fluid Mosaic Model  The cell membrane is also called the plasma membrane and is made of a phospholipid bilayer.

 The phospholipids have a hydrophillic (water attracting) head  And two hydrophobic (water repelling) tails.

 The head of the phospholipid is made of an alcohol and a phosphate group.  While the tails are made of fatty acids.

 Another substance in the membrane is cholesterol.  It maintains the membrane’s fluidity (makes it not too solid or not too liquid)

 Proteins called integral proteins go all the way through the bilayer  While peripheral proteins are only on the side.

 Integral proteins are also called channel proteins.  Large molecules use integral proteins to help move across cell membranes

TRANSPORT: THE TRAFFIC ACROSS MEMBRANES  The ability of molecules to move across the cell membrane depends on two things:  the semi-permeability of plasma membrane  and the size and charge of the particles that want to get through.

 The plasma membrane helps a cell to maintain homeostasis, or a relatively stable internal environment.  It does this by being semi-permeable (it lets certain molecules inside of the cell, while keeping others out).

 Generally speaking the membrane lets lipid molecules inside, while keeping non-lipid molecules out.  Water is a major exception to this rule!

Diffusion  But what determines the direction of the traffic across the membrane?  Most solutes (or particles dissolved in a solution) naturally want to spread out from each other.

Diffusion  So particles want to move from areas of high concentration to areas of low concentration.  This is known as diffusion. Ex:  When you spray perfume, the perfume particles will spread out in the room.  If you drop food coloring into water, the color will spread out.

Diffusion  The particles will keep spreading out until the concentration is the same throughout the solution.  Once the concentration is the same everywhere, the particles have reached what we call equilibrium.

Diffusion  So what does that have to do with the plasma membrane?  Well suppose a substance is present in unequal concentrations on either side of a cell membrane.  If the substance can move across the membrane, then its particles will move to the area of lower concentration, until equilibrium is reached.

Diffusion  This process of diffusion is how some molecules move into and out of the cell.  Since diffusion occurs naturally, the cell does not need to use energy and this is called PASSIVE transport.

Osmosis  Osmosis is the diffusion of water across a semi-permeable membrane.  It is also driven by the concentration of particles.

 Some particles cannot move across the membrane, so water moves instead to even out the concentration.

 A solution that has a high concentration of particles is said to be HYPERtonic  A solution that has low concentration of particles is said to be HYPOtonic.

 Water will move from the hypotonic solution to the hypertonic solution, until the ratio of water to particles is the same on both sides.  The solution is then said to be ISOtonic (it has reached equilibrium).

Facilitated Diffusion  Non-lipid materials cannot diffuse across the membrane as easily as lipid molecules.  These substances must rely on special proteins called channel or carrier proteins that are embedded inside of the plasma membrane.

Facilitated Diffusion  The proteins pick up the substance from one side of the membrane and carry it across to the other.  Facilitated Diffusion Facilitated Diffusion

 Since the substances are still travel from a high concentration to a low concentration,  Facilitated diffusion does not require the cell to use ATP (energy)

Active Transport  As powerful as diffusion is, cells sometime have to use energy to move substances against the concentration gradient (from low concentrations to high concentrations).  This is called active transport.

Active Transport- Transport proteins  Embedded in the plasma membrane are special transport proteins, that use ATP (energy) to pump molecules in or out of the cell.

Transport Proteins  The proteins are similar to the carrier proteins that work in facilitated diffusion, BUT they pump molecules from areas of low concentration to high concentration  They require the use of energy (ATP) because they are going against the concentration gradient.  Active Transport - Transport Protein Active Transport - Transport Protein

 Transport proteins are usually only involved in the transfer of smaller ions or molecules.

Endocytosis  Larger molecules or clumps of material can be actively transported across the membrane through a process called endocytosis.

Endocytosis  Endocytosis occurs when the cell membrane surrounds a substance and engulfs it.  The pocket that results breaks loose from the cell membrane and forms a vacuole within the cell’s cytoplasm.

Endocytosis  There are two types of endocytosis: phagocytosis and pinocytosis.

Phagocytosis  In phagocytosis (cell eating), the plasma membrane surrounds a particle and packages it in a food vacuole.

Pinocytosis  In pinocytosis, tiny pockets form along the plasma membrane, fill with liquid, and pinch off to form vacuoles within the cell.

Exocytosis  Many cells also use active transport to release large amounts of material from the cell. This is known as exocytosis.  The membrane of the vacuole surrounding the material fuses with the cell membrane, forcing the contents outside of the cell.