Cell Transport

Concentration Gradient -When a substance is more concentrated in one area than another, it forms a concentration gradient, a gradual difference in concentration of a substance as a function of distance.

Concentration Gradient -Molecules and ions are in constant motion because they have energy. They will naturally move from a region of high concentration to a region of low concentration. -This is called movement down a concentration gradient. It requires no energy input to occur as the particles already have energy.

Passive Transport -allows substances to pass through the cell membrane down the concentration gradient from high to low concentration without the cell using energy

3 Different Types of Passive Transport 1. Diffusion 2. Facilitated Diffusion 3. Osmosis

Diffusion small nonpolar molecules like oxygen and carbon dioxide cross the phospholipid bilayer, easily slipping between phospholipids in and out of the cell

Continuing movement of particles -Even when equilibrium is reached, particles of a solution will continue to move across the membrane in both directions but this movement is not considered diffusion because diffusion requires a concentration gradient to occur.

Facilitated Diffusion - special diffusion -substances diffuse across the membrane through special transport proteins -each protein only allows a specific substance to pass through the cell membrane

Facilitated Diffusion -Molecules that cannot directly diffuse across the membrane, such as large molecules, charged ions, and polar molecules, pass through special protein channels.

Osmosis -the diffusion of water through a selectively permeable membrane

Osmosis: An Example of Facilitated Diffusion -The inside of a cell’s lipid bilayer is hydrophobic—or “water-hating.” -water molecules have a tough time passing through the cell membrane

Osmosis: An Example of Facilitated Diffusion -Many cells contain channel proteins,known as aquaporins, tubes with hydrophilic centers, that allow water to pass right through them. Without aquaporins,water would diffuse in and out of cells very slowly.

Osmosis -For organisms to survive, they must have a way to balance the intake and loss of water. -The net movement of water out of or into a cell exerts a force known as osmotic pressure.

Volume changes -Because the plasma membrane is impermeable to some solutes, osmosis can change the volume of fluid inside a cell.

Water in or out? -Placing cells in solutions of different concentrations affects whether water will enter or exit the cell; there are 3 possible types of solutions a cell can be exposed to

1. Isotonic Solutions - there is an equal amount of water and solute inside the cell as outside the cell - when cells are placed in isotonic solutions, water will enter and exit the cell at the same rate -cell will remain the same size

2.Hypotonic Solutions -when cells are placed in hypotonic solutions, water will flow into the cell because there is more solute in the cell than outside the cell -causes the cell to swell

3.Hypertonic Solutions -when cells are exposed to hypertonic solutions, water will flow out of the cell because there is more solute outside of the cell than inside the cell -causes the cell to shrink

If the cell is placed in a solution that is…..

Using energy - Some cells require creation and maintenance of a concentration gradient, and others require materials too large for membrane proteins to enter or exit.

Active Transport -the movement of materials against a concentration gradient -allows substances to move across the cell membrane from an area of low concentration to an area of high concentration -requires energy in the form of ATP

-The active transport of ions across a cell membrane is generally carried out by ion pumps, proteins found in the membrane. Active Transport

Ion pumps -Many cells use such proteins to move calcium, potassium, and sodium ions across cell membranes. Ex> a nerve cell needs a high concentration of sodium ions outside the cell and a high concentration of potassium ions in the cytoplasm

Molecular Transport -Macromolecules and other large materials can also be actively transported across the cell membrane by processes known as endocytosis and exocytosis. -The transport of these larger materials sometimes involves changes in the shape of the cell membrane.

Transport into and out of the cell 1. Exocytosis 2. Endocytosis

Exocytosis -movement of materials out of the cell -The membrane of the vesicle surrounding the material fuses with the cell membrane, forcing the contents out of the cell. -Hormones and neurotransmitters are released from the cell this way to their destinations.

Exocytosis

Endocytosis -movement of materials into the cell -Endocytosis is the process of taking material into the cell by means of infoldings, or pockets, of the cell membrane. -The pocket that results breaks loose from the outer portion of the cell membrane and forms a vesicle within the cytoplasm.

Endocytosis

Types of Endocytosis 1.Phagocytosis -the plasma membrane surrounds a large solid piece of material and brings it into the cell Ex>Amoebas use this method for taking in food.

Types of Endocytosis 2. Pinocytosis -In pinocytosis, cells take in liquid from the surrounding environment by forming tiny pockets along the cell membrane. -The pockets fill with liquid and pinch off to form vesicles within the cell.