Across the Membrane Movement of cellular materials in and out of the cell.

Cell membranes help organisms maintain homeostasis by controlling what substances may enter or leave cells.

Passive vs. Active Transport: Movement of substances without energy input by the cell is a passive transport process. Movement of substances with energy input by the cell (ATPs) is an active transport process.

Diffusion Diffusion – (simplest type of passive transport) The random movement of molecules from an area of higher concentration to an area of lower concentration. It is driven by the kinetic energy of the molecules.

Diffusion movement occurs with, (or down) the concentration gradient. A concentration gradient is a difference in concentration of molecules across a space.

Diffusion will eventually cause the concentration of molecules to be the same throughout the space. When the concentration of molecules are the same throughout the space, equilibrium is reached and the concentration gradient disappears. When equilibrium is reached, there is no net movement of molecules

Diffusion & the cell membrane The ability of substances to diffuse across a cell membrane depends upon the size and type of the molecules, as well as the chemical nature of the cell membrane. Cell membranes are selectively-permeable in that they allow only certain substances to pass.

Molecules that dissolve in lipids (such as CO 2 O 2 ), diffuse through the cell membrane. Small molecules that aren’t soluble, move through membrane pores (such as H 2 O).

Osmosis The process by which a solvent (such as water) diffuses across a selectively- permeable membrane (such as a cell membrane) is called osmosis.

Osmosis always occurs with the concentration gradient, since it is a form of diffusion.

THREE DIFFERENT SOLUTIONS: Isotonic, Hypotonic, & Hypertonic

Norway’s Osmosis Energy Power Plant – Time (Dec. 13, 2010) Norway’s Statkraft state-owned power company has opened a prototype power plant that uses the principle of osmosis to develop water pressure to move an electricity generating turbine.

The plant opened in November of 2009 in Tofte, Norway, along the Oslo Fjord. Water from the Tofte River falls into a vessel separated from salty fjord water by a thin, selectively-permeable membrane. The membrane lets freshwater force its way through to the enclosed saltwater side, where pressure builds, pushing water through a pipe to drive a turbine.

The plant generates less that a watt of energy per square mile (5 watts per square mile make it cost effective). A new membrane will be tested soon to increase efficiency. Stein Erik Skillhagen, (head of the program) foresees 30 osmotic powerplants by 2030.

Facilitated Diffusion It is a type of passive transport in which molecules that cannot diffuse rapidly into the cell, pass through pores in the cell membrane.

Carrier proteins help move molecules across the membrane. They bind to the molecule they carry. The carrier molecule changes shape to shield the molecule from the interior of the cell membrane.

DIFFUSION THROUGH ION CHANNELS: Ion channels provide small passageways across the membrane for the purpose of ion diffusion. Ions such as Na+ & Cl-, that are not soluble in lipids, can move through specific ion channels (Na+ moves through Na+ channels)

Gated Ion Channels Some ion channels are always open, while others are called gated channels and may open or close in response to stimuli such as: stretching of the cell membrane chemicals in the cell cytoplasm chemicals in the cell’s external environment electrical signals