PASSIVE TRANSPORT This lesson meets the following DoE Specific Curriculum Outcome for Biology 11: 314-1, 314-3, 314-8

CELL TRANSPORT All cells must exchange materials with their environment to maintain homeostasis. The cell can use a variety of methods to move things from the internal environment to the external environment and vice versa. There are two main types of cell transport; passive transport and active transport.

CELL TRANSPORT

PASSIVE TRANSPORT Passive transport is the movement of materials across the plasma membrane without the use of cellular energy and it includes diffusion, osmosis, and facilitated diffusion.

DIFFUSION Diffusion is the movement of materials into or out of the cell (across the cell membrane) from areas of high concentration to areas of low concentration. Does NOT require energy - follows the concentration gradient. Generally involves small non-polar and lipid-soluble particles.

BROWNIAN MOTION This can be defined as the random movement of molecules. Molecules will move about randomly in a solution or gas and collide with one another with equal frequency. This random motion is the reason why molecules move from an area of high concentration to one of low concentration.

CONCENTRATION GRADIENT The difference in concentration between a region of high concentration and a region of low concentration.

EQUILIBRIUM A condition in which all acting influences are balanced resulting in a stable environment. Once equilibrium has been reached there are now as many molecules moving into an area as there are moving out.

OSMOSIS Diffusion of WATER into or out of cell from areas of high concentration to areas of low concentration. Passive - does not require energy. Process continues until state of dynamic equilibrium is obtained - concentration of water is the same on both sides of membrane.

DIFFUSION AND THE PLASMA MEMBRANE Particles that are soluble in lipids or small enough to fit through the spaces between the polar heads will diffuse across a plasma membrane IF a concentration gradient exists. Diffusion will continue until equilibrium is reached.

What will happen to the water levels over a period of time?

OSMOSIS The diffusion of water across a semi-permeable membrane from an area of high concentration to one of low. Water will also move as a result of osmotic pressure from high to low. In the image on the right water is represented by the small red dots.

OSMOTIC BALANCE Water molecules are small and can diffuse easily across the plasma membrane. The direction of osmosis will depend on the concentration gradient. When equilibrium is reached in osmosis it is known as osmotic balance.

EFFECTS OF OSMOSIS Cells will react differently to different solutions. Plant cells and animal cells will also react differently to different solutions. There are three types of solutions that have an effect on cells; isotonic, hypotonic and hypertonic solutions.

ISOTONIC SOLUTION A solution where the concentration of water and solute molecules outside a cell is equal to the concentration of water and solute molecules inside the cell. Isotonic solutions are the ideal solution for cells to be contained in. A cell will be at equilibrium in this solution.

HYPOTONIC SOLUTION A solution where the concentration of water outside a cell is greater than that found inside the cell. (so less solutes) Cells placed in this solution will experience a net movement of water into the cell by osmosis. This will result in increased pressure inside the cells.

Effects of a hypotonic solution Animal cells cannot withstand a high amount of osmotic pressure and the plasma membrane will burst. Plant cells build up turgor pressure. Plant cells can stand up to the pressure due to the presence of the cell wall. Turgor pressure helps keep a plant cell rigid.

HYPERTONIC SOLUTION A solution where the concentration of water molecules outside a cell is lower than that found inside the cell. (more solutes) Cells placed in this solution will experience a net movement of water out of the cell by osmosis. The cytoplasm of the cell will shrink due to the loss of water. The shrinking of the cytoplasm in cells is known as plasmolysis.

Effect of Solutions on Plant and Animal Cells

FACILITATED DIFFUSION The cell membrane contains glycoproteins which aid in the passage of certain particles across the membrane. These proteins are known as transport proteins. Certain particles that a cell needs are either too large or are not soluble in lipids to diffuse easily across the cell membrane.

FACILITATED DIFFUSION The transport proteins aid or facilitate these particles as they move from an area of high concentration to an area of low concentration (passive and selective). Transport proteins are selective because each one has a distinctive shape and charge to allow only one kind of particle to enter or leave the cell.

FACILITATED DIFFUSION 1. CHANNEL PROTEINS: Also called pores. Water-filled passages through which small dissolved particles, such as ions, can diffuse. Ions pass through different pores depending on size and charge.

FACILITATED DIFFUSION 2. CARRIER PROTEINS: Change shape to allow different molecules to cross the plasma membrane.

FACILITATED DIFFUSION 3. GATE PROTEINS : Certain chemicals combine with the transport protein to signal it to “open”. Glucose diffuses into a cell this way.