 Homeostasis – Process by which organisms maintain a relatively stable internal environment; All organisms have ranges that are tolerated (i.e. pH and temperature)  Example - Paramecium contain contractile vacuoles that collect and remove excess water, thereby helping to achieve homeostasis

 Composed of a double-layered sheet called the lipid bilayer which includes: 1. Two layers of lipids (phosphate group + two fatty acids)  Creates a strong barrier that is flexible 2. Proteins that are embedded in the bilayer  Some form channels and pumps that help to move material across the cell membrane 3. Carbohydrate molecules that are attached to the proteins  Allows individual cells to identify one another

 Concentration - the mass of solute in a given volume of solution, or mass/volume  Example:  12 grams of salt in 3 liters of water  Concentration = 12 g/3 L (4 g/L)  If you had 12 grams of salt in 6 liters of water  concentration = 12 g/6 L (2 g/L) ***The 1st solution is 2X as concentrated as the 2 nd.

 Passive transport – Transport of materials from a higher to lower concentration. Energy is not required for this to occur.  Active transport – Transport of materials from a lower to higher concentration. Energy is needed for this to occur. high low Weeee!!! high low This is gonna be hard work!!

Three Types of Passive Transport: 1. Diffusion - Transport of substances 2. Osmosis – Transport of water 3. Facilitative Diffusion – Transport with the help of carrier proteins (for molecules too large to pass through the membrane without help)

 Diffusion - Process by which molecules tend to move from an area where they are more concentrated to an area where they are less concentrated  when the concentration of a solute is the same throughout a solution it has reached equilibrium

 ecnotes/diff.html ecnotes/diff.html

 Because diffusion depends upon random particle movements, substances diffuse across membranes without requiring the cell to use energy.  Even when equilibrium is reached, particles of a solution will continue to move across the membrane in both directions.  Particles are always in motion!

 Many substances can diffuse across biological membranes, but some are too large or too strongly charged to cross the lipid bilayer.  If a substance is able to diffuse across a membrane, the membrane is said to be permeable to it.  A membrane is impermeable to substances that cannot pass across it.  Most biological membranes are selectively permeable, meaning that some substances can pass across them and others cannot.

 Water passes easily across most membranes, but most solute does not.  Osmosis - Diffusion of water through a selectively permeable membrane  High to low concentration

 There are more sugar molecules on the right. That means that the concentration of water is lower on the right than the left.  The membrane is permeable to water but not to sugar. As a result, there is a net movement of water from the area of high concentration to the area of low concentration.

 Water will tend to move across the membrane to the left until equilibrium is reached.  Isotonic - when the concentration of two solutions is the same  Hypotonic - when comparing two solutions, the solution with the lesser concentration of solutes  Hypertonic - when comparing two solutions, the solution with the greater concentration of solutes

 For organisms to survive, they must have a way to balance the intake and loss of water.  Osmosis exerts a pressure known as osmotic pressure on the hypertonic side of a selectively permeable membrane.  Osmotic Pressure - The force exerted by osmosis that “pushes” water through a selectively permeable membrane to make the concentration of solutes the same on both sides.

 Turgor Pressure - The force behind the cell walls of plants, which allows them to keep their shape.  Very similar to osmotic pressure, but occurs in plants.  When the cells contents press the plasma membrane against the cell wall and the cell wall in turn presses against the membrane

 The cell is filled with salts, sugars, proteins, and other molecules.  If placed in a hypertonic solution.  Cell will shrink or become shriveled ( plasmolysis )  Water moves out of cell.  If placed in a hypotonic solution  Cell will be swell or become enlarged.  Cytolysis occurs when a cell ruptures or lyses.  Water moves into the cell.

Plasmolysis occurs as the cell shrinks. Cytolysis occurs when the cell bursts

ISOTONIC ENVIRONMENT

HYPOTONIC ENVIRONMENT HYPERTONIC ENVIRONMENT