HOMEOSTASIS AND TRANSPORT I Cell Membranes A Cell

HOMEOSTASIS AND TRANSPORT I. Cell Membranes A. Cell membranes help organisms maintain homeostasis by controlling what substances may enter or leave cells B. Some substances can cross the cell membrane without any input of energy by the cell C. The movement of such substances across the membrane is known as passive transport D. To stay alive, a cell must exchange materials such as food, water, & wastes with its environment E. These materials must cross the cell or plasma membrane

• F. Small molecules like water, oxygen, & carbon dioxide can move in and out freely • G. Large molecules like proteins & carbohydrates cannot move easily across the plasma membrane • H. The Cell Membrane is semipermeable or selectively permeable only allowing certain molecules to pass through

II. Diffusion A. Diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration http: //www. biosci. ohiou. edu/introbioslab/ Bios 170/diffusion/Diffusion. html

B. Small molecules can pass through the cell membrane by a process called diffusion C. Diffusion across a membrane is a type of passive transport because it does not require energy

D. This difference in the concentration of molecules across a membrane is called a concentration gradient E. Kinetic energy keeps molecules in constant motion causing the molecules to move randomly away from each other in a liquid or a gas

• The rate of diffusion depends on temperature, size of the molecules, & type of molecules diffusing • H. Molecules diffuse faster at higher temperatures than at lower temperatures • I. Smaller molecules diffuse faster than larger molecules

• K. Solutions have two parts --- the solute which is being dissolved in the solvent • L. Water serves as the main solvent in living things

M. Diffusion always occurs down a concentration gradient (water moves from an area where it is more concentrated to an area where it is less concentrated) http: //www. tvdsb. on. ca/westmin/science/sbi 3 a 1/Cells/Osmosis. htm

N. Diffusion continues until the concentration of the molecules is the same on both sides of a membrane O. When a concentration gradient no longer exists, equilibrium has been reached but molecules will continue to move equally back & forth across a membrane http: //www. tvdsb. on. ca/westmin/science/sbi 3 a 1/Ce lls/Osmosis. htm

Osmosis • Think back to solutions: solutes and solvents • Osmosis is the process by which water molecules diffuse across a cell membrane from an area of lower solute concentration to an area of higher solute concentration. • Direction of movement depends on relative concentrations of solutions.

Direction of movement into cells • Hypotonic to the cytosol – solution outside the cell is less concentrated than inside the cell – water moves into the cell • Hypertonic to the cytosol – solution outside the cell is more concentrated than inside the cell – water moves out of the cell • Isotonic – concentrations are equal http: //www. tvdsb. on. ca/westmin/science/sbi 3 a 1/Cells/Osmosis. htm

Red blood cells are placed into 3 different types of solutions: figure out the types of solution in the beakers by looking at the RBC ISOTONIC HYPERTONIC HYPOTONIC

How do freshwater organisms deal with osmosis? • What would be the problem? – Water would diffuse into the cells • What would organisms do to deal with the problem? – Pump water out or excrete large amounts of water

How do marine organisms deal with osmosis? • What would be the problem? – Water would diffuse out of the cells • What would organisms do to deal with the problem? – Pump water out or excrete large amounts of salt

Finish the statements • • Diffusion is… Osmosis is… A hypertonic solution has… A hypotonic solution has… A cell placed in a hypertonic solution will… A cell placed in a hypotonic solution will… Both osmosis and diffusion are _______ transport because they do not require _______.

Active Transport • Transport Notes • Active Transport Movie 1 • Active Transport Movie 2