How stuff gets in & out of the cell. You learned in section 1 that the membrane does a lot to control what goes in & out of the cell. Today you will learn the specific ways materials enter & exit the cell. You will see that the way a cell gets each material is unique and specialized. Also, that there are ways the cell gets its needed materials by both using and not using energy.

Objectives: Passive & Active Transport Identify what determines the direction in which passive transport occurs. Understand osmosis and why it’s important. Illustrate how substances move against a concentration gradient in active transport.

Vocabulary Equilibrium Concentration gradient Diffusion Carrier protein Osmosis Sodium-potassium pump

Before we get started… You’ll need to know these terms before we begin. Define these in your notebooks. Solution: Solute: Solvent: Concentration: an amount of a substance within a given volume

Passive Transport: Diffusion & Equilibrium In a solution, randomly moving molecules tend to fill up a space. Watch as I drop some food coloring into the beaker. What happens? The process that causes this dispersion of polar color molecules is diffusion. When the space is filled evenly with the particles, a state called equilibrium is reached. A state that exists when the concentration of a substance is the same through-out a space.

Equilibrium Temperature Water temp Ice Temp http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_diffusion_works.html Equilibrium Temperature As the ice melts the water temperature drops. The low temperature of the ice equalizes with the warmer water temp. Ice Temp Water temp

Passive Transport: Concentration Gradient The amount of a particular substance in a given volume is called the concentration of the substance. When one area has a higher concentration than another area does, a concentration gradient exists. The difference in the concentration of a substance across a distance.

Down CONCENTRATION GRADIENT Up

Visual Concept: Concentration Gradient Area of High Concentration Area of Low Concentration

Passive Transport: Diffusion The movement of particles from regions of higher density to regions of lower density is diffusion. Watch as I drop yellow food coloring into the beaker. Would you all agree that the concentration of the food coloring is highest right where it is dropped? Over time, the particles of color naturally diffuse through the water, without any need of physical movement.

Diffusion

Diffusion One of the main jobs of the cell membrane is to separate the cytoplasm from the fluid outside the cell. But the cell still needs an abundance of materials that comes from outside the cell. Some substances that the cell needs can enter and leave the cell by diffusing across the cell membrane. The direction of movement depends on the concentration gradient, meaning that the particles will naturally flow where there is less of them, & usually where more is needed. The greatest part of this is… DIFFUSION DOES NOT REQUIRE ENERGY!

Diffusion is Passive Transport In cells, diffusion is called passive transport. In passive transport, substances cross the cell membrane down their concentration gradient. Some substances diffuse through the lipid bilayer. Other substances diffuse through transport proteins.

Passive Transport… Simple Diffusion Diffusion through the membrane is SIMPLE DIFFUSION Simple Diffusion Small, nonpolar molecules can pass directly through the lipid bilayer. This type of movement is called simple diffusion. EXAMPLES OF SIMPLE DIFFSION Oxygen moves down its concentration gradient into the cell. Carbon dioxide diffuses out of the cell. Natural steroid hormones, which are nonpolar and fat soluble, can also diffuse across the lipid bilayer.

Passive Transport & Not So Simple Diffusion Facilitated Diffusion Many ions and polar molecules that are important for cell function do not diffuse easily through the nonpolar lipid bilayer. During facilitated diffusion, transport proteins help these substances diffuse through the cell membrane. Two types of transport proteins are channel proteins and carrier proteins.

Facilitated Diffusion: Passive Transport Ions, sugars, and amino acids can diffuse through the cell membrane through channel proteins. These proteins, sometimes called pores, serve as tunnels through the lipid bilayer. Each channel allows the diffusion of specific substances that have the right size and charge.

Visual Concept: Diffusion Through Ion Channels

Passive Transport, continued Facilitated Diffusion Carrier proteins transport substances that fit within their binding site. A protein that transports substances across a membrane A carrier protein binds to a specific substance on one side of the cell membrane. This binding causes the protein to change shape. As the protein’s shape changes, the substance is moved across the membrane and is released on the other side.

Visual Concept: Passive Transport: Facilitated Diffusion Link to McGrawHill online (a good textbook) for animations and explanations of biology concepts. Take the quizzes! http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_diffusion_works.html

Osmosis. Water can diffuse across a selectively permeable membrane in a process called osmosis. Osmosis in cells is a form of facilitated diffusion. Polar water molecules do not diffuse directly through the bilayer. But the cell membrane contains channel proteins that only water molecules can pass through. Osmosis allows cells to maintain water balance as their environment changes.

Osmosis/ How the Environment Changes how_osmosis_works.html Osmosis/ How the Environment Changes When ions and polar substances dissolve in water, they attract and bind some water molecules. The remaining water molecules are free to move around. If a concentration gradient exists across a membrane for solutes, a concentration gradient also exists across the membrane for free water molecules. Osmosis occurs as free water molecules move down their concentration gradient into the solution that has the lower concentration of free water molecules.

Visual Concept: Osmosis

Osmosis The direction of water movement in a cell depends on the concentration of the cell’s outside environment. If the solution is hypertonic, or has a higher solute concentration than the cytoplasm does, water moves out of the cell. The cell loses water and shrinks. If the solution is hypotonic, or has a lower solute concentration than the cytoplasm does, water moves into the cell. The cell gains water and expands in size. If the solution is isotonic, or has the same solute concentration that the cytoplasm does, water diffuses into and out of the cell at equal rates. The cell stays the same size.

Hypertonic, Hypotonic, and Isotonic Solutions Another way to say it… with “Free Water Molecules”

Hypertonic Hypotonic Isotonic

Example of Osmosis Onion Cells in Hypotonic Solution Cell Swells Onion Cells in Hypertonic Solution Cell Shrinks

Questions on Passive Transport? What is a concentration gradient? What is diffusion? What is passive transport? What kinds of passive transport are there? Does passive transport use energy?

Active Transport Area of High Concentration Area of Low Concentration

Active Transport The opposite of diffusion is active transport. In order to move substances against their concentration gradients, cells must use energy. Active transport requires energy to move substances against their concentration gradients. Most often, the energy needed for active transport is supplied directly or indirectly by ATP.

Visual Concept: Comparing Active and Passive Transport Link on sodium potassium pump http--www.stolaf.edu-people-giannini-flashanimat-transport-secondary%20active%20transport.swf

Active Transport, continued Pumps Pumps are carrier proteins that require energy to move substances UP their concentration gradient. The sodium-potassium pump is a carrier protein that actively transports three sodium ions out of the cell and two potassium ions into the cell. This pump is one of the most important carrier proteins in animal cells. It prevents sodium ions from building up in the cell, resulting in osmosis into the cell… which could burst the cell. The concentration gradients of sodium ions and potassium ions also help transport other substances, such as glucose, across the cell membrane.

Sodium-Potassium Pump

Visual Concept: Sodium-Potassium Pump

Active Transport Vesicles Many substances, such as proteins and polysaccharides, are too large to be transported by carrier proteins altogether. Instead, they cross the cell membrane in vesicles, which are membrane-bound (lipid bi-layer) sacs. The vesicle membrane is a lipid bilayer, like the cell membrane. Therefore, vesicles can bud off from the membrane, fuse with it, or fuse with other vesicles.

Active Transport, continued Vesicles The movement of a large substance into a cell by means of a vesicle is called endocytosis. During endocytosis the cell membrane forms a pouch around the substance. The pouch then closes up and pinches off from the membrane to form a vesicle inside the cell. Vesicles that form by endocytosis may fuse with lysosomes or other organelles.

Active Transport, continued Vesicles Vesicles help the movement of large molecules two ways: Endocytosis Exocytosis

Endocytosis = “Into” Cell ingests large macromolecules or other cells Vesicle

Exocytosis = “Exits” Opposite of Endocytosis Release of contents in the cell to the external environment Vesicle

Summary Questions Does Passive Transport require Energy? Does Active Transport require Energy? What is the energy required to Active Transport? What is the difference between active and passive transport? Is diffusion Passive or Active? Is Osmosis Passive or Active? Is the Sodium-Potassium pump Passive or Active?

Summary In passive transport, substances cross the cell membrane down their concentration gradient. Osmosis allows cells to maintain water balance as their environment changes. Active transport requires energy to move substances against their concentration gradients.