Chapter 5.3 Diffusion, Membranes, and Metabolism AP Biology Fall 2010

Bell Ringer

Objectives Differentiate between the factors that influence the rate and direction of diffusion Understand how substances cross membranes Know what a concentration gradient is State the definition of diffusion

What Is A Concentration Gradient? Concentration Gradient: difference in the number of molecules or ions of a substance in a given volume of fluid between two adjoining regions In absence of other forces, molecules move from a region where they are more concentrated to a region where they are not as concentrated

What Is A Concentration Gradient? Thermal energy keeps molecules in motion Collide at random and bounce off each other Happens more in regions where molecules are most concentrated  net movement is toward the region where they are not colliding

What Is A Concentration Gradient? Molecules flow down their concentration gradient High concentration  low concentration

What Is A Concentration Gradient? Diffusion: net movement of like molecules or ions down a concentration gradient – From high to low concentration Each substance diffuses independently (ex. Dye molecules in water)

The Rate of Diffusion Size – Smaller molecules diffuse faster than larger ones – Smaller = faster

The Rate of Diffusion Temperature – More heat energy makes molecules move faster – Higher = faster

The Rate of Diffusion Steepness of the concentration gradient – Rates are high with steep gradients

The Rate of Diffusion Charge – A difference in electric charge between adjoining regions – Ex. Each ion dissolved in fluids bathing a cell membrane contributes to a local electric charge – Opposite charges attract – The fluid having more negative charge overall exerts the greatest pull on positively charged substances

The Rate of Diffusion Pressure – Difference in exerted force per unit areas in two adjoining areas

The Rate of Diffusion Dynamic Equilibrium: when gradients no longer exist there is no net movement – Still movement, but “equal”

Membrane Crossing Mechanisms Selective permeability: allow some substances but not others to enter and leave a cell – All cell membranes structured to show this – Control when and how much crosses

Membrane Crossing Mechanisms Lipids and nonpolar molecules pass easily through cell membrane Glucose and other large polar molecules cannot pass through the bilayer directly – Rely on passage through interior of transport proteins

Membrane Crossing Mechanisms Passive transport- material passes through interior of transport proteins without use of energy – Known as facilitated diffusion Active transport- proteins become activated to move a solute against its concentration gradient

Membrane Crossing Mechanisms Substances move in bulk across cell membrane by exocytosis and endocytosis Endocytosis: vesicle forms around particles when a patch of plasma membrane sinks inward and seals back on itself Exocytosis: vesicle that formed in the cytoplasm fuses with the plasma membrane, so that its contents are released to the outside

Membrane Crossing Mechanisms

Membrane Permeability Cells keep extracellular fluid contents separate from the contents of the cell with membranes that are selectively permeable Raw materials enter the cell to be used in metabolism Wastes are expelled from the cell into the extracellular fluid

Membrane Permeability Cell volume is adjusted and maintained within normal ranges as the environment around the cell changes pH is adjusted to maintain homeostasis by movement of substances into and out of the cell

Review 1.T/F With diffusion, molecules move from areas of low concentration to high concentration. 2.T/F Endocytosis is the process of moving large particles out of the cell. 3.T/F With dynamic equilibrium, there is a net movement of molecules.

Answers 1.False 2.False 3.False