Download - Chapter 5.3
Chapter 5.3
Diffusion, Membranes, and Metabolism
AP BiologyFall 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. False2. False3. False