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