1. The fundamental life processes of plants and animals depend on a variety of chemical
reactions that occur in specialized areas of the organism’s cells. As a basis for
understanding this concept:
a. Students know cells are enclosed within semi-permeable membranes that regulate their
interaction with their surroundings.
The plasma membrane consists of two layers of lipid molecules organized with the polar (globular)
heads of the molecules forming the outside of the membrane and the nonpolar (straight) tails forming the
interior of the membrane. Protein molecules embedded within the membrane move about relative to
one another in a fluid fashion. Because of its dynamic nature the membrane is sometimes referred to as
the fluid mosaic model of membrane structure.
Cell membranes have three major ways of taking in or of regulating the passage of materials into and
out of the cell: simple diffusion, carrier-facilitated diffusion, and active transport. Osmosis of water is a
form of diffusion. Simple diffusion and carrier-facilitated diffusion do not require the expenditure of
chemical bond en¬ergy, and the net movement of materials reflects a concentration gradient or a volt-
age gradient or both. Active transport requires free energy, in the form of either chemical bond energy
or a coupled concentration gradient, and permits the net transport or “pumping� of materials
against a concentration gradient.

1. b. Students know enzymes are proteins that catalyze biochemical reac¬tions without
altering the reaction equilibrium and the activities of enzymes depend on the temperature,
ionic conditions, and the pH of the surroundings.
Almost all enzymes are protein catalysts made by living organisms. Enzymes speed up favorable
(spontaneous) reactions by reducing the activation energy re¬quired for the reaction, but they are not
consumed in the reactions they promote. To demonstrate the action of enzymes on a substrate, the
teacher can use liver ho¬mogenate or yeast as a source of the enzyme catalase and hydrogen
peroxide as the substrate. The effect of various environmental factors, such as pH, temperature, and
substrate concentration, on the rate of reaction can be investigated. These in¬vestigations should
encourage student observation, recording of qualitative and quantitative data, and graphing and
interpretation of data.

1. c. Students know how prokaryotic cells, eukaryotic cells (including those from plants and
animals), and viruses differ in complexity and general structure.
All living cells are divided into one of two groups according to their cellular structure. Prokaryotes have
no membrane-bound organelles and are represented by the Kingdom Monera, which in modern
nomenclature is subdivided into the Eubacteria and Archaea. Eukaryotes have a complex internal
structure that allows thousands of chemical reactions to proceed simultaneously in various organelles.
Viruses are not cells; they consist of only a protein coat surrounding a strand of genetic material, either
RNA or DNA.