Cell membranes are made up of a phospholipid bilayer and various proteins. Phospholipds are amphipathic: one region is hydrophobic (the fatty acid tails) and one is hydrophilic (the cycerole/phosphate head). In an aqueous environment, they congregate into a bilayer to protect the hydrophobic tails. Peripheral and integral proteins can be embedded all around and within the bilayer in what is known as the fluid mosaic model, and cholesterol molecules can sit within the layer to improve its fluidity. The membrane proteins can function as enzymes, improving adhesion to neighboring cells, transporting molecules into and out of the cell (actively or passively), acting as receptors for cell signaling, and recognizing other molecules. Transport can occur by simple or facilitated diffusion via a transport protein due to osmotic pressure. Active transport occurs via a pump powered by ATP or cotransport. Endocytosis (pinocytosis and phagocytosis) and exocytosis (receptor mediated) are other methods of exchange.
fluid mosaic model describes the cell membrane as a phospholipid
bilayer embedded with proteins.
are amphipathic; they have hydrophobic fatty acid tails and
hydrophilic heads composed of a phosphate group and its attachments.
cell membrane is selectively permeable. It is most permeable to
small, nonpolar molecules and less permeable or impermeable to
larger, nonpolar molecules and ions.
transport does not require energy. Both simple diffusion and
facilitated diffusion are methods of passive transport.
is the diffusion of water down its concentration gradient.
transport requires energy in order to move a substance against its
concentration gradient. Cotransport uses the energy of one substance
moving down its concentration gradient to move a second substance
against its concentration gradient.
substances can enter the cell via endocytosis, in which the cell
engulfs materials by pinching off a portion of the cell membrane to
form a vesicle. Molecules may leave the cell via exocytosis.
Cell Membranes and Transport
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The book and CD package include two full length practice exams that are presented with all questions answered and explained. The author also presents detailed reviews of all test topics and includes multiple-choice and free-response questions with answers at the end of each topic's chapter.
This book includes a review of the AP Biology exam format and scoring, proven strategies for answering multiple-choice questions, and hints for tackling the essay questions. The practice tests include answers and explanations and there is also proven test-taking strategies for both the multiple choice and essay parts.