文档介绍：USMLE Step 1
Physiology
Lecture Notes
medicaKAPLAN' . _l
SECTION I
General Topics
Membrane Transport 1
MEMBRANE STRUCTURE What the USMLE
Requires You to Know
General Features
• Factors affecting the rate of dif-
Biological membranes are bilayers that are assembled from a mixture of lipids
fusion
and proteins. The general structure of a membrane is shown in Figure I-1-1.
• General characteristics of pro-
tein-mediated transport
Intrinsic • The differences among the vari-
Protein Outside
ous membrane transport mech-
anisms
Hydrophilic End ^
> Phospholipid
Hydrophobic End J
Cytoplasmic Side
Figure 1-1-1. The Structure of Biological Membranes
KAPLAN MEDICAL 405
USMLE Step 1: Physiology
Lip ids
The ponent posed primarily of a bilayer of phospholipids, with
the hydrophilic (water-soluble) ends facing the aqueous environment and the
hydrophobic (water-insoluble) ends facing the interior of the membrane. Other
ponents include unesterified cholesterol and glycolipids.
Proteins
The proteins are responsible for the dynamic aspects of membrane function.
There are two main types of membrane proteins.
Integral membrane proteins: They are embedded in the lipid bilayer and
cannot be removed without disrupting the bilayer. They include channels,
pumps, carriers, and receptors. ~~
Peripheral proteins: They bind to the hydrophilic polar heads of the lipids
or to the integral proteins. Peripheral proteins contribute to the cytoskele-
ton and the glycocalyx (glycolipid and glycoprotein that cover the cell mem-
brane).
MEMBRANE TRANSPORT
Diffusion
Factors that affect the rate of diffusion (D) of a substance between part-
ments separated by a membrane are given in the following formula:
AP X SA X SOL
Doc
rx
AP = concentration gradient across the membrane. The greater the concentration
gradient, the greater the rate of diffusion.
SA = surface area of the membrane. The greater the surface area, th