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Lesson 12 - Taking a Deeper Look at Cellular Membranes
(Optional: Integrating knowledge of lipids and proteins to construct cellular membranes)
Student Performance Objectives
1. Explain the contribution by weight of the lipid and protein components of cellular membranes.
2. List and describe the functions for the following three types of membrane lipids:
phospholipids, cholesterol, and glycolipids.
3. Define glycocalyx.
4. Describe the functions of peripheral and integral (transmembrane) proteins of cellular
membranes.
5. List and describe 5 functions of proteins associated with cellular membranes.
Lesson Outline
A. Membranes as the basis for cellular structure - cell membrane, nuclear membrane, organelle
membranes. The remaining items refer specifically to the cell membrane or plasma
membrane surrounding a living cell of the human body.
B. Lipids molecules account for about 98% of all molecules found in cellular membranes,
however, these molecules are light therefore making up only about 50% of the membrane's
weight.
C. Phospholipids are the major lipid type in membrane structure (75% of the lipid molecules in
the membrane). They give the membrane fluidity.
D. Cholesterol as the second major lipid type in membranes (20% of lipid molecules in the
membrane. Stabilize and solidify the basic fluidity established by the phospholipids.
E. Glycolipids are the final lipid type in membranes (5% of lipid molecules in the membrane).
These are lipid-carbohydrate complexes: phospholipids + a short string of sugars that form
part of the cellular glycocalyx acting as a cellular recognition system and anchoring system,
among other things.
F. Proteins make up only about 2% of the molecules found in cellular membranes but they are
heavy and account for about 50% of the membrane's weight.
G. Proteins have many functions in strengthening, anchoring and giving functionality to the
cell membranes.
1. Peripheral proteins: are attached only to the intracellular (inner) surface of the cell
membrane and also attach to cytoskeletal elements of the cytoplasm thus anchoring
the membrane to the inner cytoplasmic mass. They also attach to the next type of
membrane protein.
2. Integral (transmembrane) proteins: these protrude on both sides of the membrane (i.e.,
the cytoplasmic side and the extracellular side) and pass completely through the
membrane. The part protruding into the cytoplasmic side of the membrane typically
binds to the cellular cytoskeleton thus tightening the cell membrane against the
cytoplasmic mass, as did the peripheral proteins. The parts protruding into the extra-
cellular fluid have, typically, carbohydrate chains attached to them (like the glycolipids,
above). Thus, these glycoproteins serve along with the glycolipids as part of the
cellular recognition system.
H. Proteins serve the following functions in and on membranes:
1. Receptors (e.g., for hormones).
2. Channels (forming pores for movements of hydrophilic substances like water, Na+ and
K+ into and out of the cell. Some channels have gates that open in response to certain
stimuli as seen in nerve and muscle cells.
3. Enzymes (for digestion as seen in the small intestine).
4. Carriers and Pumps (for movement, sometimes energy-dependent, of molecules like
glucose, amino acids and some ions into and out of cells).
5. Cell-identity markers, which are glycoproteins (working along with the glycolipids,
they form the cell's glycocalyx).
6. Cell-adhesion molecules (which permit cells to adhere to each other as they form
tissues (or as sperm and egg initially bind to each other), and to adhere or anchor to molecules in the
intercellular matrix surrounding them.
7. Molecular motors (which pull on other membrane proteins changing their shape and
actually causing the cell to move as in white blood cell movements through tissues).
8. For an in-depth look at the mitochondrial membrane and all its functions, see
http://www.johnkyrk.com/mitochondrion.swf
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