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Lecture 1

BIOL 4004 Lecture Notes - Microtubule, Peptide, Epb41

5 Pages
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Fall 2015

Department
Biology
Course Code
BIOL 4004
Professor
Matthes David
Lecture
1

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MBC5 Study Guide – Chapter 10 (Membrane Structure)
All living cells have a boundary called the plasma membrane that separates the cytoplasm from
the external environment. Eukaryotic cells also have internal membranes that create organelles
such as the cell nucleus. In this chapter, we will focus on structural aspects of membranes and
membrane proteins. Later chapters will consider functional issues.
THE LIPID BILAYER
When exposed to water, certain types of lipids form a bilayer (Figure 10-1). Membrane proteins
may be embedded in the bilayer structure. In this section, we will consider structural features of
lipids that cause them to form a bilayer, and we will examine the dynamic aspects of lipid
bilayers and biological membranes.
Phosphoglycerides, Sphingolipids, and Sterols are the Major Lipids in Cell Membranes
The most abundant membrane lipids are phospholipids, and the most abundant phospholipids are
phosphoglycerides (Figure 10-2). Phosphoglycerides are amphipathic because they have a
polar head and two nonpolar tail regions. The backbone of phosphoglycerides is the three-
carbon compound glycerol, whereas the backbone of the phospholipid sphingomyelin is
sphingosine. Many membranes contain cholesterol, a sterol that contains a rigid ring structure
attached to a single polar hydroxyl group (Figure 10-4).
Phospholipids Spontaneously Form Bilayers
The nonpolar tails of phospholipids do not favorably interact with water (Figure 10-7). This
unfavorable interaction decreases entropy, because water forms an ordered structure around the
nonpolar tail in an attempt to minimize interactions. To avoid a decrease in entropy, which is
energetically unfavorable, phospholipids in water will spontaneously form a bilayer in which the
nonpolar tails interact with each other and polar heads are exposed to water. This minimizes
entropy because water is more disordered.
The Lipid Bilayer Is a Two-Dimensional Fluid
When exposed to water, phospholipids form a spherical structure called a liposome (Figures 10-
8. 10-9). The bilayer is semi-fluid, allowing certain types of movements and preventing others
(Figure 10-11). Lipids can diffuse laterally (e.g., from left to right) and rotationally (rotate 360o).
However, it is energetically unfavorable for them to flip from one leaflet of the bilayer to the
other.
The Fluidity of a Lipid Bilayer Depends on Its Composition
An optimal fluidity of a biological membrane is needed to ensure the proper integrity of the
membrane and to promote the function of membrane proteins. Fluidity can be altered by altering
the lipid composition. As noted in Table 10-1, certain categories of phospholipids are common.
You should be aware that structural features of lipids make the bilayer more or less fluid. These
include the following:
Double bonds in the lipid tails make the bilayer more fluid. This is because double bonds
diminish packing between the tails.
Shorter lipids tails make the bilayer more fluid. Shorter tails are freer to diffuse laterally
and rotationally.
At high concentrations, cholesterol makes membranes more fluid because it inhibits
packing between phospholipid tails.
High temperature makes the bilayer more fluid.
Despite their Fluidity, Lipid Bilayers can Form Domains of Different Compositions
A lipid raft is a grouping of lipid and protein molecules that are somewhat stuck together (Figure
10-14). They diffuse within a lipid bilayer as a single unit. Researchers speculate that lipid rafts
may promote the function of certain membrane proteins. Future research is likely to shed
additional light on the functional importance of lipid rafts.
Lipid Droplets are Surrounded by a Phospholipid Monolayer
The Asymmetry of the Lipid Bilayer Is Functionally Important
With regard to the two leaflets of a membrane, asymmetry exists in the following ways (Figures
10-16):
The composition of different types of lipids varies in the two leaflets.
The orientations of membrane proteins are asymmetrical.
As we will learn later in the course, this asymmetry is very important with regard to membrane
function.
Glycolipids Are Found on the Surface of All Plasma Membranes
Glycolipids are lipids that have carbohydrates attached to the polar head group (Figure 10-18).
When found in the plasma membrane, the carbohydrate portion projects to the outside of the
cell. When found in organellar membranes, the carbohydrate projects away from the cytosol; it
projects into the lumen of organelles. As we will learn later in the course, glycolipids play a role
in cell recognition and help to protect the outer surface of cells.
MEMBRANE PROTEINS
Proteins that are embedded within the lipid bilayer or attached to its surface are termed
membrane proteins. In this section, we will survey some of the structural aspects of membrane
proteins.
Membrane Proteins Can Be Associated with the Lipid Bilayer in Various Ways
As shown in Figure 10-19, there are different ways that proteins can associate with membranes:
The structure of a portion of a protein can be embedded in the bilayer. This is shown for the
first, second, third, and fourth protein in Figure 10-19. These are integral membrane
proteins.
A protein can be anchored in a membrane by a lipid molecule. The fifth and sixth proteins
are examples of this. This is also shown in Figure 10-18.
A protein can be attached to the polar region of a membrane. This is shown for the seventh
and eighth (green) protein in Figure 10-19. Such a protein is termed a peripheral
membrane protein.
Lipid Anchors Control the Membrane Localization of Some Signaling Proteins
Some membrane proteins are covalently attached to a fatty acid chain or a prenyl group (Figure
10-20). This anchors the protein to one side of the membrane where it carries out its biological

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Description
MBC5 Study Guide Chapter 10 (Membrane Structure) All living cells have a boundary called the plasma membrane that separates the cytoplasm from the external environment. Eukaryotic cells also have internal membranes that create organelles such as the cell nucleus. In this chapter, we will focus on structural aspects of membranes and membrane proteins. Later chapters will consider functional issues. THE LIPID BILAYER When exposed to water, certain types of lipids form a bilayer (Figure 101). Membrane proteins may be embedded in the bilayer structure. In this section, we will consider structural features of lipids that cause them to form a bilayer, and we will examine the dynamic aspects of lipid bilayers and biological membranes. Phosphoglycerides, Sphingolipids, and Sterols are the Major Lipids in Cell Membranes The most abundant membrane lipids are phospholipids, and the most abundant phospholipids are phosphoglycerides (Figure 102). Phosphoglycerides are amphipathic because they have a polar head and two nonpolar tail regions. The backbone of phosphoglycerides is the three carbon compound glycerol, whereas the backbone of the phospholipid sphingomyelin is sphingosine. Many membranes contain cholesterol, a sterol that contains a rigid ring structure attached to a single polar hydroxyl group (Figure 104). Phospholipids Spontaneously Form Bilayers The nonpolar tails of phospholipids do not favorably interact with water (Figure 107). This unfavorable interaction decreases entropy, because water forms an ordered structure around the nonpolar tail in an attempt to minimize interactions. To avoid a decrease in entropy, which is energetically unfavorable, phospholipids in water will spontaneously form a bilayer in which the nonpolar tails interact with each other and polar heads are exposed to water. This minimizes entropy because water is more disordered. The Lipid Bilayer Is a TwoDimensional Fluid When exposed to water, phospholipids form a spherical structure called a liposome (Figures 10 8. 109). The bilayer is semifluid, allowing certain types of movements and preventing others o (Figure 1011). Lipids can diffuse laterally (e.g., from left to right) and rotationally (rotate 360). However, it is energetically unfavorable for them to flip from one leaflet of the bilayer to the other. The Fluidity of a Lipid Bilayer Depends on Its Composition An optimal fluidity of a biological membrane is needed to ensure the proper integrity of the membrane and to promote the function of membrane proteins. Fluidity can be altered by altering the lipid composition. As noted in Table 101, certain categories of phospholipids are common. You should be aware that structural features of lipids make the bilayer more or less fluid. These include the following: Double bonds in the lipid tails make the bilayer more fluid. This is because double bonds diminish packing between the tails. Shorter lipids tails make the bilayer more fluid. Shorter tails are freer to diffuse laterally and rotationally.
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