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Cell Membranes

Structure, Chemistry and Characteristics

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Introduction

• This presentation consists of 18 slides • Use it a an outline for review of what you need to know

about cell membranes, including the plasma membrane, the outer boundary of the cell.

• Study artist rendition of the plasma membrane in the next slide and try to identify: cholesterol, the phospholipids, the glycolipids, the proteins and the glycoproteins. Notice that all the oligosaccharides are in the layer facing the extracellular fluid.

• For additional help consult a college biology text, college A&P text or a biochemistry text.

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The Plasma Membrane(Guyton Ch. 2.3 or Marieb ch. 2)

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Significance of Carbohydrate Coat on Cell Surface (Alberts Fig. 11.39)

• Most plasma membrane proteins have bound to them oligosaccharide residues that face the extracellular fluid.

• These sugars play a key role in cell to cell recognition and function as markers for some cells.

• Example: Blood types A, B and AB

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Different Views of Cell MembranesNotice Thickness in nanometer!

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Functions of Membrane Proteins

This summarizes some of the major functions carried out by membrane proteins. Transporters move solutes across the membrane, or form passageways/channels for ions. Anchors attach cells to each other, to the extracellular matrix and to the cytoskeleton. Receptors bind specific ligands and enzymes like adenylyl cyclase and phospholipase A catalyze the synthesis of second messengers like cAMP, inositol triphosphate (IP3) and diacylglycerol (DAG)

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Plasma membrane: The Lipid Bilayer (I)

• As illustrated in the previous slides, the lipid bilayer is composed of a variety of phospholipids, glycolipids, cholesterol (in animal cells but other sterols in plant cells and in bacteria). The type of phospholipid and the relative abundance of a phospholipid varies with the membrane within the cell and also among different cells. (Will illustrate in lecture).

• The next slide lists some of the major phospholipids found in cell membranes

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Plasma Membrane Lipid Bilayer (II)Composition

• Some of the major phospholipids in the plasma membrane include:– Phosphatidylcholine (24%)– Phosphatidylserine (4%)– Phosphatidylethanolamine (17%)– Phosphatidylglycerol– Sphingomyelin (19%)– Phosphatdylinositol (Significance in cell-cell communication

• Cardiolipin (diphosphatidylglycerol is found in inner mitochondrial membrane and in chloroplasts, but NOT in the plasma membrane)

• Glycolipid (like the gangliosides and cerebrosides)• Cholesterol (in animal cells but phytosterols and

hopanoids in bacteria)– As illustrated in the next slide, all of these lipids are amphipathic

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Membrane Lipids Are Amphipathic

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Amphipathic Phospholipids form a Bilayer in Water (Fig. 11.11)

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Arrangement of Chlesterol in Lipid Bilayer: Significance (Fig. 11.16)

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Phospholipids Aggregate to Form Cell Membranes (Fig. 2.20)

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Phospholipid Bilayers Form Sealed Compartments (Fig. 11.12)

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Lipososmes: Significance in Medicine (Fig. 11.13)

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Significance & Characteristics of the Lipid Bilayer

• The lipid bilayer forms a permeability barrier for solutes that are relatively large, hydrophilic and charged

• The lipid bilayer is:1. Fluid (because its components are NOT static (see # 16)2. Assymetrical with respect to composition of each monolayer (see

#17)3. Impermeable to solutes that are polar, charged and relatively

large4. Selectively permeable implies that the lipid will select

what solutes diffuses through the membrane on the basis of 3 solute criteria: a) partition coefficient, b) MW, c) charge and/or polarity.

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Types of Movements Within the Plane of the Membrane (Fig. 11.15)

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Membrane Assymetry (Alberts et al, fig. 11.17)

• Assymetry refers to the unequal distribution of the lipids between the two monolayers as illustrated by the different colors (different lipids) in the monolayer facing the cytosol vs that facing the extracellular space.

• Thin Layer Chromatography (TLC) is a t echnique used to isolate & purify membrane lipids: Ask Dr. Faitar for some detail and/or illustration

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THE END

Cell Membranes