Lipids and Proteins Are Associated in Biological Membranes

Chapter 8

What is a lipid? Lipids:Lipids: consists of nonpolar groups

- Classified on the basis of common solubility properties

- insoluble in water- soluble in organic solvents like chloroform and

acetone- Amphipathic in nature

What are the two groups of lipids?

Lipids include: Open Chain forms fatty acids, triacylglycerols, sphingolipids,

phosphoacylglycerols, and glycolipids, lipid-soluble vitamins prostaglandins, leukotrienes, and thromboxanes

Cyclic forms cholesterol, steroid hormones, and bile acids

What are Fatty Acids?

- Carboxyl group at polar end and hydrocarbon chain at non polar tail

- Length of fatty acid plays a role in its chemical character

- Usually contain even numbers of carbons (can contain odd, depending on how they are biosynthesized)

What are saturated fatty acids?

- Contain only C-C bonds - saturated

What are unsaturated fatty acids? In most unsaturated fatty acids, the cis isomer

predominates; the trans isomer is rare FA that contain C=C, are unsaturated Unsaturated fatty acids have lower melting points

than their saturated counterparts

What are triacylglycerols? Triacylglycerol (triglyceride):Triacylglycerol (triglyceride): an ester of glycerol with three

fatty acids natural soaps are prepared by boiling triglycerides (animal

fats or vegetable oils) with NaOH, in a reaction called saponification (Latin, sapo, soap)

What are soaps?

Soaps form water-insoluble salts when used in water containing Ca(II), Mg(II), and Fe(III) ions (hard hard waterwater)

Results in characterisitc scum left on insides of bathtubs and sinks

What are phosphoacylglycerols? When one alcohol group of glycerol is esterified by a phosphoric When one alcohol group of glycerol is esterified by a phosphoric

acid rather than by a carboxylic acid, acid rather than by a carboxylic acid, phosphatidic acidphosphatidic acid producedproduced

PhosphoacylglycerolsPhosphoacylglycerols (phosphoglycerides) are the second most abundant group of naturally occurring lipids

Other important members

Phosphatidyl ethanolamine (cephalin), phosphatidyl serine, phosphatidyl choline (lecithin), phosphatidyl inositol, phosphatidyl glycerol and diphosphatidyl glycerol (cardiolipin)

What are Waxes? A complex mixture of esters of long-chain carboxylic

acids and long-chain alcohols Found as protective coatings for plants and animals

What are Sphingolipids? Contain sphingosine, a long-

chain amino alcohol sphingosine

Found in plants and animals

Abundant in nervous system

Bares structural similarity to phospholipids

What are Glycolipids? a compound in which a

carbohydrate is bound to an -OH of the lipid many glycolipids are

derived from ceramides

Gangliosides are Glycolipids with complex carbohydrate moiety that contains more than 3 sugars

What are Steroids? Steroids:Steroids: a group of

lipids that have fused-ring structure

- of 3 six-membered rings

- and 1 five-membered ring

Sex Hormones

Androgens:Androgens: male sex hormonessynthesized in the testes responsible for the development of male secondary

sex characteristicsTestosterone

Estrogens:Estrogens: female sex hormonessynthesized in the ovaries responsible for the development of female secondary

sex characteristics and control of the menstrual cycle

What is Cholesterol? Present in biological

membrane Does not occur in

prokaryotes Precursor of vitamin D3 Harmful - when present

in excess - Atherosclerosis

What is the structure of lipid bilayers?

The polar surface of the bilayer contains charged groups

The hydrophobic tails lie in the interior of the bilayer

How does the composition of bilayer affect its properties?

Both inner and outer layers of bilayer contain mixtures of lipids

Compositions on inside and outside of lipid bilayer can be different

How does the composition of bilayer affect its properties?

In saturated fatty acids, a linear arrangement of hydrocarbon chains leads to close packing of molecules in bilayer

Provides rigidity

How does the composition of bilayer affect its properties? (Unsaturated)

Kink in hydrocarbon chain

Causes disorder in packing against other chains

This disorder causes greater fluidity in membranes with cis-double bonds vs...... saturated FA chains

Biological Membranes Plant membranes have a higher percentage of unsaturated

fatty acids than animal membranes

The presence of cholesterol is characteristic of animal rather than plant membranes

Animal membranes are less fluid (more rigid) than plant membranes

The membranes of prokaryotes are fluid - contain no steroids

Temperature Transition in Lipid Bilayer

• With heat, the transition temperature is higher for more rigid membranes

- it is lower for less rigid membranes

Membrane Proteins

FunctionsFunctions: transport substances across membranes; act as receptor sites, and sites of enzyme catalysis

Peripheral proteins bound by electrostatic interactions can be removed by raising the ionic strength

Membrane Proteins

Integral proteins bound tightly to the interior of the membrane can be removed by treatment with detergents or

ultrasonification removal generally denatures them

Fluid Mosaic Model

FluidFluid: there is lateral motion of components in the membrane Proteins float in the membrane and can move along its

plane of membrane MosaicMosaic: components in the membrane exist side-by-side as

separate entities Forms a lipid bilayer with proteins, glycolipids, and

steroids such as cholesterol embedded in it no lipid-protein complexes, are formed

Fluid Mosaic Model of Membrane Structure

How does transport through membranes take place?

Passive transportPassive transport driven by a

concentration gradient (from higher to lower)

Simple diffusionSimple diffusion:: a molecule or ion moves through an opening

Facilitated diffusionFacilitated diffusion:: a molecule or ion is carried across a membrane by a carrier/channel protein

How does transport through membranes take place?

Active transport Active transport a substance is moved against a concentration

gradient Primary active transportPrimary active transport:: transport is linked to

the hydrolysis of ATP or other high-energy molecule; for example, the Na+/K+ ion pump (Figure 8.24)

Secondary active transportSecondary active transport:: driven by H+ gradient

How do membrane receptors work?

Membrane receptors

oligomeric proteins

binding of a biologically active substance to a receptor initiates an action within the cell

Lipid-Soluble Vitamins Vitamins are divided into two classes: lipid-soluble and water-

soluble

How are prostaglandins related to lipids?

Prostaglandins:Prostaglandins: a family of compounds that have the 20-carbon skeleton of prostanoic acid

First detected in seminal fluid…from prostate

The metabolic precursor is arachidonic acid (20 carbon atoms: 4 double bonds)

How are leukotrienes related to lipids?

Compounds also derived from arachidonic acid

Found in white blood cells (leukocytes)

Consists of 3 conjugated double bonds

Constriction of smooth muscles, especially in the lungs

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