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
This project is funded by a grant awarded under the President’s Community Based Job Training Grant as implemented by the U.S. Department of Labor’s Employment and Training Administration (CB-15-162-06-60). NCC is an equal opportunity employer and does not discriminate on the following basis:
against any individual in the United States, on the basis of race, color, religion, sex, national origin, age disability, political affiliation or belief; and
against any beneficiary of programs financially assisted under Title I of the Workforce Investment Act of 1998 (WIA), on the basis of the beneficiary’s citizenship/status as a lawfully admitted immigrant authorized to work in the United States, or his or her participation in any WIA Title I-financially assisted program or activity.
Disclaimer This workforce solution was funded by a grant awarded
under the President’s Community-Based Job Training Grants as implemented by the U.S. Department of Labor’s Employment and Training Administration. The solution was created by the grantee and does not necessarily reflect the official position of the U.S. Department of Labor. The Department of Labor makes no guarantees, warranties, or assurances of any kind, express or implied, with respect to such information, including any information on linked sites and including, but not limited to, accuracy of the information or its completeness, timeliness, usefulness, adequacy, continued availability, or ownership. This solution is copyrighted by the institution that created it. Internal use by an organization and/or personal use by an individual for non-commercial purposes is permissible. All other uses require the prior authorization of the copyright owner.