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Al - Iraqia university
Prof. Dr. Samia –Alshahwani
College of medicine
Y1 L7
. 12. 2015
Human Biology/ Molecules of life
(Lipids, proteins & nucleic acids)
 2.5 Lipids
Objectives: -Compare the structure of fats, phospholipids, & steroids.
-State the function of each lipid class.
Lipids are, diverse in structure & function, do not dissolve in water because of
absence of polar group, contain little oxygen & consist of carbon & hydrogen
atoms, contain more energy per gram than other molecules; therefore, fats & oils
function as energy-storage. Others (phospholipids) form cell membrane. Steroids
are large class of lipids that includes sex hormones.
Fats & Oils: Fat, is animal origin (e.g. butter), solid at room temperature. Oil, of
plant origin (e.g., corn oil & soybean oil), are liquid at room temperature. Fat uses:
 Long-term energy storage.
 Insulates against heat loss.
 Forms protective cushion around organs.
Steroids: Smaller lipid molecules function as chemical messengers. Emulsifiers:
cause fats to mix with water. They contain molecules with non polar & polar ends.
The molecules position themselves about oil droplet so that their polar ends project
outward. The droplet disperses in water, means emulsification occurred. Prior to
digestion of fatty foods, fats are emulsified by bile. Liver manufactures bile &
gallbladder store it. Fats & oils formed when one glycerol molecule reacts with 3 fatty
acid.(Fig. 2.16). A fat is called triglyceride, because it’s three-part structure, or term
neutral fat, because the molecule is non polar & carries no charges. Waxes are
molecules made of one fatty acid combined with another single organic molecule,
usually alcohol. Wax prevents loss of moisture. cerumen or ear wax is thick wax
produced by glands lining outer ear channel, it protects ear canal from irritation &
infection by trapping particles, bacteria & viruses when ear wax is washed by
swimming the result is painful swimmer ear.
Fig. 2.16 Structure of a triglyceride. Triglycerides formed when 3 fatty acids combine with
glycerol by dehydration synthesis reactions. Reverse reaction starts digestion of fat; hydrolysis
introduces water, & fatty acid–glycerol bonds are broken.
Saturated, Unsaturated, & Trans-Fatty Acids: A fatty acid is carbon–hydrogen chain
ends with acidic group —COOH (Fig. 2.16, left). Most of fatty acids contain 16 or 18
carbon atoms per molecule, smaller with fewer carbons known. Fatty acids are
saturated or unsaturated. Saturated fatty acids have no double bonds between
carbon atoms. The chain is saturated, so to speak, with all the hydrogen it can hold.
Unsaturated fatty acids have double bonds in carbon chain wherever number of
hydrogen less than two per carbon Fig2.17.Oils, present in cooking oils, are liquids at
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room temperature because of double bond creates bend in fatty acid chain. Such kinks
prevent close packing between hydrocarbon chains & account for oils fluidity;
saturated fats have no double bonds between carbons in fatty acid. Unsaturated
fats have one or more double bond.
mmmmore double bonds in fatty acid.
Fig. 2.17 Comparison of saturated, unsaturated, & trans fats.
For a fat to be trans, hydrogen need to be on opposite sides of carbon–carbon double
bond, butter contains saturated fatty acids & no double bonds, solid at room temp.
Saturated fats, contribute to disease atherosclerosis. Atherosclerosis caused by
formation of lesions, or atherosclerotic plaques, inside blood vessels. The plaques
narrow blood vessel diameter, choking off blood & oxygen supply to tissues.
Atherosclerosis is a cause of cardiovascular disease (heart attack & stroke). More
harmful than naturally occurring saturated fats are trans fats (Fig. 2.17), created
artificially using vegetable oils. Trans fats may be partially hydrogenated to make
them semisolid. Complete hydrogenation of oils causes all double bonds to become
saturated. Partial hydrogenation does not saturate all bonds. It reconfigures some double
bonds, but some of hydrogen atoms end up on different sides of the chain. Trans fats are
found in shortenings & solid margarines. in processed foods (snack foods, baked goods,
& fried foods). Dietary guidelines from American Heart Association (AHA) advise
replacing trans fats with unsaturated oils. In particular, monounsaturated oils (like olive
oil, with one double bond in carbon chain). Polyunsaturated oils (many double bonds in
the carbon chain) as corn oil, canola oil, & sunflower oil also fit AHA guidelines.
Dietary Fat:For health, the diet should include some fat. The total recommended fat in
a 2,000-calorie diet is 65 grams
The Omega-3 Fatty Acids: (n-3 fatty acids).Some fat is essential to health, omega-3
fatty acids, Three- lanoline acid (ALA), docosahexaenoic acid (DHA), & eicosapenaenc
acid (EPA). Omega-3 fatty acids are major component of fatty acids in the brain
adequate amounts of them are important for children & adults. It offers protection
against cardiovascular disease, DHA reduce risk of Alzheimer disease. DHA & EPA
manufactured from APA in small amounts within our bodies, best sources of omega-3
fatty acids salmon & sardines. Flax oil, called linseed oil, is excellent plant-based
source of omega-3 fatty acids, not overdo diet with excessive supplements as omega3s may cause health issues when taken in large doses.
Phospholipids: Have phosphate group (Fig. 2.19), constructed like fats, & except that in
place of third fatty acid, there is phosphate group contains both phosphate & nitrogen.
These molecules are not electrically neutral, as are fats, because phosphate & nitrogencontaining groups are ionized. They form polar (hydrophilic) head of molecule, & the
rest becomes nonpolar (hydrophobic) tails. Phospholipids are primary components of
cellular membranes. They spontaneously form a bilayer (a sort of molecular
“sandwich”) in which the hydrophilic heads (sandwich “bread”) face outward toward
watery solutions, & tails ( sandwich “filling”) form hydrophobic interior
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Steroids: Are lipids that have different structure from fats. Have a backbone of 4 fused
carbon rings. Each one differs by the attached molecules, called functional groups.
Cholesterol is a component animal cell’s plasma membrane & is the precursor of
several other steroids, such as sex hormones estrogen & testosterone. The liver makes
all cholesterol the body needs.
Figure 2.19 Structure of a phospholipid.
a- Phospholipids are structured like fats with one fatty acid is replaced by a polar phosphate group.
Therefore, the head is polar, whereas the tails are nonpolar.
b- This causes the molecules to arrange themselves in a “sandwich” arrangement when exposed to water—
polar phosphate groups on the outside of the layer, nonpolar lipid tails on the inside of the layer.
Dietary sources should be restricted because elevated levels of cholesterol, saturated
fats, & trans fats are linked to atherosclerosis, Male sex hormone, testosterone, is
formed in testes; female sex hormone, estrogen, is formed in ovaries, they differ by
functional groups attached to same carbon backbone.
Good and Bad Cholesterol
Blood tests to analyze lipid profile are part of annual medical exams, total cholesterol
need to be below 200, threshold of a healthy diet, LDLreferred to as “bad” cholesterol,
and HDL is “good” cholesterol, these are not forms of cholesterol; are types of
proteins. The lipoproteins in the body serve as a form of fat & cholesterol carrier,
moving these nutrients around as needed.LDL is lipoprotein that is full of triglycerides
& cholesterol, HDL is basically empty. Thus, a high LDL value indicates that carriers
were usually full, meaning that diet must be providing too many of these nutrients.
Other factors, as amount of dietary fiber, daily exercise, & genetics, play a role in
regulating “good” & “bad” levels of these lipoproteins. Concentrations in mg/dL).
2.6 Proteins; Important in cell structure & function. functions are:
Support: Structural proteins. Keratin, in hair & nails. Collagen support ligaments,
tendons, & skin.
Enzymes: Enzymes bring reactants together & thereby speed chemical reactions. They
are specific for one type of reaction & only function at body temperature.
Transport: Channel & carrier proteins in plasma membrane allow substances to enter
& exit cells. Other proteins transport molecules in blood. hemoglobin in red blood
cells is a complex protein that transports oxygen.
Defense: Antibodies are proteins, combine with foreign substances, antigens prevent
it from destroying cells & upsetting homeostasis.
Hormones: Regulatory proteins, serve as intercellular messengers that influence
metabolism. Insulin regulates the content of glucose in blood & in cells. Growth
hormone determines individual height.
Motion: The contractile proteins actin & myosin allow parts of cells to move & cause
muscles to contract. Muscle contraction facilitates movement from place to place.
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Amino Acids: Proteins are macromolecules with amino acid subunits. The central
carbon atom in an amino acid bonds to a hydrogen atom and 3 other groups of atoms.
named amino acid because one of these groups is an —NH2 (amino group) and another
is a —COOH (carboxyl group, an acid). The third group is the R group for amino acid:
Fig. 2.21 The structure of some amino acids.
Amino acids have amine group (H3N+), acid group (COO−), & R group, attached to central carbon
atom. R groups (screened in blue) are different. Some R groups are nonpolar & hydrophobic; others
are polar & hydrophilic. Still others are polar & ionized.
Amino acids differ according to their R group. R groups range from a single hydrogen
atom to a complicated ring compound. Some R groups are polar & some are not. Also,
amino acid cysteine ends with an —SH group, which often serves to connect one chain
of amino acids to another by a disulfide bond, —S—S—. Several amino acids
commonly found in cells are shown Fig.2.21.
Peptides : Two amino acids join by a dehydration reaction between carboxyl group of
one & amino group of another. Covalent bond between two amino acids is called a
peptide bond. When three or more amino acids linked by peptide bonds, polypeptide
result. The atoms associated with peptide bond share electrons unevenly because
oxygen attracts electrons more than nitrogen. Therefore, hydrogen attached to nitrogen
has a slightly (δ+), whereas oxygen has a slightly (δ−)
Proteins shape: Proteins cannot function unless they have a specific shape. When
proteins are exposed to extreme heat & pH, they undergo irreversible change in shape
called denaturation, e.g., addition of vinegar (an acid) to milk causes curdling, heating
causes coagulation of egg whites, which contain a protein called albumin.
Denaturation occurs because the normal bonding between R groups has been
disturbed. Once a protein loses its normal shape, it is no longer able to perform its
usual function. Change in protein shape is responsible for both Alzheimer disease
and Creutzfeldt–Jakob disease (the human form of mad cow disease).
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Levels of Protein Organization: Protein structure has at least 3 or 4 levels (Fig. 2.23).
The first level, primary structure, is linear sequence of amino acids joined by peptide
bonds. Each polypeptide has its own sequence of amino acids. The secondary structure
comes when polypeptide takes on a certain orientation in space. Once amino acids are
assembled into a polypeptide, the resulting C=O section between amino acids in the
chain is polar, having a partially negative charge. Hydrogen bonding is possible
between the C=O of one amino acid & N—H of another amino acid in a polypeptide.
chain coiling results in α (alpha) helix, or a right-handed spiral & folding of chain
results in a pleated sheet. Hydrogen bonding between peptide bonds holds the shape in
place. The tertiary structure of a protein is its final, three dimensional shapes. In
enzymes, polypeptide bends & twists in different ways. hydrophobic portions are
packed on inside & hydrophilic portions on outside make contact with water. Tertiary
structure of enzymes determines what types of molecules with which they will interact.
The tertiary shape of polypeptide maintained by various types of bonding between R
groups; covalent, ionic, & hydrogen bonding all occur.
Fig. 2.23 Levels of protein structure. structure of proteins differ significantly. Primary structure,
sequence of amino acids, determines secondary & tertiary structure. Quaternary structure is
created by assembling smaller proteins into a large structure.
Some proteins have one polypeptide, others have more than one, each with its own
primary, secondary, and tertiary structures. These separate polypeptides are arranged
to give proteins a fourth level of structure, termed the quaternary structure.
Hemoglobin is a complex protein having a quaternary structure; many enzymes
also have quaternary structure. Each of 4 polypeptides in hemoglobin is associated
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with a nonprotein heme group. heme group contains iron (Fe) atom that binds to
oxygen; in that way, hemoglobin transports O2 to tissues.
2.7.Nucleic Acids. 2 types DNA (deoxyribonucleic acid) & RNA (ribonucleic acid)
(Fig. 2.24), called nucleic acids because first detected in nucleus. DNA structure
discovery has influence on biology. DNA stores genetic information in cell & organism.
DNA replicates & transmit information when each cell & each organism—reproduces.
Researchers are beginning to understand how genes function & are working on ways to
manipulate them. Biotechnology is devoted to altering genes.
Function of DNA & RNA: DNA molecule contains many genes, & genes specify
sequence of amino acids in proteins. RNA is intermediary that conveys DNA’s
instructions regarding amino acid sequence in a protein. If DNA’s information is faulty,
illness results. Relation between a gene, a protein, & illness illustrated by sickle-cell
disease, red blood cells are sickle-shaped it occur because in hemoglobin molecule,
amino acid valine substitutes for an amino acid glutamine
Fig.2.24 The structure of DNA & RNA.
a- In DNA, adenine & thymine are a complementary base pair. hydrogen bonds join them (like
the “steps” in a spiral staircase). Likewise, guanine and cytosine pair.
b- RNA has uracil instead of thymine, so complementary base pairing isn’t possible
Exchanging one amino acid for another—a seemingly small change—makes red blood
cells lose their normal round, flexible shape & become weak & easily torn. Effects on
health result,when abnormal red blood cells go through small blood vessels, they clog
blood flow & break apart. Sickle-cell disease is another cause of anemia, & it results in
pain & organ damage.
Structure of DNA and RNA: DNA & RNA are polymers of nucleotide, there are
differences in types of subunits ,differences give DNA & RNA unique functions.
Nucleotide Structure : Complex of 3 types of subunit molecules—phosphate
(phosphoric acid), a pentose (5-carbon) sugar, & nitrogen-containing base:
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Nucleotides in DNA contain sugar deoxyribose, & nucleotides in RNA contain sugar
ribose; this difference accounts for their respective names (Table 2.1). There are 4 types
of bases in DNA: adenine (A), thymine (T), guanine (G), and cytosine (C)
The base can have 2 rings (adenine or guanine) or one ring (thymine or cytosine). In
RNA, the base uracil (U) replaces base thymine. These structures are called bases
because their presence raises the pH of a solution.
Polynucleotide Structure: Nucleotides link to make a polynucleotide, called a strand,
which has a backbone made up of phosphate–sugar–phosphate–sugar. The bases project
to one side of the backbone. The nucleotides of a gene occur in a definite order, & so do
the bases. Sequence of bases in DNA is known, human genome. Leading to improved
genetic counseling, gene therapy, & medicines to treat causes of many illnesses. DNA
is double-stranded, with two strands twisted about each other in form of double helix
Fig. 2.24a. In DNA, two strands are held together by hydrogen bonds between bases.
When coiled, DNA resembles a spiral staircase. When unwound, resembles stepladder.
The uprights (sides) of the ladder are made of phosphate & sugar molecules, & the
rungs of the ladder are made only of complementary paired bases. Thymine (T ) pairs
with adenine (A), & guanine (G) pairs with cytosine (C). Complementary bases have
shapes that fit together. Complementary base pairing allows DNA to replicate in a way
that ensures that the sequence of bases will remain the same. This is important because
it is the sequence of bases that determines the sequence of amino acids in a protein.
RNA is single-stranded. When RNA forms, complementary base pairing with one DNA
strand passes the correct sequence of bases to RNA (Fig. 2.24b). RNA is nucleic acid
directly involved in protein synthesis.
ATP: An Energy Carrier: In addition to being subunits of nucleic acids, nucleotides
have metabolic functions. When adenosine (adenine plus ribose) is modified by the
addition of three phosphate groups instead of one, it becomes ATP (adenosine
triphosphate), which is an energy carrier in cells.
ATP Structure Suits Its Function: ATP is high-energy molecule because the last two
phosphate bonds are unstable & easily broken. Last phosphate bond is hydrolyzed,
leaving molecule ADP (adenosine diphosphate) & & a molecule of inorganic
Fig. 2.25 ATP is the universal energy currency of cells. (Called a riphosphate). When cells need energy, ATP is
hydrolyzed forming ADP & P . Energy released. To recycle ATP, energy from food is required & reverse reaction occurs:
ADP & P join to form ATP, & water is given off
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phosphate P (Fig. 2.25). energy released by TP breakdown used to synthesize
macromolecules, as carbohydrates & proteins. In muscle, energy used for muscle
contraction; & in nerve cells, used for conduction of nerve impulses. After ATP
breaks down, it recycled by adding P to ADP. Fig. 2.25 input of energy is required to reform ATP.
Glucose Breakdown Leads to ATP Buildup; Glucose contains energy used as direct
energy source in cellular reactions. Instead, energy of glucose is converted to that of
ATP molecules. ATP contains an amount of energy that makes it usable to supply
energy for chemical reactions. Muscles use ATP energy & produce heat when they
contract. This is the heat that warms the body. Insufficient oxygen limits glucose
breakdown and limits ATP buildup.
Summary
The 4 categories of organic molecules, examples, monomers &functions, are shown
below:
Monomer molecule; Bind chemically to other molecule to form a polymer.
Polymer; Macro (large) molecule composed of many repeated subunits.
Thank you
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Samia
2015