Date post: | 22-Dec-2015 |
Category: |
Documents |
View: | 251 times |
Download: | 1 times |
2
Lipids
Lipids are• biomolecules that contain fatty acids or a steroid
nucleus.• soluble in organic solvents, but not in water.• named for the Greek word lipos, which means
“fat.”• extracted from cells using organic solvents.
3
Types of Lipids
The types of lipids containing fatty acids are• waxes• fats and oils (triacylglycerols)• glycerophospholipids• prostaglandins
The types of lipids that do not contain fatty acids are• steroids
5
Fatty Acids
Fatty acids are• long-chain carboxylic acids.• typically 12-18 carbon atoms.• insoluble in water.• saturated or unsaturated.
Olive oil contains 84% unsaturated fatty acids and 16% saturated fatty acids.
6
Saturated and Unsaturated Fatty Acids
Fatty acids are• saturated with
all single C–C bonds.
• unsaturated with one or more double C=C bonds.
palmitic acid, a saturated acid
palmitoleic acid, an unsaturated acid
O
C OH
O
C OH
7
Properties of Saturated Fatty Acids
Saturated fatty acids
• contain only single C–C bonds.
• are closely packed.
• have strong attractions between chains.
• have high melting points.
• are solids at room temperature.
COOHCOOHCOOH
8
Properties of Unsaturated Fatty Acids
Unsaturated fatty acids• contain one or more cis double C=C
bonds.• have “kinks” in the fatty acid chains.• do not pack closely. • have few attractions between chains.• have low melting points.• are liquids at room temperature.
COOHH
H CC
HOOC
H
H
C
C
“kinks” in chain
11
Learning Check
Assign the melting points of –17 °C, 13 °C, and 69 °C to the correct fatty acid. Explain.
stearic acid (18 C) saturated
oleic acid (18 C) one double bond
linoleic acid (18 C) two double bonds
12
Solution
Stearic acid is saturated and would have a higher melting point than the unsaturated fatty acids. Because linoleic has two double bonds, it would have a lower mp than oleic acid, which has one double bond.
stearic acid mp 69 °C saturated
oleic acid mp 13 °C
linoleic acid mp -17 °C most unsaturated
14
Waxes
Waxes are• esters of saturated fatty acids and long-chain alcohols.• coatings that prevent loss of water by leaves of plants.
(pronounced “ho-HO bah”
C O
O
15
Fats and Oils: Triacylglycerols or Triglycerides
Fats and oils are• also called triacylglycerols.• esters of glycerol.• produced by esterification.• Formed when the hydroxyl
groups of glycerol react with the carboxyl groups of fatty acids.
16
In a triacylglycerol, glycerol forms ester bonds with three fatty acids.
Triacylglycerols (Triglycerides)
Ester Bonds
17
Formation of a Triacylglycerol
Glycerol + three fatty acids triacylglycerol
OHCH2
OH
OHCH2
CHO
(CH2)14CH3CHO
O
(CH2)14CH3CHO
O
(CH2)14CH3CHO
+ 3H2O
O
O
C (CH2)14CH3
CH O
O
C (CH2)14CH3
CH2 O
O
C (CH2)14CH3
CH2
+
18
Melting Points of Fats and Oils
A fat • is usually solid at room temperature.• is prevalent in meats, whole milk, butter, and cheese.
An oil• is usually liquid at room temperature.• is prevalent in plants such as olive and safflower.
19
Oils with Unsaturated Fatty Acids
Oils• have more unsaturated fats.• have cis double bonds that cause “kinks” in the fatty
acid chains.• with “kinks” in the chains do not allow the
triacylglycerol molecules to pack closely.• have lower melting points than saturated fatty acids.• are liquids at room temperature.
20
Diagram of Triacylglycerol with Unsaturated Fatty Acids
Unsaturated fatty acid chains with kinks cannot pack closely.
23
The chemical reactions of triacylglycerols are similarto those of alkenes and esters.
• In hydrogenation, double bonds in unsaturated fatty acids react with H2 in the presence of a Ni or Pt catalyst.
• In hydrolysis, ester bonds are split by water in the presence of an acid, a base, or an enzyme.
Chemical Properties of Triacylglycerols (Triglycerides)
24
Hydrogenation of Oils
The hydrogenation of oils
• adds hydrogen (H2) to the carbon atoms of double bonds.• converts double bonds to single bonds. • increases the melting
point.• produces solids, such as
margarine and shortening.
25
Hydrogenation
Ni + 3H2
Glyceryl tripalmitoleate
(tripalmitolean)
Glyceryl tripalmitate
(tripalmitin)
O
(CH2)14CH3C
O
(CH2)14CH3C
O
(CH2)14CH3C
O
O
OCH2
CH2
CHCH(CH2)7CH3(CH2)5CH
O
C
CH(CH2)7CH3(CH2)5CH
O
C
CH(CH2)7CH3(CH2)5CH
O
C
O
O
OCH2
CH2
CH
26
OlestraTM, a Fat Substitute
Olestra is• used in foods as an artificial fat.• sucrose linked by ester bonds to several long-chain fatty
chains.• not broken down in the intestinal tract.
27
Cis and Trans Fatty Acids
Unsaturated fatty acids can be • cis with bulky groups on same side of C=C.CH3─ (CH2)5 (CH2)7─ COOH cis
C=C H H
• trans have bulky groups on opposite sides of C=C. CH3─ (CH2)5 H
C=C trans
H (CH2)7─ COOH
28
Hydrogenation and Trans Fatty Acids
Most naturally occurring fatty acids have cis doublebonds.• During hydrogenation, some cis double bonds are converted to
trans double bonds.
• In the body, trans fatty acids behave like saturated fatty acids.
• It is estimated that 2-4% of our total Calories is in the form of trans fatty acid.
• Several studies reported that trans fatty acids raise LDL-cholesterol and lower HDL-cholesterol.
30
Learning Check
(1) True or (2) false:
A. There are more unsaturated fats in vegetable oils.
B. Vegetable oils have higher melting points than fats.
C. Hydrogenation of oils converts some cis-double bonds to trans-double bonds.
D. Animal fats have more saturated fats.
31
Solution
(1) True or (2) false:
A. T There are more unsaturated fats in vegetable oils.
B. F Vegetable oils have higher melting points than fats.
C. T Hydrogenation of oils converts some cis-double bonds to trans- double bonds.
D. T Animal fats have more saturated fats.
32
Hydrolysis
In hydrolysis, • triacylglycerols split into glycerol and three fatty acids.• an acid or enzyme catalyst is required.
OCH2
OCH
OCH2
OHCH2
OHCH
OHCH2
O
(CH2)14CH3CHO
H2OO
(CH2)14CH3C
O
(CH2)14CH3C
O
(CH2)14CH3C
H+
+
+
33
Saponification and Soap
Saponification• is the reaction of a fat with a strong base.• splits triacylglycerols into glycerol and the salts of fatty
acids. • is the process of forming “soaps” (salts of fatty acids).• with KOH gives softer soaps.
34
Saponification
O
O
C (CH2)14CH3
CH O
O
C (CH2)14CH3
CH2 O
O
C (CH2)14CH3
CH2
+ 3NaOH
Na+ -O
O
C (CH2)14CH33
OH
CH OH
CH2 OH
CH2
+
“soap”
35
Learning Check
Write the product of the following reaction:
OCH2
O
O
CH2
CH
CH(CH2)7CH3(CH2)5CH
O
CO
CH(CH2)7CH3(CH2)5CHCO
C CH(CH2)7CH3(CH2)5CH
H2
Ni+ 3
38
Glycerophospholipids are
• the most abundant lipids in cell membranes.
• composed of glycerol, two fatty acids, phosphate, and an amino alcohol.
Glycerol
PO4Amino alcohol
Glycerophospholipids
Fatty acid
Fatty acid
39
Polarity of Glycerophospholipids
A glycerophospholipid has
• two nonpolar fatty acid chains.
• a phosphate group and a polar amino alcohol.
CH3
│+ +
HO−CH2−CH2−N−CH3 HO−CH2−CH2−NH3
│ Choline CH3 Ethanolamine +
NH3
│ Amino alcohols HO−CH2−CH−COO− Serine
41
Lecithin and cephalin are glycerophospholipids• abundant in brain and nerve tissues.• found in egg yolk, wheat germ, and yeast.
Lecithin and Cephalin
43
Steroid Nucleus
A steroid nucleus consists of
• 3 cyclohexane rings.
• 1 cyclopentane ring.
• no fatty acids.
steroid nucleus
44
Cholesterol
Cholesterol• is the most abundant steroid in the body.
• has methyl CH3- groups, alkyl chain, and -OH attached to the steroid nucleus.
CH3
CH3CH3
CH3
HO
CH3
45
Cholesterol in the Body
Cholesterol • is obtained from meats, milk, and
eggs. • is synthesized in the liver.• is needed for cell membranes,
brain and nerve tissue, steroid hormones, and vitamin D.
• clogs arteries when high levels form plaque.
An artery clogged by cholesterol plaque
A normal, open artery.
46
Cholesterol in Foods
Cholesterol• is considered
elevated if plasma cholesterol exceeds 200 mg/dL.
• is synthesized in the liver and obtained from foods.
47
Learning Check
Match the components of the cholesterol molecule with
the following:
___ carbon chain ___hydroxyl group
___ steroid nucleus ___methyl group
CH3
CH3CH3
CH3
HO
CH3
AC
B
D
48
Solution
Match the components of the cholesterol molecule with
the following:
_D carbon chain _A_hydroxyl group
_C steroid nucleus _B_methyl group
CH3
CH3CH3
CH3
HO
CH3
AC
B
D
49
Lipoproteins
Lipoproteins• combine lipids
with proteins and phospholipids.
• are soluble in water because the surface consists of polar lipids.
50
Types of Lipoproteins
Lipoproteins• differ in density, composition, and function.• include low-density lipoproteins (LDLs) and high-
density lipoproteins (HDLs).
51
Steroid Hormones
Steroid hormones are• chemical messengers in cells.• sex hormones.
- androgens in males (testosterone)
- estrogens in females (estradiol)• Adrenocortical hormones from adrenal glands.
- mineralocorticoids (electrolyte balance)
- glucocorticoids (regulate glucose level)
52
Steroid Hormones
Steroid hormones• are produced from
cholesterol.
• include sex hormones such as androgens (testosterone) in males and estrogens (estradiol) in females.
53
Adrenal Corticosteroids
Steroid hormones called adrenal corticosteroids • are produced by the adrenal glands located on the top
of each kidney.• include aldosterone, which regulates electrolytes and
water balance by the kidneys.• include cortisone, a glucocorticoid, which increases
blood glucose level and stimulates the synthesis of glycogen in the liver.
54
Anabolic Steroids
Anabolic steroids• are derivatives of testosterone.• are used illegally to increase muscle mass.• have side effects including fluid retention, hair growth,
sleep disturbance, and liver damage.
56
Learning Check
Identify each lipid as a(1) fatty acid, (2) steroid, or (3) triacylglycerol.
A. cholesterolB. stearic acidC. glyceryl tristearateD. contains no fatty acids
57
Solution
Identify each lipid as a
(1) fatty acid, (2) steroid, or (3) triacylglycerol.
A. 2 cholesterol
B. 1 stearic acid
C. 3 glyceryl tristearate
D. 2 contains no fatty acids
59
Cell Membranes
Cell membranes • separate cellular contents from the external
environment.
• consist of a lipid bilayer made of two rows of phospholipids.
• have an inner portion made of the nonpolar tails of phospholipids with the polar heads at the outer and inner surfaces.
60
Fluid Mosaic Model of Cell Membranes
The lipid bilayer
• contains proteins, carbohydrates, and cholesterol.
• has unsaturated fatty acids that make cell membranes fluid-like rather than rigid.
• has proteins and carbohydrates on the surface that communicate with hormones and neurotransmitters.
62
Transport Through Cell Membranes
The transport of substances through cell membranes
involves
• simple diffusion (passive transport), which moves particles from a higher to a lower concentration.
• facilitated transport, which uses protein channels to increase the rate of diffusion.
• active transport, which moves ions against a concentration gradient.
64
Learning Check
The transport of particles across a cell membrane from
high concentration to low concentration is called
1. facilitated transport.
2. simple diffusion.
3. active transport.