PRT3402- Agricultural Biochemistry
PJJ UPM / UPMET
UNIT 4
Structure and function of lipids
Introduction to Unit
In this unit lipids are introduced as another important biomolecules. Lipids are the
third major class of biomolecules. Unlike protein and carbohydrates, lipids are not
polymers but have varied structures yet still essential component of living organisms.
Lipids are water insoluble compounds in biological systems. They form the largest
fraction in the cells as part of the membrane structures separating and dividing
compartments from one another and the surroundings. Fats which are lipids also
function as energy storage molecules and provide heat insulation.
Learning Outcomes
At the end of this unit the students will be able to:
1. Describe the chemistry of lipids and fatty acids.
2. Describe and understand the structure and properties of different categories
of lipids including triacylglycerides, phospholipids, waxes, terpenes, steroids
and sphingolipids
3. Describe and understand the structure and properties of bilipid biological
membrane and transport across membranes
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TOPIC 1: STRUCTURE AND CHEMICAL PROPERTIES OF FATTY ACIDS AND
ITS DERIVATIVES
Main Points
1.1
Lipids are compounds which have low solubility in water highly soluble in nonpolar solvents. Lipids are either hydrophobic (containing only nonpolar
groups) or amphipathic (contain both polar and nonpolar groups). Oxidation of
lipids will yield large amount of energy.
1.2
Unlike proteins, nucleic acids and polysaccharides, lipids are not polymers.
They are quite small molecules that have a strong tendency to associate
through non-covalent forces.
1.3
Lipids carry out multiple functions in plant and animal cells. Some lipids are
fats which are used for energy storage and heat insulation. The largest
fraction of lipid in most cells is used to form membranes – the hydrophobic
nature acts as barrier to polar molecules. Some lipids function as vitamins
(e.g. vitamin D, vitamin E) and others are hormones including steroid
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hormones and prostaglandins Lipids also act as signalling molecules within
the tissues and organs.
1.4
Some major catogeries of lipids include fats / oils, phospholipids, waxes,
terpenes, steroids and prostaglandins.
1.5
Fatty acids are composed of long hydrocarbon chain (tail) and a terminal
carboxyl group (head).Polar head and nonpolar tail readily form micelles in
water. Most fatty acids found in nature have even number carbon atoms (1424). The saturated fatty acids are single bond of carbon-carbon while the
unsaturated fatty acids have one or more double bond in hydrocarbon chains.
1.6
Micelles are aggregate of molecules dispersed in a liquid with the hydrophilic
"head" regions in contact with surrounding solvent, while the hydrophobic
single tail regions are in the micelle centre.
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PRT3402- Agricultural Biochemistry
1.7
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Fatty acid can be monounsaturated i.e. fatty acids with no double bond or
polyunsaturated - fatty acids with one or more than one double bond.
No. of carbon
Common name
Systematic name
12
14
16
18
Saturated
Lauric acid
Myristic
Palmitic
Stearic
Dodecanoic acid
Tetradecanoic acid
Hexadecanoic acid
Octadecanoic acid
16
18
18
Non-saturated
Palmitoleic
Oleic
Linoleic
9-Hexadecenoic acid
9-Octadecenoic acid
9,12-Octadecenoic acid
1.8
Fatty acids are important metabolic fuels of cells and the oxidation of fatty
acid yields more energy (~37 kJ/g) than oxidation of proteins or carbohydrates
(~16 kJ/g each). Fatty acids in plants and animals exist in the form of
triacylglycerols (triglycerides).
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TOPIC 2: BIOLOGICAL MEMBRANES
Main Points
2.1
Biological Membranes define the external boundaries of cells and
compartments in cells and they have a wide variety of complex functions. The
membranes are composed of proteins embedded in or associated with lipid
bilayer. The protein and lipid composition is also widely variable.
2.2
Lipid Bilayers consist of amphipatic molecules i.e having both hydrophobic
and hydrophilic properties such as detergents, glycerophospholipids and
glycoshingolipids and thus can form bilayers. The layers are typically about 5
to 6 nm havind two leaflets. Structurally the polar heads of amphipatic
molecules are in contact with the aqueous medium and the hydrophobic
groups hidden in the interior.
2.3
A model known as Fluid Mosaic Model was proposed by Singer and
Nicholson in 1972. This model proposed that the membrane is a dynamic
structure in which proteins and lipids can move laterally or transversely (flipflop). The range of lipid is about 25% to 50% while protein is about 50 to 75%.
Different membranes will have different composition of lipid and protein myelin membrane contains very little protein while the inner mitochondrial
membrane is rich in proteins.
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2.4 The movement of protein and lipids within the membranes is indicated below.
2.5
Freeze-fracture electron microscopy showed that the bilayer can be split into
the leaflets. This technique showed that the distribution of proteins is asymmetric
across the membrane and the phase transition temperature varies based on the
composition of bilayers. A pure phospholipid bilayer will have a sharp temperature
range of transition fron gel to liquid crystalline phase, whereas a bilayer containing
80% phosphlipid and 20% cholesterol will have a broader temperature range as
indicated below.
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2.6
PJJ UPM / UPMET
Membrane is a barrier to polar molecules and only allows hydrophobic
molecules and small molecules to pass through. The traffic of polar molecules
controlled by 3 types of integral proteins: 1) Channels and pores, 2) Passive
transporters and 3) Active transporters. The table below indicates the characteristics
of different membrane transport mode.
Transport
mode
Protein carrier
Saturable with Movement
Energy
input
substrate
relative
to required
concentration
gradient
Simple diffusion
No
No
Down
No
Channel
pores
Yes
No
Down
No
Yes
Yes
Down
No
Primary
Yes
Yes
Up
Secondary
Yes
Yes
Up
Yes (direct
source
and
Passive
transport
Active transport
Yes (ion
gradient)
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2.7
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Membrane pore or channels will have a central passage by which molecules
will pass through but only the molecules or ions of the right size, charge and
geometry will pass through. Movement can be of both directions. The diagram below
showed the differences between different types of active transport but bear in mind
that the passage is not a straightforward movement through the pore but involves
conformational changes of the transporters during transportation.
Uniport
Symport
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Antiport
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TOPIC 3: STRUCTURE AND ROLE OF TRIGLYCERIDES AND PHOSPHOLIPIDS
Main Points
3.1
Triacylglycerols are formed from glycerol and fatty acids. Triacylglycerols are
also known as Triglycerides (or fats) are the main constituents of vegetable
oils and animal fats. Triacylglycerols – rich in reduced carbon, during
metabolism or oxidative reactions yield high amount of energy. Fats are used
to store energy.
3.2
When three fatty acids are esterified to glycerol, the resulting molecule is a fat
(butter) (if solid at room temperature) or oil (if liquid at room temperature).
Fats rich in unsaturated fatty acids (like olive oil) are liquids at room
temperature but those with higher content of saturated fatty acids (like butter)
are more solid.
3.3
The reason is long saturated chains can pack closely together, to form
regular, semi-crystalline structures. The bend impose by one or more cis
double bonds in unsaturated fatty acids make molecular packing more
difficult.
3.4
Glycerol (or Glycerin) is a trihydric alcohol. It contains three -hydroxyl groups
which can combine with fatty acids to form:
a.
Monoglycerides (with one fatty acid)
b.
Diglycerides (with two fatty acids)
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c.
3.5
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Triglycerides (with three fatty acids)
The reaction between acids (fatty acids) + alcohols (glycerol) release water
(H2O) as by-product.
If
all three fatty acids
are
the
same
e.g.
Tristearoylglycerol (common name tristearin) will be formed.
1.6
Most vegetable oils such as olive oil, palm oil and maize and also dairy
products and animal fat contain triacylglycerols. Vertebrates and human have
specialized cells known as adipocytes or fat cells for storing triacylglycerol as
stored energy. Oxidation of triaclyglycerols will yield twice the amount of
energy compared with oxidation of starch. In some animals living in almost
permanent arctic conditions have triacylglycerols stored under the skin for
insulation as in seals and penguins.
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PRT3402- Agricultural Biochemistry
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TOPIC 4: STRUCTURE AND ROLE OF SPHINGOLIPIDS, WAX, TERPENES
AND STEROIDS
4.1
Waxes are organic compounds, produced by plants and animals. They are
long chain fatty acid esterified to a long chain alcohol. This results in a head
group that is only weakly hydrophilic, attached to two hydrocarbon chains. As
a consequence, the wax is completely water insoluble.
4.2
They serve as water repellent as in birds’ feathers and on some leaves. In
some marine microorganisms waxes are used for energy storage. As with
triacylglycerols, the hardness of waxes increased with chain length and
degree of hydrocarbon saturation.
4.3
Many tropical plants are coated with thick layer of waxes to prevent the loss of
water and for protection. Waxes are also used for making lotions, polishes as
part of consumers’ goods.
4.4
Lipids are also an essential structural component of biological membranes.
Membrane consists of double layer of lipids also known as membrane
bilayers. Membrane lipids have amphipathic properties i.e. one end is
hydrophobic while the other end is hydrophilic. It acts as a barrier to passage
of polar molecules and ions.
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Lipids bilayers: hydrophilic head and two hydrophobic tails
4.5
There are many types of membrane lipids and these are listed below:
a) Glycerophospholipids - also known as phosphoglycerides. It has two fatty
acids attached to 1st and 2nd carbon of glycerol. A highly polar or charged
group attached to 3rd carbon through phosphodiester linkages.
b) Galactolipids: - membrane consisting of lipids and galactose residue.
Mainly localised in the thylakoid membrane of protoplast. It may have a
role in photosynthesis. A general chemical structure of a galactolipid
(monogalactosyl diacylglycerol - MGDG), is shown below. R1 and R2 are
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fatty chains. There is some evidence that galactolipid from plant extracts
acan act as an inhibitory agent of Brugia malayi, a human lymphatic filarial
parasite.
c) Sulpholipids are sulfonated glucose residue joined to diacylglycerol. In
other words it is a class of lipid with sulfur functional group.
d) Sphingolipids consists of no glycerol. It contain one long chain of amino
alcohol called sphingosine, one molecule of long chain fatty acid
and a polar head group. Plays an important role in signal transmission
and cell recognition. The second most abundant lipid in plant and animal
membranes and also abundant in tissues of the Central Nervous System.
•
3 major families of sphingolipids
1. Sphingomyelins (contain phosphates -phospholipids) present in
plasma membrane of mammalian cells and myelin sheaths
surrounding nerve cells
2. Cerebrosides (contain carbohydrate – glycosphingolipids) present
in nerve tissues especially myelin sheath
3. Gangliosides (contain carbohydrate – glycosphingolipids) – present
on cell surfaces (ABO blood grouping)
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e) Glycosphingolipids contain one or more sugars connected to the
ceramide. Galactosylceramide is found in the brain tissue of vertebrate
and glucosylceramide is found as a major component in the outer layer of
plasma membrane of plants.
4.6
Terpenes are another class of lipid formed from molecules of 2 methyl-1,3
butadiene, known as isoprene (5 carbon units). It has the general molecular
formula of (C5H8)n. They are produced by many plants such as conifers,
flowering plants and some insects like termites. Terpenes are also
components of essential oils which are used toattract pollinating insects.
Some examples of terpenes include:
 Monoterpene (C10, H16)- Consists of 2 isoprene units
 Sesquiterpene (C15, H24)- Consists of 3 isoprene units
 Diterpene (C20, H32) – Consists of 4 isoprene units
 Tetraterpene (C40, H64) – Consists of 8 isoprene units
 Polyterpene - Many units of isoprene e.g. in rubber latex
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PRT3402- Agricultural Biochemistry
4.7
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Steroid is another member of the lipid class. It consists of 3 six membered
ring and 1 five membered ring fused together. Cholesterol is a common
steroid in animal and also a principle component of animal cell plasma
membrane. Steroid hormones are derived from cholesterol. Examples of
these hormones include estrogens, a female sex hormone and androgens
which stimulate the male characteristics of human and animals.
4.7
Prostaglandin belongs to the group of eicosanoids acids, consisting of 20
carbon atoms, including a 5 member ring. They are considered as locally
acting hormones but the actions of a given prostaglandins varies in different
tissues. Some of the effects that have been recorded include lowering of
blood pressure, affect clotting of blood, lowers gastric secretions and promote
inflammation causing pain (but can countered by aspirin, which inhibits
prostaglandin biosynthesis).
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