Structure classification and function of lipids
•Lipids are made of the elements Carbon, Hydrogen and Oxygen, but have
a much lower proportion of water than other molecules such as
carbohydrates.
•Unlike polysaccharides and proteins, lipids are not polymers they lack a
repeating momomeric unit.
•They are made from two molecules: Glycerol and Fatty Acids.
•Fatty acids consist of an acid group at one end of the molecule and a
hydrocarbon chain, which is usually denoted by the letter ‘R’.
•Fatty acids may be saturated or unsaturated. A fatty acid is saturated if
every possible bond is made with a Hydrogen atom, such that there exist
no C=C bonds (no carbon to carbon double bonds).
•Unsaturated fatty acids on the other hand do contain C=C bonds.
Monounsaturated fatty acids have one C=C bond, and polyunsaturated
have more than one C=C bond.
•The chemical and physical properties of the different fatty acids, such as
their solubility in nonpolar solvents and the melting point, depend on the
number of carbon atoms of the molecule (the size of R)
When fatty acids are classified on the basis of the number of
carbon atoms in the molecule four classes are formed:
•4 to10 carbon atoms are referred as short-chain fatty acids,
•12 to 14 atoms as medium-chain fatty acids
•16 to18 carbon atoms are long-chain fatty acids.
•20 or more carbons are very long-chain fatty acids.
Structure of Triglycerides
•Triglycerides are lipids consisting of one glycerol
molecule bonded with three fatty acid molecules.
•They are also known as glycerolipids or triacylglycoerol
•The bonds between the molecules are covalent and are
called Ester bonds.
•They are formed during acondensation reaction.
•Triglycerides are insoluble in water.
Classification of lipids
Lipids can be classified according to their hydrolysis products and according to
similarities in their molecular structures. Three major subclasses are recognized:
simple lipids, compound lipids and sterols
Simple lipids
Fats and oils which yield fatty acids and glycerol upon hydrolysis.
(b) Waxes, which yield fatty acids and long-chain alcohols upon hydrolysis.
Fats and Oils
•These are called triacylglycerols because they are esters composed of three fatty
acids joined to glycerol.
•Those that are solid state at room temperature is called a fat these are mostly
found in animals.
•Lipids that are liquid at room temperature are called oils. These are mainly
found in plants
•These differences in melting points reflect differences in the degree of
unsaturation of the constituent fatty acids.
•Waxes are lipids with long carbon chains.In
nature, they serve mostly as protective and
structural coatings.
•Some plants have wax on the outside of their
leaves, giving them ashiny appearance.The wax
helps cut down on the evaporation of water.
•AWax is an ester of long-chain alcohol and a
fatty acid.The acids and alcohols normally
found in waxes have chains of the order of 12-34
carbon atoms in length.
b. Waxes
Compound lipids
phospholipids
•These lipids are like triglycerides, but they have two fatty acid chains
called “tails” and one charged group called the “head” that contains
phosphate and oxygen atoms.Because it is charged, the head is polar and
therefore attracts water molecules.
•The long fatty acid tail is nonpolar and does not attract water molecules.
The polar and nonpolar parts of phospholipids allow them to form lipid
bilayers.
•The bilayer forms when the phospholipid molecules arrange themselves in
two layers with the tails facing in (facing each other) and the heads facing
out.The formed phospholipid bilayer has the tails buried inside and the
polar atoms of the heads facing out, where they can form Hbonds with
water and other molecules.
•The membranes of cells are composed of phospholipid bilayers through
which small molecules like water and oxygen can pass.For bigger
molecules, there are large proteins in the membranes that serve as channels
for the transport of substances in and out of cells.
•An important feature of cell membranes is that they are semi-permeable,
which means that some substances can pass through them, but others
cannot.This way, acell can control what it needs to allow in (nutrients,
oxygen, water) and out (waste products from reactions).
———
Phosphate
Glycerol
Hydrophilic
head
Saturated
fatty
acid
Unsaturated
fatty
acid
Hydrophobic
tails
Sterols
•These are also amphiphilic (molecule having both
hydrophilic and hydrophobic parts), the hydroxyl
groups form the hydrophilic head and the side chain
serves as the hydrophobic tail.
•Plants contain large variety sterols as membrane
constituents.They are contained primarily in the
outer membrane of mitochondria, in the membranes
of the endoplasmic reticulum, and in the plasma
membrane.
•Sterols determine to alarge extent the properties of
these membranes.
•The insertion of sterol molecules into the membrane
increases the structural order of the hydrophobic
region
Glycolipids
•These are membrane lipids with short-chain
carbohydrates attached to them.
•They are essential components cell membranes.
•They play arole in the regulation of cellular
interactions, growth, and development.
•Glycolipids resemble phospholipids.The molecule is
amphipathic
•The polar is agalactosyl derivative of adiglyceride,
the non-polar part of the molecule being apair of
long, straight-chain fatty acids.
•They are characteristic of photosynthetic tissues since
they are the major lipid component of chloroplast
lamellae, largely replacing the phospholipids.
Sphingolipids
•These are structural elements in the lipid bilayer that contribute to the dynamic
nature of the membrane.
•They have been shown to be central to many essential processes in plants such as:
Pollen development, signal transduction and response to biotic and abiotic stress.
•A sphingolipid is any member of a class of lipids containing the organic aliphatic
amino alcohol sphingosine or a substance structurally similar to it.
•*sphingosine=an 18-carbon amino alcohol with an
•unsaturated hydrocarbon chain.
•Among the simplest sphingolipids are the ceramides (sphingosine + a fatty acid).
•Ceramides are widely distributed in small amounts in plant and animal tissues.
The other sphingolipids are derivatives of ceramides. Sphingosine is one of the
most crucial elements in both intra-and inter-cellular signalling, especially in
animal cells, with very many biological effects.
Functions of lipids in plants
Lipids are very important in the normal function of plant cells.
They perform many functions, such as:
•Energy Storage
•Making Biological Membranes
•Protection – e.g. protecting plant leaves from drying up
•Acting as hormones
•Act as the structural component of the body and provide the
hydrophobic barrier that permits partitioning of the aqueous
contents of cell and sub cellular structures.
•Lipids are major sources of energy in high lipid-containing
seeds.
•Activators of enzymes-some enzymes require certain lipid
molecules for activation.
Triacylglycerols as storage substances in plants
•Triacylglycerols are contained primarily in seeds but
also in some fruits such as olives or avocados.The
purpose of triacylglycerols in fruits is to attract
animals to consume these fruits for distributing the
seeds.
•The triacylglycerols in seeds function as acarbon
store to supply the carbon required for biosynthetic
processes during seed germination.
•Triacylglycerols have an advantage over
carbohydrates as storage compounds, because their
weight/carbon content ratio is much lower.
•This is because triacylglycerides are not hydrated.
Lower weight is advantageous for seed dispersal.
Amino acids and Proteins
•Amino acids the monomers that make up proteins.Amino acid have
the same fundamental structure, which consists of acentral carbon
atom, also known as the alpha (α) carbon, bonded to an amino group
(NH2), acarboxyl group (COOH), bonded to ahydrogen atom, and
an side chain.
•In the aqueous environment of the cell, both the amino group and the
carboxyl group are ionized under physiological conditions, and so
have the structures -NH3+ and -COO–, respectively.
•Every amino acid also has another atom or group of atoms bonded to
the central atom known as the Rgroup.
•This Rgroup, or side chain, gives each amino acid proteins specific
characteristics, including size, polarity, and pH.
Based on the R group, amino acids
can be classified into five categories
•Amino acids with non
polar aliphatic R groups
•Amino acids with polar
uncharged R groups
•Amino acids with
positively charged R
groups
•Amino acids with
negatively charged R
groups
•Amino acids with
aromatic R groups
Role of amino acids in plants
Plant cells contain low levels of protein in comparison with animal
cells mainly because of the high amount of carbohydrate (cellulose
and others) that compose most of aplant’s structure.
•Proteins and their building blocks (amino acids) are critical for
plant growth.
•The major role of amino acids in plants is in the synthesis of
proteins.
•Amino acids are also involved in cellular reactions and therefore
influence physiological processes such as plant growth and
development
•They are involved control of intracellular pH
•Generation of metabolic energy or redox power,
•Resistance to both abiotic and biotic stress
•They are also precursors for the synthesis of secondary metabolites
•They can also be metabolized of energy generation
Proteins
•Proteins are synthesized from amino acid through aprocess called
translation that is carried out by the ribosomes.The information for
protein synthesis comes from the DNA in the nucleus.
•The segment of DNA coding for the synthesis of aprotein is called a
gene.
•The gene is first used to synthesize an RNA molecule called messenger
RNA (mRNA), which is then transported to the cytoplasm where
protein synthesis occur.
•Protein synthesis involved joining of different amino acids together in a
particular order through formation of bonds called peptide bonds.
•After formation of peptide bonds, functional proteins are formed
through folding, removal of sequences, combination of different
peptides chains and addition of other compounds such as
carbohydrates.
•In plants proteins play various roles such as:
•Hormones, enzymes, structural proteins, and source of energy,
•Peptides are short polymers of amino acids.
•The peptide bond is formed when the α- carboxyl group of
one amino acid reacts with the α- amino group of another
amino acid with the release of water molecule.
•Polypeptides vary in size from small to very large ones
comprising of thousands of amino acid residues of the
chain.
•These groups ionize like free amino acids but their pK
value differs.For instance,
•Apeptide contains afree amino group on one end (the
amino terminal) and afree carboxyl group on the other
end (the carboxy terminal)
Structural organization of proteins
•Proteins are built up with increasingly complex
organizational units.
Primary structure of protein
•This is the linear sequence of amino acids and the
location of disulphide bonds.The amino acid
sequence of aprotein dictates the three-
dimensional structure, which is biologically active
confirmation and more stable than the unfolded
state.The length of the primary protein varies
from protein to protein
Secondary structure
•Refers to regular, local structural units, usually held
together by hydrogen bonding.
•This is the structure formed after the folding of the
primary structure.Va r i o us types of folding include:
•Helical structures, β-Sheet structure, β-turn, coils and
loops
Helical structures β-Sheet structure β-turn
Tertiary structures
•These are formed when secondary structure
undergoes tertiary folding.This gives rise to the
formation of tertiary structure.
•The tertiary structure is stabilized by long range
interactions such as disulfide bonds, hydrogen
bonding, hydrophorbic interactions,electrostatic
interactions and van der Waals interactions.
•Amino acids that are far apart are brought together
and different types of secondary structure may
interact to form tertiary structure.
•In tertiary structure the hydrophorbic amino acids
are buried into the structure while the polar ones
appear on the surface.
Quaternary structure
•This is the rearrangement of protein subunits in three-
dimensional structure that is biologically functional.
•The subunits may be similar or different thus producing
homogeneous or heterogeneous quaternary structures.
•At this level structure proteins may be classified as
fibrous or globular.
•Fibrous proteins contain single type of secondary
structure such as strands or sheets and their tertiary
structure is relatively simple.
•In globular proteins, the polypeptide chains are
arranged into spherical or globular shape and with
several types of secondary structures.
Figure 2.8
Hierarchy
of
protein
structure.
(A)
Primary
structure:
peptide
bond.
(B
and
C)
Secondary
structure:
«
helix
(B)
and
antiparallel
§
pleated
sheet
(C).
(D)
Tertiary
structure:
a
helices,
pleated
sheets,
and
random
coils.
(E)
Quaternary
structure:
four
subunits.
(C)
Secondary
structure
(ji
pleated
sheet)
(R
groups
not
shown)
(D)
Tertiary
structure
Source: Sinauer, 2002
