B1.1 Carbohydrates and lipids Flashcards
B1.1.1—Chemical properties of a carbon atom allowing for the formation of diverse compounds upon
which life is based
Students should understand the nature of a covalent bond. Students should also understand that a carbon
atom can form up to four single bonds or a combination of single and double bonds with other carbon
atoms or atoms of other non-metallic elements. Include among the diversity of carbon compounds
examples of molecules with branched or unbranched chains and single or multiple rings.
NOS: Students should understand that scientific conventions are based on international agreement (SI
metric unit prefixes “kilo”, “centi”, “milli”, “micro” and “nano”).
Carbon atoms consist of six electrons orbiting a nucleus of six protons and six, seven or eight neutrons depending on the isotope. Four of the six electrons are in the outer shell, so carbon atoms can form four covalent bonds. Carbon atoms can bond covalently with other carbon atoms or other types of atom.
n a methane molecule (shown right) there are single covalent bonds between a carbon atom and four hydrogen atoms (so the formula is CHa). In a carbon dioxide molecule, a carbon atom has two double covalent bonds with oxygen atoms (so the formula is CO,).
Carbon can bond covalently to other non-metals, with hydrogen, oxygen, nitrogen and phosphorus all occurring commonly in carbon compounds made by living organisms.
Carbon’s ability to link to four other atoms allows complex molecular structures to be formed, which can include rings or chains of atoms. The rings can be single or multiple and the chains can extend to any number of atoms and may be branched or unbranched.
All these possibilities result in a diverse range of carbon compounds, which form the basis of life in all organisms.
covalent bond consisting of a shared pair of electrons
methane molecule
1 electron in a –hydrogen atom’s inner and only shell
* 2 electrons in a carbon atom’s inner shell
B1.1.2—Production of macromolecules by condensation reactions that link monomers to form a polymer
Students should be familiar with examples of polysaccharides, polypeptides and nucleic acids
Living organisms produce macromolecules (very large molecules) from subunits that are linked up using covalent bonds. The subunits are monomers and a chain of 10 or more monomers is a polymer.
For example, a polypeptide is a polymer, consisting of a chain of amino acids, which are linked together by covalent bonds. Nucleic acids (DNA and RNA) are polymers, with nucleotides as the subunits. A polysaccharide is also a polymer with monosaccharides (single sugar units) as subunits.
The covalent bonds that link the monomers in these polymers are made by condensation reactions. In a condensation reaction, two molecules are joined together to form a larger molecule, plus a molecule of water. The water is made by removing an -OH (hydroxyl group) from one of the molecules and a hydrogen from the other. This allows a new covalent bond to be made to link the two molecules.
The diagrams show two glucose molecules being linked by condensation to form a disaccharide (maltose). A polysaccharide is produced by linking on more glucose molecules by condensation.
B1.1.3—Digestion of polymers into monomers by hydrolysis reactions
Water molecules are split to provide the -H and -OH groups that are incorporated to produce monomers,
hence the name of this type of reaction.
Hydrolysis reactions are the reverse of condensation
reactions. In a hydrolysis reaction a large molecule is
broken down into smaller molecules and water is used
up in the process. Water molecules are split into -H
and -OH groups, hence the name hydrolysis (lysis =
B1.1 Molecules
3. Hydrolysis reactions
bonds after a bond in the large molecule has been
broken. Hydrolysis reactions are used to digest food.
Examples of hydrolysis reactions used to digest
* a m i n o a c i d s
- monosaccharides
* fatty acids + glycerol
splitting). The -H and -OH are needed to make new
polymers:
polypeptide + water —
polysaccharide + water -
glyceride + water -
The diagrams show the bond that links two nucleotides
being broken by hydrolysis.
ОН
O H + H
H,0
* C-O bond is broken
* OH from water bonds to the C
* H from water bonds to the O
B1.1.4—Form and function of monosaccharides
Students should be able to recognize pentoses and hexoses as monosaccharides from molecular diagrams
showing them in the ring forms. Use glucose as an example of the link between the properties of a
monosaccharide and how it is used, emphasizing solubility, transportability, chemical stability and the
yield of energy from oxidation as properties.
Monosaccharides are sugars that cannot be split
into simpler sugars, so they are monomers. Most
monosaccharides used by living organisms contain three,
five or six carbon atoms (trioses, pentoses and hexoses).
They contain only atoms of carbon, hydrogen and
oxygen, in the ratio 1:2:1, so pentoses have the formula
CHoOs and hexoses C6H2%
* One of the oxygen atoms in monosaccharides is
bonded only to carbon, either by a double bond to
one carbon, or more usually by two single bonds
to different carbon atoms, in a ring of atoms. The
other oxygens are part of OH groups. Molecules of
pentose and hexose usually exist in a ring form and
can be recognized by their shared features:
one or two side chains with one OH and
two H groups bonded to a carbon atom
a ring of atoms all of
H
ý which are carbon apart
from one oxygen
H ↑ O H
a single H group
on the carbon atom
to which the s i d e
chain is a t t a c h e d OH OH
Hand OH groups attached to carbon atoms in
the ring (apart from those with a CHOH side chain)
The molecule shown in the diagram above is D-ribose. The D
indicates that this is the right-handed form. Left-handed forms
of ribose and glucose can exist but living organisms do not
use them. D-glucose can exist in two forms, alpha and beta.
A numbering system is used so the carbon atoms can be
referred to directly. The alpha and beta forms of glucose differ
in whether the OH groups on C 1 and C 4 face in the same
direction or opposite directions:
6CH20H ©CH2OH
-O. H
١١
ェ OH
CO
H c o
⼯
OH
1 ‘ 8
- С ® ОНС
с е н с
OH
x-D-glucose ß-D-glucose
There are clear links between the properties of glucose and
how living organisms use this monosaccharide:
Property Function of glucose
Soluble in
Transport of carbohydrate in blood, where
water
glucose is dissolved in the plasma
Can be
Source of energy, released when glucose is
oxidized
used as a substrate in cell respiration
Chemically Energy storage, usually after conversion to astable polysaccharide
B1.1.5—Polysaccharides as energy storage compounds
Include the compact nature of starch in plants and glycogen in animals due to coiling and branching
during polymerization, the relative insolubility of these compounds due to large molecular size and the
relative ease of adding or removing alpha-glucose monomers by condensation and hydrolysis to build or
mobilize energy stores.
Plants use starch as an energy store. Mammals use
glycogen (in liver and muscle). Some fungi and bacteria
use glycogen. Both starch and glycogen are composed
C * can also be linked to C 6 , making 1-6 glycosidic
of alpha glucose. Glucose is linked to these
bonds which give the molecule a branched structure.
polysaccharides with a glycosidic bond, made by a
condensation reaction:
Approximately 25% of starch molecules have no 1-6
bonds so are u n b r a n c h e d - t h e y are amylose. About
75% of starch molecules have some 1-6 bonds so they
1-6 glycosidic bond
COOH
OH 3
H/
{ ② 앤
HI
OHI
hydrogen is removed *
from the OH group on C 0
of the glucose
H from glucose and
OH from the polysaccharide
combine to produce water
- a n OH is removed from a
carbon atom on a glucose
that is already part of
the polysaccharide
a single covalent bond is
formed linking the glucose
to the polysaccharide
C of the extra glucose is most frequently linked to
is helical.
are branched-they are amylopectin.
Glycogen molecules are similar to amylopectin, but with
twice as much branching. The diagram shows part of a
glycogen or amylopectin molecule.
CH,OH
СН , ОН
O. H
H HI
⼯
OH
H
OH
H ОН H OH
1-4 glycosidic bond CH2
CHOH CH2
H
OH OH
HI OH HI
Starch and glycogen function well as energy stores:
* The coiled and branched form of the molecules
makes them compact, so they do not take up much
space in cells.
* They are relatively insoluble so do not draw an
excessive amount of water into cells by osmosis.
* When in surplus, glucose can easily be added and
when scarce it can be removed. This can be done
at more points in branched chains so addition or
removal is more rapid.
0. H
C 4 of the terminal glucose in the polysaccharide. This
is 1-4 glycosidic bonding.
A chain of alpha glucose monomers linked by 1-4 bonds
B1.1.6—Structure of cellulose related to its function as a structural polysaccharide in plants
Include the alternating orientation of beta-glucose monomers, giving straight chains that can be grouped
in bundles and cross-linked with hydrogen bonds.
Cellulose is composed of beta glucose molecules, linked by 1-4 glycosidic bonds. The glucose molecules alternate
in their orientation (up-down-up-down). This is due to the OH groups on C and C 4 of beta-glucose facing in
opposite directions. The consequence is that whereas amylose made from alpha-glucose is helical, cellulose made
from beta-glucose is a straight chain. This allows groups of cellulose molecules to be packed together in parallel, with
hydrogen bonds forming cross links. These structures are called cellulose microfibrils. They have enormous tensile
strength and are the main component of plant cell walls.
B1.1.7—Role of glycoproteins in cell–cell recognition
Include ABO antigens as an example.
Glycoproteins and glycolipids are components of plasma membranes, with short chains of sugars (oligosaccharides) projecting outwards from the membrane. The carbohydrate is attached either to protein in the membrane (glycoproteins) or to lipid (glycolipids). Interactions between the oligosaccharides and carbohydrate-binding proteins in adjacent cells allow cell-cell recognition.
Oligosaccharides in the membranes of adjacent cells can become linked, binding the cells together into a tissue. Cells in a multicellular organism can recognize foreign cells or infected body cells by the oligosaccharides of their glycoproteins and glycolipids. ABO blood groups in humans are due to a glycoprotein and glycolipid in red blood cells.
humans with only H
antigens have type O blood
A
antigen
antigen
antigen
humans with an A allele, have an enzyme that adds one extra sugar to some of the glycoprotein and glycolipid
humans with a B allele, have an enzyme that adds a different sugar to some of the glycoprotein and glycolipid
B1.1.8—Hydrophobic properties of lipids
Lipids are substances in living organisms that dissolve in non-polar solvents but are only sparingly soluble
in aqueous solvents. Lipids include fats, oils, waxes and steroids.
Lipids are defined by solubility-they dissolve in non-
polar solvents such as toluene but are sparingly soluble
or insoluble in water. This is because lipid molecules
have few charged groups (+ or -) and few groups that
form hydrogen bonds.
They are chemically diverse, with fats, oils, waxes and
steroids as the main groups made by organisms. The
C:O and H:O ratios are higher than in carbohydrates
and other hydrophilic carbon compounds.
The table shows CHO composition of four lipids and
glucose for comparison:
Example Formula Function
Glucose Sugar in blood plasma
Palmitic acid G6H320 г Animal fat component
Linoleic acid G8H3202 Plant oil component
Octacosanoic acid C25602 Wax on leaves (cutin)
Cholesterol Membrane component
B1.1.9—Formation of triglycerides and phospholipids by condensation reactions
One glycerol molecule can link three fatty acid molecules or two fatty acid molecules and one phosphate
group.
Triglycerides (fats and oils) are made by combining fatty
—COOH
acids and glycerol. Fatty acids have a carboxyl group which is
acidic. Two ways of representing it are shown.
The fatty acids also have an unbranched hydrocarbon chain. The example shown below is stearic acid, a saturated fatty acid.
Glycerol is an alcohol with three hydroxyl (OH) groups.
A fatty acid is linked to glycerol by a condensation reaction, using the COOH group of the fatty acid and a hydroxyl group of glycerol. The molecule produced is a monoglyceride. Because glycerol has three OH groups, two more fatty acids can be linked to it, producing a triglyceride. The new bonds that link the fatty acids to the glycerol are ester bonds.
3 fatty acids
glycerol
HO -CH2
HO
* CH
HO— CH2
triglyceride (a fat or oil)
condensation reaction
CH2
* CHI
CH2
+ 3H,0
wwww
hydrocarbon chain
ester bond
of fatty acid
Phospholipids are made by combining two fatty acids and one phosphate group with glycerol. As with production of triglycerides, condensation reactions create ester bonds and water is produced.
Because the phosphate is group is hydrophilic and the hydrocarbon chains of the fatty acids are hydrophobic, they are on opposite sides of the molecule:
ーP
—0
CH
CH2
www
hydrocarbon chain of fatty acid
phosphate
ester hond
B1.1.10—Difference between saturated, monounsaturated and polyunsaturated fatty acids
Include the number of double carbon (C=C) bonds and how this affects melting point. Relate this to the
prevalence of different types of fatty acids in oils and fats used for energy storage in plants and
endotherms respectively.
Fatty acids vary in the number of carbon atoms in the hydrocarbon chain and in the bonding of the carbon and hydrogen atoms.
Saturated fatty acids-all the carbon atoms are connected by single covalent bonds so the number of hydrogen atoms bonded to the carbons cannot be increased.
Monounsaturated fatty acids-there is a double bond between two of the carbon atoms and if this was replaced by a single bond, more hydrogen could be incorporated.
oleic acid
(omega 9 monounsaturated)
Polyunsaturated fatty acids-there are two or more double bonds. The position of the double bond nearest to the CH, terminal is significant. In omega-3 fatty acids, it is the third bond from CH and in omega-6 fatty acids it is the sixth.
linoleic acid
(omega-6 polyunsaturated)
Fatty acid
Double bonds Melting point (°C)
Stearic acid
18
69
Oleic acid
18
131
Linoleic acid
18
-5
A double bond puts a kink into the hydrocarbon chain.
A consequence is that unsaturated fatty acids do not pack together as neatly as saturated fatty acids, so they change from solid to liquid (melt) at a lower temperature.
Triglycerides with mostly unsaturated fatty acids are liquid at room temperature (20°C)—they are oils.
Triglycerides with mostly saturated fatty acids are solid at 20°C and liquid at 37°C (body temperature)-they are fats.
Stores of triglyceride must remain liquid, so birds and mammals with constant high body temperatures can store fats. Plants and other organisms must use oils instead as their tissues are sometimes below the melting point of fats
B1.1.11—Triglycerides in adipose tissues for energy storage and thermal insulation
Students should understand that the properties of triglycerides make them suited to long-term energy
storage functions. Students should be able to relate the use of triglycerides as thermal insulators to body
temperature and habitat.
Adipose cells can accumulate large amounts of
triglyceride. Fats and oils are inert and coalesce into
compact droplets, which do not cause osmosis.
Fats and oils are very efficient energy stores because
they release twice as much energy per gram as
carbohydrates when used in cell respiration. Adipose
tissue can be used to store energy anywhere in the
body, but much of it is positioned next to the skin. This
is because fats and oils are poor heat conductors, so
Molecules B1.1
they function as thermal insulators and reduce loss of
body heat to the environment.
B1.1.12—Formation of phospholipid bilayers as a consequence of the hydrophobic and hydrophilic
regions
Students should use and understand the term “amphipathic”.
Phospholipids are the basic component of all biological membranes. Phospholipid molecules are amphipathic. This means that part of the molecule is attracted to water (hydrophilic) and part is not attracted (hydrophobic). The phosphate head is hydrophilic and the two fatty acid tails, which are composed of hydrocarbon chains, are hydrophobic.
When phospholipids are mixed with water they naturally become arranged into bilayers, with the hydrophilic heads facing outwards and making contact with the water and the hydrocarbon tails facing inwards away from the water. This is a very stable structure because hydrophobic tails in the centre of the bilayer are more attracted to each other than to water outside the membrane and the hydrophilic heads of the phospholipids are more attracted to the water.
phospholipid bilayer
aqueous solution
aqueous solution
hydrophobic
- hydrocarbon tails hydrophilic phosphate heads
B1.1.13—Ability of non-polar steroids to pass through the phospholipid bilayer
Include oestradiol and testosterone as examples. Students should be able to identify compounds as
steroids from molecular diagrams.
Steroids are a group of lipids with molecules similar to that of sterol. They have four fused rings of carbon atoms, three with six and one with five carbon atoms.
There are hundreds of different steroids, which differ in the position of C=C double bonds and the functional groups such as -OH that are attached to the four-ring structure. Steroids are mostly hydrocarbon and therefore hydrophobic. This allows them to pass through phospholipid bilayers and therefore enter or leave cells.
testosterone (male sex hormone)
CHa
