Lipids and Carbohydrates
Revision PowerPoint
Lipids
• At room temperature, a solid lipid is called a fat and a
liquid lipid called an oil
• Lipid functions include: energy source for respiration,
energy storage as adipose cells, cell membranes,
insulation e.g. blubber in whales, protection e.g. cuticle
of leaf and hormones
• Lipids contain carbon, hydrogen and oxygen
• Lipids are insoluble in water (they don’t dissolve)
Glycerol and Fatty Acids
• Found in all storage fats and oils, including
membranes
• The glycerol molecule is always the same, but the
fatty acid differs
• Most fatty acids can be made, except for ones
called essential fatty acids which must be eaten
Fatty Acids
• All fatty acids have an acid group (part) at one
end, like an amino acid, the rest of the
molecule is a hydrocarbon chain (a chain
made of carbons and hydrogens)
• The hydrocarbon chain can be 2 to 20 carbons
long, but most have around 18
Acid Group
Hydrocarbon chain
Saturated vs. unsaturated
• These terms refer to the hydrocarbon chain and whether it
is ‘saturated’ (full) of hydrogen or not.
• Unsaturated fatty acids have C=C double bonds, so fewer
hydrogen atoms can be bonded to the molecule.
• If there is on C=C bond it is called monounsaturated, two or
more makes it polyunsaturated (poly means many)
• The presence of C=C bonds changes the shape of the
hydrocarbon chain and makes the molecules in the lipid
push apart making them more fluid e.g. olive oil
Triglycerides
• A triglyceride is made of one glycerol molecule bonded to three fatty acid
molecules
• They are joined by a condensation reaction between the acid part of a
fatty acid and an OH group (called the hydroxyl part) by a covalent bond
• The bond is called an ester bond.
• It is called a monoglyceride at this stage, however two more fatty acid
chains form ester bonds causing it to become a triglyceride. (tri means
three, so it is one glycerol molecule with 3 fatty acids joined)
• It is insoluble in water (hydrophobic)
Phospholipids
• Almost the same as a triglyceride, but the third fatty acid is
not added, instead a phosphate joins to the 3rd OH by a
condensation reaction
• The phosphate head is hydrophilic, and the fatty acids are
hydrophobic.
• As the majority of the molecule is insoluble, but the
phosphate head is hydrophilic it is able to form membranes
Phospholipids in membranes
• Phospholipids may still be
saturated or unsaturated.
• Organisms can control the
fluidity of their membranes
using this feature
• Organisms living in colder
climates have more
unsaturated fatty acids in
their phospholipid
molecules ensuring their
membranes remain fluid in
low temperatures
Lipids and respiration
• Hydrolysis of the ester bonds then molecular breakdown releases
water, carbon dioxide and energy which is used to generate ATP
• The respiration of one gram of lipid gives out twice as much energy
as the respiration of a carbohydrate
• As they are insoluble they can be stored in a compact way and
don’t affect water potential of surrounding cells
• As the respiration of lipids releases more water than carbohydrates,
some organisms use stored fat as a water supply
Cholesterol and Steroid Hormones
•
•
•
•
Cholesterol is a type of lipid
It is made of four carbon based rings
It is found in all membranes
Its small, narrow hydrophobic nature
allows it to sit between phospholipid
hydrocarbon tails and help regulate the
strength and fluidity of membranes
• Testosterone, oestrogen and vitamin D are
made from cholesterol
• The lipid nature means they can pass
through the phospholipid bilayer to reach
their target receptor (site) usually inside
the nucleus, they can also pass through
the nuclear envelope
Cholesterol Dangers
• Many cells can make cholesterol as it is essential e.g. the liver, but
excess cholesterol can:
• Stick together in bile to form gallstones
• Cause atherosclerosis by depositing in inner linings of blood vessels
• FHC (familial hypercholesterolaemia)is a genetic disorder meaning
cholesterol is made even if there is enough in the blood. The cells
don’t have a receptor that tells them when the ideal amount has
been made. People with this genetic disease can suffer heart
attacks and strokes by the time they are 2 years old.
Summary Table
Lipid
Structure
Main Role
Other Features
Triglyceride
Glycerol and
Compact energy store,
three fatty acids insoluble in water so
doesn’t affect water
potential
Stored as fat, used
for thermal
insulation and
protective properties
Phospholipd
Glycerol plus
two fatty acids
and a
phosphate
group
Molecule is part
hydrophobic, part
hydrophilic, ideal for
membranes
Phosphate parts
have carbohydrate
parts attached called
glycolipids for cell
signalling
Cholesterol
Four carbon
based ring
structures
joined together
Forms a small, thin
molecule that fits to a
lipid bilayer giving
strength and stability
Used to form steroid
hormones
Carbohydrates
• Functions: energy source from respiration,
energy store e.g. starch, structure e.g.
cellulose cell walls
• Can form nucleic acids and glycoproteins (cell
signalling)
• Contain carbon, hydrogen and oxygen
• Have the general formula Cn(H2O)n
• This means for every 1 carbon and oxygen
atoms, there are 2 Hydrogen atoms
Simple Sugars
• Called monosaccharides which are monomers (basic
units)
• Larger carbohydrates made by joining
monosaccharides together
• They are all: soluble in water, sweet and form crystals
• Triose sugars have 3 carbons, pentose have 5 carbons
and hexose have 6 carbons
• Hexose sugars are the most common e.g. glucose and
fructose
• They occur in ring structures
Two Forms of Glucose
• Glucose can be in
chain or ring form
• Ring glucose can
also be in 2 forms
called alpha and
beta glucose
Alpha (α) glucose
Beta (β) glucose
Joining monosaccharides
• Condensation reaction forming a disaccharide (2
saccharides)
• Covalent bond forms called a glycosidic bond
• One water molecule is released
• Starch, glycogen and cellulose are all polysaccharides made
this way
• Disaccharides are still called sugars
Carbohydrates and Energy
• In respiration, glucose is broken down to release energy used to
make ATP
• The equation for respiration is:
• Glucose + oxygen  carbon dioxide + water
• Each step in respiration is controlled by enzymes
• Animals and plants only have enzymes that can break down alpha
(α) glucose.
• Due to its shape, beta (β) glucose cannot be broken down
Remember: the numbers
stand for Carbon atoms
that are not drawn in on
these diagrams
Carbohydrates for Storage
• Two alpha glucose molecules joined are called maltose (a
disaccharide). If more are joined, it is known as amylose
• Amylose can be made of many thousands of glucose
molecules bonded together
• As the glycosidic bonds are between carbon number 1 and
carbon number 4, it is called a 1,4- glycosidic bond
Amylose coils into a spring making it compact- iodine molecules become trapped in
the coils and turn blue/black which is the basis of the starch test
Starch
• A mixture of a long,
straight chain of
spring like amylose,
and a branched
molecule called
amylopectin
• It is stored in
chloroplasts in plant
cells and as starch
grains
• Starch can be broken
down to glucose and
used for respiration
Glycogen
• Sometimes called
animal starch
• Made up of alpha
glucose
• Different from starch
as the 1-4 glucose
chains are shorter and
have more 1-6
branches
• It is more compact
and forms glycogen
granules in the liver
and muscle cells
Starch and glycogen
• Described as energy storage molecules as they
are so long
• Do not dissolve so does not affect water
potential of cell
• Hold glucose in chains so they can easily be
broken off at the ends fro respiration when
required
Cellulose
• Made of beta glucose (the H is below Carbon 1 and the
OH is above, the opposite of alpha glucose)
• When beta glucose forms glycosidic bonds, they are
long and straight and are not spring like
• They are stronger than amylose chains
• So many beta glucose joined together forms cellulose
and is only found in plants
Cellulose in Plants
• Arranged in a specific way to form plant cell walls
• Many hydrogen bonds form as there are so many OH groups
• 60-70 cellulose molecules become cross linked with hydrogen bonds to
form microfibril bundles
• These are held together by more hydrogen bonds forming macrofibrils
• Almost as strong as steel
• They are embedded in a polysaccharide ‘glue’ of substances called pectins,
to form cell walls
Structure and Function of Plant Cell Walls
• Gives strength to each cell
• Supports plant
• Macrofibril arrangement allows water to pass in
and out of cell easily
• Prevents bursting when cell is turgid (full of
water)
• Allows cells to be different shapes e.g. guard cells
opening and closing stoma
• Can be reinforced with other substances to make
the walls waterproof
Other Structural Carbohydrates
• Chitin: polysaccharide forming insect
exoskeleton
• Peptidoglycan: polysaccharide forming
bacterial cell walls
Carbohydrate
Examples
Monosaccharides Glucose
(monomers)
(6 carbon)
Features
Role
Small, soluble, sweet and
crystalline
Provides energy via
respiration
Deoxyribose
(5 carbon)
Part of DNA
Disaccharides
(dimers)
Maltose
(glucose +
glucose)
Small, soluble, sweet and
crystalline
A sugar obtained when
starch is broken in
hydrolysis reactions. It
can be split further to
glucose for respiration
Polysaccharides
(polymers)
Starch and
Glycogen
Large molecules, α-glucose
joined by condensation.
Insoluble in water, forms
grains/ granules
Energy storage
carbohydrates- starch in
plants, glycogen in
animals and fungi
Cellulose
Large molecules, β-glucose Structural, only in plants
joined by condensation.
forming cell walls
Insoluble in water. Very
strong