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Plant Metabolism
Photosynthesis
The Most Important Equation in Biology
Primary Metabolites
• Primary metabolites are compounds that
are commonly produced by all plants and
that are directly used in plant growth and
development.
• The main primary metabolites are
carbohydrates, proteins, nucleic acids,
and lipids.
Carbohydrates
• Carbohydrates are the sugars made up of glucose
and its isomers
• Carbohydrates come in many different sizes:
• Monosaccharides made up of one sugar unit
(glucose or fructose)
• Disaccharides made up of two sugar units (sucrose
is a glucose and a fructose)
• Polysaccharides are polymers made up of more
than two sugar units
Harvesting Sucrose
Sugar Cane
Maple Syrup
Refining Sucrose
Maltose and Sucrose
Malting Process
Malt Varieties
Polysaccharides
• Structural polysaccharides are used to
support plants
• Storage polysaccharides are used to store
energy for later use by the plant
Structural Polysaccharides
• The most common structural polysaccharide
in plants is cellulose. It makes up 40 to
60% of the cell wall. It is also the most
common polymer on earth
• Cellulose is extremely strong due to its
chemical organization. It is made of a long
chain of beta-glucose molecules – 100 to
15,000 glucose molecules
Cotton Boll – Pure Cellulose
Gluey Polysaccharides
• Pectins are mainly polymers of galacturonic acid.
• Hemicelluloses are highly variable and are not
related to cellulose.
• Grass hemicelluloses are high in xylose, with
small amounts of arabinose, galactose, and urionic
acids. But pea family (Fabaceae) hemicelluloses
are high in arabinose, galactose and urionic acid,
but low in xylose.
• Some of the most interesting hemicelluloses are
not actually used structurally, but rather are
exuded from stems, leaves, roots, or fruits in a
sticky mixture called a gum
Pectin and Hemicellulose
Gum Arabic from Acacia senegal
Storage Polysaccharides
• The most important storage polysaccharides are
amylose and amylopectin. Amylose is a long
chain of alpha-glucose, several hundred to several
thousand molecules long. Amylopectin is more
complex, often made up of 50,000 molecules.
• These two polymers are both used in making
starch grains. Most starch grains are about 20%
amylose and 80% amylopectin, but this varies
with the plant species.
Inulin – another storage carbohydrate –
long chain of fructose
Jerusalem artichoke
Digestability
• The properties of alpha-glucose and beta-glucose affect
their digestability. Alpha-glucose polymers don’t form
fibrils and thus are not as strong as cellulose from betaglucose. Even more important, almost all organisms have
alpha-amylase, a digestive enzyme that breaks down alphaglucose bonds. So starches can be digested easily. Plants
have alpha-amylase so they can harvest energy from
starches in their seeds, roots, and tubers.
• Very few organisms have cellulases – just some fungi and
bacteria, earthworms, and a few insects can digest
cellulase.
Proteins
• Proteins make up most of the remaining biomass
of living plant cells.
• A protein consists of one or more polypeptides
made up of amino acids. Plants make amino acids
from the products of photosynthesis through a
very complex process involving the acquisition of
N, usually in the form of NH4, and involving the
use of large amounts of energy, in the form of
ATP and NADPH.
Structural Proteins
• Structural proteins make up 2 to 10% of the cell
wall in plants. Expansins help increase the surface
area of cell walls. Extensins help protect or repair
damaged cell walls. The plant cell membrane is
about 50% structural proteins.
Storage Proteins
• Storage proteins are used mostly in seeds
and are used as source of nutrition for the
early development of seedlings. Storage
proteins used in seeds vary considerably
between plant species. Corn produces a
storage protein called ZEIN. Wheat
produces a storage protein called GLIADIN
Enzymes
• Enzymes catalyze biochemical reactions.
Most proteins in living cells are enzymes.
• Pure enzymes that maintain their activity
when removed from plants are
commercially important to us.
Papaya – Papain and Chymopapain
Pineapple - Bromelain
Nucleic Acids
• The most complex
biological polymers are
the nucleic acids that
make up RNA and
DNA. The basic
content of bases
(adenine, thymine,
gaunine and cytosine)
are similar in all plants
Lipids
• Unlike other biological polymers, lipids are
not defined by specific, repeating
monomeric units. Rather they are defined
by their water-repelling properties. The
only structure they share is that they mostly
are made up of nonpolar hydrocarbon
groups (CH3, CH2, and CH).
• Oils are fats that are liquid at room
temperature.
Oils
• Oils occur in all parts of a plant, but are most
common in seeds. Some seeds have so much oil
that it can be commercially harvested. The most
commonly used oils are cotton, sesame, safflower,
sunflower, olive, coconut, peanut, corn, castor
bean, canola, and soybean oils.
• The most common seed oil fatty acids are oleic
acid (one double bond), linoleic acid (two double
bonds), and linolenic acid (three double bonds).
Linoleic and linolenic are essential fatty acids –
we can’t make them ourselves.
Olive Oil
Canola Oil
Waxes
• Waxes are complex mixtures of fatty acids linked
to long-chain alcohols. Waxes comprise the
outermost layer of leaves, fruits, and herbaceous
stems and are called EPICUTICULAR waxes.
Waxes embedded in the cuticle of the plant are
cuticular waxes. Cutin is another wax in the
cuticle and it makes up most of the cuticle.
Suberin is a similar wax that is found in cork cells
in bark and in plant roots. Both help prevent
water loss by the plant.
• Structures of waxes vary depending on which
plant produced them.
• Waxes are usually harder and more water repellant
than other fats.
Bayberry Wax
Euphorbia antisyphilitica
Candelilla wax
Jojoba Wax
Plant Secondary Metabolites
• Plants make a variety of less widely distributed
compounds such as morphine, caffeine, nicotine,
menthol, and rubber. These compounds are the
products of secondary metabolism, which is the
metabolism of chemicals that occurs irregularly or
rarely among plants, and that have no known
general metabolic role in plants.
• Secondary metabolites or secondary compounds
are compounds that are not required for normal
growth and development, and are not made
through metabolic pathways common to all plants.
• Most plants have not been examined for secondary
compounds and new compounds are discovered
almost daily.
Plant Secondary Metabolites
• Secondary compounds are grouped into classes
based on similar structures, biosynthetic pathways,
or the kinds of plants that make them. The largest
such classes are the alkaloids, terpenoids, and
phenolics.
• Secondary compounds often occur in combination
with one or more sugars. These combination
molecules are known as glycosides. Usually the
sugar is a glucose, galactose or rhamnose. But
some plants have unique sugars. Apiose sugar is
unique to parsley and its close relatives.
Functions of Secondary Compounds
• The most common roles for secondary compounds
in plants are ecological roles that govern
interactions between plants and other organisms.
• Many secondary compounds are brightly colored
pigments like anthocyanin that color flowers red
and blue. These attract pollinators and fruit and
seed dispersers.
• Nicotine and other toxic compounds may protect
the plant from herbivores and microbes.
• Other secondary compounds like rubber and
tetrahydrocannabinil (THC) from cannabis plants
have no known function in plants.
Alkaloids
• Alkaloids generally include alkaline substances
that have nitrogen as part of a ring structure.
More than 6500 alkaloids are known and are the
largest class of secondary compounds. They are
very common in certain plant families, especially:
• peas – Fabaceae
• sunflower – Asteraceae
• poppy – Papaveraceae
• tomato – Solanaceae
• dogbanes – Apocynaceae
• milkweeds - Asclepiadaceae
• citrus – Rutaceae.
Terpenoids
• Terpenoids are dimers and polymers of 5 carbon
precursors called isoprene units (C5 H8).
• Terpenoids often evaporate from plants and
contribute to the haze we see on hot sunny days.
They are expensive to make; they often take 2% of
the carbon fixed in photosynthesis; carbon that
could otherwise be used for sugars.
Phenolics
• Compounds that contain a fully unsaturated six
carbon ring linked to an oxygen are called
phenolics.
• Salicylic acid (basic part of aspirin) is a simple
phenol.
• Myristicin is a more complex phenol that provides
the flavor of nutmeg.
• Flavonoids are complex phenolics. They are often
sold in health food stores as supplements to
vitamin C. The most commonly available
flavonoid is rutin from buckwheat.
• Anthocyanins are a type of flavonoid that give
flowers red and blue pigments.
More Phenolics
• Some phenolics form polymers.
• Tannins are astringent to the taste. They give
dryness (astringency) to dry wines. They can also
be used to tan leather. They often give water a
tea-colored look. Tannins are common in pines
and oaks.
• Lignin is a major structural component of wood.
The exact structure of lignin is complex and not
known.
Minor Secondary Metabolites
• Mustard oil glycosides are nitrogen-sulfur containing
compounds that occur in cabbage, broccoli, horseradish,
watercress and other members of the mustard family
(Brassicaceae). They give the group its characteristic taste
and odor.
• Cyanogenic glycosides occur in several families of plants,
but are especially common in roses (Rosaceae) and peas
(Fabaceae). They are sugar containing compounds that
release cyanide gas when hydrolyzed.
• Cardiac glycosides effect vertebrate heart rate. Especially
common in milkweeds Asclepiadaceae.
• The parsley/carrot family Apiaceae is noted for having
aromatic and poisonous 17 carbon polyacetylenes, though
a few species have alkaloids like Coniium.
Mustard Oil