Plant Metabolism

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Plant Metabolism
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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.

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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

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Harvesting Sucrose
Sugar Cane Maple Syrup
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Refining Sucrose
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Polysaccharides

• Structural polysaccharides are used to support
  plants
• Storage polysaccharides are used to store energy
  for later use by the plant

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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

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Cotton Boll Pure Cellulose
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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) 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

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Pectin and Hemicellulose
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Gum Arabic from Acacia senegal
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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.

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Inulin another storage carbohydrate
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Jerusalem artichoke
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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.

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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.

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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

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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.

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Papaya Papain and Chymopapain
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Pineapple - Bromelain
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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

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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.

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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, 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 cant make them ourselves.

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Olive Oil
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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.

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Bayberry Wax
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Jojoba Wax
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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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Mustard Oil