Molecular composition of plants

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• Molecular composition of plants
• Chapter 2 in your text

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Eight checkpoints

• What are the four main types of organic molecules
  found in plant cells?
• What are the principal functions?
• How are they split, how are they joined?
• Energy storing versus structural polysaccharides
• What is an enzyme
• ATP, ADP etc.
• Primary versus secondary metabolite
• What are the main types of secondary metabolites

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Four main types of organic molecules that make up
living things

• Organic
• originally meant related to living matter
• now refers to carbon-based chemistry
• Carbohydrates
• Lipids
• Proteins
• Nucleic acids

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Carbohydrates

• Carbon, hydrogen, and oxygen (more than just a
  trace)
• Best known example, sugars, especially glucose
• Glucose is the most important sugar in our blood
  but is not very important in plant metabolism
• When is a sugar not a sugar?
• When it is a sugar alcohol, for example in
  sugar-free gum

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Sugars (-ose)

• (CH2O)n carbon with water, three carbons or more
• Each carbon has one hydrogen and one hydroxyl
• H-C-OH, except for one carbon
• CO, double bonded oxygen, called a carbonyl
• Carbonyls and hydroxyls make sugars hydrophilic

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Sugars

• (CH2O)n
• n 3 example, glyceraldehyde
• n 4 example, erythrose
• n 5 example, ribose
• n 6 examples, glucose and fructose
• Five and six carbon sugars often exist in a ring
  form

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Sugars

• Monosaccharides
• Disaccharides formed by adding two
  monosaccharides together
• -OH HO- ? -O- H2O
• Condensation reaction, dehydration synthesis
• Reversal is called hydrolysis
• The disaccharide of sucrose (glucose fructose)
  is stabilized by the bond between the
  monosaccharides. Sucrose is called a nonreducing
  sugar because it does not react with (reduce)
  copper in a famous test for sugars

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Important disaccharides

• Sucrose
• Table sugar, beet sugar, cane sugar
• A nonreducing sugar, more stable
• Primary transport sugar in plants
• Maltose
• Two glucoses (G2)
• Product of starch hydolysis
• Made by starting grains (eg barley) toward
  germination

• Lactose (non-botanical side light)
• Glucose plus galactose, important in milk
• People often lose the enzyme needed to hydrolyze
  lactose, said to be lactose-intolerant

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Important polysaccharides

• Starch
• Glucose based, 1-4 linkages mostly with a few
  branches (1-6 linkages)
• Regular branching pattern
• (Animal equivalent is glycogen but the branching
  pattern is irregular in glycogen)
• When boiled in water, starch gelatinizes by
  taking up water, becomes soluble but the solution
  is thick making gravy

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Starch structure
?-1,6 linked branch point
?-1,4 linked polyglucan
Different branch lengths etc. give starches
different properties. For example, corn starch is
good for making gravy.
Other terms you may see amylose is all straight
chain (?-1,4), amylopectin is the branched
structure shown here. These terms are not
required for Botany 130.
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Important polysaccharides

• Cellulose (structural)
• Glucose based, 1-4 linkages
• The 1-4 linkage is upside down in cellulose
  compared to starch (?-1,4)
• Makes long straight molecules compared to the
  spirals of starch
• Very resistant to breakdown, cows use bacteria in
  their rumen to breakdown cellulose
• Fructans
• stored in many cereals in cold climates

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Storage versus structural polyglucans
? - linkages are kinked
? - linkages result in a straight chain
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Lipids

• A class of compounds defined by properties, not
  composition
• Lipids are fats and fat-like substances
• Generally insoluble in water
• Typically have just carbon and hydrogen but can
  have small amounts of oxygen
• Two primary types
• Fats and oils
• Sterols (and other isoprenoids)

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Fats and oils

• Triglycerides
• Three carbon backbone with fatty acids attached
• Fatty acids are long carbon/hydrogen chains
• Made from two-carbon units so fatty acids are
  often 16 or 18 carbons but rarely 17

COO-
Stearic acid, an 18 carbon fatty acid

• Fats are solid at room temperature
• Oils are liquid at room temperature

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Lard
Linseed oil (Flax)
Olive oil
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Membrane-forming lipids
P
C

• Phospholipids

C
C-O
C-O
C
Gal
C
Galactolipids
C
C-O
C-O
C
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Waxes, cutin, and suberin

• Derived from fatty acids
• Complex chemical mixtures
• Prevent water loss and probably pathogen and
  herbivore attacks
• Especially important on leaf surface and
  Casparian strip in the root

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Sterols as lipids

• Cholesterol in animals, stigmasterol in plants
• Sterols play a crucial role in membranes
• Both plants and animals have also adapted sterols
  for use as chemical messengers

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Sterols
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Four main types of organic molecules that make up
living things

• Carbohydrates
• Lipids
• Proteins
• Nucleic acids

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Proteins

• Chemically defined
• Amino acids linked together by peptide bonds
• Bond formed by removal of water, opposite
  reaction is hydrolysis of proteins
• Properties of proteins depend on which amino
  acids are used and in what order

• Many amino acids exist, but only twenty are used
  in proteins (in all organisms, the same 20)
• In a few cases some amino acids are modified
• Amino acid selection is one thing that is common
  to all life forms

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Start here Friday
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Physiological form
R
R
Amino acids are linked together by peptide bonds.
Linked amino acids are called peptides.
H N-C-C-O- O
H H-N-C-C H O

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By convention, proteins are read from the
amino end to the carboxy end
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?-helix
Linus Pauling
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Four levels of protein structure

• Primary structure the order of amino acids
• Secondary first folding of the chain.
• Examples ?-helix, ?-sheets
• Tertiary folding of secondary structures on
  themselves
• Quaternary more than one string of amino acids
• Many proteins do not have quaternary structure

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Essential amino acids

• Animals cannot make some amino acids, these are
  called essential
• In plants, essential amino acids are made in
  plastids
• Not all proteins have the right mix of amino
  acids for good animal nutrition
• Eating grains with legumes helps
• have beans on your taco
• Efforts underway to modify corn to make more
  balanced protein

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Proteins as catalysts

• Enzymes are proteins (mostly) that catalyze
  reactions
• If it ends in ase it is most likely an enzyme
• Enzymes do not cause reactions, enzymes allow
  reactions to proceed

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Four main types of organic molecules that make up
living things

• Carbohydrates
• Lipids
• Proteins
• Nucleic acids

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Nucleic acids

• Cells contain a lot of DNA, RNA, etc.
• The large amount made it easy to isolate long
  before it was known how the nucleic acids
  function
• DNA and RNA are made up of individual
  nucleotides, the A, T(U), C, G bases that encode
  genes
• These nucleotides are also involved in metabolism
• Best example, ATP, the A in the four letter
  alphabet of DNA

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(Ribose)
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• Best known role of nucleic acids is as DNA
• DNA uses deoxyribose instead of the ribose of RNA
• The lack of oxygen at carbon 2 of ribose reduces
  the reactivity, therefore increases stability
• Increased stability is good for storage of
  information
• The higher reactivity in RNA makes it good for
  processing information

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RNA and DNA is made by attaching to the number
3 carbon of the ribose or deoxy- ribose to make a
long string
Some vocabulary - Nucleotide - Nucleotide
triphosphate - Nucleoside - sugar plus
base (adenine ribose adenosine)
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Carbons numbered without primes
A
5
1
Carbons labeled with a prime.
4
3
2
Direction of DNA and RNA is, by convention,
read 5 to 3, ATG, not GTA
T
G
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Many cofactors in metabolism are nucleotides

• ATP is called the energy currency of the cell
• Adenine Ribose P P P
• General carrier
• Adenine Ribose P P X
• X P, S, glucose, etc.
• Other nucleotides also participate in metabolism
• Example UDPglucose in cellulose synthesis

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Secondary metabolism

• What is secondary metabolism?
• Things made by some, but not all plants
• Often involved in defense
• Not required for the plant to survive
• Three families
• Two pathways
• Phenylpropanoids
• Isoprenoids (includes terpenes and sterols)
• One functionally defined group
• Alkaloids

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Alkaloids
- When potatoes are exposed to light they make
chlorophyll and solanine solanine is
poisonous - Dont eat green potatoes!
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Alkaloids
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Isoprenoids include terpenes and sterols
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Frits Went and the Blue Mountains
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Salicylic acid is made by the phenylpropanoid
pathway. Other phenolics make leaves taste
bitter, discouraging herbivores (plants dont
want to be eaten).
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Summary

• Four major components of cells
• Carbohydrates - structural (cellulose) or storage
  (starch), simple sugars (fruit sugar-fructose),
  disaccharides
• Lipids - fats, oils, isoprenoids etc., fatty
  acids, triglycerides, phospholipids,
  galactolipids
• Proteins - four levels of structure, peptide
  bond, NCC backbone and R goups, directionality,
  enzymes
• Nucleic acids, DNA, RNA, plus many cofactors in
  metabolism
• Secondary compounds
• Often defensive, poisonous (but we like to live
  on the edge) alkaloids, isoprenoids,
  phenylpropanoids