Chapter 2 The Molecular Composition of Plant Cells

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Chapter 2The Molecular Composition of Plant
Cells
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Outline

• Intro
• Organic molecules
• Carbohydrates
• Lipids
• Proteins
• Nucleic acids
• Secondary metabolites

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Intro

• All living/nonliving on Earth composed of
  elements
• element substance of 1 atom
• major elements in living organisms C, H, O, N,
  P, S
• 92 naturally occurring elements
• molecule smallest unit of cmpd, 2 atoms
• non-woody plant tissues
• gt90 wt is H2O, 9 wt is organic molecules, 1
  is ions
• organic molecule (OM) contains C-C bond

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Organic Molecules

• 4 types OM comprise most of dry wt of living
  organisms
• carbohydrates
• lipids
• proteins
• nucleic acids
• Macromolcules made of smaller subunits called
  polymers
• polymers made of monomers
• polymerization linking of monomers into
  polymers

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Carbohydrates

• Most abundant OM in nature
• cell walls (CW)
• Composed of sugars
• 3 types of carbohydrates
• monosaccharides
• ribose, glucose, fructose
• disaccharides
• sucrose, maltose, lactose
• polysaccharides
• cellulose, starch

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Carbohydrates Monosaccharides

• Simplest carbohydrate
• Source of S and monomer of complex carbohydrates
• Specific proportion of C, H, O (1C 2H 1O)
• (CH2O)n where n3 to 7
• C skeleton w/ -OH attached to every C except one
• other C has -CO (carbonyl) attached to it
• hydrophilic thus most carb. dissolve in water
• pentose and hexose most common
• chain or ring form

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Common monosaccharides glyceraldehyde used as
C skeleton ribose used in DNA, RNA, ATP
glucose used structure, transport
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Carbohydrates Disaccharides

• Disaccharide
• 2 monosaccharides
• water removed via dehydration syn. or
  condensation rxn
• requires S
• hydrolysis is opposite of condensation rxn
• splitting of polymer by
• water added, releases S
• sucrose is transport molecule in plants
• glucose fructose
• table sugar

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Carbohydrates Polysaccharides

• Storage/transport form of S and structural
  material
• Long chains of monosaccharides
• Starch is 1 storage polysaccharide in plants
• glycogen is storage polysaccharide in
  animals/fungi
• polymer of ?-glucose linked by ?-1,4-glycosidic
  bonds
• 2 forms of starch
• amylose (unbranched)
• amylopectin (branched)

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Carbohydrates Polysaccharides

• Cellulose is 1 structural polysaccharide in
  plants
• component of CW and fibers
• very abundant
• wood ½ cellulose
• cotton almost pure cellulose
• ½ organic C is cellulose
• composed of monomers of ß-glucose
• unbranched, ß-1,4-glycosidic bonds
• flipping thus H-bonding btwn strands
• not easily broken down
• CW also contains pectins and hemicelluloses
• branched polysaccharides
• pectins middle lamella, cements CWs
• hemicelluloses stabilize CW by H-bonding

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Carbohydrates Polysaccharides

• Chitin structural polysaccharide in fungal CW
  and exoskeletons of arthropods
• polymer of N-acetyl-glucosamine monomers linked
  by ß-1,4-glycosidic bonds
• similar flipping and H-bond as cellulose

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Lipids

• S storage (fats, oils) and structural func.
  (phospholipids, waxes)
• Hydrophobic, insoluble in water
• Not macromolcules b/c not polymers of monomer
  subunits

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Lipids Fats and Oils

• Store S, esp. in seeds and fruits
• Higher proportion of C-H bonds than carbohydrates
  
• fat 9.1 kcal/g
• carbohydrate 3.8 kcal/g
• protein 3.1 kcal/g

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Lipids Fats and Oils

• Structure
• 3 fatty acid (FA) chains bound to one glycerol
  (thus a.k.a. triglyceride) by dehydration syn.
• no hydrophilic (polar) groups thus hydrophobic
• forms droplets/clusters in water
• saturated vs. unsaturated
• nature determined
  by
  saturation and
  FA chain
  length
• melting pt.

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Lipids Phospholipids

• Plasma/cell membranes (PM)
• Structure
• 2 FA chains 1 PO4-/R group bound to 1 glycerol
• forms by dehydration syn.

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Lipids Phospholipids

• PO4- has (-) charge and polar R group thus
  hydrophilic head and hydrophobic tail
• hydrophilic heads face water
• hydrophobic tails face inward

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Lipids Cutin, Suberin and Wax

• Part of cell wall
• barrier for water mvmt, prevent water loss
• Cuticle covers epidermal cells of leaves and
  stems
• composed of wax embedded in cutin (cuticular wax)
• cuticle covered byepicuticular wax

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Lipids Cutin, Suberin and Wax

• Suberin part of CW of cork
• suberized having suberin, layered look
  (lamellar)
• light band wax, dark band suberin
• Wax most water repellent

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Lipids Steroids

• Steroid 4 interconnected hydrocarbon rings
• Many functions
• stabilize PM, hormones, growth
• sterol hydroxyl attached to C3, stabilize
  phospholipid tails