Water, Carbohydrates and Lipids

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Water, Carbohydrates and Lipids

• Molecular characteristics and interactions

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Just enough biochemistry?

• The idea for the next couple of lectures is to
  review just enough of the structure and behavior
  of biologically important molecular classes to
  allow you to make sense of their function in the
  cellular context -

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Water the universal solvent of biological
systems

• 75-85 of the typical cell weight is H2O.
• Polarity is the result of an uneven distribution
  of electrons within a molecules structure.
  Polarity of water allows formation of hydrogen
  bonds between water molecules or with other polar
  molecules.
• Membrane components interact with the polar
  nature of water to determine cell boundaries.

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Polar and Ionic Solutes are hydrophilic, or
readily soluble in water

• Water-loving or hydrophilic molecules have
  prosthetic groups that can form hydrogen bonds
  with water.
• Examples hydroxyl, amino and carboxyl groups
• Ions cannot form hydrogen bonds with water, but
  their charges attract a shell of water molecules
  oriented to oppose their charge. This is called
  the ions hydration shell.

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• Hydrophobic molecules are excluded by water
• Water molecules simply minimize their contact
  with nonpolar, nonionic substances, which are
  thus poorly soluble in water
• Examples of such groups aromatic rings and long
  aliphatic chains
• Amphipathic molecules contain both polar
  (hydrophilic) and non-polar (hydrophobic) groups
• They can thus interact both with solvent water
  and with each other

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Carbohydrates

• The Formula for CarbohydratesCnH2nOn

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Carbohydrates Simple sugars and polysaccharides

• Smaller carbohydrates sugars such as
  monosaccharides illustrated below and the
  disaccharides, maltose, lactose, galactose and
  sucrose

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Sugars are joined by glycosidic bonds in a
dehydration reaction to yield short or long
polymers
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Other dehydration examples
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Oligosaccharides

• Oligosaccharides are short chains of sugars
  they may be linked to other molecules to serve as
  address labels
• Examples protein-oligosaccharide links can allow
  proteins to be delivered to the right organelle
  or part of the cell membrane, and
  oligosaccharides extending from cell membranes
  label the cell in ways that other cells, such as
  the cells of the immune system, can read. The
  electron micrograph below shows the surface of an
  erythrocyte with its thick (up to 1400A)
  carbohydrate coat, called the glycocalyx.

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Polysaccharides result from polymerization of
sugar molecules

• Glycogen synthesis (typical of animal cells) and
  synthesis of starch (a common storage form in
  plants), involves long a(1-4) bonds with
  occasional a(1-6) branch points. (Both are
  highly digestible.)
• Cellulose synthesis (typical of higher plants)
  involves ß(1-4) bonds. The polymers associate to
  form rigid structures such as plant cell walls
  and wood. (Digestion is typically accomplished by
  microbes.)
• Chitin (present in the arthropod exoskeleton and
  also in cell walls of lower plants) is a ß(1-4)
  bond polymer of glucosamine residues

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Polymer types
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Lipids

• Roles in Cells
• 1. Energy storage/retrieval
• 2. Major components of cell membranes
• 3. Information molecules
• a) between cells (steroid hormones)
• b) within cells (membrane phospholipids
  hydrolyzed to yield second messengers)

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The nature of lipids

• Lipids are organic compounds that possess long
  chains consisting of hydrogen and carbon. Fatty
  acids have a carboxyl (acid) group (COO-) at the
  end. The chains may be either saturated
• or unsaturated

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Fatty Acids Double bonds between the carbons
store additional energy and give the molecule a
kink.
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Fatty acids linked to glycerol make triglycerols,
otherwise known as fats.

• Triglycerols are the form in which fatty acids
  are stored. A given amount of chemical energy
  can be stored in half the weight if stored as fat
  rather than carbohydrates. This makes fats
  superior to carbohydrates as a form of energy
  storage, especially for organisms that move
  around (e.g., animals rather than plants.)

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Variations on the triglyceride structure

• What if one of the fatty acids is replaced by
  another kind of molecule.?
• This can result in
• Glycolipids and
• Phospholipids
• Both of these molecules are important in the
  composition of membranes.

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Examples of Phospholipids
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An example of a Glycolipid
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Cholesterol A membrane component in many animal
cells and the starting point for steroid hormone
synthesis
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How cholesterol looks in a membrane..
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Features of a membrane containing only
phospholipids

• 1. The stable bilayers have fluidity, and the
  individual lipid molecules can spin and drift
  around while maintaining their orientation (polar
  group toward the water, hydrophobic fatty acids
  away from water). In artificial membranes the
  lipids almost never shift from one half of the
  bilayer to the opposite side.
• 2. The hydrophobic interior repels polar
  molecules or ions, but very small molecules (like
  O2 or CO2), with molecular weights below 100,
  diffuse through even if they are polar (like H2O,
  EtOH or urea)

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Features, Cont.

• 3. The proportion of different phospholipids
  affects fluidity/rigidity, and can be adjusted in
  living cells as one aspect of temperature
  acclimation.
• Determining Factors
• A. the length of the hydrocarbon chain as it
  moves from 10 to 20, the membrane becomes less
  fluid.
• B. For a given number of carbons, the presence of
  unsaturation increases the fluidity of the
  membrane, because the fatty acids do not pack as
  tightly.

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Lipid packing potential
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Different membranes are composed of different
lipids
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Membrane asymmetry

• The types of lipids are distributed unequally
  between the outer and inner membranes.
• In general, the lipids with carbohydrate groups,
  the glycolipids, protrude from the outer side of
  the bilayer, where they are involved in signaling
  and recognition, and the inner monolayer is
  predominantly composed of phospholipids.

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Changing concepts of biological membranes

• 1926 The original proposal of membranes as lipid
  bilayers.
• 1943 the addition of the concept of protein on
  membrane surfaces.
• 1972 Understanding that proteins are
• 1) anchored in the lipid bilayer to extend on one
  side
• 2) Integral to the protein and detectable on both
  surfaces
• Current Advances include understanding of
  protein structure that allow proteins to anchor
  or extend through the hydrophobic interior.

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Membrane concepts illustrated
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Summary

• In reviewing the biochemistry of cells, we are
  focusing on molecules as sources of energy, as
  structural components of the cell, and as the
  elements of a living and functioning system.
• The roles of carbohydrates for animal cells are
• 1. quick sources of energy (sugars obtained by
  release from polymers or digestion)
• 2. energy storage (the polysaccharide glycogen)
• 3. cell recognition (in association with lipids
  or proteins)
• The roles of lipids in animal cells are
• Energy supply and storage (triglycerides)
• Membrane components
• Information/communication molecules