Transcript Carbohydrates and Lipids - Washington State University

Water, Carbohydrates and Lipids
Molecular characteristics and
interactions
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 -
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 molecule’s
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.
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 ion’s
hydration shell.
• 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
Carbohydrates
The Formula for Carbohydrates:CnH2nOn
•
Smaller carbohydrates – sugars such as monosaccharides illustrated below and
the disaccharides, maltose, lactose, galactose and sucrose
Carbohydrates: Simple
sugars and
polysaccharides
Sugars are joined by glycosidic bonds in a dehydration
reaction to yield short or long polymers
Other dehydration examples
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.
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 α(1-4) bonds with
occasional α(1-6) branch points. (Both are highly digestible.)
• Cellulose synthesis (typical of higher plants) involves β(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 β(1-4) bond polymer of glucosamine residues
Polymer types
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)
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:
Fatty Acids: Double bonds between the carbons store additional
energy and give the molecule a “kink”.
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.)
Variations on the triglyceride structure:
What if one of the fatty acids is
replaced by another kind of
molecule….?
This can result in
1. Glycolipids and
2. Phospholipids
Both of these molecules are
important in the composition
of membranes.
Examples of Phospholipids
An example of a Glycolipid
Cholesterol: A membrane component in many animal cells
and the starting point for steroid hormone synthesis
How cholesterol looks in a membrane…..
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)
Features, Con’t.
• 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.
Lipid packing potential
Different membranes are composed of different lipids
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.
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.
Membrane
concepts illustrated:
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
1. Energy supply and storage (triglycerides)
2. Membrane components
3. Information/communication molecules