III. Molecules
A. Water
1. Structure
- H2O - polar molecule: partially positive hydrogen ends, partially negative oxygen end
- thus, water forms hydrogen bonds with other water molecules
2. Properties and Their Importance
a. liquid at common earth temperatures
can be a medium for life processes on earth
b. universal solvent - polar and ionic compounds
chemicals can dissolve (and then interact)
c. cohesive/adhesive
'sticks' to itself and other polar/charged surfaces...capillary action in living systems
d. high specific heat - takes lots of energy to change its temperature
the temperature of terrestrial organisms changes less rapidly than air,
giving the organism time to respond
aquatic habitats are thermally stable - easy to adapt to
evaporation causes a large exchange of heat energy (cooling a surface when water is warmed and
evaporated from it)
e. maximum density at 4 degrees C. ice floats, and can be melted again in spring. Water circulation
f. it dissociates - 1 x 10^-7 molecules in pure water = pH of 7. HCl dissociates more frequently (because Cl rips the electron away from H more easily) - in fact, about 1 in 100 molecules of HCl is dissociated (1 x 10^-2) for a pH = 2.0
B. Carbohydrates
1. Structure:
- monomer - monosaccharide (simple sugar) - CnH2nOn
(glucose, galactose, fructose are 6 carbon sugars; ribose, ribulose, deoxyribose are 5 carbon sugars)
- disaccharides: sucrose (glucose + fructose); maltose - (2 glucose)
- polymer - polysaccharide - chain of sugars
starch, glycogen, chitin, cellulose
- monomers are linked together into polymers using dehydration synthesis - a removal of a water molecule (dehydration) and the synthesis of a bond. This requires energy and is catalyzed by enzymes in living systems.
2. Function:
a. energy storage:
- all large biomolecules have lots of bonds and thus store lots of energy.
But, the larger the molecule, the more time it takes to harvest all the
energy by metabolic breakdown (catabolism). So, polysaccarides serve better as
'longer-term' energy storage than monosaccharides, whereas monosaccarides,
because they can be metabolized more quickly, serve better as a short term
energy supply. (starch in plants andglycogen in the liver of animals are longer term storage molecules; glucose is the short-term energy molecule in all of life)
b. Structural:
- cellulose is just a long chain of glucose. And decomposers break
down wood to create the sugars they will use for metablic energy.
- chitin is the primary component of exoskeletons in arthropods.
C. Proteins
1. Structure:
monomer - amino acid - amine (NH2) group at one end and carboxyl
group (COOH) at other
there are 20 different amino acids that are found in living systems.
polymer - polypeptide - 100 to 300 amino acids long. The AA's are linked by dehydration synthesis reactions into a long linear chain. Because there are 20 Amino Acids ("letters") that can be used in their construction, proteins can have a limitless number of different combinations (like letters in different combinations make differnt "words"). This variety in form means variety in function.
**Higher levels of structure:
1. the primary structure of a polypeptide/protein is the linear sequence of amino acids
2. this linear sequence can take a helical or "pleated" sheet shape, depending on bond angles and soforth. These are secondary levels structure
3. some proteins then fold upon themselves, taking a globular shape. This globular shape is maintained by bonds between different functional groups of differnt amino acids. Enzymes and cell membrane proteins are common globular proteins.this is called tertiary structure.
4. Sometimes, single proteins are not functional on their own - they must be combined with other proteins to forma a protein with a quaternary structure. Hemoglobin, with 2 alpha and 2 beta globular polypeptides, is one example. collagen is another, composed of several helical polypeptides.
2. Function:
a. Energy Storage: (all biomolecules can be broken down for energy harvest.
Typically, since proteins are doing something else, too, they are broken
down last so that the organism can maintain this function that the protein
performs for as long as possible).
b. Structural:
after water, animals are largely proteinaceous
collagen, elastin, muscle proteins, etc.
c. Metabolic:
all biological reactions are catalyzed. Most biological catalysts
are proteinaceous ENZYMES
d. transport:
cell membrane - there are proteins that assist transport across the membrane
organism - hemoglobin, for instance, transport oxygen
e. Immunity:
antibodies are proteins.
Study Questions:
1. Draw the structure of a water molecule, showing all electron "shells" and shared electrons, and use your diagram to explain why water is cohesive.
2. What are three functions of carbohydrates?
3. Why are proteins so variable in structure and function?
4. Draw the basic structure of 2 amino acids and show how they are linked in a dehydration synthesis reaction.
5. List four major functions of proteins.