Essay Plan E6A “The Importance of Carbohydrates in Living Organisms”
INTRODUCTION
What is carbohydrate? Monomers/polymers containing C, H and O (H:O ratio of 2:1 hence 'hydrate' = water).
Condensation of monomers to form polymers with the release of water. Glycosidic bonds. The monomers differ
and so the polymers can form different shapes. Monosaccharides, dissaccharides and polysaccharides.
ANIMALS: Storage and liberation of energy
PLANTS: Storage and liberation of energy AND structural function
Energy source
Monosaccharides:
Glucose:
Mostly important carbohydrate in aerobic respiration to release energy (ATP). The large number
of carbon-hydrogen bonds broken to release a lot of energy which is transferred to form ATP.
Easily transported round the body. Plants transport glucose in the form of the disaccharide,
sucrose (=glucose + fructose), which is very soluble and so efficiently transported. Carbohydrate
polymers can be broken down to their monomers before being oxidised during respiration.
Storage function
Polysaccharides:
The following are polymers of alpha glucose which are insoluble in water & compact, and act as energy stores.
Polymers are stored rather than simple monomers, which would otherwise lower the water potential of the cell
and cause an osmotic imbalance. Polymers can be broken down by a hydrolysis reaction to liberate monomers.
Starch:
Found in most parts of a plant, also reserve food formed from any excess glucose molecules
produced during photosynthesis. Can form an unbranching, helical chain (amylose) and a highly
branched form (amylopectin) -- compact. Observed as starch grains under the microscope.
Glycogen:
Major storage material in animals and fungi, stored in the liver and muscles. When glycogen
stores are full, fat is formed via intermediates of aerobic respiration. Very similar structure to
amylose, but is also highly branched (more so than amylopectin) -- very compact. The
hormones insulin and glucagon are released from the pancreas in response to blood glucose
levels.
Structural function
Monosaccharides:
Deoxyribose: A pentose monomer used to form DNA
Ribose:
A pentose monomer used to form RNA to provide structural support to the molecules, also
constituent of hydrogen carriers (NAD & NADP). Ribose is used in the ATP molecule.
Ribulose:
is a carbon dioxide acceptor in the light-independent stages of photosynthesis.
Polysaccharides:
Cellulose:
Typically 50% of plant walls, structural strength, polymer of beta glucose. Long straight chains
held together by weak hydrogen bonds. Collectively the weak hydrogen bonds provide
enormous strength to the molecule. This strength also means that plant cells can withstand the
large pressures that develop due to osmosis. Without cellulose the cell wall would burst if the
cell was placed in a dilute solution. Turgor pressures provide support to the plant. The position
of the cellulose in the cell wall determines the shape of a developing plant cell.
Important food source for some organisms (bacteria and fungi). These organisms release the
enzyme, cellulase. However, cellulose is indigestible to many organisms and so forms fibre in
the diet - provides bulk to food as it passes along the gut and so gives the muscles something to
push against in peristalsis. Symbiotic bacteria found in ruminant are able to digest cellulose and
in turn act as a food source for the ruminant.
CONCLUSION