Monosaccharides- simple sugars
The most abundant monosaccharide is the six carbon glucose. Glucose is the major fuel for most organisms and the basic building block of the most abundant polysaccharides, such as starch and cellulose. In the biosphere, there is probably more carbohydrate than all other organic matter combined, largely due to the abundance in the plant world of two polymers of glucose: starch and cellulose.
Oligosaccharide - from two to ten monosaccharide units joined in glycosidic linkage.
Polysaccharides - contain many monosaccharide units joined in long linear or branched chains. Most polysaccharides contain recurring monosaccharide units of only a single kind or two alternating kinds.
Polysaccharides have two major biological functions:
a storage form of fuel
structural elements
Starch is the chief form of fuel storage in most plants, whereas cellulose is the main extracellular structural component of the rigid cell walls and the fibrous and woody tissues of plants.
Glycogen, which resembles starch in structure, is the chief storage carbohydrate in animals.
In addition, some polysaccharides and shorter polymers of sugars act as markers for a variety of cell recognition processes, including the adhesion of cells to their neighboring cells and the transport of proteins to appropriate intracellular locations.
>MONOSACCHARIDES Structures of some representative sugars are as follows:
see FIGURE in text
Basic formula (CH2O)n
C= carbo H2O = hydrate
With glucose n = 6 C6 H12 O6
Principle source of cellular energy
Other simple sugars have between 3 and 7 carbons
Sugars containing five or more carbons can form ring structures - which are the predominant forms of these molecules within cells.
alpha and beta ring structures both occur
see FIGURE in text
These two ring forms depend on the configuration of the first Carbon.
Two monosaccharides can be joined by dehydration synthesis which involves the removal of a molecule of water and the formation of a glycosidic bond.
see FIGURE in text
Glycogen and starch are composed entirely of glucose molecules in the alpha configuration in which the principal linkage is between the C1 of one unit and the C4 of the second unit. In addition, both glycogen and one form of starch (amylopectin) contain occasional a (1-6) linkages, in which carbon 1 of one glucose is joined to carbon 6 of a second glucose. These linkages lead to the formation of branches.
Cellulose in contrast has quite a distinct function as the principal structural component of the plant cell wall. Perhaps surprisingly, then, cellulose is also composed entirely of glucose molecules. The glucose molecules in cellulose are in beta linkage and cellulose is unbranched. This beta (1-4) linkage causes cellulose to form long extended chains that pack side by side to form fibers of great mechanical strength.
Lipids have 3 major roles in cells.
1- They provide an important form of energy storage.
2- They are the major components of cell membranes.
3- They play important roles in cell signaling, both as steroid hormones (e.g. testosterone and estrogen) and as messenger molecules that convey signals from cell surface receptors to targets within the cell.
Fatty Acids
simplest lipids
Fatty acids consist of long hydrocarbon chains, most frequently containing 16 or 18 carbon atoms, with a carboxyl group at one end.
see FIGURE in text
UNSATURATED fatty acids contain one or more double bonds between carbon atoms
SATURATED fatty acids all of the carbon atoms are bonded to the maximum number of hydrogen atoms.
The long hydrocarbon chains of fatty acids contain only nonpolar C-H bonds, which are unable to interact with water. The hydrophobic nature of these fatty acid chains is responsible for much of the behavior of complex lipids, particularly in the formation of biological membranes.
Fatty acids are stored in the form of TRIACYLCLYCEROLS, or fats, which consist of three fatty acids linked to a glycerol molecule. These molecules are insoluble in water and therefore accumulate as fat droplets in the cytoplasm. When required, they can be broken down for use in energy-yielding reactions.
Fats are actually a more efficient form of energy than carbohydrates, yielding more than twice as much energy per weight of material broken down.
Well over 100 different kinds of fatty acids have been isolated from various lipids of animals, plants, and microorganisms. Bacteria contain fewer and simpler types of fatty acids than higher organisms -- fatty acids with more than one double bond have not been found in bacteria.
PHOSPHOLIPIDS principle components of cell membranes, consist of two fatty acids joined by a polar head group. In the glycerol phospholipids, the 2 fatty acids are bound to carbon atoms in glycerol but the third C is bound to a phosphate group, which in turn is often attached to another small polar molecule, such as choline, serine, inositol, or ethanolamine.
SEE FIGURE in text
SPHINGOMYELIN , the only nonglycerol phospholipid in cell membranes, contains two hydrocarbon chains linked to a polar head group formed from serine rather than glycerol.
ALL phospholipids have POLAR head groups and NON-POLAR fatty acid tails.
AMPHIPATHIC- part water-soluble and part water -insoluble. This property is the basis for the formation of biological membranes
Many cell membranes also contain GLYCOLIPIDS and CHOLESTEROL. Glycolipids consist of two hydrocarbon chains linked to polar head groups that contain carbohydrates.
see FIGURE in text
Therefore they are quite similar to phospholipids in terms of general chemical properties.
CHOLESTEROL in contrast, consists of four hydrocarbon rings rather than linear hydrocarbon chains
see FIGURE in text
The hydrocarbon rings are strongly hydrophobic, but the hydroxyl group attached to one end of cholesterol is weakly hydrophilic, so cholesterol is amphipathic.
Steroid hormones are derivative of cholesterol
see FIGURE in text
These hormones are a diverse group of chemical messengers, all of which contain four hydrocarbon rings to which distinct functional groups are attached.