B. Carbohydrates

I. MOLECULAR COMPOSITION OF CELLS

Content

A. General information. 2

B. Carbohydrates. 3

C. Lipids. 4

D. Proteins. 5

E. Nucleic acids. 7

A. General information

1. Biology that deals with the properties and life phenomena of living matter, the study of life usually begins with cellular composition

2. Although organisms are tremendously varied, all cells are remarkably similar in their basic structures, particularly at the molecular level

3. About 99% (by weight) of all living matter is composed of carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur

4. Water makes up more than 50% of all living tissue and more than 90% of most plant tissues

5. There are 92 different types of naturally occurring atoms, but only a few are used to form the complex, highly organized molecules of living things

6. Molecules are groups of atoms joined together by chemical bonds

a. Chemical bonds are links of pure energy that result from the sharing or complete transfer of electrons between atoms

b. Bonds hold together similar or different types of atoms to form the many kinds of molecules

c. Chemical bonds are classified according to the type of interaction between the atoms

(1) Covalent bonds result from a sharing of electrons by two atoms

(2) Ionic bonds result from the complete transfer of electrons from one atom to another; an atom either gains or loses electrons

7. All organic compounds contain carbon, and they may also contain hydrogen, oxygen, and nitrogen

a. Carbon atoms tend to form covalent bonds rather than ionic bonds

b. Organic molecules, which are classified according to their chemical structure and properties, may consist of long chains of carbon atoms

8. The four classes of biologically important molecules are the carbohydrates, lipids, proteins, and nucleic acids

a. Molecules belonging to these four classes are called macromolecules because of their large size

b. Most macromolecules consist of smaller repeated units, or building blocks

9. Monomers are the building blocks of large molecules; many monomers link together to form large molecules known as polymers

a. Through the process of dehydration synthesis, monomers are chemically joined by the removal of water to form polymers

b. Through hydrolysis, polymers are split chemically into their component monomers by the addition of water

B. Carbohydrates

Carbohydrates all have the empirical formula

CH₂O.

Carbohydrates are polyhydroxy aldehydes, polyhydroxy ketones, or compounds that can be hydrolyzed to them. A carbohydrate that cannot be hydrolyzed to a simpler compound is called a “monosaccharide.” If it can be hydrolyzed into two monosaccharides, it called a “disaccharide.” A “polysaccharide” can be hydrolyzed into many monosaccharides. Upon further classification, if a carbohydrate contains an aldehyde group, it is known as an “aldose”, if it contains a ketone group, it is known as a “ketose.”

Monosaccharides are also classified by the number of carbon atoms they contain. A triose, tetrose, and pentose carbohydrate would have three, four, and five carbon atoms, respectively. An aldopentose would be a five-carbon monosaccharide with an aldehyde group. A ketohexose would be a six-carbon monosaccharide containing a ketone group. A carbohydrate that reduces Fehling’s reagent is known as a “reducing sugar.” All monosaccharides are reducing sugars. Most disaccharides are reducing sugars (sucrose is one of the exceptions). Carbohydrates can exist either in open chain form or a closed ring. It is easy to change from one to the other.

1. Carbohydrates, the most abundant organic compounds in nature, are typically composed of carbon, hydrogen, and oxygen in a ratio of 1:2:1 (C:2 H:O)

2. Carbohydrates include sugars, starches, and related substances

3. They may be used as energy sources for the cell, such as energy storage units, or as structural components, such as membranes and organelles

4. Carbohydrates, like other macromolecules, are classified according to their structure and function:

a. Monosaccharides are simple sugars

(1) They usually consist of five or six carbon atoms that form a ring

(2) Common ones include glucose, ribose, deoxyribose, and fructose

a) Glucose has six carbon atoms and the formula C₆H₁₂O₆

b) Ribose has five carbon atoms and the formula C₅H₁₀O₅

c) Deoxyribose also has five carbon atoms but lacks one oxygen atom; its formula is C₅H₁₀O₄

d) Fructose, or "fruit sugar," has six carbon atoms and a chemical formula identical to that of glucose

(1) Many monosaccharides, such as glucose and fructose, have a short life span in the cell because they are either metabolized to free energy for use in cellular reactions or are linked together to form disac-charides or polysaccharides

b. Disaccharides are two monomer units joined by dehydration synthesis

(1) Common disaccharides include sucrose (table sugar), which consists of one unit of glucose joined to one unit of fructose, and maltose (malt sugar), which consists of two units of glucose

(2) When the cell needs energy, disaccharides are commonly converted to monosaccharides by hydrolysis

(3) In plants, disaccharides are the type of sugar that is commonly transported from one area to another

c. Polysaccharides are long chains of monosaccharides used as energy storage molecules and structural components of cells

(1) Starch, the chief storage polysaccharide in plants, consists of long chains of glucose molecules

(a) It may occur as a coiled, unbranched chain of up to 1,000 glucose subunits

(b) It more commonly occurs as huge branched chains of up to 500,000 glucose subunits

(2) Glycogen, a common storage molecule in fungi and bacteria, is a polymer of glucose; it is usually smaller than starch, with small branches every 10 to 12 glucose subunits

(3) Cellulose, the principal structural polysaccharide in plants, is found in cell walls

a) It is the most abundant natural polysaccharide

b) Cellulose is composed of chains of glucose, as is starch and glycogen, but the chains are oriented differently (every other glucose subunit is upside down), thereby giving cellulose different biologic functions

(4) Chitin, a structural polysaccharide found in the cell walls of fungi, consists of a six-carbon sugar with many nitrogen groups attached

(5) Pectin, another important structural polysaccharide in plants, serves as a glue to hold adjacent cells together, particularly in fruits

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