Chapter 3The Molecules of Cells

By Dr. Par Mohammadian

Overview:-Carbon atom-Functional Groups-Major Biomolecules

Life’s molecular diversity is based on the properties of

carbon– Cell consists mostly of carbon-based

molecules– Organic chemistry: Study of carbon

compounds

Carbon:– It has four electrons in an outer shell that

holds eight– Carbon can share its electrons with other

atoms to form up to four covalent bonds

• The simplest organic compounds are hydrocarbons– These are organic molecules containing only carbon

and hydrogen atoms– The simplest hydrocarbon is methane

• The unique properties of an organic compound depend not only on its carbon skeleton but also on the atoms attached to the skeleton– These atoms are called functional groups

• Functional groups are the groups of atoms that participate in chemical reactions– Hydroxyl groups are characteristic of alcohols– The carboxyl group acts as an acid

Functional groups help determine the properties of organic

compounds

• Most of the large molecules in living things are macromolecules called polymers (e.g. proteins, DNA)– Polymers are long chains of smaller

molecular units called monomers– A huge number of different polymers can

be made from a small number of monomers

Cells make a huge number of large molecules from a small set of

small molecules

• Cells link monomers to form polymers by dehydration synthesis

• Polymers are broken down to monomers by the reverse process, hydrolysis

• There are four categories of large molecules in cells

BIOLOGICAL MOLECULES

– Carbohydrates– Lipids– Proteins– Nucleic acids

• Carbohydrates include

Carbohydrates

– Small sugar molecules in soft drinks– Long starch molecules in pasta and potatoes

Monosaccharides are the simplest carbohydrates

• Monosaccharides are simple sugars

• The monosaccharides glucose and fructose are isomers

– Their atoms are arranged differently

• The most common disaccharide is sucrose, common table sugar

– It consists of a glucose linked to a fructose– Sucrose is extracted from sugar cane and the roots of

sugar beets

• Simple sugars and double sugars dissolve readily in water

– They are hydrophilic

• A disaccharide is a double sugarDisaccharides

– It is constructed from two monosaccharides

• Disaccharides are joined by the process of dehydration synthesis

• Complex carbohydrates are called polysaccharides

Polysaccharides

– They are long chains of sugar units– They are polymers of monosaccharides

• These large molecules are polymers of hundreds or thousands of monosaccharides linked by dehydration synthesis

• Starch and glycogen are polysaccharides that store sugar for later use

• Cellulose is a polysaccharide in plant cell walls

• Lipids are hydrophobic

Lipids

– They do not mix with water– Examples: fats and steroids

• Fats are lipids whose main function is energy storage– They are also called triglycerides

• A triglyceride molecule consists of one glycerol molecule linked to three fatty acids

• The fatty acids of unsaturated fats (plant oils) contain double bonds– These prevent them from solidifying at room

temperature

• Saturated fats (lard) lack double bonds– They are solid at room temperature

• Phospholipids are a major component of cell membranes

• Waxes form waterproof coatings

• Steroids are often hormones

Phospholipids, waxes, and steroids are lipids with a variety of functions

• Steroids are very different from fats in structure and function

Steroids

– The carbon skeleton is bent to form four fused rings

• Cholesterol: your body produces other steroids

• Proteins are involved in – cellular structure– movement– defense– transport– communication

• Mammalian hair is composed of structural proteins

• Enzymes regulate chemical reactions

PROTEINSProteins are essential to the structures and activities

of life

• Each amino acid consists of

– A central carbon atom bonded to four covalent partners

– an amino group

– a carboxyl group– A side group that

is variable among all 20

• All proteins are constructed from a common set of 20 kinds of amino acids

The Monomers: Amino Acids

• Cells link amino acids together by dehydration synthesis

Proteins as Polymers

– The resulting bond between them is called a peptide bond

• Your body has tens of thousands of different kinds of protein

– The arrangement of amino acids makes each one different

• A protein, such as lysozyme, consists of polypeptide chains folded into a unique shape– The shape determines the protein’s function – A protein loses its specific function when its

polypeptides unravel

Overview: A protein’s specific shape determines its function

A protein’s primary structure is its amino acid sequence

Secondary structure is polypeptide coiling or folding produced by hydrogen bonding

Tertiary structure is the overall shape of a polypeptide

Quaternary structure is the relationship among multiple polypeptides of a protein

• A protein’s shape is sensitive to the surrounding environment

What Determines Protein Structure?

– Unfavorable temperature and pH changes can cause a protein to unravel and lose its shape

– This is called denaturation

• Nucleic acids such as deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) serve as the blueprints for proteins

• They ultimately control the life of a cell

NUCLEIC ACIDSNucleic acids are information-rich polymers of nucleotides

• Nucleic acids are polymers of nucleotides

– Each nucleotide is composed of a sugar, phosphate, and nitrogenous base (RNA has A, C, G and instead of T, it has uracil (U).

• Each DNA nucleotide has one of the following bases

– Adenine (A)– Guanine (G)– Thymine (T)– Cytosine (C)