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Samenvatting Biology: A Global Approach Global Edition Chapter 5, 6, 7, 8, 10
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Hoofdstuk 5: p 116-139: Biological Marcromolecules and LipidsChapter 5
The Molecules of Life
Large molecules found in all living things can be sorted into 4 main classes: carbohydrates,
lipids, proteins, and nucleic acids -> macromolecules.
5.1
Macromolecules are polymers, built from monomers
A polymer is a long molecule consisting of many similar or identical building blocks linked by
covalent bonds. The repeating units that serve as the building blocks of polymer are
monomers. In addition to forming polymers, some monomers have functions of their own.
The Synthesis and Breakdown of Polymers
Enzymes are specialized macromolecules that speed up chemical reactions. The reaction
connecting monomers is a dehydration reaction, a reaction in which two molecules are
covalently bonded to each other with the loss of a water molecule. One monomer provides a
hydroxyl group (-OH), while the other provides a hydrogen (-H). This reaction is repeated as
monomers are added to the chain one by one, making a polymer (polymerization).
Polymers are disassembled to monomers by hydrolysis. Reverse of dehydration.
The Diversity of Polymers
Small molecules common to all organisms act as building blocks that are ordered in to
unique macromolecules.
For each class, the large molecules have emergent properties not found in their individual
components.
5.2
Carbohydrates serve as fuel and building material
Carbohydrates include sugars and polymers of sugars. The simplest carbohydrates are the
monosaccharides. Disaccharides are double sugars, consisting of two monosaccharides
joined by a covalent bond. Carbohydrate macromolecules are polymers called
polysaccharides, composed of many sugar building blocks.
Sugars
Monosaccardies generally have molecular formulas that are some multiple of the unit CH2O.
The molecule has a carbonyl group (C = O) and multiple hydroxyl groups (-OH). Depending
on the location of the carbonyl group, a sugar is either an aldose (aldehyde sugar) or a
ketose (ketone sugar).
Another criterion for classifying sugars is the size of the carbon skeleton, which ranges from
3 to 7 carbons long.
Carbons from rings because they are the most stable form of these sugars under
physiological conditions.
In the process, cellular respiration, cells extract energy from glucose molecules by breaking
them down in a series of reactions. Sugar molecules are a major fuel for cellular work, but
their carbon skeletons also serve as raw material for the synthesis of other types of small
organic molecules, such as amino acids and fatty acids. Sugar molecules that are not
immediately used in these ways are generally incorporated as monomers into disaccharides
or polysaccharides.
,A disaccharide consists of 2 monosaccharides joined by a glycosidic linkage, a covalent
bond formed between 2 monosaccharides by a dehydration reaction (glyco refers to
carbohydrate).
Lactose intolerance is a common condition in humans who lack lactase, the enzyme that
breaks down lactose.
Polysaccharides
Some polysaccharides serve as storage material, hydrolyzed as needed to provide sugar for
cells. Other polysaccharides serve as building material for structures that protect the cell or
the whole organism. The architecture and function of a polysaccharide are determined by its
sugar monomers and by the positions of its glycosidic linkages.
Storage Polysaccharides
Plants store starch, a polymer of glucose monomers, as granules within cellular structures
known as plastids.
Animals store a polysaccharide glycogen, a polymer of glucose that is extensively branched.
Vertebrates store glycogen mainly in liver and muscle cells. Extensively branched structure
of glycogen fits its function: more free ends are available for hydrolysis.
Structural Polysaccharides
Cellulose is a major component of the tough walls that enclose plant cells.
When glucose forms a ring, the hydroxyl group attached to the number 1 carbon is
positioned either below or above the plane of the ring. These two ring forms for glucose are
called alpha and beta. In starch, all the glucose monomers are in the alpha configuration.
The glucose monomers of cellulose are all in the beta configuration.
Chitin is the carbohydrate used by arthropods to build exoskeletons.
5.3
Lipids are a diverse group of hydrophobic molecules
The hydrophobic behavior of lipids is based on their molecular structure, they consist mostly
of hydrocarbon regions.
Fats
A fat is constructed from two kinds of smaller molecules: glycerol and fatty acids. Glycerol is
an alcohol; each of its three carbons bears a hydroxyl group. A fatty acid has a long carbon
skeleton (16 or 18 carbon atoms). The carbon at one end of the skeleton is part of a carboxyl
group, the functional group that gives these molecules the name fatty acid. The rest of the
skeleton consists of a hydrocarbon chain.
In making fat, 3 fatty acid molecules are each joined to glycerol by an ester linkage, a bond
formed by a dehydration reaction between a hydroxyl group and a carboxyl group. The
resulting fat, triacylglycerol, consists of 3 fatty acids linked to a glycerol molecule.
If there are no double bonds between carbon atoms composing a chain, then as many
hydrogen atoms as possible are bonded to the carbon skeleton -> saturated. An unsaturated
fatty acid has one or more double bonds, with one fewer hydrogen atom on each double
bonded carbon. Nearly every double bond is a cis bond.
Saturated fats are solid at room temperature. In contrast, the fats of plants and fishes are
generally unsaturated. Usually liquid at room temperature (oils).
The major function of fats is energy storage. The hydrocarbon chains of fats are similar to
gasoline molecules and just as rich in energy. Humans and other mammals stock their long-
term food reserves in adipose cells, which swell and shrink as fat is deposited and withdrawn
from storage. In addition to storing energy, adipose tissue also cushions such vital organs as
the kidneys, and a layer of fat beneath the skin insulates the body.
, Phospholipids
Phospholipids are essential for cells because they are major constituents of cell membranes.
A phospholipid is similar to a fat molecule but has only 2 fatty acids attached to glycerol
rather than 3. The 3e hydroxyl group of glycerol is joined to a phosphate group, which has a
negative electrical charge in the cell. Typically, an additional small charged or polar molecule
is also linked to the phosphate group.
The two ends of phospholipids show different behaviors with respect to water. The
hydrocarbon tails are hydrophobic and are excluded from water. However, the phosphate
group and its attachments form a hydrophilic head that has an affinity for water. When
phospholipids are added to water, they self-assemble into a double-layered sheet called a
bilayer that shields their hydrophobic fatty acid tails from water.
Steroids
Steroids are lipids characterized by a carbon skeleton consisting of four fused rings. Different
steroids are distinguished by the particular chemical groups attached to this ensemble of
rings. Cholesterol, a type of steroid, is a crucial molecule in animals. It is a common
component of animal cell membranes and is also the precursor from which other steroids are
synthesized. In vertebrates, cholesterol is synthesized in the liver and is also obtained from
the diet.
5.4
Proteins include a diversity of structures, resulting in a wide range of functions
Enzymatic proteins regulate metabolism by acting as catalysts, chemical agents that
selectively speed up chemical reactions without being consumed in the reaction.
Proteins are all constructed from the same set of 20 amino acids, linked in unbranched
polymers. The bond between amino acids is called a peptide bond, so a polymer of amino
acids is called a polypeptide. A protein is a biologically functional molecule made up of one
or more polypeptides, each folded and coiled into a specific 3D structure.
Amino Acid Monomers
An amino acid is an organic molecule with both an amino group and a carboxyl group. At the
center of the amino acid is an asymmetric carbon atom called alpha carbon. Its four different
partners are an amino group, a carboxyl group, a hydrogen atom and a variable group
symbolized by R. The R group, also called the side chain, differs with each amino acid.
• Enzymatic proteins -> Selective acceleration of chemical reactions
• Defensive proteins -> Protection against disease
• Storage proteins -> Storage of amino acids
• Transport proteins -> Transport of substances
• Hormonal proteins -> Coordination of an organism’s activities
• Receptor proteins -> Response of cell to chemical stimuli
• Contractile and motor proteins -> Movement
• Structural proteins -> Support
The physical and chemical properties of the side chain determine the unique characteristics
of a particular amino acid, thus affecting its functional role in polypeptide.
The amino acids are grouped according to the properties of their side chains. One group
consists of amino acids with nonpolar side chains (hydrophobic). Another group consists of
amino acids with polar side chains (hydrophilic). Acidic amino acids have side chains that are
generally negative in charge due to the presence of a -OH group, which is usually
dissociated (ionized) at cellular pH. Basic amino acids have amino groups in their side chains
that are generally positive in charge. Because they are charged, acidic and basic side chains
are also hydrophilic.
Polypeptides (Amino Acid Polymers)
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