Chapter 5 Bi 212
Which of the following is a polymer?
Cellulose, a plant cell wall component Explanation: The macromolecules in three of the four classes of life's organic compounds—carbohydrates, proteins, and nucleic acids—are chain-like molecules called polymers. A polymer is a long molecule consisting of many similar or identical building blocks linked by covalent bonds, much as a train consists of a chain of cars. The repeating units that serve as the building blocks of a polymer are smaller molecules called monomers. The polysaccharide cellulose is a major component of plant cell walls. It is a polymer composed of many glucose monomers joined together by glycosidic linkages
A shortage of phosphorus in the soil would make it especially difficult for a plant to manufacture __________.
DNA
Which is the term for compounds that do not mix with water?
Hydrophobic
The sex hormones estrogen, progesterone, and testosterone belong to which class of molecules?
Lipids
__________ is always involved in hydrolysis reactions.
Water
Sugars are molecules that have __________ C:H:O and are called __________.
a 1:2:1 ratio of; carbohydrates
The peptide bond is __________.
a covalent bond joining amino acids together to form a polypeptide
The components of nucleic acids are __________.
a nitrogenous base, a pentose sugar, and a phosphate
Sickle-cell anemia is a disease that is caused by __________ in the __________ of the protein.
a single amino acid change; primary structure explanation: Even a slight change in primary structure can affect a protein's shape and ability to function. For instance, sickle-cell disease, an inherited blood disorder, is caused by the substitution of one amino acid (valine) for the normal one (glutamic acid) at a particular position in the primary structure of hemoglobin, the protein that carries oxygen in red blood cells. Normal red blood cells are disk-shaped, but in sickle-cell disease, the abnormal hemoglobin molecules tend to aggregate into chains, deforming some of the cells into a sickle shape. A person with the disease has periodic "sickle-cell crises" when the angular cells clog tiny blood vessels, impeding blood flow. The toll taken on such patients is a dramatic example of how a simple change in protein structure can have devastating effects on protein function.
Nitrogenous bases are classified as either purines or pyrimidines. Examples of purines are __________.
adenine and guanine
Protein molecules are polymers (chains) of __________.
amino acid molecules
At a conference, the speaker's grand finale was sautéing mealworms (insect larvae) in butter and serving them to the audience. They were crunchy (like popcorn hulls) because their exoskeletons contain the polysaccharide __________.
chitin
The molecule with four fused rings that is found in animal membranes and is the precursor of vertebrate sex hormones is __________.
cholesterol
When comparing saturated and naturally occurring unsaturated fats, the unsaturated fats have __________ and are __________ at room temperature.
cis double bonds; liquids
The type of bond that forms to join monomers (such as sugars and amino acids) into polymers (such as starch and proteins) is a(n) __________ bond.
covalent Explanation: Monomers are joined together by a dehydration reaction in which two molecules are covalently bonded to each other through the loss of a water molecule.
In living organisms, DNA exists as a __________ with the strand(s) __________.
double helix; running antiparallel
Carbohydrates are used in our bodies mainly for __________.
energy storage and release
A polysaccharide that is used for storing energy in human muscle and liver cells is __________.
glycogen
The subunits (monomers) in cellulose are linked together by __________.
glycosidic linkages
The proper three-dimensional shape and folding of a protein is assisted by _________.
molecules called chaperonins
The tertiary structure of a protein includes all of the following interactions except _________ bonds.
peptide
The lipids that form the main structural component of cell membranes are __________.
phospholipids Explanation: Phospholipids have a hydrophilic head and two hydrophobic tails. This permits the phospholipids to be arranged in a bilayer, or double layer, which forms a boundary between the cell and its external environment.
One characteristic shared by sucrose, lactose, and maltose is that __________.
they are all disaccharides
Macromolecules, the molecules of life, include all of the following except __________.
trace elements; proteins, carbs, and nucleic acids are macromolecules
Which type of protein shields a newly forming protein from cytoplasmic influences while it is folding into its functional form?
Chaperonins Explanation: A polypeptide chain of a given amino acid sequence can spontaneously arrange itself into a three-dimensional shape determined and maintained by the interactions responsible for secondary and tertiary structure. This folding normally occurs as the protein is being synthesized in the crowded environment within a cell, aided by other proteins. Crucial to the folding process are chaperonins (also called chaperone proteins), protein molecules that assist in the proper folding of other proteins. Chaperonins shield proteins from "bad influences" (interactions with other molecules in the cytoplasm) while they are folding into their functional forms.
Generally, animals cannot digest (hydrolyze) the glycosidic linkages between the glucose molecules in cellulose. How then do cows get enough nutrients from eating grass?
Microorganisms in their digestive tracts hydrolyze the cellulose to individual glucose units. Explanation: The polysaccharide called cellulose is a major component of the tough walls that enclose plant cells. Like starch, cellulose is a polymer of glucose, but the glycosidic linkages in these two polymers differ. In starch, all the glucose monomers are in the α configuration. In contrast, the glucose monomers of cellulose are all in the β configuration, making every glucose monomer "upside down" with respect to its neighbors. Enzymes that digest starch by hydrolyzing its α linkages are unable to hydrolyze the β linkages of cellulose because of the distinctly different shapes of these two molecules. In fact, few organisms possess enzymes that can digest cellulose. Cows have digestive chambers populated by microorganisms that can produce certain hydrolytic enzymes that cows cannot. The enzymes hydrolyze (digest) the cellulose polymer into glucose monomers.
Sugars have a(n) __________ group that interacts with a _________ group that forms ring structures when the dry molecule is placed in water.
carbonyl (-C=O); hydroxyl (-OH) Explanation: Monosaccharides (from the Greek monos, single, and sakchar, sugar) generally have molecular formulas that are some multiple of the unit CH2O. Glucose (C6H12O6), the most common monosaccharide, is of central importance in the chemistry of life. In the structure of glucose, we can see the trademarks of a sugar: The molecule has a carbonyl group (CO) 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). Glucose, for example, is an aldose; fructose, an isomer of glucose, is a ketose. (Most names for sugars end in the suffix -ose.) Another criterion for classifying sugars is the size of the carbon skeleton, which ranges from three to seven carbon molecules long. Glucose, fructose, and other sugars that have six carbon molecules are called hexoses. Trioses (three-carbon sugars) and pentoses (five-carbon sugars) are also common.
The secondary structure of a peptide backbone is stabilized by __________ forming either a(n) __________ or a(n) __________.
hydrogen bonds; α helix; β pleated sheet Explanation: Most proteins have segments of their polypeptide chains repeatedly coiled or folded in patterns that contribute to the protein's overall shape. These coils and folds, collectively referred to as secondary structure, are the result of hydrogen bonds between the repeating constituents of the polypeptide backbone (not the amino acid side chains). Within the backbone, the oxygen atoms have a partial negative charge, and the hydrogen atoms attached to the nitrogen atoms have a partial positive charge; therefore, hydrogen bonds can form between these atoms. Individually, these hydrogen bonds are weak, but because there are so many of them over a relatively long region of the polypeptide chain, they can support a particular shape for that part of the protein. One such secondary structure is the α helix, a delicate coil held together by hydrogen bonding between every fourth amino acid, as shown below. Although each transthyretin polypeptide has only one α helix, other globular proteins have multiple stretches of α helix separated by nonhelical regions. Some fibrous proteins, such as α-keratin, the structural protein of hair, have the α helix structure over most of their length. The other main type of secondary structure is the β pleated sheet. As shown below, in this structure two or more segments of the polypeptide chain lying side by side (called β strands) are connected by hydrogen bonds between parts of the two parallel segments of the polypeptide backbone.
The sequence of amino acids in a protein is called the __________ structure of the protein.
primary Explanation: As an example, let's consider transthyretin, a globular blood protein that transports vitamin A and one of the thyroid hormones throughout the body. Transthyretin is made up of four identical polypeptide chains, each composed of 127 amino acids. Shown here is one of these chains unraveled to provide a closer look at its primary structure. Each of the 127 positions along the chain is occupied by one of the 20 amino acids, indicated here by its three-letter abbreviation. The primary structure is like the order of letters in a very long word. If left to chance, there would be 20,127 different ways of making a polypeptide chain that is 127 amino acids long. However, the precise primary structure of a protein is determined not by the random linking of amino acids, but by inherited genetic information. The primary structure in turn dictates secondary and tertiary structures, by means of the chemical nature of the backbone and the side chains (R groups) of the amino acids along the polypeptide.
In a dehydration synthesis reaction, __________ is always formed as a by-product of the reaction.
water explanation: Although each class of polymer is made up of a different type of monomer, the chemical mechanisms by which cells make and break down polymers are basically the same in all cases. In cells, these processes are facilitated by enzymes, specialized macromolecules that speed up chemical reactions. Monomers are connected by a reaction in which two molecules are covalently bonded to each other, with the loss of a water molecule; this is known as a dehydration reaction. When a bond forms between two monomers, each monomer contributes part of the water molecule that is released during the reaction: One monomer provides a hydroxyl group (-OH), while the other provides a hydrogen group (-H). This reaction is repeated as monomers are added to the chain one by one, making a polymer.
