Chemical Composition in a Cell, practice questions

C, The Biuret test detects protein and works at room temperature, turning purple in the presence of protein, unlike the Benedict's test, which requires heating.

C, The joining of two amino acids by a peptide bond is a condensation reaction, which always releases one water molecule at the new bond formed.

B, A lipid (triglyceride) forms when one glycerol molecule condenses with three fatty acid molecules, releasing three water molecules.

C, Denaturation is a permanent change in a protein's three-dimensional shape, usually caused by heat or extreme pH, that destroys its function without breaking the peptide bonds joining its amino acids.

B, A carbon-to-carbon double bond puts a kink in the fatty acid chain, which prevents the molecules from packing tightly together, so unsaturated fats remain liquid oils at room temperature.

B, Nucleic acids such as DNA and RNA are polymers of nucleotides, and each nucleotide consists of a sugar, a phosphate group and a nitrogenous base.

B, Starch (in plants) and glycogen (in animals) are both compact, coiled storage polysaccharides that release glucose by hydrolysis when energy is needed, unlike cellulose, which provides structural support.

C, The two strands of a DNA double helix are held together by hydrogen bonds that form between complementary bases on opposite strands.

C, Hydrolysis adds a water molecule across a bond to split a polymer back into its monomers; this is the reverse of condensation, which releases water when joining monomers.

B, Water's high specific heat capacity means it can absorb or release a large amount of heat energy with only a small change in its own temperature, which buffers a cell or organism against sudden temperature changes.

• Starch is present in the sample.
• Reducing sugar, protein and lipid are all absent from the sample.
• The Benedict's test remaining blue after heating shows no reducing sugar reacted with the Benedict's solution, so no reducing sugar is present.
• The iodine test turning blue-black shows starch is present, since iodine only produces this colour change when it binds to starch molecules.
• The Biuret test remaining blue (rather than turning purple) shows no protein is present to produce the characteristic colour change.
• No milky-white layer forming in the emulsion test shows no lipid is present in the sample.
• A suitable food is plain cooked rice or a slice of plain bread, since both are mostly starch with negligible amounts of reducing sugar, protein and lipid.
• Any other named food that is almost pure starch with little sugar, protein or fat present would also be an acceptable answer.

• P is cellulose and Q is starch (or glycogen).
• Both P and Q are polysaccharides made entirely of glucose monomers joined by condensation, but the type of glycosidic bond linking the glucose units differs between them, producing different shapes.
• Cellulose's long, straight chains lie parallel to each other and are cross-linked by hydrogen bonds into strong fibres, which gives plant cell walls the tensile strength to resist the internal pressure of the cell without bursting.
• This straight, fibrous structure makes cellulose well suited to a structural role rather than to compact storage.
• Starch's compact, branched, coiled structure packs a large number of glucose units into a small volume, which is efficient for storing energy inside a plant cell without taking up excessive space.
• The many branch points on starch (or glycogen) also provide many free ends at which enzymes can act simultaneously, allowing glucose to be released quickly by hydrolysis when the plant or animal needs energy.

• The enzyme worked fastest at pH 7, since the protein was digested in the shortest time (6 minutes) at this pH.
• An enzyme is itself a protein folded into a specific three-dimensional shape, including a precisely shaped active site that binds its substrate.
• At pH 7, close to the enzyme's optimum pH, this shape is maintained, so the active site continues to bind the substrate efficiently and the reaction proceeds quickly.
• At the extreme pH values of pH 2 and pH 10, the abnormal concentration of hydrogen or hydroxide ions breaks the bonds holding the enzyme's three-dimensional shape together, denaturing the enzyme.
• Once denatured, the active site's shape no longer matches the substrate, so the enzyme can no longer bind and catalyse the reaction efficiently, which is why digestion took far longer at pH 2 and pH 10.
• If the enzyme were exposed to pH 2 for ten minutes, it would very likely become permanently denatured, since denaturation is generally irreversible.
• Testing this denatured enzyme again at pH 7 would therefore most likely still show a very slow digestion time, similar to the result at pH 2, rather than returning to the fast 6-minute result, because the enzyme's original shape cannot be restored simply by returning it to its optimum pH.

Water makes up most of the mass of a living cell and is the medium in which nearly all reactions occur. It dissolves ionic and polar substances so nutrients and wastes can be transported, acts as a reactant in hydrolysis reactions, and helps keep cell temperature stable because it absorbs heat without a large rise in its own temperature.

Carbohydrates range from single sugar units (monosaccharides such as glucose and fructose) to two linked units (disaccharides such as maltose and sucrose) to long chains of hundreds or thousands of units (polysaccharides such as starch, glycogen and cellulose). Their main role is to supply and store energy, though cellulose instead gives plant cell walls structural strength.

Proteins are polymers of amino acids joined by peptide bonds and folded into a specific three-dimensional shape that determines their function. They form enzymes that catalyse reactions, antibodies that fight infection, and structural material such as keratin and collagen. Heat and extreme pH break the bonds holding the shape together, denaturing the protein and destroying its function.

Lipids are fats and oils formed when one glycerol molecule joins with three fatty acid molecules by condensation. They store roughly twice as much energy per gram as carbohydrates, form the phospholipid bilayer of cell membranes, and provide insulation and protection around organs; saturated fatty acids have only single bonds while unsaturated fatty acids contain one or more double bonds.

Nucleic acids are polymers of nucleotides, each nucleotide made of a sugar, a phosphate group and a nitrogenous base. DNA stores the genetic instructions for an organism as a sequence of bases arranged along a double helix, while RNA carries a copy of part of this code to build proteins in the cell.

Four food tests identify the main biomolecules in a sample: Benedict's solution turns brick-red on heating with a reducing sugar, iodine solution turns blue-black with starch, Biuret solution turns purple in the presence of protein without heating, and the emulsion or Sudan III test shows a milky-white layer or red colouration for lipids.

Monomers join to form a polymer through condensation, a reaction that releases one water molecule at every new bond formed; digestion reverses this process through hydrolysis, in which a water molecule is added to break a bond and split the polymer back into its monomers. The same two reactions build and break down carbohydrates, proteins and lipids.

DNA is a double helix made of two strands of nucleotides twisted around each other, held together by base pairing between the two strands. The sequence of bases along one strand is the genetic code, and it determines the sequence of amino acids joined together when a cell builds a particular protein.

Saturated fatty acids contain only single bonds between their carbon atoms, pack closely together and are usually solid at room temperature, as in most animal fats. Unsaturated fatty acids contain one or more carbon-to-carbon double bonds, do not pack as closely, and are usually liquid oils from plant sources such as corn or olive oil.

Beyond storing energy, three groups of biomolecules build physical structure: cellulose strengthens plant cell walls, structural proteins such as collagen and keratin form connective tissue, hair and nails, and phospholipids arrange into the bilayer of every cell membrane. Comparing this structural role against the energy-storage role of starch, glycogen and fat is a common way exam questions test understanding across the whole chapter.