Flashcards · 7402 · AS Topic 1

Biological molecules (AS Level) flashcards

81 cards on AS Topic 1, subtopics 3.1.1 to 3.1.8. Pick a subtopic or study them all.

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3.1.1 Monomers and polymers

Monomer: A smaller unit from which larger molecules are made.
Polymer: A molecule made from a large number of monomers joined together.
Condensation reaction: Joins two molecules with the formation of a chemical bond and eliminates a molecule of water.
Hydrolysis reaction: Breaks a chemical bond between two molecules and uses a water molecule.
Name the monomer of a polysaccharide, a polypeptide and a polynucleotide: Polysaccharide: monosaccharide. Polypeptide: amino acid. Polynucleotide: nucleotide.
Which bond joins the monomers in a polysaccharide, a polypeptide and a nucleic acid?: Polysaccharide: glycosidic bond. Polypeptide: peptide bond. Nucleic acid: phosphodiester bond.
n monomers are joined into one molecule. How many water molecules are released, and how many are used in complete hydrolysis?: n − 1 released (one per bond formed), and n − 1 used in complete hydrolysis. Count the bonds, not the monomers.
Are lipids such as triglycerides polymers?: No. A triglyceride is one glycerol and three fatty acids joined by ester bonds; there is no repeating monomer unit.
A branched polysaccharide contains 60 glucose units. How many glycosidic bonds does it have?: 59 glycosidic bonds, so 59 water molecules were released in making it and 59 are used to hydrolyse it completely.
Glucose Mr = 180, water Mr = 18. What is the Mr of a chain of 12 glucose units?: 12 × 180 − 11 × 18 = 2160 − 198 = 1962. Eleven condensations each remove one water.

3.1.2 Carbohydrates

Name three monosaccharides and give their molecular formula: Glucose, galactose and fructose. All are C₆H₁₂O₆.
Which monosaccharides make maltose, sucrose and lactose?: Maltose: glucose + glucose. Sucrose: glucose + fructose. Lactose: glucose + galactose.
Glycosidic bond: The bond formed between two monosaccharides by a condensation reaction.
How do α-glucose and β-glucose differ?: The position of the OH group on carbon 1: below the ring in α-glucose, above the ring in β-glucose.
Amylose and amylopectin: structure and advantage: Amylose: unbranched α-glucose chain coiled into a helix, so very compact. Amylopectin: branched, so it has many ends for rapid hydrolysis.
How is glycogen suited to its role in animal cells?: It is highly branched with short chains, giving many ends for enzymes to hydrolyse, so glucose is released faster than from starch. This suits the higher activity of animals. It is also compact and insoluble.
Structure of cellulose: Straight, unbranched chains of β-glucose, with alternate molecules rotated 180°. Hydrogen bonds between parallel chains form microfibrils.
Why do hydrogen bonds between cellulose chains matter to a plant cell?: Many hydrogen bonds give the microfibrils high tensile strength, so the cell wall withstands the pressure of water entering by osmosis.
Why is being insoluble useful for a storage polysaccharide?: It has no effect on water potential, so water does not enter the cell by osmosis, and it cannot diffuse out of the cell.
Test for a reducing sugar: Add Benedict's solution and heat. Positive: blue → green, yellow, orange, then brick-red precipitate. Negative: stays blue. All monosaccharides, maltose and lactose are reducing sugars.
Test for a non-reducing sugar (for example sucrose): Benedict's on the sample first stays blue. Boil a fresh sample with dilute hydrochloric acid to hydrolyse it, neutralise (for example with sodium hydrogencarbonate), then add Benedict's and heat: brick-red precipitate.
Test for starch: Add iodine in potassium iodide solution. Positive: yellow-brown → blue-black.

3.1.3 Lipids

How is a triglyceride formed?: One glycerol and three fatty acids join in three condensation reactions, forming three ester bonds and releasing three water molecules.
Ester bond: Forms between an OH of glycerol and the COOH of a fatty acid.
Saturated and unsaturated fatty acids: Saturated: no C=C double bonds in the hydrocarbon chain. Unsaturated: at least one C=C double bond.
Why are unsaturated fats usually liquid at room temperature?: C=C double bonds put kinks in the chains, so the molecules cannot pack closely together.
Structure of a phospholipid: Glycerol, two fatty acids and a phosphate-containing group in place of the third fatty acid. The phosphate head is hydrophilic; the fatty acid tails are hydrophobic.
How do phospholipids behave in water, and why?: They form a bilayer: hydrophilic heads face the water, hydrophobic tails face inward. The core blocks water-soluble substances; this is the basis of cell membranes.
Why are triglycerides a good energy store?: Long chains with many C–H bonds release more energy per gram than carbohydrate. They are non-polar and insoluble, so they do not affect water potential. Their high hydrogen content gives metabolic water when oxidised.
Hydrophilic and hydrophobic: Hydrophilic: attracted to water; polar or charged. Hydrophobic: repelled by water; non-polar.
Emulsion test for lipids: Shake the sample with ethanol, then pour into water. Positive: a cloudy white emulsion. Negative: stays clear. The result is cloudiness, not a colour change.
Exam trap: how many fatty acids does a phospholipid have?: Two. A triglyceride has three; in a phospholipid one is replaced by a phosphate-containing group.

3.1.4 Proteins

General structure of an amino acid: A central carbon carrying an amine group (NH₂), a carboxyl group (COOH), a hydrogen atom and an R group. The twenty amino acids common to all organisms differ only in R.
Peptide bond: A C–N bond formed by condensation between the carboxyl group of one amino acid and the amine group of the next.
Primary structure of a protein: The sequence of amino acids in a polypeptide, held by peptide bonds. It determines all the higher levels of structure.
Secondary structure: The α-helix or β-pleated sheet, held by hydrogen bonds along the polypeptide backbone.
Tertiary structure, and the bonds that hold it: The specific three-dimensional fold of one polypeptide, held by hydrogen bonds, ionic bonds and disulfide bridges between R groups (with hydrophobic interactions).
Quaternary structure: The arrangement of two or more polypeptides in one protein, held by the same kinds of bond as tertiary structure. Example: haemoglobin, four polypeptides.
Biuret test for protein: Add sodium hydroxide, then dilute copper(II) sulfate. Positive: blue → lilac or purple. It detects peptide bonds, not free amino acids.
Enzyme: A globular protein that acts as a biological catalyst. It lowers the activation energy of the reaction by forming an enzyme–substrate complex.
Induced-fit model of enzyme action: The active site is not rigid: it changes shape slightly as the substrate binds to become complementary, putting strain on the substrate's bonds and lowering the activation energy.
Competitive and non-competitive inhibitors: Competitive: similar shape to the substrate, binds the active site; effect reduced by more substrate. Non-competitive: binds elsewhere and changes the active site's shape; more substrate does not overcome it.
Why does the rate level off as substrate concentration rises?: All the active sites are occupied, so enzyme concentration is now the limiting factor.
Denaturation (of an enzyme): Loss of the tertiary structure, so the active site changes shape and is no longer complementary to the substrate. Never write that the enzyme is killed.

3.1.5 Nucleic acids are important information-carrying molecules

Nucleotide: A pentose, a nitrogen-containing organic base and a phosphate group.
DNA and RNA: pentose and bases: DNA: deoxyribose; bases adenine, cytosine, guanine, thymine. RNA: ribose; bases adenine, cytosine, guanine, uracil.
Phosphodiester bond: The bond formed by a condensation reaction between two nucleotides. It makes the sugar–phosphate backbone.
Structure of a DNA molecule: A double helix of two polynucleotide chains, held together by hydrogen bonds between specific complementary base pairs: A with T, C with G.
Structure and job of RNA (compared with DNA): A relatively short single polynucleotide chain. It transfers genetic information from DNA to the ribosomes. Ribosomes themselves are made of RNA and proteins.
Double-stranded DNA is 36% adenine. What percentage is guanine?: T = 36%, so A + T = 72%. C + G = 28%, so G = 14%.
Semi-conservative replication: Each new DNA molecule has one original strand and one newly made strand.
Role of DNA helicase in replication: Breaks the hydrogen bonds between complementary bases, unwinding the double helix so each strand can act as a template.
Role of DNA polymerase in replication: Catalyses the condensation reactions that join adjacent nucleotides of the new strand (forming phosphodiester bonds), after free nucleotides have base-paired with the template.
Meselson–Stahl: results after one and two generations in ¹⁴N: Generation 1: one intermediate band (rules out conservative). Generation 2: intermediate and light bands in equal amounts (rules out dispersive; fits semi-conservative).
Why did many scientists once doubt that DNA carries genetic information?: It has only four different bases, so it looked too simple; proteins, with many amino acids, seemed more likely. The information is in the order and length of the base sequence.

3.1.6 ATP

Structure of ATP: A nucleotide derivative: ribose, adenine and three phosphate groups.
Hydrolysis of ATP: equation and enzyme: ATP + H₂O → ADP + Pi, catalysed by ATP hydrolase.
How is ATP resynthesised?: ADP + Pi → ATP by condensation, catalysed by ATP synthase, during photosynthesis or respiration.
How is the energy released by ATP hydrolysis used?: It is coupled to energy-requiring reactions in cells.
What is the inorganic phosphate (Pi) from ATP hydrolysis used for?: It is added to other compounds (phosphorylation), often making them more reactive.
ATP compared with an RNA nucleotide: Both contain ribose. An RNA nucleotide has one of four bases (A, C, G, U) and one phosphate; ATP has adenine and three phosphates.
Exam trap: ATP hydrolase and ATP synthase: ATP hydrolase breaks ATP down (hydrolysis, uses water). ATP synthase makes ATP (condensation, releases water). Not 'ATP synthetase'.
ATP hydrolysis releases 30.5 kJ per mole (data given). How much energy is released by 0.20 mol?: 0.20 × 30.5 = 6.1 kJ

3.1.7 Water

Why are water molecules attracted to each other?: Water is polar (δ− oxygen, δ+ hydrogens), so hydrogen bonds form between neighbouring molecules. Each is weak, but there are very many.
Metabolite: A substance taking part in, or made by, a metabolic reaction. Water is a reactant in hydrolysis and a product of condensation.
Why is water a good solvent, and why does that matter?: Polar water molecules surround ions and polar molecules, so they dissolve. Reactions happen in solution, and substances are transported in blood, tissue fluid and xylem.
Why does water have a relatively high heat capacity, and what is the benefit?: Energy supplied first disrupts hydrogen bonds, so temperature rises slowly. This buffers temperature change in cells, blood and aquatic habitats.
Explain how evaporation of water cools an organism: Hydrogen bonds must be broken for water to evaporate, which takes a lot of energy. Escaping molecules carry it away, so a little water removes a lot of heat (sweating, panting, transpiration).
Latent heat of vaporisation: The energy needed to turn a liquid into a gas at constant temperature.
Cohesion: Attraction between molecules of the same kind. In water it is due to hydrogen bonding and allows continuous columns of water in the xylem.
Surface tension: The skin-like surface where water meets air, caused by cohesion. It can support small organisms such as pond skaters.
200 g of water absorbs 8.36 kJ (specific heat capacity 4.18 J g⁻¹ °C⁻¹). What is the temperature rise?: 8360 ÷ (200 × 4.18) = 8360 ÷ 836 = 10 °C. Convert kJ to J first.
15 g of water evaporates (latent heat 2.4 kJ g⁻¹). How much energy is removed?: 15 × 2.4 = 36 kJ

3.1.8 Inorganic ions

Where are inorganic ions found in organisms?: In solution in cytoplasm and body fluids, some at high concentrations and others at very low concentrations. Each ion's role depends on its properties.
Role of hydrogen ions (H⁺): Their concentration sets pH. H⁺ affects the charges on R groups, so ionic and hydrogen bonds in a protein change, altering the shape of an enzyme's active site.
pH 6 compared with pH 8: which has more H⁺, and by what factor?: pH 6, with 100 times the H⁺ concentration. pH = −log₁₀[H⁺], so each unit is a factor of 10.
Role of iron ions in haemoglobin: Each of the four haem groups contains an Fe²⁺ ion, which binds one O₂ reversibly. So one haemoglobin carries up to four O₂ molecules.
How do sodium ions help glucose enter an ileum epithelial cell?: The sodium–potassium pump actively removes Na⁺ into the blood, keeping Na⁺ low inside the cell. Na⁺ diffuses in from the lumen through a co-transporter protein, carrying glucose (or an amino acid) in with it.
Roles of phosphate ions: Part of DNA, RNA and ATP nucleotides; joins nucleotides by phosphodiester bonds; Pi released by ATP hydrolysis phosphorylates other compounds.
Why does glucose absorption in the ileum slow if the sodium–potassium pump is inhibited?: Less Na⁺ leaves the cell, so the Na⁺ concentration gradient into the cell is lost and less glucose is co-transported.
Exam trap: does co-transport of glucose use ATP directly?: No. ATP is used by the sodium–potassium pump; co-transport uses the Na⁺ gradient the pump sets up.