Biological Molecules (OCR A Level Biology): Flashcards

Hydrogen bonds hold water molecules together in a continuous column, so water can be pulled up through the xylem.

Solvent (for reactions and transport), transport medium, coolant (via evaporation), and a habitat.

Hydrogen bonds form between the slightly positive hydrogen atom of one water molecule and the slightly negative oxygen atom of another.

Because water is a polar molecule, it can dissolve ions and other charged substances.

It lets metabolic reactions take place in solution and lets dissolved substances be transported.

True

False — water has a high specific heat capacity, because its many hydrogen bonds need a lot of energy to break.

A small unit that joins with others of its kind to build a larger molecule (a polymer).

A large molecule made of many identical or similar monomers joined together in a chain.

A reaction that joins two molecules together by forming a chemical bond, releasing a molecule of water.

A reaction that breaks the chemical bond between two molecules by adding a molecule of water.

A condensation reaction joins monomers and releases water; a hydrolysis reaction breaks bonds by adding water.

• Polysaccharide (e.g. starch) ← monosaccharides

• Protein ← amino acids

• Nucleic acid ← nucleotides

They are not built from many identical repeating monomers joined into a chain — a triglyceride is just glycerol joined to fatty acids, not a repeating chain.

Carbon, hydrogen and oxygen (C, H, O).

Carbon, hydrogen and oxygen (C, H, O) — but with a lower proportion of oxygen than carbohydrates.

Carbon, hydrogen, oxygen and nitrogen (C, H, O, N); some proteins also contain sulfur (S).

Carbon, hydrogen, oxygen, nitrogen and phosphorus (C, H, O, N, P).

Each carbon atom can form four stable covalent bonds and can join into straight chains, branched chains or rings, allowing a huge variety of molecules to form.

A condensation reaction joins two monomers together and releases a molecule of water.

Proteins contain carbon, hydrogen, oxygen and nitrogen, and some also contain sulfur.

False — a polymer's subunits must be the same repeating units, so not all macromolecules are polymers.

True

A single sugar unit — the monomer from which larger carbohydrates are built (e.g. glucose).

A monosaccharide containing six carbon atoms (e.g. glucose).

A monosaccharide containing five carbon atoms (e.g. ribose and deoxyribose).

Molecules with the same molecular formula but a different arrangement of atoms.

C₆H₁₂O₆

It is soluble, so it can be transported in water (e.g. blood); and it holds chemical energy that is released during respiration.

They are isomers (both C₆H₁₂O₆). On carbon 1, the hydroxyl (-OH) group points below the ring in α-glucose and above the ring in β-glucose.

A hexose has six carbon atoms (e.g. glucose); a pentose has five carbon atoms (e.g. ribose).

Ribose (in RNA) and deoxyribose (in DNA).

Deoxyribose has one fewer oxygen atom (at carbon 2).

Glucose, fructose and galactose.

The two forms build different polysaccharides with different functions — α-glucose forms storage molecules (starch, glycogen) and β-glucose forms structural cellulose.

Glucose is a hexose sugar, meaning it contains six carbon atoms.

Ribose and deoxyribose are pentose sugars, each containing five carbon atoms.

True

False — deoxyribose has one fewer oxygen atom (at carbon 2).

The covalent bond that links two monosaccharides together, formed between two hydroxyl (–OH) groups.

A carbohydrate made of two monosaccharides joined by a glycosidic bond.

By a condensation reaction between two hydroxyl (–OH) groups on adjacent monosaccharides, which releases one molecule of water.

By hydrolysis — a water molecule is added to break the bond (catalysed by an enzyme).

Glucose + glucose.

Glucose + fructose.

Glucose + galactose.

One.

A disaccharide has only one glycosidic bond, but a polysaccharide has many glycosidic bonds, and each one must be hydrolysed.

Digestion of food in the gut, and the breakdown of stored carbohydrates (e.g. glycogen in muscle/liver) for respiration.

A glycosidic bond is formed by a condensation reaction, which releases one molecule of water.

Sucrose is a disaccharide made from glucose and fructose.

True

False — hydrolysis is catalysed by different enzymes from those used in condensation reactions.

A polymer made of many monosaccharides joined together by glycosidic bonds.

Amylose and amylopectin.

An unbranched, helix-shaped chain of α-glucose joined by 1,4 glycosidic bonds.

A branched chain of α-glucose with 1,4 glycosidic bonds along the chain and 1,6 glycosidic bonds at the branch points.

A highly branched polymer of α-glucose with 1,4 and 1,6 glycosidic bonds — similar to amylopectin but with many more branches.

Long, straight, unbranched chains of β-glucose joined by 1,4 glycosidic bonds, with alternate glucose molecules rotated 180°.

They are compact (so large amounts can be stored) and insoluble (so they have no osmotic effect on the cell).

Branches create many terminal (free) ends, so glucose can be added or removed quickly by condensation and hydrolysis to meet the cell's demands.

It is more branched (so more compact) than starch, and its many free ends let glucose be released rapidly to meet the high respiration rate of active animals.

Alternate β-glucose molecules are rotated 180°, so many hydrogen bonds form between parallel chains, bundling them into strong microfibrils.

Its high tensile strength lets the cell wall resist turgor pressure and support the plant, and the fibres are freely permeable to water and solutes.

As granules inside plastids, such as amyloplasts and chloroplasts.

Cellulose is made of long, straight chains of β-glucose joined by 1,4 glycosidic bonds.

Starch is made of two polysaccharides: amylose and amylopectin.

False — cellulose is made from β-glucose.

True

A covalent bond formed between two monosaccharides in a condensation reaction.

A sugar that can donate electrons (be oxidised) and so gives a positive Benedict's test without hydrolysis (e.g. glucose, fructose, maltose).

A sugar that cannot donate electrons, so it must be hydrolysed before it will give a positive Benedict's test (e.g. sucrose).

Add an excess of (blue) Benedict's reagent to the sample and heat in a boiling water bath for a few minutes.

The blue solution changes through green, yellow and orange to a brick-red precipitate (the more sugar, the further towards brick-red).

The final colour gives an approximate indication of how much reducing sugar is present, without giving an exact concentration.

First hydrolyse it by heating with dilute hydrochloric acid, then neutralise with sodium hydrogencarbonate, then carry out the Benedict's test as normal.

Hydrolysis breaks it into monosaccharides, which can donate electrons (reduce the copper), so a positive result can then form.

The Benedict's test needs alkaline conditions to work, so the added acid must first be neutralised.

Add a few drops of orange-brown iodine in potassium iodide solution to the sample. If starch is present, the colour changes to blue-black.

The solution stays blue.

In a positive iodine test, the orange-brown iodine solution turns blue-black when starch is present.

Sucrose is a non-reducing sugar, so it must be hydrolysed before it gives a positive Benedict's test.

False — it must first be hydrolysed into monosaccharides before it can give a positive result.

True

A lipid made of one glycerol molecule joined to three fatty acids by ester bonds.

A lipid made of a glycerol molecule joined to two fatty acids and a phosphate group.

The covalent bond formed between a hydroxyl (-OH) group of glycerol and the carboxyl (-COOH) group of a fatty acid.

A fatty acid whose hydrocarbon chain contains no carbon-carbon (C=C) double bonds.

A fatty acid whose hydrocarbon chain contains one or more carbon-carbon (C=C) double bonds.

One glycerol molecule and three fatty acids.

By three condensation reactions between glycerol and three fatty acids, forming three ester bonds and releasing three water molecules.

Three — one for each ester bond.

By hydrolysis — three water molecules are added to break the three ester bonds, releasing glycerol and three fatty acids.

In a phospholipid one of the three fatty acids is replaced by a phosphate group, so it has glycerol, two fatty acids and a phosphate group.

A saturated fatty acid has no C=C double bonds; an unsaturated fatty acid has one or more C=C double bonds, which add kinks to the chain.

A saturated fatty acid has no C=C double bonds; an unsaturated fatty acid has one or more C=C double bonds, which add kinks to the chain.

A triglyceride is formed from one glycerol molecule and three fatty acids joined by ester bonds.

An unsaturated fatty acid contains one or more C=C double bonds in its hydrocarbon chain.

True

False — it is a condensation reaction that releases water.

A lipid found in the cell membranes of eukaryotic cells that regulates membrane fluidity and is used to make steroid hormones.

Having both a hydrophilic (water-attracting) region and a hydrophobic (water-repelling) region — e.g. a phospholipid.

Their many carbon-hydrogen bonds release a large amount of energy when oxidised (about twice as much per gram as carbohydrate), and being hydrophobic they cause no osmotic water uptake, so large amounts can be stored.

Thermal insulation (adipose tissue under the skin), protection of organs, and buoyancy (low density aids floating). They also release metabolic water when respired.

It has a hydrophilic (polar) phosphate head and two hydrophobic (non-polar) fatty acid tails.

The hydrophilic phosphate heads face the water while the hydrophobic tails point inwards away from water, forming a bilayer — the basis of cell membranes.

It acts as a barrier to water-soluble (polar) molecules, helping control what enters and leaves the cell.

It sits between phospholipids and regulates membrane fluidity, making the membrane more rigid and less permeable to water-soluble substances.

To make steroid-based hormones such as oestrogen, testosterone and progesterone.

More unsaturated tails make the membrane more fluid; more saturated tails make it less fluid.

Phospholipids are amphipathic, meaning they have both a hydrophilic head and hydrophobic tails.

Cholesterol is used to make steroid hormones such as oestrogen and testosterone.

True — they store about 37 kJ per gram compared with 17 kJ for carbohydrates, due to their many carbon-hydrogen bonds.

False — a phospholipid has only two fatty acids; the third position is replaced by a phosphate group.

The test for lipids: a sample is dissolved in ethanol and then mixed with water; a milky-white emulsion forms if lipid is present.

Add ethanol to the sample and shake to dissolve any lipid, then pour the mixture into a test tube of water.

A milky-white emulsion forms; the more lipid present, the more obvious the milkiness.

The solution stays clear.

Lipids are insoluble in water but dissolve in ethanol; when the ethanol solution is added to water, the lipid comes out of solution as tiny droplets, forming the milky emulsion.

Qualitative — it shows whether lipid is present but not how much.

In a positive emulsion test, a milky-white emulsion forms when lipid is present.

Describes a molecule, such as a lipid, with no charge separation, so it does not dissolve in water but does dissolve in organic solvents like ethanol.

False — lipids are nonpolar and do not dissolve in water, but they do dissolve in ethanol.

False — it is a qualitative test that shows whether lipid is present but not how much.

The monomer of proteins: a molecule with a central carbon bonded to an amine group, a carboxyl group, a hydrogen atom and an R group.

The covalent bond formed between two amino acids in a condensation reaction.

Two amino acids joined together by a peptide bond.

Many (three or more) amino acids joined together by peptide bonds.

A central carbon atom bonded to:

• an amine group (-NH₂)

• a carboxyl group (-COOH)

• a hydrogen atom

• a variable R group

The R group (side chain), which differs in each amino acid and gives it its particular properties.

20.

By a condensation reaction between the carboxyl group of one amino acid and the amine group of another, releasing a molecule of water.

By hydrolysis — adding water breaks the peptide bond, releasing the individual amino acids.

The carboxyl (-COOH) group of one amino acid and the amine (-NH₂) group of another.

The part of an amino acid that varies and gives it its properties is the R group.

A peptide bond forms by a condensation reaction between two amino acids, releasing water.

The R group is not involved.

True

False — a peptide bond is broken by hydrolysis; condensation is how it forms.

The sequence of amino acids in a polypeptide chain, joined by peptide bonds.

The folding of a polypeptide into α-helices and β-pleated sheets, held by hydrogen bonds between the amine and carboxyl groups of the backbone.

The overall 3D folding of a polypeptide, held by bonds between R groups (hydrogen, ionic, disulfide bonds and hydrophobic interactions).

The arrangement of two or more polypeptide chains (subunits) in a protein — e.g. haemoglobin.

The sequence of amino acids joined by peptide bonds.

The α-helix and the β-pleated sheet.

Hydrogen bonds, ionic bonds, disulfide bonds and hydrophobic interactions — all between R groups.

In secondary structure they form between the amine and carboxyl groups of the backbone; in tertiary structure they form between R groups.

A strong covalent bond.

Between oppositely charged R groups (e.g. -NH₃⁺ and -COO⁻).

The sequence of amino acids (and their R groups) determines which bonds and interactions form as the protein folds, giving the specific 3D shape needed for its function.

Disulfide bonds form between the R groups of two cysteine amino acids.

A protein has a quaternary structure when it contains two or more polypeptide chains.

The DNA (gene) that codes for the protein.

Hydrogen bonds between the carboxyl and amine groups of the polypeptide backbone.

Between two cysteine R groups (cysteine is the only amino acid containing sulfur).

pH changes.

True

False — in secondary structure hydrogen bonds form between the amine and carboxyl groups of the backbone; R-group bonds belong to tertiary structure.

A compact, roughly spherical, water-soluble protein with a specific tertiary structure (e.g. enzymes, haemoglobin, insulin).

A protein with a non-protein prosthetic group attached (e.g. haemoglobin with its haem group).

A non-protein component permanently attached to a conjugated protein (e.g. the haem group in haemoglobin).

As they fold into their tertiary structure, hydrophilic R groups face outwards (interacting with water) and hydrophobic R groups face inwards, making the molecule soluble.

It lets them be transported around organisms and take part in metabolic reactions.

It lets them carry out precise roles — e.g. enzymes catalysing specific reactions and antibodies binding specific antigens.

A globular protein with a quaternary structure: four polypeptide (globin) subunits — two α and two β — each with a haem prosthetic group containing an Fe²⁺ ion.

Each of its four haem groups contains an Fe²⁺ ion that reversibly binds one oxygen molecule, so one haemoglobin molecule can carry four O₂ molecules.

Oxygen is not very soluble in water, so binding it to soluble haemoglobin allows much more oxygen to be transported in the blood.

A globular protein hormone made of two polypeptide chains held together by disulfide bonds; it helps control blood glucose concentration.

They are soluble, roughly spherical proteins with a specific tertiary shape that forms an active site where the substrate binds.

Haemoglobin is a conjugated protein because it contains a non-protein haem prosthetic group.

In globular proteins, hydrophilic R groups face outwards, making the protein soluble in water.

True

False — the hydrophobic R groups face inwards; the hydrophilic R groups face outwards.

A long, insoluble protein with a highly repetitive amino acid sequence, suited to structural roles (e.g. collagen, keratin, elastin).

They are long and strand-like, insoluble in water, and strong, with a highly repetitive amino acid sequence.

They have a large number of hydrophobic R groups.

It provides structural support and strength in connective tissue — e.g. in tendons, ligaments, bone, skin and blood vessel walls.

A tough structural protein that makes up hair, nails, horns and feathers.

Found in connective tissue, it allows tissues to stretch and then recoil back to their original shape.

Globular proteins are soluble with metabolic/transport roles, whereas fibrous proteins are insoluble with structural roles.

Fibrous proteins are insoluble in water, which suits them to structural roles.

Keratin is the tough fibrous protein that makes up hair, nails and feathers.

True

False — collagen molecules are too long to dissolve, making it insoluble.

Its triple helix structure contains many hydrogen bonds, letting it withstand large pulling forces without stretching or breaking.

A charged particle that does not contain carbon, involved in many essential cellular processes.

The pH of a solution.

They stimulate muscle contraction and are needed for transmission of nerve impulses at synapses (they are also involved in blood clotting).

They are needed for the co-transport of glucose and amino acids across cell membranes (e.g. in the small intestine); they are also involved in nerve impulse transmission.

Transmission of nerve impulses (repolarisation of the axon); they also help open the stomata in plants.

They provide a source of nitrogen for making amino acids/proteins and nucleotides, which are needed for growth.

During the deamination of excess amino acids in the liver.

They are a component of DNA, RNA, ATP and phospholipids — e.g. allowing nucleotides to join and storing energy in ATP.

Hydrogencarbonate ions (HCO₃⁻) and chloride ions (Cl⁻).

Calcium Ca²⁺, phosphate PO₄³⁻, ammonium NH₄⁺, nitrate NO₃⁻.

Sodium Na⁺, potassium K⁺, chloride Cl⁻, hydrogencarbonate HCO₃⁻.

The concentration of hydrogen ions (H⁺) determines the pH of a solution.

Phosphate ions (PO₄³⁻) are a component of DNA, RNA, ATP and phospholipids.

pH affects enzyme activity — extreme pH can change an enzyme's structure and denature it.

True — the more H⁺ ions present, the lower the pH.

False — nitrate ions provide a source of nitrogen for protein synthesis.

A non-protein chemical compound required for a protein to function; some inorganic ions act as cofactors.

The test for proteins: sodium hydroxide is added, then copper(II) sulfate; a colour change from blue to lilac/purple shows protein (peptide bonds) is present.

Add sodium hydroxide solution to make the sample alkaline, then add a few drops of blue copper(II) sulfate solution and mix.

The solution changes colour from blue to lilac/purple (mauve).

Peptide bonds — so the sample must contain at least two peptide bonds for a positive result.

The solution stays blue.

The test detects peptide bonds; single amino acids (and dipeptides) do not have enough peptide bonds to give a positive result.

Qualitative — it shows whether protein is present, not how much.

A positive biuret test produces a colour change from blue to purple, showing that protein is present.

True — the sample is first made alkaline with sodium hydroxide, then copper(II) sulfate is added.

The colour change can be very subtle, so a white tile background makes it easier to see.

An instrument that measures how much light of a particular wavelength is absorbed by a coloured solution.

A stepwise series of dilutions of a stock solution, each of known concentration, used to create a set of standards.

A graph of a measured value (e.g. absorbance) against known concentrations, used to find the concentration of an unknown sample.

Compare the sample's colour against a set of standard solutions of known reducing sugar concentration (made by serial dilution), all treated in exactly the same way.

The absorbance (or transmission) of light through a coloured solution, giving a quantitative measure of colour intensity, which relates to concentration.

To set the absorbance reading to zero, so the readings measure only the colour due to the sample.

Plot absorbance against the known concentrations of the standards, then read the unknown sample's concentration off the curve using its measured absorbance.

It selects the wavelength of light used, so a single known colour of light passes through the sample.

To create a set of standards of known, evenly decreasing concentrations, which an unknown sample can be compared against.

Time how long it takes for the first colour change to occur — the higher the reducing sugar concentration, the shorter the time taken.

A colorimeter measures the absorbance of light through a coloured solution to give a quantitative result.

An unknown concentration is read from a calibration curve of absorbance against known concentrations.

Choose a colour that contrasts with (is complementary to) the solution so it is absorbed most — e.g. a blue filter for an orange solution.

True

False — a higher concentration gives a brick-red colour; green indicates a low concentration.

A technique that separates a mixture into its individual components based on differences in their solubility.

In chromatography, the solvent that moves and carries the components of the mixture (e.g. water or ethanol).

In chromatography, the material that stays still while the solvent moves through it (e.g. the chromatography paper or TLC plate).

The distance moved by a solute divided by the distance moved by the solvent; used to help identify a substance.

To separate a mixture into its individual components — e.g. proteins, carbohydrates, vitamins and nucleic acids.

Differences in their solubility in the mobile phase (solvent).

They spend more time dissolved in the mobile phase, so they are carried further before the solvent stops moving.

Rᶠ = distance moved by the solute ÷ distance moved by the solvent

A solute cannot travel further than the solvent front, so the top of the fraction is always smaller than the bottom.

It is less soluble (and usually larger), so it travelled only a short distance up the paper.

Compare its Rᶠ value (or the distance it travelled) with those of known standard substances run under the same conditions.

By spraying the dried paper with a locating agent such as ninhydrin, which stains amino acids blue-violet.

The origin line is where the samples were spotted at the start; the solvent front is the furthest point the solvent reached.

The Rᶠ value is the distance moved by the solute divided by the distance moved by the solvent.

Colourless amino acid spots can be made visible by spraying the chromatogram with ninhydrin.

It is a ratio of two distances, so the units cancel out.

False — a solute cannot travel further than the solvent, so the Rᶠ value is always less than one.

True