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OCR A A-Level Biology > Unit 3 - Biological Molecules > Flashcards

Unit 3 - Biological Molecules Flashcards

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1
Q

Define elements.

A

Elements are different types of atoms distinguished by the number of protons in their atomic nuclei.

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2
Q

Define molecule.

A

A molecule is two or more atoms bonded together.

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3
Q

Define covalent bond.

A

A covalent bond is when two atoms share a pair of electrons.

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4
Q

How many bonds can carbon atoms make?

A

Four.

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5
Q

How many bonds can nitrogen atoms make?

A

Three.

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6
Q

How many bonds can oxygen atoms make?

A

Two.

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7
Q

How many bonds can hydrogen atoms make?

A

One.

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8
Q

Define ion.

A

An ion is an atom/molecule where the total number of protons are not equal to the total number of electrons.

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9
Q

What is a positive ion called?

A

A positive ion is called a cation.

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10
Q

What is a negative ion called?

A

A negative ion is called an anion.

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11
Q

Which elements make up carbohydrates?

A

Carbon, hydrogen and oxygen.

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12
Q

Which elements make up proteins?

A

Carbon, hydrogen, oxygen, nitrogen and (sometimes) sulfur.

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13
Q

Which elements make up lipids?

A

Carbon, hydrogen, oxygen and (sometimes) phosphorus.

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14
Q

Which elements make up nucleic acids?

A

Carbon, hydrogen, oxygen, nitrogen and phosphorus.

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15
Q

What is the formula for a calcium ion?

A

Ca 2+

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16
Q

What are calcium ions required for?

A

Nerve impulses and muscle contraction.

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17
Q

What is the formula for a sodium ion?

A

Na +

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18
Q

What are sodium ions required for?

A

Nerve impulses and kidney function.

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19
Q

What is the formula for a potassium ion?

A

K +

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20
Q

What is a potassium ion required for?

A

Nerve impulses and stomata.

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21
Q

What is the formula for a hydrogen ion?

A

H +

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22
Q

What are hydrogen ions required for?

A

Catalysts and pH determination.

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23
Q

What is the formula for an ammonium ion?

A

NH4 +

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24
Q

What are ammonium ions required for?

A

Making nitrate ions.

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25
Q

What is the formula for a nitrate ion?

A

NO3 -

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26
Q

What are nitrate ions required for?

A

Amino acid formation.

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27
Q

What is the formula for a hydrogen carbonate ion?

A

HCO3 -

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28
Q

What are hydrogen carbonate ions required for?

A

Maintaining blood pH.

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29
Q

What is the formula for a chloride ion?

A

Cl -

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30
Q

What are chloride ions required for?

A

Balancing sodium and potassium ions in cells.

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31
Q

What is the formula for a phosphate ion?

A

PO4 3-

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32
Q

What are phosphate ions required for?

A

Cell membranes, nucleic acids and ATP formation and bone formation.

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33
Q

What is the formula for a hydroxide ion?

A

OH -

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34
Q

What are hydroxide ions required for?

A

Catalysts and pH determination.

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35
Q

Define electronegativity.

A

Electronegativity is the ability of an atom to attract the bonding electrons in a covalent bond.

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36
Q

Define polymer.

A

Polymers are long-chain molecules made up by the linking of multiple individual molecules in a repeating pattern.

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37
Q

Define monomer.

A

Monomers are individual molecules which can link to form polymers.

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38
Q

Define polar molecule.

A

A polar molecule is a covalently bonded molecule where the electrons spend more time closer to one of the atoms than the other.

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39
Q

What are the atoms in a polar molecule called?

A

The atom with the greater share of electrons has a slightly negative charge and is called d- (delta negative). The other atom has a slightly positive charge and is called d+ (delta positive).

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40
Q

Why is water a polar molecule?

A

Water contains oxygen and hydrogen atoms covalently bonded together. Oxygen atoms have more protons than hydrogen atoms and so attract the electrons in the bond.

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41
Q

Define hydrogen bonds.

A

Hydrogen bonds are formed when polar molecules interact, as the positive and negative atoms are attracted to each other. Hydrogen bonds are intermolecular forces.

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42
Q

Are hydrogen bonds strong?

A

Compared to intramolecular forces, hydrogen bonds are quite weak, but they are relatively stronger than other intermolecular forces.

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43
Q

What is the difference between hydrophilic and hydrophobic?

A

Hydrophilic = dissolves in water
Hydrophobic = doesn’t dissolve in water

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44
Q

Why does ice float?

A

When water freezes into ice, the hydrogen bonds fix the positions of the molecules far apart from each other (further than in liquid water), forming a lattice. This means ice is less dense than water.

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45
Q

Why is the fact that ice floats important?

A

When ice forms on a body of water, it forms an insulating layer, stopping the water below from freezing. This means aquatic organisms don’t freeze.

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46
Q

Why is water very cohesive?

A

The hydrogen bonds mean that water molecules stick together and can move as one mass.

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47
Q

Why is water’s cohesion important?

A

Cohesion helps water to flow, making it better for transporting substances. It also explains why the water column doesn’t break in the xylem during transpiration.

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48
Q

What is adhesion?

A

The attraction of (water) molecules to other molecules.

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49
Q

What is surface tension in water?

A

Water molecules are more cohesive (attracted to each other) than adhesive to air, so water has a ‘skin’ of surface tension.

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50
Q

Why is water a good solvent?

A

Because water is a polar molecule, it is attracted to charged substances like ions and other polar molecules, so these molecules form hydrogen bonds with water and can dissolve.

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51
Q

Which substances dissolve in water?

A

Ions and polar molecules:
- amino acids
- proteins
- nucleic acids

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52
Q

Which substances don’t dissolve in water?

A

Non-polar molecules like lipids. Out of the important biological molecules in the spec, only triglycerides and large polymers don’t dissolve in water.

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53
Q

Why is water being a good solvent important?

A

Many biological reactions take place in solution.
This also helps water transport substances in the blood and in and out of cells.

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54
Q

What is capillary action?

A

Capillary action is where water can rise up a narrow tube against gravity due to cohesion and adhesion.
This happens in transpiration.

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55
Q

Why does water have a high specific heat capacity?

A

Because hydrogen bonds take a lot of energy to break, water can absorb a lot of heat before its temperature changes.

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56
Q

Why is the fact that water has a high specific heat capacity important?

A

Water doesn’t experience rapid temperature changes, making it a good habitat (as many organisms need a stable temperature). This also makes water an important component in blood for regulating body temperature, making sure enzymes have the right temperature to work properly.

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57
Q

Why is it important that water has a high specific latent heat of evaporation?

A

This makes water a good coolant. When mammals are warm, they sweat, and the evaporation of sweat cools the skin down.

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58
Q

Define specific heat capacity.

A

The specific heat capacity of a substance is the amount of thermal energy required to raise the temperature of 1kg of that substance by 1°C.

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59
Q

Define latent heat of evaporation.

A

This is how much thermal energy a substance can absorb before it evaporates.

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60
Q

Give two functions of carbohydrates.

A

Any two of:
- Energy source
- Energy store
- Structure
- Forming larger molecules

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61
Q

What is the common molecular formula for carbohydrates?

A

Cn(H2O)n

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62
Q

Define monosaccharide.

A

Monosaccharides are carbohydrates made from only one sugar molecule.

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63
Q

What is a hexose sugar?

A

A monosaccharide made from 6 carbons.

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64
Q

Give three examples of hexose sugars.

A
  • Glucose
  • Fructose
  • Galactose
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65
Q

What is a pentose sugar?

A

A monosaccharide made of 5 carbons.

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66
Q

Give two examples of pentose sugars.

A
  • Ribose
  • Deoxyribose
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67
Q

Give three properties of monosaccharides.

A
  • Soluble in water
  • Small so they can diffuse across cell membranes
  • Easily respired to release energy/produce ATP
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68
Q

Why are glucose molecules hydrophilic?

A

Because they’re polar. Hydrogen bonds form between the hydroxyl groups and the water molecules.

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69
Q

What is the difference structurally between alpha and beta glucose?

A

In alpha glucose, the hydroxyl group at carbon 1 is below the plane of the ring. In beta glucose, the hydroxyl group at carbon 1 is above the plane of the ring.

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70
Q

What is alpha glucose’s role in the body?

A
  • Respiratory substrate
  • The form in which carbohydrate is transported in mammalian blood
  • Component of starch and glycogen
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71
Q

What is beta glucose’s role in the body?

A
  • Respiratory substrate
  • Component of cellulose
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72
Q

What is fructose’s role?

A
  • Helps attract animals for pollination and fruit dispersal (found in nectar and fruit)
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73
Q

What is ribose’s role in the body?

A
  • Component of RNA
  • Component of ATP
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74
Q

What is deoxyribose’s role in the body?

A
  • Component of DNA
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75
Q

Define disaccharide.

A

A disaccharide is a carbohydrate made of two monosaccharides joined together.

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76
Q

Define condensation reaction.

A

A condensation reaction joins two monosaccharides to form a disaccharide, releasing a water molecule.

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77
Q

Define glycosidic bond.

A

A glycosidic bond is a covalent bond linking two monosaccharides, formed during a condensation reaction.

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78
Q

What are glycosidic bonds in glucose-glucose reactions called?

A

They are called 1-4 glycosidic bonds because carbon 1 of one glucose molecule joins to carbon 4 of the other molecule.

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79
Q

Define hydrolysis reaction.

A

A hydrolysis reaction is the separation of two monosaccharides with the addition of water.

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80
Q

What is the word equation for the formation of maltose?

A

Glucose + Glucose = Maltose + Water

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81
Q

What is the role of maltose?

A

Formed from the breakdown of starch in germinating seeds, where it provides energy for the growing embryo.

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82
Q

What is the word equation for the formation of sucrose?

A

Glucose + Fructose = Sucrose + Water

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83
Q

What is the role of sucrose?

A

The form in which sugars are transported in the phloem in plants.

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84
Q

What is the word equation for the formation of lactose?

A

Glucose + Galactose = Lactose + Water

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85
Q

What is the role of lactose?

A

The sugar found in milk; energy source for suckling mammals.

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86
Q

Define polysaccharide.

A

A polysaccharide is thousands of monosaccharides joined together by glycosidic bonds.

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87
Q

Which two polysaccharides make up starch and in what proportions?

A

Starch is made from two polysaccharides of alpha glucose. It is 25% amylose and 75% amylopectin.

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88
Q

Which type of glycosidic bond is used in amylose?

A

Only alpha 1-4 glycosidic bonds.

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89
Q

What is the overall shape of an amylose molecule?

A

Amylose is a long unbranched chain in a coiled helix shape.

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90
Q

Is starch soluble in water?

A

Starch is insoluble.

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91
Q

Which type of glycosidic bond is used in amylopectin?

A

Alpha 1-4 glycosidic bonds and alpha 1-6 glycosidic bonds.

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92
Q

What is the overall shape of an amylopectin molecule?

A

Amylopectin is a long chain with side branches in a coiled helix shape.

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93
Q

What monosaccharide is used to make cellulose?

A

Beta glucose.

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94
Q

Which type of glycosidic bond is used in cellulose?

A

Beta 1-4 glycosidic bonds. Because the hydroxyl groups on C1 and C4 of beta glucose are too far apart, the molecules are at 180 degrees to them alternately.

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95
Q

What is the overall shape of a cellulose molecule?

A

Cellulose is made of long unbranched chains. Hydrogen bonds between the chains form microfibrils.

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96
Q

Is cellulose soluble in water?

A

Cellulose is insoluble.

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97
Q

What monosaccharide is used to make glycogen?

A

Alpha glucose.

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98
Q

Which type of glycosidic bond is used to make glycogen?

A

Alpha 1-4 glycosidic bonds and alpha 1-6 glycosidic bonds.

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99
Q

What is the overall shape of glycogen?

A

Glycogen is a coiled helix structure with side branches - more side branches than in amylopectin.

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100
Q

Is glycogen soluble in water?

A

Glycogen is insoluble.

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101
Q

What is the function of starch?

A

Starch is a chemical energy store in plants.

102
Q

How does the structure of starch relate to its function? (4 points)

A

Amylose’s helix shape makes it compact - meaning more glucose can be stored.
Side branches in amylopectin - enzymes can easily reach glycosidic bonds, speeding up the release of glucose
Insoluble - doesn’t affect water potential
Starch can be hydrolysed and built up quickly.

103
Q

What is the function of glycogen?

A

Glycogen is a chemical energy store in animals. It is found in the liver and muscle cells.

104
Q

How does the structure of glycogen relate to its function? (4 points)

A

Insoluble - doesn’t affect water potential
Glycogen can be hydrolysed and built up quickly
Compact - has a high energy content for mass and less space is needed to store glycogen
Side branches - glucose released quicker for respiration

105
Q

What is the function of cellulose?

A

Cellulose is used to make plant cell walls.

106
Q

How does the structure of cellulose relate to its function?

A

Fibrous (microfibrils) - helps provide structural support for cells
Insoluble
Enzymes can’t reach the glycosidic bonds to break them down

107
Q

What are reducing sugars?

A

Sugars which can reduce another compound in a reduction reaction by giving away electrons.

108
Q

Are monosaccharides reducing or non-reducing sugars?

A

Monosaccharides are reducing sugars

109
Q

How do you test for reducing sugars and what indicates a positive result?

A

Add Benedict’s reagent and heat. If reducing sugars are present, the solution will turn from blue to brick red. Depending on the concentration of reducing sugar present, the solution can turn green, yellow or orange as well.

110
Q

Why does Benedict’s reagent change colour when heated with reducing sugars?

A

The colour change occurs because the reducing sugars give electrons to Benedict’s reagent.
Benedict’s reagent contains CuSO4, made up of Cu 2+ ions. This compound is blue in colour. When heated with reducing sugars, the copper ions gain electrons to become Cu+, forming a coloured (brick red if there is enough sugar) precipitate.

111
Q

What type of test is the Benedict’s test?

A

The Benedict’s test is a qualitative test. It could be considered semi-quantitative as the colour change is different depending on the concentration of reducing sugar present.

112
Q

How do you test for non-reducing sugars?

A

After a negative Benedict’s test, add hydrochloric acid to the sample and boil. Once the solution has cooled, neutralise it with sodium hydrogencarbonate (an alkali) before adding Benedict’s reagent and heating the solution. If non-reducing sugars are present, the solution will turn brick red or orange.

113
Q

How would you test for reducing sugars quantitatively?

A

By using a colourimeter, which can measure how much light a substance absorbs. First, use the colourimeter on samples with known concentrations of reducing sugars to create a calibration curve, then use it on the test sample and compare results to the calibration curve.

114
Q

Which elements do lipids contain?

A

Carbon, hydrogen and oxygen. Lipids contain less oxygen than carbohydrates.

115
Q

What are the three types of lipids?

A

Triglycerides (fats and oils), phospholipids (modified triglycerides containing phosphorus) and sterols (steroids and cholesterol).

116
Q

What is the structure of fatty acids?

A

Fatty acids consist of a long hydrocarbon chain with a carboxyl group at one end.

117
Q

What is the difference between saturated and unsaturated fatty acids?

A

Saturated fatty acids contain only single bonds between the carbon atoms and are usually found in animals. Unsaturated fatty acids contain one or more double bonds between the carbon atoms and are usually found in plants.

118
Q

Are saturated fatty acids solid or liquid at room temperature and why?

A

Saturated fatty acids are solid at room temperature because the hydrocarbon chain is straight (no double bonds), so the molecules can pack closely together.

119
Q

What is the difference between cis and trans fats?

A

Cis and trans fats are both unsaturated. In cis fats, the hydrogen atoms are on the same side of the double bond. In trans fats, the hydrogen atoms are on opposite sides of the double bond.

120
Q

What is the structure of triglycerides?

A

Triglycerides are formed by a condensation reaction between three fatty acid molecules and one glycerol molecule.

121
Q

What bond is formed in triglycerides?

A

Triglycerides have an ester bond between the carboxyl group of the fatty acid and the hydroxyl group of the glycerol.

122
Q

Are triglycerides hydrophilic or hydrophobic?

A

Triglycerides are hydrophobic because there are no spare oxygen molecules for water to bond with.

123
Q

Why are triglycerides good for storage?

A

Because the hydrocarbon chains of the fatty acid molecules contain lots of chemical energy which is released when they are broken down and they are insoluble, so they don’t affect the cells’ water potential.

124
Q

What is the structure of phospholipids?

A

Phospholipids have a glycerol molecule bonded to two fatty acid molecules and a phosphate group.

125
Q

Are phospholipids hydrophilic or hydrophobic?

A

The phosphate group is ionised and so hydrophilic, while the hydrocarbon chain is hydrophobic.

126
Q

How do phospholipids form cell membranes?

A

Phospholipids form bilayer structures - a two-layered sheet with their hydrophobic fatty acid tails pointing inside and the phosphate group outside. This separates the aqueous environment outside cells from the aqueous cytosol inside, and also acts as a barrier to water-soluble substances, preventing them from entering the cell.

127
Q

What is the structure of sterols?

A

Sterols have a four carbon ring structure attached to a hydroxyl group and a hydrocarbon tail.

128
Q

Are sterols hydrophilic or hydrophobic?

A

The hydroxyl group (polar) is hydrophilic, but the rest of the molecule is hydrophobic.

129
Q

Are sterols soluble in blood?

A

Sterols are insoluble in blood and are carried around the body by lipoproteins.

130
Q

How is cholesterol used to make cell membranes?

A

Cholesterol is used to give cell membranes stability and regulate their fluidity by interacting with the phospholipid bilayer. The molecules are positioned between the phospholipids, with the hydroxyl group at the periphery of the cell membrane. Cholesterol has a small size and flattened shape, allowing it to fit between the phospholipid molecules. At higher temperatures, the cholesterol molecules bond to the hydrophobic tails of the phospholipids, making the membrane less fluid, while at lower temperatures it prevents the phospholipids from packing too close together, increasing the membrane’s fluidity.

131
Q

What is cholesterol used for?

A

To manufacture Vitamin D, steroid hormones and bile.

132
Q

How would you test for lipids?

A

Mix the sample with ethanol, then mix the resulting solution with water and shake. If lipids are present, a white emulsion will form over the solution.

133
Q

How does the emulsion test for lipids work?

A

Lipids are soluble in ethanol - therefore when the sample is mixed with ethanol any lipid present will dissolve.
Lipids are insoluble in water - so when water is added the lipid will precipitate out and form an emulsion.

134
Q

What structures are made of proteins?

A

Enzymes, hormones and antibodies.

135
Q

What is a polypeptide?

A

A protein made of multiple amino acids.

136
Q

What is a dipeptide?

A

Two amino acids joined together.

137
Q

What are proteins made of?

A

Proteins are made of amino acids.

138
Q

What is the general structure of an amino acid?

A

An amino acid is made of a carbon atom bonded to a hydrogen atom, an amine group (NH3), a carboxylic acid group (COOH) and an R group.

139
Q

What is an R group in an amino acid?

A

An R group is a side chain from the alpha carbon atom which can be simple or complex. Each amino acid has a different R group.

140
Q

What reaction joins amino acids together?

A

Amino acids join in condensation reactions.

141
Q

What bond is formed between amino acids?

A

A peptide bond is formed between the H on the amine group of one amino acid and the OH on the carboxyl group of another amino acid. These molecules combine to form an H2O molecule.

142
Q

What is the primary structure of a protein?

A

The unique sequence of amino acids, which is directed by information in DNA. The primary structure helps determine how the polypeptide chain folds to make its final shape and therefore the protein’s function.

143
Q

What bonds are in the primary structure of a protein?

A

The primary structure only contains peptide bonds between the amino acids.

144
Q

What is the secondary structure of a protein?

A

The coiling or bending of the polypeptide chains. Can form:
Alpha helices - hydrogen bonds within the chains
Beta pleated sheets - the polypeptide chains lie parallel to each other and are joined by hydrogen bonds

145
Q

What bonds are in the secondary structure of a protein?

A

The secondary structure contains peptide bonds and hydrogen bonds.

146
Q

What is the tertiary structure of a protein?

A

The folding back of the secondary structure on itself, making the protein 3D.

147
Q

What bonds are in the tertiary structure of a protein?

A

Hydrogen bonds
Ionic bonds - between oppositely charged R groups
Disulfide bonds - between the R groups of cystine (these contain S)
Hydrophobic and hydrophilic interactions - when hydrophobic amino acids cluster together on the centre of the molecule and hydrophilic amino acids on the surface

148
Q

What is the quaternary structure of a protein?

A

Proteins have a quaternary structure if they are made of multiple polypeptide subunits joined together.

149
Q

What are the two types of proteins?

A

Globular proteins and structural/fibrous proteins

150
Q

What are globular proteins?

A

Proteins in a compact globe/ball structure, formed by the amino acids with hydrophobic R groups turning inwards and the hydrophilic amino acids on the surface.

151
Q

What are structural/fibrous proteins?

A

Proteins with a long, thin structure and a repetitive primary structure (sequence of amino acids) allowing them to form fibres.

152
Q

What are conjugated proteins?

A

Globular proteins containing a non-protein component called a prosthetic group.

153
Q

What is the structure of haemoglobin?

A

Haemoglobin is a globular protein made of two alpha and two beta polypeptide chains. Each chain is attached to a haem group (prosthetic group) containing an iron ion - Fe 2+.

154
Q

What is the function of haemoglobin?

A

Haemoglobin is found in red blood cells and transports oxygen around the body. Because it contains a haem group, haemoglobin can bond to four oxygen molecules (each ion bonds to one), increasing the blood oxygen capacity.

155
Q

Which proteins are soluble in water?

A

Globular proteins are soluble in water because the amino acids with hydrophilic R groups are on the surface of the protein. Fibrous proteins are insoluble in water because they have a high proportion of amino acids with hydrophobic R groups.

156
Q

What is the structure of insulin?

A

Insulin is a globular protein made of two polypeptide chains - one begins with alpha helix and the other ends with beta pleated sheet. The chains are joined by disulfide bonds.

157
Q

What is the function of insulin?

A

Insulin is secreted by the pancreas and maintains blood glucose levels. It does this by binding to receptors on muscle and fat cells to increase their uptake of glucose from the blood.

158
Q

What is the structure of pepsin?

A

Pepsin is a globular protein made of one polypeptide chain folded into a symmetrical tertiary structure, which is held together by hydrogen bonds and two disulfide bonds. Most of the amino acids have acidic R groups.

159
Q

What is the function of pepsin?

A

Pepsin is an enzyme found in the stomach which helps to digest proteins. It can survive in the stomach because it has mostly amino acids with acidic R groups - basic R groups would bind to the H+ ions in the stomach and disrupt the enzyme’s structure.

160
Q

What is the structure of catalase?

A

Catalase is a globular protein containing four haem groups.

161
Q

What is the function of catalase?

A

Catalase is an enzyme which digests hydrogen peroxide, a toxic substance.

162
Q

What is the structure of elastin?

A

Elastin is a fibrous protein made of tropoelastin molecules linked together. These molecules join through interactions between hydrophobic areas. Elastin contains crosslinking and coiling, making it strong and extendable.

163
Q

What is the function of elastin?

A
  • helps the skin stretch around bones and muscle + keeps it elastic
  • helps the bladder expand to hold urine
  • helps the lungs inflate and deflate
  • helps the blood vessels stretch and recoil + maintains pressure
164
Q

What is the structure of collagen?

A

Collagen is a fibrous protein made of three intertwined polypeptide chains in a triple helix. Hydrogen bonds form between the polypeptide chains, which join end to end to form tropocollagen fibrils. Tropocollagen fibrils crosslink to form fibres. Every third amino acid is glycine, allowing the molecules to pack closely together. Has high proportions of proline and hydroxyproline, which repel each other.

165
Q

What is the function of collagen?

A

A component of bones, along with calcium phosphate
Cartilage and connective tissue
Prevents arteries bursting
Tendons - connect muscles and bones

166
Q

What is the structure of keratin?

A

Keratin is a fibrous protein containing a high amount of cystine, meaning there are disulfide bonds between the polypeptide chains.

167
Q

What is the function of keratin?

A

Keratin makes body parts like hair, skin and nails hard and strong. It provides mechanical protection and a barrier to infection.

168
Q

What structure does DNA have?

A

DNA has a double helix structure - two polynucleotide strands are coiled tightly to form a long spiral.

169
Q

What are the components of a DNA nucleotide?

A
  • phosphate group
  • deoxyribose sugar
  • base (could be adenine, cytosine, guanine, thymine)
170
Q

What reaction joins nucleotides together?

A

A condensation reaction.

171
Q

What bond forms between nucleotides?

A

A phosphodiester bond. This forms between the phosphate group of one nucleotide and the hydroxyl group of the pentose sugar on the other nucleotide.

172
Q

What is the chain of phosphodiester bonds called?

A

The sugar-phosphate backbone.

173
Q

How do the two DNA strands join together?

A

Hydrogen bonds form between bases on each strand, and the strands run antiparallel to each other.

174
Q

What is complimentary base pairing?

A

Each base will only pair with one other base.

175
Q

What are purines?

A

Purines are bases with double carbon ring structure.

176
Q

Which bases are purines?

A

Adenine and guanine.

177
Q

What are pyrimidines?

A

Pyrimidines are bases with single carbon ring structures.

178
Q

Which bases are pyrimidines?

A

Cytosine and thymine.

179
Q

Which bases pair and how?

A

Adenine and thymine (with two hydrogen bonds)
Cytosine and guanine (with three hydrogen bonds)

180
Q

How many strands does RNA have?

A

One.

181
Q

What sugar does RNA contain?

A

Ribose.

182
Q

What four bases does RNA contain?

A

Adenine, cytosine, guanine and uracil.

183
Q

When extracting DNA, why do you add salt solution?

A

To break hydrogen bonds between DNA and water molecules.

184
Q

When extracting DNA, why do you add detergent solution?

A

To break down the plasma membrane and nuclear membrane, releasing the DNA into solution.

185
Q

When extracting DNA, why do you add protease solution?

A

To break down the histone proteins associated with DNA.

186
Q

After DNA is extracted, why does DNA precipitate out when mixed with ethanol?

A

DNA is a polar membrane and ethanol is a non-polar membrane.

187
Q

Why must DNA be replicated before cell division?

A

So that each daughter cell has a complete set of DNA.

188
Q

What does DNA helicase do?

A

DNA helicase breaks hydrogen bonds between the complementary base pairs on each DNA strand, unwinding the double helix and separating the strands.

189
Q

What is the replication fork?

A

When the two strands are partly separated and partly still attached in DNA replication.

190
Q

What are the two strands of the replication fork called in DNA replication?

A

The strand uncoiled from the 3’ end is called the leading strand, while the strand uncoiled from the 5’ end is called the lagging strand.

191
Q

What does the enzyme primase do?

A

Primase makes small pieces of RNA called primers on each strand, which act as a starting point for replication.

192
Q

What are activated nucleotides?

A

Nucleotides containing three phosphate groups rather than one.

193
Q

What do free nucleotides in the nucleus do once the double helix has separated?

A

Free nucleotides are attracted to their complementary base pairs on the template strands and form hydrogen bonds.

194
Q

What does DNA polymerase do?

A

DNA polymerase moves along the strands checking that the base pairings are complementary and catalysing the formation of phosphodiester bonds between adjacent nucleotides. It also fills in the gaps once the RNA primers have been removed.

195
Q

Why do the free nucleotides in DNA replication have extra phosphate groups?

A

The phosphate groups are used up to provide energy for the formation of phosphodiester bonds between adjacent nucleotides.

196
Q

What direction does DNA polymerase move in?

A

From 5’ to 3’ on the newly synthesised strand (3’ to 5’ on the original strand)

197
Q

Is DNA replication on the leading strand continuous or discontinuous?

A

Continuous - DNA polymerase moves from the 3’ end on the leading strand towards the replication fork.

198
Q

Is DNA replication on the lagging strand continuous or discontinuous?

A

Discontinuous - DNA polymerase moves from the 3’ end on the lagging strand away from the replication fork.

199
Q

What are Okazaki fragments?

A

The small segments of DNA formed by DNA polymerase on the lagging strand.

200
Q

What does the enzyme exonuclease do in DNA replication?

A

Exonuclease removes the RNA primers from the newly synthesised DNA strands.

201
Q

What does the enzyme DNA ligase do?

A

DNA ligase joins Okazaki fragments on the newly synthesised DNA strand (the strand bonded to the lagging strand).

202
Q

Why is DNA replication described as semi-conservative?

A

DNA replication is described as semi-conservative because each new DNA molecule contains one strand of parental DNA and one strand of newly synthesised DNA.

203
Q

What is a mutation?

A

A change in DNA structure caused by replication errors or environmental factors such as radiation, viruses or chemicals.

204
Q

Do all mutations cause ill effects?

A

No - not all DNA codes for proteins in every cell. Also, sometimes a change in the order of bases does not affect which amino acid is produced.

205
Q

What is conservative replication?

A

The idea that during DNA replication, the original molecule remains intact and the two new strands join together to form a new molecule.

206
Q

What did the Meselson-Stahl experiment prove?

A

That DNA replicates semi-conservatively.

207
Q

What did Meselson and Stahl do?

A

They grew bacteria in a growth medium containing 15N, before moving it to a medium containing 14N. After each replication, they spun the DNA in a centrifuge.

208
Q

What are 15N and 14N?

A

Isotopes of nitrogen (different numbers of neutrons). 15N is heavier as it has more neutrons.

209
Q

What does growing bacteria in a medium containing 15N do?

A

Bacteria will take in the nitrogen to make nucleotides for DNA replication. Therefore, all of the DNA molecules will contain two strands of 15N nucleotides.

210
Q

What is a centrifuge?

A

A machine which spins around very quickly. This separates material based on density - heavier material (15N) will settle at the bottom, while lighter material (14N) will settle at the top.

211
Q

Meselson-Stahl:
After one replication in a medium containing 14N, what was the composition of DNA molecules?

A

Each molecule had one strand of 15N nucleotides and one strand of 14N nucleotides.

212
Q

Meselson-Stahl:
After two replications in a medium containing 14N, what was the composition of DNA molecules?

A

50% of molecules had two strands of 14N nucleotides and the other 50% had one strand of 15N nucleotides and one strand of 14N nucleotides.

213
Q

Meselson-Stahl:
After three replications in a medium containing 14N, what was the composition of DNA molecules?

A

25% of molecules had one strand of 15N nucleotides and one strand of 14N nucleotides, while 75% had two strands of 14N nucleotides.

214
Q

What is a gene?

A

A length of DNA/sequence of nucleotide bases that codes for one or more polypeptides.

215
Q

How many genes do humans have?

A

25000

216
Q

Where are genes found in humans?

A

Mostly in the nucleus, but some genes are found in the mitochondria and chloroplasts.

217
Q

What is a codon?

A

A sequence of three bases on DNA or RNA that codes for one amino acid.

218
Q

Why is the genetic code degenerate?

A

There are more codons possible than amino acids, so most amino acids are coded for by multiple codons.

219
Q

What codon always starts off the gene base sequence?

A

AUG - codes for the amino acid methionine.

220
Q

Which codons mark the end of the polypeptide chain?

A

UAA, UAG and UGA.

221
Q

Why is the genetic code non-overlapping?

A

The triplets are read together in groups and don’t overlap.

222
Q

What are the two stages of protein synthesis?

A

Transcription and translation.

223
Q

Where does transcription happen?

A

In the nucleus.

224
Q

Why does an RNA copy need to be made of DNA for protein synthesis?

A

Because DNA is fixed in the nucleus and can’t move into the cytoplasm.

225
Q

Where does translation happen?

A

On the ribosomes attached to the rough endoplasmic reticulum.

226
Q

What are the three types of RNA involved in protein synthesis?

A

Messenger RNA, transfer RNA and ribosomal RNA.

227
Q

What is the role of ribosomal RNA in protein synthesis?

A

Ribosomes are made of rRNA. It also makes up the enzyme peptidyl transferase.

228
Q

What happens in transcription?

A

The DNA double helix unwinds and the sequence on one strand is used to produce mRNA from free RNA nucleotides. The nucleotides bond to form an mRNA strand which leaves the nucleus.

229
Q

Which enzyme catalyses the breaking of hydrogen bonds between complementary base pairs on the DNA double helix in transcription?

A

DNA helicase.

230
Q

What is the sense strand in transcription?

A

The strand from 5’ to 3’. This strand contains the code for the protein.

231
Q

What is the antisense strand in transcription?

A

The strand from 3’ to 5’. This acts as the template strand so that the mRNA strand has the same base sequence as the sense strand.

232
Q

Which enzyme catalyses the formation of phosphodiester bonds between RNA nucleotides in transcription?

A

RNA polymerase.

233
Q

Where does the mRNA strand go after transcription?

A

The strand leaves the nucleus through the nuclear pore and travels to a ribosome for translation. It attaches to a specific site on the small subunit of the ribosome.

234
Q

How is tRNA composed?

A

tRNA is a strand of RNA folded so that three bases (an anticodon) are at the end of the molecule.

235
Q

What does tRNA do in translation?

A

The tRNA molecule will bind to a codon on mRNA complementary to the anticodon. It will bring an amino acid corresponding to the codon.

236
Q

How many tRNA molecules can bind to the mRNA strand at a time?

A

One, maximum two.

237
Q

What enzyme catalyses the formation of peptide bonds between amino acids in translation?

A

Peptidyl transferase.

238
Q

What is the role of the ribosome in translation?

A

The ribosome holds the mRNA molecule in position while the tRNA molecule brings amino acids. It also moves along the molecule, releasing each tRNA molecule as the amino acids are attached, until it reaches a stop codon at the end of the mRNA. Multiple ribosomes can also follow the first, allowing multiple polypeptide chains to be synthesised simultaneously.

239
Q

What three main types of activity do cells require energy for?

A

Synthesis, transport and movement.

240
Q

What three components make up ATP?

A

ATP is a ribose sugar attached to an adenine base and three phosphate groups.

241
Q

What is the equation for the hydrolysis of ATP?

A

adenosine triphosphate + water - adenosine diphosphate + inorganic phosphate + energy

242
Q

Why does the hydrolysis of ATP release energy?

A

More energy is released when the free phosphate group undergoes other reactions than is required to break the bond between the phosphate group and the rest of the molecule.

243
Q

What is phosphorylation?

A

When a phosphate group is attached to a molecule of ADP in a condensation reaction, forming a molecule of ATP.

244
Q

Why is the fact that ATP is small useful to its role in the cell?

A

It means that ATP can move easily between and within cells.

245
Q

Why is the fact that ATP is soluble in water useful to its role in the cell?

A

Energy-requiring processes often happen in aqueous environments.

246
Q

Why is the fact that ATP releases energy in small quantities useful to its role in the cell?

A

It releases enough energy for cellular needs, but not enough that energy is wasted as heat.

247
Q

Why is the fact that ATP is easily regenerated useful to its role in the cell?

A

It can be recharged with energy.

248
Q

Why is ATP not a good long-term energy source?

A

The bonds between the phosphate groups are unstable and it can’t be stored in cells for long periods of time.

249
Q

Why are fats and carbohydrates used in cells as long-term energy stores instead of ATP?

A

When fats and carbohydrates are broken down in respiration, the energy released is used in phosphorylation to regenerate ATP.

250
Q

Why is ATP a good immediate energy store?

A

Even though ATP isn’t stored in large amounts, phosphorylation takes place constantly so there is always enough.

OCR A A-Level Biology (3 decks)

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