3.1 Chemical elements and water
3.1.1 State that the most frequently occurring chemical elements in living things are carbon, oxygen, hydrogen and nitrogen.
3.1.2 State that a variety of other elements are needed by living organisms, including sulphur, calcium, phosphorus, iron and sodium.
3.1.3 State one role for each of the elements mentioned in 3.1.2
● Sulphur is in the amino acid, cysteine. They form the disulphide bridge in the tertiary structure of a protein.
● Phosphorus is in a phospholipid molecule.
● Calcium is in bones.
● Iron in haemoglobin binds to oxygen, which is then carried to parts of the body for cellular respiration.
● Sodium is used to establish a voltage difference in neurons (Na/K pump)
3.1.4 Draw and label a diagram showing the structure of water molecules to show their polarity and hydrogen bond formation.
3.1.5 Outline the thermal, cohesive and solvent properties of water.
3.1.6 Explain the relationship between the properties of water and its uses in living organisms as a coolant, medium for metabolic reactions and transport medium.
| Property | Outline of the property | Relationship between property and its use in living things |
| Cohesion | water molecules stick to each other because of the hydrogen bonding that occurs between them | cohesive forces can be observed in the xylem of angiosperms--strong pulling forces (cohesion) between water molecules help draw the transpiration stream up and ensure that it is an uninterrupted stream; water is the transport medium in the xylem of plants |
| Solvent properties | water is an effective solvent and allows charged inorganic particles (eg. sodium), organic substances with polar molecules (eg. glucose), or enzymes to dissolve in it | water is the medium for metabolic reactions since most chemical reactions in living things take place dissolved in water; water is also a transport medium since substances can be dissolved and carried in it, such as in blood or sap |
| Thermal properties: heat capacity | water has a very large heat capacity, and large amounts of energy are required to raise its temperature due to the presence of hydrogen bonding between water molecules | blood, which is mainly composed of water, is a transport medium for heat and can carry heat from warmer to cooler parts of the body larger organisms have stable temperatures aquatic environments can maintain a stable temperature |
| Thermal properties: boiling point | much energy is needed to change water from a liquid to a gas due to the presence of hydrogen bonding between water molecules | evaporation causes marked cooling, such as in sweating, and can be used to cool down the body |
| Thermal properties: freezing point | much heat needs to be removed before freezing occurs in water | cell contents and water in aquatic environments are slow to freeze in the winter |
3.2 Carbohydrates, lipids and proteins
3.2.1 Distinguish between organic and inorganic compounds.
● Compounds containing carbon that are found in living organisms (excluding hydrocarbonates, carbonates and oxides of carbon) are organic.
3.2.2. Identify amino acids, glucose, ribose and fatty acids from diagrams showing their structure.
pg 45 of textbook - glucose and ribose
pg 48 of textbook - fatty acid
pg 51 of textbook - amino acid
3.2.3 List three examples each of monosaccharides, disaccharides, and polysaccharides.
● Monosaccharides: glucose, galactose, and fructose
● Disaccharides: lactose, maltose, and sucrose
● Polysaccharides: cellulose, glycogen, starch
3.2.4 State one function of glucose, lactose and glycogen in animals, and of fructose, sucrose and cellulose in plants.
● Animals
○ glucose: used as a respiratory substrate in cellular respiration
○ lactose: produced in mammary glands and secreted into the milk for the diet of very young mammals
○ glycogen: storage carbohydrate made from glucose
● Plants
○ fructose: used in the production of sucrose; produced in cellular respiration as an intermediate of glucose breakdown
○ sucrose: sugar is transported in plant solution as sucrose, within the vascular bundles
○ cellulose: main component of plant cell walls
3.2.5 Outline the role of condensation and hydrolysis in the relationships between monosaccharides, disaccharides, and polysaccharides; between fatty acids, glycerol and triglycerides; and between amino acids and polypeptides.
This can be dealt with using equations with words or chemical formulas.
● Carbohydrates
○ glycosidic linkage
○ pg 46
● Lipids
○ ester linkages
○ pg 49
● Amino acids
○ peptide linkage
○ pg 52
3.2.6 State three functions of lipids
● Energy storage - in the form of fat in humans and oil in plants
● Thermal insulation - layers of fat under skin reduces heat loss
● Buoyancy - lipids are less dense than water and so they help animals to float
3.2.7 Compare the use of carbohydrates and lipids in energy storage
| Lipids | Role | Carbohydrate |
| lipids have more energy per gram than carbohydrates | energy store | less energy per gram than lipids |
| much metabolic water is produced on oxidation | metabolic water source | less metabolic water is produced on oxidation |
| are insoluble, so osmotic water uptake is not caused | solubility | are soluble, so they cause osmotic uptake of water |
| not as easily digested | ease of breakdown | more easily digested than lipids so energy can be transferred more quickly |
3.3 DNA structure
3.3.1 Outline DNA nucleotide structure in terms of sugar (deoxyribose), base and phosphate.
Chemical formulas and the purine/pyrimidine subdivision are not required. Simple shapes can be used to represent the component parts. Only the relative positions are required.
3.3.2 State the names of the four bases in DNA.
● Adenine
● Guanine
● Cytosine
● Thymine
3.3.3 Outline how DNA nucleotides are linked together by covalent bonds into a single strand.
Only the relative positions are required.
3.3.4 Explain how a DNA double helix is formed using complementary base pairing and hydrogen bonds.
● DNA nucleotides (one nucleotide = phosphate, sugar, and base) are linked together by a covalent bond between the sugar of one molecule and the phosphate group of another molecule
● DNA molecules consist of two strands of nucleotides wound together in a double helix
● Hydrogen bonding link the strands
● Complementary base pairing occurs between adenine and thymine, and between guanine and cytosine
3.3.5 Draw and label a simple diagram of the molecular structure of DNA.
An extension of the diagram in 3.3.3 is sufficient to show the complementary base pairs of A–T and G–C, held together by hydrogen bonds and the sugar–phosphate backbones. The number of hydrogen bonds between pairs and details of purine/pyrimidines are not required.
3.4 DNA Replication
3.4.1 Explain DNA replication in terms of unwinding the double helix and separation of the strands by helicase, followed by formation of the new complementary strands by DNA polymerase.
It is not necessary to mention that there is more than one DNA polymerase.
Okazaki fragments, ligase, RNA primer, etc., mentioned in AHL material.
● The DNA double helix is first unzipped by helicase, which breaks the hydrogen bonds between the two strands.
● Each strand acts as templates for new strands.
● Free-floating nucleotides are present in the nucleoplasm. The nitrogenous bases of these nucleotides line up against the complementary base on the parent strand. The bases of the two strands are aligned in opposite directions, so they are antiparallel.
○ Adenine pairs with thymine, and guanine pairs with cytosine.
● Hydrogen bonds form between the complementary bases.
● The sugar and phosphate groups of adjacent nucleotides of the new strand are condensed together by DNA polymerase.
● The daughter DNA molecules each rewind into a double helix.
3.4.2 Explain the significance of complementary base pairing in the conservation of the base sequence of DNA.
● Complementary base pairing between the parent strand and the free-floating nucleotides results in identical copies of DNA.
3.4.3 State that DNA replication is semi-conservative.
3.5 Transcription and translation
3.5.1 Compare the structure of RNA and DNA.
Limit this to the names of sugars, bases and the number of strands.
| DNA | RNA |
| contains a 5-carbon sugar | contains a 5-carbon sugar |
| its sugar is deoxyribose | its sugar is ribose |
| each nucleotide has one of four nitrogenous bases | each nucleotide has one of four nitrogenous bases |
| its bases are cytosine, guanine, thymine, and adenosine | its bases are cytosine, guanine, uracil, and adenosine |
| double-stranded molecule | single-stranded molecule |
3.5.2 Outline DNA transcription in terms of the formation of an RNA strand complementary to the DNA strand by RNA polymerase.
● RNA polymerase unwinds the section of DNA that is to be transcribed (length of a gene). The hydrogen bonds “unzip”.
● Free floating nucleotides are used by RNA polymerase, which catalyzes synthesis of mRNA against the template strand of DNA.
●
3.5.3 Describe the genetic code in terms of codons composed of triplets of bases.
● The genetic code is a triplet code. Three bases code for one amino acid.
● Most amino acids have degenerate codes, which is when multiple codons code for the same amino acid. This is because there are 64 possible combinations of codons but only 20 different amino acids.
● Some codons are “stop” or “start” codons.
3.5.4 Explain the process of translation, leading to polypeptide formation.
Include the roles of messenger RNA (mRNA), transfer RNA (tRNA), codons, anticodons, ribosomes and amino acids.
● mRNA binds to the small sub unit of a ribosome. The mRNA contains a series of codons, each which codes for one amino acid.
● tRNA molecules are free-floating and present in the cytoplasm. Each tRNA has a special triplet of bases called an anticodon, and carries the amino acid corresponding to the anticodon.
● tRNA with the anticodon that is complementary to the codon on the mRNA can bind to the ribosome. The bases on the codon and anticodon link together through hydrogen bonding. Two tRNA molecules can bind to the ribosome at a time, in the first and second binding site.
● The two amino acids carried by the two tRNA molecules are bonded together by a peptide linkage. The polypeptide chain is held at on the second tRNA molecule.
● The first tRNA molecule is discarded and the ribosome moves one codon down the mRNA molecule.
○ This brings the position of the second tRNA at the site of the first tRNA that was discarded.
● Another tRNA molecule then binds to the vacant site and these stages are repeated until a polypeptide is formed.
3.5.5 Discuss the relationship between one gene and one polypeptide.
Originally, it was assumed that one gene would invariably code for one polypeptide, but many exceptions have been
discovered.
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