4, 5 and 6 markers

Describe how the structures of starch and cellulose molecules are related to their functions (5 marks)

Biological molecules

Starch:

• Helical shape so compact

• Large molecule/ insoluble so osmotically inactive

• Branched so glucose is easily released for respiration

• Large molecule so cannot leave cell-surface membrane

Cellulose:

• Long, straight/ unbranched chains of B glucose

• Joined by hydrogen bonding

• To form microfibrils

• Provides rigidity/ strength

Mucus produced by epithelial cells in the human gas exchange system contains triglycerides and phospholipids.

Compare and contrast the structure and properties of triglycerides and phospholipids (5 marks)

Biological molecules

• Both contain ester bonds (between glycerol and fatty acid)

• Both contain glycerol

• Fatty acids on both may be saturated or unsaturated

• Both are insoluble in water

• Both contain C, H and O but phospholipids also contain P

• Triglyceride has 3 fatty acids and phospholipid has 2 fatty acids plus phosphate group

• Triglyercides are hydrophobic/ non=polar and phospholipids have hydrophilic and hydrophobic region

• Phospholipids form monolayer on surface/ micelle/ bilayer (in water) but triglycerides don’t

Mucus also contains glycoproteins. One of these glycoproteins is a polypeptide with the sugar, lactose, attached.

Describe how lactose is formed and where in the cell it would be attached to a polypeptide to form a glycoprotein (4 marks)

Biological molecules

• Glucose and galactose

• Joined by condensation (reaction)

• Joined by glycosidic bond

• Added to polypeptide in gogli apparatus

Describe the biochemical tests you would use to confirm the presence of lipid. non-reducing sugar and amylase in a sample (5 marks)

Biological molecules

Lipid:

• Add ethanol then add water and shake

• White emulsion

Non-reducing sugar:

• Do benedict’s test and stays blue/ negative

• Boil with acid then neutralise with alkali

• Heat with benedict’s and becomes red/ orange (precipitate)

Amylase:

• Add biuret reagent and becomes lilac

• Add starch, (leave for a time), test for reducing sugar

Compare and contrast the structure of starch and the structure of cellulose (6 marks)

Biological molecules

• Both polysaccharides/ polymers of glucose

• Both contain glycosidic bonds between monomers

• Both contain carbon, hydrogen and oxygen

• Starch has a glucose and cellulose has B glucose

• Starch molecule is helical and cellulose molecule is straight

• Starch molecule is branched and cellulose is unbranched

• Cellulose has microfibrils and starch does not

Describe how an ATP molecule is formed from its component molecules (4 marks)

Nucleic acids

• Adenine, ribose pentose sugar, and 3 phosphates

• Condensation reaction

• ATP synthase

Describe the process of semi-conservative replication of DNA (5 marks)

Nucleic acids

• DNA helicase unwinds DNA double helix/ breaks hydrogen bonds

• Both strands act as templates

• Free DNA nucleotides line up in complementary pairs/ A-T and C-G

• DNA polymerase joins nucleotides (of new strand)

• Forming phosphodiester bonds

• Each new DNA molecule consists of one original strand and one new strand

Describe the structure of DNA (5 marks)

Nucleic acids

• Polymer of nucleotides

• Each nucleotide formed from deoxyribose, a phosphate group and an organic base

• Phosphodiester bonds (between nucleotides)

• Double helix/ 2 strands held by hydrogen bonds

• (Hydrogen bonds/ pairing) between adenine, thymine and cytosine, guanine

Describe the process of translocation (5 marks)

Plant transport

• (At source_ sucrose is actively transported into the phloem/ sieve element/ tube

• By companion/ transfer cells

• Lowers water potential in phloem and water enters by osmosis

• (Produces) high (hydrostatic) pressure

• Mass flow/ transport towards sink

• At sink/ roots sugars are removed/ unloaded

Describe the cohesion- tension theory of water transport in the xylem (5 marks)

Plant transport

• Water lost from leaf because of transpiration/ evaporation of water from mesophyll cells

• Lowers wtaer potential of mesophyll

• Water pulled up xylem (creating tension)

• Water molecules cohere/ stick together by hydrogen bonds

• (forming continuous) water column

• Adhesion of water (molecules) to walls of xylem

Describe the transport of carbohydrate in plants (5 marks)

Plant transport

• Sucrose actively transported into phloem cells

• By companion cells

• Lowers water potential in phloem and water enters from xylem by osmosis

• Produces higher hydrostatic pressure

• Mass flow to respiring cells

• Unloaded/ removed from phloem by active transport

Describe the roles of iron ions, sodium ions, and phosphate ions in cells (5 marks)

Ions/ water

Iron ions:

• Haemoglobin binds/ associates with oxygen OR Haemoglobin transports/ loads oxygen

Sodium ions:

• Co-transport of glucose/ amino acids into cells

• Because sodium moved out by active transport/ Na- K pump

• Creates a sodium concentration/ diffusion gradient

• Affects osmosis/ water potential

Phosphate ions:

• Affects osmosis/ water potential

• Joins nucleotides/ in phosphodiester bod/ in backbone of DNA/RNA/ in nucleotides

• Used to produce ATP

• Phosphorylates other compounds making them more reactive

• Hydrophilic/ water soluble part of phospholipid bilayer

Explain five properties that make water important for organisms (5 marks)

Ions/ water

• A metabolite in condensation/ hydrolysis/ photosynthesis/ respiration

• A solvent so metabolic reactions can occur quicker

• High heat capacity so buffers changes in temperature

• Large latent heat of vapourisation so provides a cooling effect through evaporation

• Cohesion between water molecules so support columns of water in plants

• Cohesion between water molecules so produces surface tension supporting small organisms

Describe the chemical reactions involved in the conversion of polymers to monomers and monomers to polymers (5 marks)

Polymers

• A condensation reaction joins monomers together and forms a chemical bond and releases water

• A hydrolysis reaction breaks a chemical bond between monomers and uses water

• As suitable example of polymers and the monomers from which they are made

• A second suitable example of polymers and the monomers from which they are made

• Reference to a correct bond within a named polymer

Describe how mRNA is formed by transcription in eukaryotes (5 marks)

Polymers

• Hydrogen bonds between DNA bases break

• Only one DNA strand acts as a template

• Free RNA nucleotides align by complementary base pairing

• In RNA Uracil base pairs with adenine on DNA OR in RNA Uracil is used in place of thymine

• RNA polymerase joinds adjacent RNA nucleotides

• By phosphodiester bonds between adjacent nucleotides

• Pre-mRNA is spliced to form mRNA OR introns are removed

Describe how a polypeptide is formed by translation of mRNA (6 marks)

Polymers

• mRNA attaches to ribosomes

• tRNA anticodons bind to complementary mRNA codons

• tRNA brings a specific amino acid

• Amino acids join by peptide bonds

• Amino acids join together with the use of ATP

• tRNA released after amino acid joined to polypeptide

• The ribosome moves along the mRNA to form the polypeptide

Describe how the structure of glycogen is related to its function (4 marks)

Polymers

• Helix/ coiled/ branched so compact

• Polymer of glucose so easily hydrolysed

• Branches so more ends for faster hydrolysis

• Glucose (polymer) so provides respiratory substrate for energy release

• Insoluble so not easily lost from cell OR insoluble so does not affect water potential

Starting with mRNA in the cytoplasm, describe how translation leads to the production of a polypeptide.

Do not include descriptions of transcription and splicing in your answer (5 marks)

Polymers

• mRNA associates with a ribosome/ ribosome attaches to mRNA

• Ribosome moves to/ finds the start codon/ AUG

• tRNA brings/ carries specific amino acid

• Anticodon on tRNA complementary to codon on mRNA

• Ribosome moves along to next codon

• Process repeated and amino acids join by peptide bonds/ condensation reaction to form polypeptide

Describe how mRNA is produced in a plant cell (5 marks)

Polymers

• DNA strands separate by breaking H bonds OR H bonds broken between complementary DNA bases

• Only one of the strands is used to make mRNA

• Complementary base pairing so A-U, T-A, C-G, G-C

• RNA nucleotides joined by RNA polymerase

• Pre-mRNA formed

• Splicing/ introns removed to form mRNA

Describe how the structure of a protein depends on the amino acids it contains (5 marks)

Polymers

• Structure is determined by relative position of amino acid

• Primary structure is sequence/ order of amino acids

• Secondary structure formed by hydrogen bonding between amino acids

• Tertiary structure formed by interactions between R groups

• Creates active site in enzymes

• Quaternary structure contains more than 1 polypeptide chain

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