AS Biology Paper 2 - Extended response

Starch and Cellulose - Structure related to function

Starch

• Coiled so compact

• Insoluble so doesn’t affect water potential

• Branched so more ends for faster hydrolysis to glucose

• Large molecule so cannot leave cell

Cellulose

• Every other beta glucose rotated 180 degrees

• Long straight chains joined by H bonds

• Forms microfibrils which provide strength to cell wall

Transport of sugars

• In source, sucrose is actively transported into phloem by companion cells.

• This lowers water potential in phloem.

• Water moves into phloem via osmosis from xylem.

• Increase in volume of water causes increase in hydrostatic pressure.

• Causes mass flow of sucrose to sinks.

• Sucrose unloaded at sinks and used in respiration etc.

Cohesion-tension theory

• Water lost from leaf because of transpiration

• Lowers water potential in leaf cells

• Water pulled up xylem creating tension

• Water molecules cohere by H bonds

• Adhesion of water molecules to walls of xylem

• Continuous water column formed

mRNA produced in plant cell

• DNA strands are separated by breaking H bonds between complementary bases.

• One strand acts as template

• RNA nucleotides align by complementary base pairing (state pairing)

• pre-mRNA formed

• Splicing

DNA in eukaryotic vs prokaryotic

• Nucleotides joined by phosphodiester bonds

• State structure

• ED linear PD circular

• ED longer CD shorter

• ED contains introns CD doesn’t

• ED DNA associated with histones CD no

Difference in primary structure of Hb as evidence of phylogeny

• Mutation causes change in base sequence of DNA

• So change in sequence of amino acids/primary structure

• Mutations build up over time

• More mutations between distantly related species

• Distantly related species have earlier common ancestor

Iron Sodium and Phosphate ions

Iron

• Hb associates with oxygen

Sodium

• Co-transport of glucose/amino acids

• Na+ moved out by active transport

• Creates Na+ concentration gradient

Phosphate

• Phosphorylates other compounds making them more reactive

• Used to produce ATP

Absorption and transport of lipids

• Micelles contain bile salts and fatty acids

• Makes fatty acids more water soluble

• Fatty acids absorbed by diffusion

• Triglycerides reformed in cells

• Vesicles move to cell membrane

Protein structure dependent on amino acid

• Structure determined by position of amino acid

• Primary structure sequence of amino acids

• Secondary structure formed by H bonding

• Tertiary structure formed by interaction between R groups

• Creates active site in enzymes

• Quaternary structure contains more than one polypeptide chain

Five properties of water

• Metabolite in hydrolysis reactions

• High SHC so buffers changes in temperatures

• Large LHOV so provides cooling effect via evaporation

• Cohesion so forms continuous water columns

• Cohesion so provides surface tensions, supporting small organisms

• Polar so good solvent so metabolic reactions can occur

Semi-conservative replication

• DNA helicase unwinds DNA double helix and breaks H bonds between complementary base pairs

• Each strand acts as a template

• Free floating DNA nucleotides align by complementary base pairing

• DNA polymerase joins adjacent DNA nucleotides by phosphodiester bonds via condensation reaction using ATP

• Each new DNA molecule contains one strand of parental DNA and one strand of newly synthesised DNA

Structure of insect gas exchange system

• Spiracles, tracheae, tracheoles

• Tracheoles are thin so short diffusion pathway

• Tracheoles are highly branched so provides large SA for exchange

• Tracheoles are permeable to oxygen

• Spiracles allow diffusion of oxygen

• Spiracles can open and close, reducing water loss.

• Hairs around spiracles which reduces water loss.

• Exoskeleton impermeable to water so reduces water loss.

How humans breath in and out

Breathing in

• Diaphragm contracts and diaphragm flattens

• External intercostal muscles contract and ribcage moves up and out

• Volume increases and pressure decreases in thoracic cavity to below atmospheric pressure

• Air moves down pressure gradient

Breathing out

• Diaphragm relaxes and diaphragm moves up

• External intercostal muscles contract and ribcage moves down and in

• Volume decreases and pressure increases in thoracic cavity to above atmospheric pressure

Similarity and differences between DNA and RNA

Similarity

• Both polymer of nucleotides

• Both contain phosphodiester bonds

• Both have base, pentose sugar and phosphate group

Difference

• DNA deoxyribose RNA ribose

• DNA double stranded RNA single stranded

• DNA longer RNA shorter

• DNA thymine RNA uracil

Similarity and differences in chloroplast and mitochondria

Similarity

• Both have double membrane

• Both have ribosomes

• Both have circular DNA

Differences

• Chloroplast has pigment, mitochondria doesn’t

• Chloroplast has stroma, mitochondria has matrix

• Chloroplast has thylakoids, mitochondria has cristae

role of Hb

• Hb loads oxygen in the lungs at high pO2

• Binding of one oxygen molecule makes binding of a second oxygen molecule easier due to cooperative binding (detail)

• Oxygen transported as oxyhaemoglobin in RBCs

• Hb unloaded in respiring cells and tissues at low pO2

Translation

• mRNA attaches to ribosomes

• tRNA anticodon binds to complementary mRNA codon

• tRNA carries/brings a specific amino acid

• Amino acids are joined together by peptide bonds via condensation reaction using ATP

• tRNA is released

• Ribosome moves along mRNA to form polypeptide

Meiosis resulting in haploid cells and genetic variation

• 2 divisions

• In meiosis 1 homologous chromosomes separate

• In meiosis 2 sister chromatids separate

• 4 haploid cells produced

• Independent segregation of homologous chromosomes causes genetic variation

• Crossing over leads to exchange of parts of alleles between homologous chromosomes

How tissue fluid is formed and returned to blood by lymphatic system

• Contraction of ventricles produces high hydrostatic pressure at arterial end

• This forces out small molecules like water out of the blood

• Plasma proteins remain in blood which lowers water potential in the blood

• Water moves into the blood by osmosis

• Excess tissue fluid is absorbed and enters lymph system where it’s returned to blood by lymphatic system