Essays paras

AO1: Digestion of carbohydrates

• Saliva contains the enzyme amylase that hydrolyses starch into maltose

• Pancreatic amylase allows hydrolysis of any undigested starch molecules into maltose

• Maltase is an enzyme found in the cell membranes of cells lining the small intestine

• Maltase catalyses the hydrolysis of maltose into molecules of glucose

AO2 links→ glycolysis

Relevant titles→ proteins, hydrogen bonds, nitrogen-containing substances, bonds and bonding, complementary shapes, membranes, enzymes, condensation and hydrolysis

AO1: Digestion of proteins

• Endopeptidases- hydrolyses peptide bonds between amino acids in the central region forming smaller polypeptide moelcules

• Exopeptidases- hydrolyse the peptide bonds between the terminal amino acids to release dipeptides or single amino acids

• Dipeptidases are membrane bound- hydrolyse peptide bonds between 2 amino acids of a dipeptide producing amino acids

AO2 links→ translation

Relevant titles→proteins, hydrogen bonds, nitrogen-containing substances, bonds and bonding, variation and diversity, complementary shaped, enzymes, condensation and hydrolysis

AO1: Cotransport (ileum)

• Active transport of sodium ions out of the epithelial cell into the blood

• This provides the concentration gradient which allows sodium ions to diffuse into the epithelial cells from the gut lumen by facilitated diffusion

• As sodium ions diffuse from the lumen into epithelial cells, the glucose or amino acid is also taken in with it (co-transport). The glucose/ aa does not diffuse)

• As the glucose or amino acid builds up inside the epithelial cells it moves into the blood by facilitated diffusion using a carrier protein

AO2 links→ amino acid absorption, translation/ protein function, glucose absorption, glycolysis

Relevant titles→ proteins, hydrogen bonds, movement, nitrogen-containing substances, diffusion, bonds and bonding, complementary shapes, ions, ATP, membranes, transport, phosphorus containing substances, condensation and hydrolysis

AO1: DNA replication

• DNA helicase unwinds the DNA double helix, by breaking the weak hydrogen bonds between complementary bases in the polynucleotide strands

• Both strands of DNA can now act as a template for the formation of a new strand

• New DNA nucleotides are attracted to the exposed bases and attach by complementary base pairing

• New hydrogen bonds form between the bases

• DNA polymerase joins the adjacent nucleotides together with phosphodiester bonds via condensation reactions

AO2 links→ mitosis and cell cycle, clonal expansion of T/ plasma B cells, meiosis

Relevant titles→ Proteins, DNA, hydrogen bonds, nitrogen-containing compounds, bonds and bonding, complementary shapes, cycles, enzymes, phosphorus-containing substances, condensation and hydrolysis

AO1: Mitosis/ Cell cycle

• Each phase of cycle incolves specific cell activities: G1= cells prepare for DNA replication. S= DNA replication occurs. G2= a relatively short gap before mitosis. M= mitosis

• Interphase- this is when DNA replicated, protein synthesis occurs, there is an increase in number of organelles and ATP production (respiration)

• Prophase- chromosomes condense/ shorten due to coiling up of DNA and become visible. Nuclear membrane breaks down

• Metaphase- chromosomes line up along equator of cell and each attaches to an individual spindle fibre by its centromere

• Anaphase- centromere divides, spindle fibres contract pulling apart the chromatids of each chromosome. Sister chromatids go to opposite poles

• Telophase- chromosomes uncoil and become threadlike again, no longer visible

• Cytokinesis- division of the cytoplasm usually follows mitosis fairly quickly. For cells with a cell wall a new cell wall forms in the middle of the cell

AO2 links→ clonal expansion of T/ plasma B cells

Relevant titles→ movement, nitrogen-containing substances, bonds and bonding, cycles, membranes, phosphorus-containing substances, transport, condensation and hydrolysis, responding to changes in internal and external environments

AO1: Transcription

• The hydrogen bonds between DNA bases are broken which separates the 2 strands of DNA

• One strand of the DNA acts as the template strand upon which pre-mRNA is built

• Free RNA nucleotides are found in the nucleoplasm. They are attracted to the exposed DNA bases on the DNA template strand and align by complementary base pairing

• RNA polymerase join RNA nucleotides to make an RNA polynucleotide chain via phosphodiester bonds

• Introns are removed and exons are spliced

AO2 links→ Translation

Relevant titles→proteins, DNA, hydrogen bonds, nitrogen-containing substances, bonds and bonding, complementary shapes, enzymes, phosphorus-containing substances, condensation and hydrolysis

AO1: Translation

• mRNA attaches to ribosomes

• tRNA anticodons bind to mRNA codons by complementary base pairing

• Each tRNA brings a specific amino acid

• 2 tRNA molecules are held together in a ribosome at any one time

• A peptide bond forms between adjacent amino acids. This requires the use of ATP

• tRNA molecules are released after their amino acids have been joined to the growing polypeptide chain

• The ribosome moves along the mRNA forming the polypeptife until a stop codon is reached, at which point the ribosome and mRNA dissociate

AO2 links→ protein function, transport proteins, transcriptional factors etc

Relevant titles→ proteins, hydrogen bonds, nitrogen-containing substances, bonds and bonding, complementary shapes, ATP, enzymes, phosphorus-containing substances, condensation and hydrolysis

AO1: Nitrogen cycle

Nitrogen fixing bacteria:

• They reduce the nitgrogen gas (N2) into ammonium ions

• Can be free living in the soil or within root nodules of legumes

Nitrifying bacteria:

• Stage 1- the oxidation of ammonium ions to nitrite ions

• Stage 2- the oxidation of nitrites to nitrates by nitrifying bacteria

• Plants take up the nitrates through their roots by active transport

• Nitrates are converted to nitrogen gas

• The saprobiotic microorganisms secrete enzymes on to the nitrogen-containing compounds (extracellular digestion)

• The products are absorbed by the saprobionts but ammonium ions are released into the environment

AO2 links→nitrogen-containing compounds, DNA replication, transcription, protein function, ATP cycle

Relevant titles→ exchanges with external environment to maintain internal environment, movement, nitrogen-containing substances, interactions between cells and organisms, ions, cycles, ATP, interactions between organisms and environment

AO1: Light dependent reaction

• Light is absorbed by the chlorophyll

• A pair of electrons become excited and leave- photoionisation

• A molecule of water is split into hydrogen ions (protons), electrons and oxygen- photolysis

• These electrons are used to replace those lost from chlorophyll

• The electrons are now passed along the electron transport chain (ETC) located in the thylakoid membranes

• Via a series of REDOX reactions, the electrons release energy

• This energy is used to actively pump protons through the thylakoid membrane into the lumen of the thylakoid

• The protons have formed a proton concentration gradient between the lumen of the thylakoid and the stroma of the chloroplast

• The protons diffuse back into the stroma through ATP synthase

• As the protons pass the inorganic phosphate groups are attached to ADP molecules to form ATP- chemiosmosis

• The protons and electrons released at the end of the electron transfer chain react with NADP and reduced NADP is formed

AO2 links→ light independent reaction

Relevant titles→proteins, hydrogen bonds, receptors, movement, nitrogen-containing substances, diffusion, bonds and bonding, ions, membranes, transport, phosphorus-containing substances, condensation and hydrolysis

AO1: Light independent reaction

• CO2 combines with a 5-carbon molecule called ribulose bisphosphate (RuBP) to form 2 molecules of glycerate 3-phosphate (3C) (GP)

• This reaction is catalysed by the enzyme RuBisCo

• ATP and the reduced NADP produced in the light-dependent reactions is used to reduce the glycerate 3-phosphate (GP) to triose phosphate

• The ATP provides the necessary energy

• Reduced NADP provides the hydrogen for the reduction reaction

• 1/6 of the triose phosphate is converted into carbohydrates such as glucose

• The rest, 5/6 of the triose phosphate is used to regenerate RUBP

AO2 links→ glycolysis, structure of cellulose

Relevant titles→ proteins, hydrogen bonds, nitrogen-containing substances, bonds and bonding, complementary shapes, cycles, ATP, enzymes, phosphorus-containing substances

AO1: Glycolysis

• Phosphorylation of glucose by ATP makes the glucose more reactive and converts it to glucose phosphate

• Glucose phosphate is split into 2 molecules of triose phosphate

• Each triose phosphate is oxidised into 2 molecules of pyruvate

• NAD accepts the hydrogen and becomes reduced NAD

• 2 molecules of ATP are produced by substrate level phosphorylation for each pyruvate formed

AO2 links→link reaction/ krebs cycle, anaerobic

Relevant titles→ proteins, hydrogen bonds, nitrogen-containing substances, bonds and bonding, ATP, phosphorus-containing substances, condensation and hydrolysis

AO1: Oxidative phosphorylation

• Reduced NAD/ FAD releases hydrogen, which becomes a proton and electron

• The electron is passed along the ETC in the inner mitochondrial membrane, in a series of redox reactions- releasing energy

• The energy is used to pump protons into the intermembrane space

• Protons diffuse back into the matric through ATP synthase

• Energy released by the protons is used to combine ADP and Pi to make ATP- chemiosmosis

• Oxygen acts as the terminal electron acceptor combining with the electron and proton to make water

• Oxygen is essential for 34 ATP molecules to be made by oxidative phosphorylation

AO2 links→ Uses of ATP e.g. cotransport, translation etc

Relevant titles→ proteins, hydrogen bonds, movement, nitrogen-containing substances, diffusion, bonds and bonding, complementary shapes, ions, membranes, transport, phosphorus-containing substances, condensation and hydrolysis

AO1: Second messenger model

• Adrenaline attaches to receptor site on liver cell

• Activates the enzyme adenylate cyclase inside the membrane

• The activated adenylate cyclase converts ATP to cyclic AMP (cAMP)

• This acts as a second messenger that activates the enzyme protein kinase A

• Protein kinase A activates a chain of reactions that breaks down glycogen to glucose (glycogenolysis)

AO2 links→glycolysis

Relevant titles→proteins, hydrogen bonds, receptors, movement, nitrogen-containing substances, interactions between cells and organisms, bonds and bonding, complementary shapes, ATP, membranes, interactions between organisms and environment, enzymes, phosphorus-containing substances, responding to changes in internal and external environments

AO1: Cotransport (PCT)

• Na+ are actively transported out of the PCT cell into the blood

• This maintains the concentration gradient for Na+ between the filtrate and the PCT cells

• Glucose and Na+ enter the PCT cells together from the filtrate using specific carrier proteins. This is an example of cotransport

• Glucose leaves the PCT cell via facilitated diffusion and enters the blood

• The transport of Na+ and glucose into the PCT cells reduces the water potential in the cells. Water moved from filtrate into the cells by osmosis

AO2 links→glycolysis

Relevant titles→exchanges with external environment to maintain their internal environment, proteins, hydrogen bondsm movement, nitrogen-containing substances, diffusionm bonds and bonding, complementary shapes, ions, ATP, membranes, transport, phosphorus- containing substances, condensation and hydrolysis

AO1: Loop of Henle

Ascending limb:

• Carries filtrate up through the medulla

• Is impermeable to water and permeable to sodium ions

• Lower part- sodium ions diffuse out of the filtrate into the interstitial fluid of the medulla by facilitated diffusion

• Upper part- sodium ions are actively transported into the interstitial fluid

• This creates a concentration gradient of sodium ions down through the medulla, highest water potential at the top and lowest water potential deepest into the medulla

Descending limb:

• Carries filtrate down into the medulla

• Is permeable to water and impermeable to sodium ions

• Water leaves the filtrate by osmosis and enters the interstitial fluid

• Filtrate in the loop becomes more concentrated so water potential is reduced

The water in the tissue fluid of the medulla is reabsorbed into the vasa recta (blood capillaries) by osmosis

AO2 links→ water potential gradients leading to osmotic lysis, hydrolysis reactions, water as a cooling mechanisms

Relevant titles→proteins, hydrogen bonds, movement, nitrogen-containing substances, diffusion, bonds and bonding, complementary shapes, ions, ATP, membranes, phosphorus-containing substances, condensation and hydrolysis, transport, responding to changes in internal and external environments

AO1: ADH

• ADH is released by the posterior pituitary

• ADH increases the permeability of the cells in the collecting duct (and DCT) wall

• Increasing transcription of genes coding for aquaporins

• More aquaporins are produced and inserted into the membranes

• This means more water can move by osmosis out of the filtrate in the collecting duct, into the medulla and then into the vasa recta

AO2 links→water potential gradients leading to osmotic lysis, hydrolysis reactions, water as a cooling mechanism

Relevant titles→proteins, hydrogen bonds, movement, nitrogen-containing substances, diffusion, bonds and bonding, complementary shapes, ions, ATP, membranes, phosphorus-containing substances, condensation and hydrolysis, transport, responding to changes in internal and external environments

AO1: Resting potential

• Active transport of 3 Na+ out of axon and 2K+ into axon by sodium/ potassium pumps

• Resulting in more K+ inside the axon than out (and more Na+ outside than in)

• The membrane is more permeable to K+ ions than to Na+ ions

• K+ ions therefore diffuse back out faster than Na+ions diffuse back in

• Leading to a p.d. of -65mV inside compared to out

• Membrane is said to be POLARISED

AO2 links→ action potentials

Relevant titles→proteins, hydrogen bonds. movement, nitrogen-containing substances, diffusion, bonds and bonding, complementary shapes, ions, ATP, membranes, transport, phosphorus-containing substances, condensation and hydrolysis

AO1: Action potentials

• A stimulus causes the gated sodium ion channels to open

• Making the membrane more permeable to sodium ions

• Na+ ions rapidly diffuse into the axon, causing depolarisation

• The inside of the membrane becomes charged to +40mV

• The gated sodium ion channels close, meaning the axon is again almost impermeable to Na+ ions

• The gated potassium ion channels open (i.e. the axon’s permeability to potassium ions increases)

• K+ ions diffuse rapidly out of the axon

• So many potassium ions diffuse out of the axon that they cause a more negative potential difference than normal

• Dropping the axon interior down to about -90mV

AO2 links→ synapses or any effectors, muscle contraction, sympathetic/ parasympathetic nerve stimulation to control heart rate

Relevant titles→ proteins, hydrogen bondsm movement, nitrogen-containing substances, diffusion, bonds and bonding, complementary shapes, ions, cycles, membranes, interactions between organisms and environment, transport, phosphorus-containing substances, responding to changes in internal and external environments

AO1: Synapses (or Neuromuscular junctions)

• Action potential at the synaptic knob opens calcium ion channels which allows calcium ions (Ca2+) to diffuse in

• This causes synaptic vesicles to fuse with the presynaptic membrane and release ACh by exocytosis

• ACh diffuses across the synaptic cleft and binds to receptors on the sodium ion channels in the postsynaptic membrane

• The sodium ion channels to open and sodium ions diffuse rapidly into the post synaptic cell, causing depolarisation

• If the threshold is reached an action potential will be initiated

• To prevent the initiation of more action potentials in the postsynaptic membrane, acetylcholinesterase, hydrolyses ACh

• The products diffuse back across the cleft and can be reabsorbed into the presynaptic neurone

• ACh is re-formed and repackaged into vesicles

AO2 links→Any effectors, muscle contraction, parasympathetic/ sympathetic nerve stimulation to control heart rate

Relevant titles→proteins, hydrogen bonds, receptors, movement, nitrogen-containing substances, diffusion, interactions between cells and between organisms, bonds and bonding, complementary shapes, ions, cycles, ATP, membranes, enzymes, transport, phosphorus-containing substances, condensation and hydrolysis, responding to changes in internal and external environments

AO1: Muscle contraction

• Calcium ions diffuse into myofibrils from the sarcoplasmic reticulum

• Calcium ions cause the movement of the tropomyosin (away from the actin)

• This causes the exposure of the binding sites on the actin

• The myosin head attaches to the binding site on the actin (forming actinomyosin bridge)

• Hydrolysis of ATP (on the myosin head) causes the myosin head to bend

• This pulls the actin molecules

• Attachment of the new ATP molecule to each myosin head causes the myosin head to detach

AO2 links→ muscle movement in ventilation

Relevant titles→ proteins, hydrogen bonds, movement, nitrogen-containing substances, bonds and bonding, complementary shapes, ions, cycles, ATP, enzymes, phosphorus-containing substances, condensation and hydrolysis