KEY PROCESSES

• Muscle Contraction


  1. Calcium ions diffuse from sarcoplasmic reticulum, into myofibrils

  2. Calcium ions bind to tropomyosin, causing it to leave and expose the actin-myosin binding site

  3. Myosin head attaches to binding site, and forms actin-myosin cross bridge

  4. ATP is hydrolysed to release energy to bind the myosin head to pull the actin filament along

  5. ATP molecule attaches to myosin head and is hydrolysed to release energy to detach the myosin head from the actin myosin binding site
• Control Of Heart Rate

  1. SAN sends waves of electrical activity to the atria so they contract

  2. Non-conducting collagen tissue prevents waves of electrical activity from being passed directly from the atria to ventricles

  3. The waves of electrical activity are transferred from the SAN to AVN after a small delay, to ensure that the atria empty before the ventricles contract

  4. The waves of electrical activity travel down to the Bundle of His to Purkyne tissue which conducts the waves to the ventricles, causing them to contract
• Kidney Ultrafiltration


  1. High hydrostatic pressure in the glomerulus due to efferent arteriole being narrower than the afferent arteriole

  2. Water and other small molecules such as glucose are forced out through pores in the capillary endothelium and basement membrane into the Bowman's capsule which forms the glomerular filtrate

  3. Proteins are too large to to pass through the basement membrane so remain in the blood
• Thick Medulla


  • Thicker medulla means longer loop of Henle
  • Soo sodium ion gradient is maintained for longer in the medulla
  • So the water potential gradient is maintained for longer so more water is reabsorbed from the loop of Henle and collecting duct by osmosis
• Selective Reabsorption


  1. Epithelial cells of proximal convoluted tubule have microvilli to provide large surface area for reabsorption

  2. Useful solutes such as glucose are reabsorbed at the PCT by facilitated diffusion and active transport through channel proteins

  3. Water enters the blood by osmosis because the water potential of the blood is lower than the water potential of the glomerular filtrate

  4. water is reabsorbed from the PCT, loop of Henle, DCT and collecting duct
• Loop of Henle


  1. Sodium ions are actively transported out of the ascending limb, into the medulla

  2. The ascending limb is impermeable to water, so water remains in it

  3. This decreases the water potential in the medulla

  4. At the bottom of the descending limb, Na+ ions diffuse out into the medulla, further lowering the water potential

  5. The medulla has a lower water potential than the descending limb, so water moves into the medulla from the descending limb by osmosis

  6. Water also moves out by osmosis at the DCT and collecting duct

  7. The water in the medulla is reabsorbed into the blood by the capillary network
• ADH


  1. Osmoreceptors in hypothalamus detect low/high water potential in blood

  2. Posterior pituitary gland releases more/less ADH

  3. More/less ADH binds to collecting duct and stimulates more/less aquaporins to be inserted into the cell membrane

  4. This increases/decreases the permeability of the collecting duct to water, so more/less water is reabsorbed into the blood by osmosis

  5. This decreases/increases the volume of urine
• Light-Independent Reaction


  1. CO2 reacts with RuBp, catalysed by rubisco

  2. This forms 2 molecules of GP

  3. GP is reduced to triose phosphate using reduced NADP and energy from ATP

  4. Triose phosphate is converted into RuBP and hexose sugars
• Light-Dependent Reaction


  1. Chlorophyll absorbs light energy and electrons are excited to a higher energy level

  2. Electrons are transferred down the electron transport chain

  3. This releases energy for cyclic photophosphorylation to produce ATP

  4. The electrons from the chlorophyll combine with H+ ions and NAD to form NADH

  5. Water splits into oxygen, H+ ions, and electrons in photolysis to replace the electrons lost from the chlorophyll
• Chemiosmotic Theory (Cyclic Photophosphorylation/Oxidative Phosphorylation)

  1. H+ ions in stroma/matrix are transported into the thylakoid membrane/cristae using energy from the electron transport chain

  2. This increases the concentration of H+ ions in the thylakoid membrane/cristae and decreases the H+ ion concentration in the stroma/matrix

  3. This produces a H+ ion concentration gradient

  4. The H+ ions diffuse back into the stroma/matrix through membrane bound ATP synthase, and the ATP synthase uses the energy from the movement to combine ADP and Pi to form ATP

  5. Oxygen is the final electron acceptor (in mitochondria)
• Glycolysis


  1. Glucose is phosphorylated using ATP into hexose bisphosphate

  2. It is then converted to two molecules of triose phosphate

  3. Triose phosphate is oxidised to pyruvate

  4. There is a net gain of ATP

  5. Reduced NAD is produced
• Link Reaction


  1. Pyruvate is decarboxylated and reduced using reduced NAD to produce acetate

  2. Acetate combines with coenzyme A to produce acetylCoA
• Krebs Cycle


  1. AcetylCoA combines with a 4 carbon molecule to produce a 6 carbon molecule

  2. The 6 carbon molecule is decarboxylated and dehydrogenated to produce a 5 carbon molecule; and reduced NAD is produced

  3. The 5 carbon molecule is decarboxylated and dehydrogenated to produce a 4 carbon molecule; and ATP, reduced FAD, and two reduced NAD are produced

  4. There is a net gain of ATP, reduced NAD and reduced FAD
• Action Of Glucagon/Glycogenolysis


  1. Alpha cells in pancreas detects low glucose concentration in blood and secrete glucagon

  2. Glucagon binds to receptors on liver cells and activates adenylate cyclase

  3. Adenylate cyclase converts ATP into cAMP

  4. cAMP activates protein kinase A

  5. Protein kinase A activates a cascade that hydrolyses glycogen into glucose
• Action of Insulin/Glycogenesis


  1. Beta cells in pancreas detect high glucose concentration in blood and secrete insulin

  2. Insulin binds to receptors on liver cells

  3. This stimulates glucose channel proteins to be inserted into the cell membrane

  4. Glucose diffuses into the cells from the blood via facilitated diffusion, which reduces the concentration of glucose in the blood

  5. Enzymes in the cells convert glucose molecules into glycogen
• Nitrogen Cycle


  1. Nitrogen gas in the air is converted into ammonium compounds via nitrogen fixation by nitrogen fixing bacteria in soil and root nodules of leguminous plants, in aerobic conditions

  2. Saprobionts decompose urea and amino acids into ammonium compounds via ammonification, in aerobic conditions

  3. Ammonium compounds are converted into nitrites and then nitrates via nitrification by nitrifying bacteria, in aerobic conditions

  4. Nitrates are converted back into nitrogen gas via denitrification by denitrifying bacteria, in anaerobic conditions
• Eutrophication


  1. Nutrients are leached into a waterway via surface runoff

  2. This causes algal bloom, which blocks light from reaching plants under the water for photosynthesis

  3. The plants die and saprobionts decompose them, using oxygen for aerobic respiration

  4. There is less oxygen available in the water for fish to aerobically respire, so fish die
• Phosphorus Cycle


  1. Phosphate ions in rocks are released into the soil by weathering

  2. Phosphate ions are transported into plants through the roots

  3. Mycorrhizae increase the rate of phosphate assimilation

  4. Phosphate ions are transferred through the food chain

  5. Phosphate ions are lost from animals in waste products

  6. Saprobionts release phosphate ions into the soil when decomposing faeces, urine and dead organisms

  7. Weathering of rocks also releases phosphate ions into waterways, which are taken up by aquatic producers and passed along the food chain to birds

  8. Guano from birds returns phosphate ions to soil and is often used as a natural fertiliser
• Succession


  1. Pioneer species first inhabit the habitat

  2. They change the abiotic factors of the habitat

  3. They make the habitat less hostile for new species, by providing food

  4. Eventually, the pioneer species dies out

  5. The final, most abundant species living in the habitat is known as the climax community
• Speciation


  1. Allopatric-There is a geographical isolation between two populations

  2. Sympatric-There is a mutation in the DNA base sequence that causes reproductive isolation and there is no gene flow between the two populations

  3. Allopatric-Different selection pressures in each environment cause only individuals with advantageous alleles to survive

  4. Sympatric-There is disruptive selection

  5. There is a change in the frequency of alleles in the two populations

  6. Eventually the two species can no longer breed to produce fertile offspring
• PCR (In Vitro Cloning)


  1. Sample of DNA fragments, primers, free nucleotides, and DNA polymerase

  2. Sample heated to 95C to break hydrogen bonds between DNA strands

  3. Sample cooled to 60C so that primers can anneal to the DNA

  4. Sample heated to 72C so that DNA polymerase can join free nucleotides and form phosphodiester bonds to create complementary DNA strands
• In Vivo Cloning


  1. DNA fragments are formed via reverse transcriptase/restriction endonucleases/gene machine

  2. Promoter and terminator regions are added

  3. Plasmid is cut using restriction endonuclease which forms sticky ends complementary to the DNA fragments

  4. DNA fragments are attached to plasmid using DNA ligase and marker genes (coding for fluorescence or antibiotic resistance) are added

  5. Plasmid is inserted into a host cell, which replicates

  6. Sample is viewed under UV light or grown in an agar plate with an antibiotic to identify transformed cells
• Synaptic Transmission


  1. Action potential arrives at and depolarises presynaptic membrane

  2. This causes calcium ions to diffuse through voltage gated calcium ion channels into the presynaptic knob

  3. This stimulates vesicles containing neurotransmitter (acetylcholine) to move to and fuse with the presynaptic membrane

  4. Neurotransmitter (acetylcholine) is released and diffuses across synaptic cleft

  5. They bind to receptors (cholinergic receptors) on the postsynaptic membrane, stimulating sodium ion channels to open

  6. Sodium ions diffuse through the channels and depolarise the postsynaptic membrane

  7. Neurotransmitter (acetylcholine) still in the synaptic cleft are broken down by enzymes (acetylcholinesterase)
• RNA Interference


  1. siRNA associates with proteins

  2. siRNA is complementary to mRNA

  3. siRNA binds to mRNA and cuts it into fragments

  4. The mRNA can no longer be translated at ribosomes

  5. So less polypeptides are produced
• Reverse Transcriptase


  1. mRNA is isolated from cells

  2. The sample of mRNA is mixed with free nucleotides and reverse transcriptase

  3. Reverse transcriptase uses mRNA as a template to synthesise cDNA
• Restriction Endonuclease


  1. DNA sample is incubated with restriction endonuclease which cuts the DNA fragment at a specific recognition site via a hydrolysis reaction

  2. The cut leaves sticky ends which can be used to anneal to vectors and other DNA fragments
• Gene Machine


  1. DNA base sequence is designed

  2. The first nucleotide in the sequence is fixed to a support, such as a bead

  3. Nucleotides and protecting groups are added step by step

  4. Short sequences called oligonucleotides are produced

  5. These are broken off from the support and protecting groups are removed

  6. Oligonucleotides can be joined by DNA ligase to form longer DNA fragments