AQA A Level Biology Organisms PDF

AQA A Level Biology Topic 6 Organisms respond to changes in their internal & external environments Model answer notes by @biologywitholivia (updated for 2025) Topic Understand Memorise Practise 6.1.1 Surv...

AQA A Level Biology Topic 6 Organisms respond to changes in their internal & external environments Model answer notes by @biologywitholivia (updated for 2025) Topic Understand Memorise Practise 6.1.1 Survival and response Required practical 10 6.1.2 Receptors 6.1.3 Control of heart rate 6.2.1 Nerve impulses 6.2.2 Synaptic transmission 6.3 Skeletal muscles 6.4.1 Principles of homeostasis and negative feedback 6.4.2 Control of blood glucose concentration Required practical 11 6.4.3 Control of blood water potential These notes are for PERSONAL USE ONLY. Redistribution, reproduction, or sale of any portion of this material is prohibited. For enquiries, please email [email protected] More GCSE & A Level Biology resources available at stan.store/biologywitholivia Copyright © 2024 Biology with Olivia 6.1 Stimuli, both internal and external, are detected and lead to a response 6.1.1 Survival and response What is a stimulus? A change in an organism's internal or external environment Why is it important that organisms can respond to stimuli? Organisms increase their chance of survival by responding to stimuli What is a tropism? Growth of a plant in response to a directional stimulus Positive tropism = towards a stimulus; negative tropism = away from stimulus Summarise the role of growth factors in flowering plants Specific growth factors (hormone-like growth substances) eg. Auxins (such as IAA) move (via phloem or diffusion) from growing regions eg. shoot / root tips where they’re produced To other tissues where they regulate growth in response to directional stimuli (tropisms) Describe how indoleacetic acid (IAA) affects cells in roots and shoots In shoots, high concentrations of IAA stimulates cell elongation In roots, high concentrations of IAA inhibits cell elongation Explain gravitropism in flowering plants 1. Cells in tip of shoot / root produce IAA 2. IAA diffuses down shoot / root (evenly initially) 3. IAA moves to lower side of shoot / root (so concentration increases) 4. In shoots this stimulates cell elongation whereas in roots this inhibits cell elongation 5. So shoots bend away from gravity whereas roots bend towards gravity 2 More GCSE & A Level Biology resources available at stan.store/biologywitholivia Copyright © 2024 Biology with Olivia Explain phototropism in flowering plants 1. Cells in tip of shoot / root produce IAA 2. IAA diffuses down shoot / root (evenly initially) 3. IAA moves to shaded side of shoot / root (so conc. ↑) 4. In shoots this stimulates cell elongation whereas in roots this inhibits cell elongation 5. So shoots bend towards light whereas roots bend away from light Describe the simple responses that can maintain a mobile organism in a favourable environment 1. Taxes (tactic response) ○ Directional response ○ Movement towards or away from a directional stimulus 2. Kinesis (kinetic responses) ○ Non-directional response ○ Speed of movement or rate of direction change changes in response to a non-directional stimulus ○ Depending on intensity of stimulus (Examples: taxis - woodlice moving away from light to avoid predators; kinesis - woodlice moving faster in drier environments to increase their chance of moving to an area with higher humidity to prevent drying out) Explain the protective effect of a simple (eg. 3 neurone) reflex Rapid as only 3 neurones and few synapses (synaptic transmission is slow) Autonomic (doesn’t involve conscious regions of brain) so doesn’t have to be learnt Protects from harmful stimuli eg. escape predators / prevents damage to body tissues Exam insight: common mistakes ❌ Mistake Explanation “The root tip contains IAA.” To get this mark you need to state that the root tip produces IAA. “There is more growth / elongation on This is too vague. You need to be clear that it is the cells on [named side] of the shoot / root”. one side that are elongating more. 3 More GCSE & A Level Biology resources available at stan.store/biologywitholivia Copyright © 2024 Biology with Olivia Required practical 10 Investigation into the effect of an environmental variable on the movement of an animal using either a choice chamber or a maze. Describe how the effect of an environmental variable on the movement of an animal (eg. woodlice) can be investigated using a choice chamber 1. Set up choice chamber (different compartments) to create different environmental conditions ○ Eg. humidity → add a drying agent to one side and damp filter paper to other ○ Eg. light → shine a light but cover one half with black card 2. Control other environmental conditions ○ Eg. if investigating humidity control light intensity with a dim even light above 3. Use a teaspoon to place a set number of animals eg. 12 woodlice on centre of mesh platform and cover with lid 4. After a set amount of time eg. 10 minutes record the number of animals in each section 5. Repeat after gently moving woodlice back to centre Common questions: The woodlice were left for 15 minutes Time to establish humidity / for substance to absorb water / before their movement was recorded water from paper to evaporate when investigating the effect of Woodlice no longer affected by handling humidity. Explain why. (2) So that behaviour is typical of that humidity Explain how you would ensure the Safely - cover open wounds / wash hands with soap after safe and ethical handling of animals. ○ To minimise risk of infection (2) Ethical - handle carefully / return to habitat ASAP Explain why a mesh platform is used To keep woodlice a safe distance from drying agent when investigating the effect of humidity. (1) 4 More GCSE & A Level Biology resources available at stan.store/biologywitholivia Copyright © 2024 Biology with Olivia Describe how the effect of an environmental variable on the movement of an animal (eg. maggots) can be investigated using a maze Mazes are used to investigate turning behaviour in response to different environmental conditions: 1. Change environment at one end of T shape eg. add food source 2. Place animal eg. maggot in stem of T 3. Record whether animal turns towards or away from food source 4. Repeat with a large number of maggots ○ Wipe / clean maze between trials 5. Repeat with food on other side of T Common questions: Explain why the same organism is not Reduces stress on maggots used more than once. (2) Prevents chance of learned behaviours Explain why a clean petri dish / maze is Animals may leave chemicals / scents used each time. (2) Which influence behaviour of other animals Explain which statistical test should be used to analyse results Chi-squared As data are categorical and comparing frequencies To see if there is a significant difference between observed and expected frequencies ○ Expected = equal numbers each side 5 More GCSE & A Level Biology resources available at stan.store/biologywitholivia Copyright © 2024 Biology with Olivia 6.1.2 Receptors Describe the basic structure of a Pacinian corpuscle Describe how a generator potential is established in a Pacinian corpuscle 1. Mechanical stimulus eg. pressure deforms lamellae and stretch- mediated sodium ion (Na+) channels 2. So Na+ channels in membrane open and Na+ diffuse into sensory neurone ○ Greater pressure causes more Na+ channels to open and more Na+ to enter 3. This causes depolarisation, leading to a generator potential ○ If generator potential reaches threshold it triggers an action potential Explain what the Pacinian corpuscle illustrates Receptors respond only to specific stimuli ○ Pacinian corpuscle only responds to mechanical pressure Stimulation of a receptor leads to the establishment of a generator potential ○ When threshold is reached, action potential sent (all-or-nothing principle) 6 More GCSE & A Level Biology resources available at stan.store/biologywitholivia Copyright © 2024 Biology with Olivia Explain the differences in sensitivity to light for rods & cones in the retina Rods are more sensitive to light Cones are less sensitive to light Several rods connected to a single neurone Each cone connected to a Spatial summation to reach / overcome threshold (as enough single neurone neurotransmitter released) to generate an action potential No spatial summation Explain the differences in visual acuity for rods & cones in the retina Rods give lower visual acuity Cones give higher visual acuity Several rods connected to a single neurone Each cone connected to a single neurone So several rods send a single set of impulses Cones send separate (sets of) impulses to brain to brain (so can’t distinguish between (so can distinguish between 2 separate sources separate sources of light) of light) Explain the differences in sensitivity to colour for rods & cones in the retina Rods allow Cones allow colour vision monochromatic vision 3 types of cones - red-, green- and blue-sensitive 1 type of rod / 1 With different optical pigments → absorb different wavelengths pigment Stimulating different combinations of cones gives range of colour perception Exam insight: common mistakes ❌ Mistake Explanation “Sodium diffuses into the sensory neurone.” Sodium ions (Na+) diffuse into the sensory neurone. “Cones send separate messages / signals to the ‘Messages’ or ‘signals’ is too vague. Instead, you need to brain.” use the term ‘impulses’. *Omitting reference to the optic nerve or brain The differences in rods and cones can be explained by when explaining differences in rods and cones.* the connections they make in the optic nerve. “There are red, green and blue cones / pigments.” There are red, green and blue- sensitive cones. 7 More GCSE & A Level Biology resources available at stan.store/biologywitholivia Copyright © 2024 Biology with Olivia 6.1.3 Control of heart rate Cardiac muscle is myogenic. What does this mean? It can contract and relax without receiving electrical impulses from nerves Label the sinoatrial node (SAN), atrioventricular node (AVN), Bundle of His and Purkyne tissue on a diagram of the heart Describe the myogenic stimulation of the heart and transmission of a subsequent wave of electrical activity 1 Sinoatrial node (SAN) acts as pacemaker → releases regular waves of electrical activity across atria ○ Causing atria to contract simultaneously 2 Non-conducting tissue between atria / ventricles prevents impulse passing directly to ventricles ○ Preventing immediate contraction of ventricles 3 Waves of electrical activity reach atrioventricular node (AVN) which delays impulse ○ Allowing atria to fully contract and empty before ventricles contract 4 AVN sends wave of electrical activity down bundle of His, conducting wave between ventricles to apex where it branches into Purkyne tissue ○ Causing ventricles to contract simultaneously from the base up 8 More GCSE & A Level Biology resources available at stan.store/biologywitholivia Copyright © 2024 Biology with Olivia Where are chemoreceptors and pressure receptors located? Chemoreceptors and pressure receptors are located in the aorta and carotid arteries Describe the roles of chemoreceptors, pressure receptors, the autonomic nervous system and effectors in controlling heart rate 1. Baroreceptors detect [fall / rise] in blood pressure and / or chemoreceptors detect blood [rise / fall] in blood CO2 conc. or [fall / rise] in blood pH 2. Send impulses to medulla / cardiac control centre 3. Which send more frequent impulses to SAN along [sympathetic / parasympathetic] neurones 4. So [more / less] frequent impulses sent from SAN and to / from AVN 5. So cardiac muscle contracts [more / less] frequently 6. So heart rate [increases / decreases] Use the pink / left words in brackets for a fall in blood pressure OR rise in blood CO2 conc. / fall in blood pH Use the blue / right words in brackets for a rise in blood pressure OR fall in blood CO2 conc. / rise in blood pH Exam insight: common mistakes ❌ Mistake Explanation *Referring to the autonomic nervous The medulla oblongata in the brain controls heart rate via the system but not specifically to the autonomic nervous system. Within this, sympathetic nerves are sympathetic or parasympathetic involved in increasing heart rate, while parasympathetic nerves are pathways. * involved in decreasing heart rate. “The medulla / cardiac control This is not wrong, but in the context of controlling heart rate, the centre sends impulses to the SAN emphasis needs to be on the frequency of impulses. When more along sympathetic / frequent impulses are sent along sympathetic neurones, heart rate parasympathetic neurones.” increases. When more frequent impulses are sent along parasympathetic neurones, heart rate decreases. 9 More GCSE & A Level Biology resources available at stan.store/biologywitholivia Copyright © 2024 Biology with Olivia 6.2 Nervous coordination 6.2.1 Nerve impulses Describe the structure of a myelinated motor neurone Describe resting potential Inside of axon has a negative charge relative to outside (as more positive ions outside compared to inside) Explain how a resting potential is established across the axon membrane in a neurone Na+/K+ pump actively transports: ○ (3) Na+ out of axon AND (2) K+ into axon Creating an electrochemical gradient: ○ Higher K+ conc. inside AND higher Na+ conc. outside Differential membrane permeability: ○ More permeable to K+ → move out by facilitated diffusion ○ Less permeable to Na+ (closed channels) 10 More GCSE & A Level Biology resources available at stan.store/biologywitholivia Copyright © 2024 Biology with Olivia Explain how changes in membrane permeability lead to depolarisation and the generation of an action potential 1. Stimulus Na+ channels open; membrane permeability to Na+ increases Na+ diffuse into axon down electrochemical gradient (causing depolarisation) 2. Depolarisation If threshold potential reached, an action potential is generated As more voltage-gated Na+ channels open (positive feedback effect) So more Na+ diffuse in rapidly 3. Repolarisation Voltage-gated Na+ channels close Voltage-gated K+ channels open; K+ diffuse out of axon 4. Hyperpolarisation K+ channels slow to close so there’s a slight overshoot – too many K+ diffuse out 5. Resting potential Restored by Na+/K+ pump Draw / label a graph showing an action potential Describe the all-or-nothing principle For an action potential to be produced, depolarisation must exceed threshold potential Action potentials produced are always same magnitude / size / peak at same potential ○ Bigger stimuli instead increase frequency of action potentials 11 More GCSE & A Level Biology resources available at stan.store/biologywitholivia Copyright © 2024 Biology with Olivia Explain how the passage of an action potential along non-myelinated and myelinated axons results in nerve impulses Non-myelinated axon Myelinated axon Action potential passes as a wave of Myelination provides electrical insulation depolarisation Depolarisation of axon at nodes of Ranvier only + Influx of Na in one region increases Resulting in saltatory conduction (local permeability of adjoining region to Na+ by currents circuits) causing voltage-gated Na+ channels to open So there is no need for depolarisation along so adjoining region depolarises whole length of axon Suggest how damage to the myelin sheath can lead to slow responses and / or jerky movement Less / no saltatory conduction; depolarisation occurs along whole length of axon ○ So nerve impulses take longer to reach neuromuscular junction; delay in muscle contraction Ions / depolarisation may pass / leak to other neurones ○ Causing wrong muscle fibres to contract Describe the nature of the refractory period Time taken to restore axon to resting potential when no further action potential can be generated As Na+ channels are closed / inactive / will not open Explain the importance of the refractory period Ensures discrete impulses are produced (action potentials don’t overlap) Limits frequency of impulse transmission at a certain intensity (prevents over reaction to stimulus) ○ Higher intensity stimulus causes higher frequency of action potentials ○ But only up to certain intensity Also ensures action potentials travel in one direction – can’t be propagated in a refractory region In the second half of the refractory period an action potential can be produced but requires greater stimulation to reach threshold Describe the factors that affect speed of conductance Myelination Depolarisation at Nodes of Ranvier only → saltatory conduction Impulse doesn’t travel / depolarise whole length of axon Axon diameter Bigger diameter means less resistance to flow of ions in cytoplasm Temperature Increases rate of diffusion of Na+ and K+ as more kinetic energy But proteins / enzymes could denature at a certain temperature 12  ❌ More GCSE & A Level Biology resources available at stan.store/biologywitholivia Copyright © 2024 Biology with Olivia Exam insight: common mistakes Mistake Explanation “Resting potential is established as 2 The numbers here are the wrong way round. Although they aren’t sodium ions are transported out for usually required to get the mark, if they’re wrong you’ll still lose the every 3 potassium ions transported in.” mark. If in doubt, leave them out. “The Na+/K+ pump involves diffusion.” The Na+/K+ pump is involved in active transport. *Not mentioning that myelination This is a key point and is why depolarisation in myelinated neurones provides (electrical) insulation.* is able to occur at nodes of Ranvier only, resulting in saltatory conduction. “Impulses jump from node to node in This is often used as an analogy, but is not technically correct. The saltatory conduction.” key point is that depolarisation takes place only at nodes. 13 More GCSE & A Level Biology resources available at stan.store/biologywitholivia Copyright © 2024 Biology with Olivia 6.2.2 Synaptic transmission Describe the structure of a synapse What are cholinergic synapses? Synapses that use the neurotransmitter acetylcholine (ACh) Describe transmission across a cholinergic synapse At pre- 1. Depolarisation of pre-synaptic membrane causes opening of voltage-gated Ca2+ channels synaptic ○ Ca2+ diffuse into pre-synaptic neurone / knob neurone 2. Causing vesicles containing ACh to move and fuse with pre-synaptic membrane ○ Releasing ACh into the synaptic cleft (by exocytosis) At post- 3. ACh diffuses across synaptic cleft to bind to specific receptors on post-synaptic membrane synaptic 4. Causing Na+ channels to open neurone ○ Na+ diffuse into post-synaptic knob causing depolarisation ○ If threshold is met, an action potential is initiated Explain what happens to acetylcholine after synaptic transmission It is hydrolysed by acetylcholinesterase Products are reabsorbed by the presynaptic neurone To stop overstimulation - if not removed it would keep binding to receptors, causing depolarisation 14 More GCSE & A Level Biology resources available at stan.store/biologywitholivia Copyright © 2024 Biology with Olivia Explain how synapses result in unidirectional nerve impulses Neurotransmitter only made in / released from pre-synaptic neurone Receptors only on post-synaptic membrane Explain summation by synapses Addition of a number of impulses converging on a single post-synaptic neurone Causing rapid buildup of neurotransmitter (NT) So threshold more likely to be reached to generate an action potential Importance - low frequency action potentials release insufficient neurotransmitter to exceed threshold Describe spatial summation Many pre-synaptic neurones share one synaptic cleft / post-synaptic neurone Collectively release sufficient neurotransmitter to reach threshold to trigger an action potential Describe temporal summation One pre-synaptic neurone releases neurotransmitter many times over a short time Sufficient neurotransmitter to reach threshold to trigger an action potential Describe inhibition by inhibitory synapses Inhibitory neurotransmitters hyperpolarise postsynaptic membrane as: ○ Cl- channels open → Cl- diffuse in ○ K+ channels open → K+ diffuse out This means inside of axon has a more negative charge relative to outside / below resting potential So more Na+ required to enter for depolarisation Reduces likelihood of threshold being met / action potential formation at post-synaptic membranes Importance - both excitatory and inhibitory neurones forming synapses with the same post-synaptic membrane gives control of whether it ‘fires’ an action potential 15 More GCSE & A Level Biology resources available at stan.store/biologywitholivia Copyright © 2024 Biology with Olivia Describe the structure of a neuromuscular junction Very similar to a synapse except: Receptors are on muscle fibre sarcolemma instead of postsynaptic membrane and there are more Muscle fibre forms clefts to store enzyme eg. acetylcholinesterase to break down neurotransmitter Compare transmission across cholinergic synapses and neuromuscular junctions In both: transmission is unidirectional Cholinergic synapse Neuromuscular junction Neurone to neurone (or effectors, glands) (Motor) neurone to muscle Neurotransmitters can be excitatory or inhibitory Always excitatory Action potential may be initiated in postsynaptic Action potential propagates along sarcolemma neurone down T tubules Use examples to explain the effect of drugs on a synapse Some drugs stimulate the nervous system, leading to more action potentials, eg.: ○ Similar shape to neurotransmitter ○ Stimulate release of more neurotransmitter ○ Inhibit enzyme that breaks down neurotransmitter → Na+ continues to enter Some drugs inhibit the nervous system, leading to fewer action potentials, eg.: ○ Inhibit release of neurotransmitter eg. prevent opening of calcium ion channels ○ Block receptors by mimicking shape of neurotransmitter Exam insight: common mistakes ❌ Mistake Explanation “Calcium ions enter the synapse.” Calcium ions diffuse into the pre-synaptic neurone. “Vesicles are released and diffuse across the When vesicles fuse with the pre-synaptic membrane, they synaptic cleft.” release neurotransmitters. The vesicles aren’t released. “Acetylcholine binds to the active site of the Only enzymes have active sites. Neurotransmitters simply receptor on the post-synaptic membrane.” bind to (a binding site on) receptors. *Not specifying the location of the receptors Receptors are found only on the post-synaptic membrane. that neurotransmitters bind to.* You need to specify this to get the mark. “Sodium / calcium diffuse in.” You need to specify that these are ions. 16 More GCSE & A Level Biology resources available at stan.store/biologywitholivia Copyright © 2024 Biology with Olivia 6.3 Skeletal muscles are stimulated to contract by nerves and act as effectors Describe how muscles work Work in antagonistic pairs → pull in opposite directions eg. biceps / triceps ○ One muscle contracts (agonist), pulling on bone / producing force ○ One muscle relaxes (antagonist) Skeleton is incompressible so muscle can transmit force to bone Advantage - the second muscle required to reverse movement caused by the first (muscles can only pull) and contraction of both muscles helps maintain posture Describe the gross and microscopic structure of skeletal muscle Made of many bundles of muscle fibres (cells) packaged together Attached to bones by tendons Muscle fibres contain: ○ Sarcolemma (cell membrane) which folds inwards (invagination) to form transverse (T) tubules ○ Sarcoplasm (cytoplasm) ○ Multiple nuclei ○ Many myofibrils ○ Sarcoplasmic reticulum (endoplasmic reticulum) ○ Many mitochondria 17 More GCSE & A Level Biology resources available at stan.store/biologywitholivia Copyright © 2024 Biology with Olivia Describe the ultrastructure of a myofibril Made of two types of long protein filaments, arranged in parallel ○ Myosin - thick filament ○ Actin - thin filament Arranged in functional units called sarcomeres ○ Ends – Z-line / disc ○ Middle – M-line ○ H zone – contains only myosin Explain the banding pattern to be seen in myofibrils I-bands - light bands containing only thin actin filaments A-bands - dark bands containing thick myosin filaments (and some actin filaments) ○ H zone contains only myosin ○ Darkest region contains overlapping actin and myosin Give an overview of muscle contraction Myosin heads slide actin along myosin causing the sarcomere to contract Simultaneous contraction of many sarcomeres causes myofibrils and muscle fibres to contract When sarcomeres contract (shorten)... ○ H zones get shorter ○ I band get shorter ○ A band stays the same ○ Z lines get closer 18 More GCSE & A Level Biology resources available at stan.store/biologywitholivia Copyright © 2024 Biology with Olivia Describe the roles of actin, myosin, calcium ions, tropomyosin and ATP in myofibril contraction Use the columns to pick out key points for questions that focus on the role of actin (A), myosin (M), calcium ions (C), tropomyosin (T) or ATP: C A T M ATP 1 Depolarisation spreads down sarcolemma via T tubules causing Ca2+ ✓ release from sarcoplasmic reticulum, which diffuse to myofibrils 2 Calcium ions bind to tropomyosin, causing it to move → exposing ✓ ✓ ✓ binding sites on actin 3 Allowing myosin head, with ADP attached, to bind to binding sites on ✓ ✓ ✓ ✓ actin → forming an actinomyosin crossbridge 4 Myosin heads change angle, pulling actin along myosin, (ADP ✓ ✓ ✓ released), using energy from ATP hydrolysis 5 New ATP binds to myosin head causing it to detach from binding site ✓ ✓ ✓ 6 Hydrolysis of ATP by ATP(hydrol)ase (activated by Ca2+) releases ✓ ✓ ✓ energy for myosin heads to return to original position 7 Myosin reattaches to a different binding site further along actin ✓ ✓ Process is repeated as long as calcium ion concentration is high During muscle relaxation: 1. Ca2+ actively transported back into the endoplasmic reticulum using energy from ATP 2. Tropomyosin moves back to block myosin binding site on actin again → no actinomyosin cross bridges 19 More GCSE & A Level Biology resources available at stan.store/biologywitholivia Copyright © 2024 Biology with Olivia Describe the role of phosphocreatine in muscle contraction A source of inorganic phosphate (Pi) → rapidly phosphorylates ADP to regenerate ATP ○ ADP + phosphocreatine → ATP + creatine Runs out after a few seconds → used in short bursts of vigorous exercise Anaerobic and alactic Compare the structure, location and general properties of slow and fast skeletal muscle fibres Slow twitch Fast twitch General Specialised for slow, sustained Specialised for brief, intensive properties contractions (eg. posture, long contractions (eg. sprinting) distance running) Produce less ATP rapidly (mostly) from Produce more ATP slowly (mostly) from anaerobic respiration aerobic respiration Fatigues quickly due to high lactate Fatigues slowly concentration Location High proportion in muscles used for High proportion in muscles used for fast posture eg. back, calves movement eg. biceps, eyelids Legs of long distance runners Legs of sprinters Structure High conc. of myoglobin → stores Low levels of myoglobin oxygen for aerobic respiration Lots of glycogen → hydrolysed to provide Many mitochondria → high rate of glucose for glycolysis / anaerobic aerobic respiration respiration which is inefficient so large Many capillaries → supply high conc. of quantities of glucose required oxygen / glucose for aerobic High conc. of enzymes involved in respiration and to prevent build-up of anaerobic respiration (in cytoplasm) lactic acid causing muscle fatigue Store phosphocreatine Exam insight: common mistakes ❌ Mistake Explanation *Mixing up the A band and the H zone.* The A band is dark because it contains myosin (thick filament), but some regions also contain actin. The H zone is the central region that contains myosin but no overlapping actin. *Going into detail on the role of troponin.* The specification states that the role of troponin is not required. “Actin has an active site.” Only enzymes have active sites. Actin has a binding site. “Actin filaments move.” This is too vague. You need to link the movement of myosin heads to the pulling of actin filaments. “Phosphocreatine provides phosphorus.” Phosphorus is not the same as phosphate (Pi). 20 More GCSE & A Level Biology resources available at stan.store/biologywitholivia Copyright © 2024 Biology with Olivia 6.4 Homeostasis 6.4.1 Principles of homeostasis and negative feedback Describe homeostasis in mammals Maintenance of a stable internal environment within restricted limits By physiological control systems (normally involve negative feedback) Examples: core temperature, blood pH, blood glucose concentration, blood water potential Explain the importance of maintaining stable core temperature If temperature is too high: ○ Hydrogen bonds in tertiary structure of enzymes break ○ Enzymes denature; active sites change shape and substrates can’t bind ○ So fewer enzyme-substrate complexes If temperature is too low: ○ Not enough kinetic energy so fewer enzyme-substrate complexes Explain the importance of maintaining stable blood pH Above or below optimal pH, ionic / hydrogen bonds in tertiary structure break Enzymes denature; active sites change shape and substrates can’t bind So fewer enzyme substrate complexes Explain the importance of maintaining stable blood glucose concentration Too low (hypoglycaemia) Too high (hyperglycaemia) Not enough glucose (respiratory substrate) for Water potential of blood decreases respiration Water lost from tissue to blood via osmosis So less ATP produced Kidneys can’t absorb all glucose → more water Active transport etc. can’t happen → cell death lost in urine causing dehydration Describe the role of negative feedback in homeostasis 1. Receptors detect change from optimum Examples: control of blood glucose 2. Effectors respond to counteract change concentration, blood pH, core 3. Returning levels to optimum / normal temperature and blood water potential 21 More GCSE & A Level Biology resources available at stan.store/biologywitholivia Copyright © 2024 Biology with Olivia Explain the importance of conditions being controlled by separate mechanisms involving negative feedback Departures in different directions from the original state can all be controlled / reversed Giving a greater degree of control (over changes in internal environment) Describe positive feedback 1. Receptors detect change from normal Not involved in homeostasis. 2. Effectors respond to amplify change Examples: onset of contractions 3. Producing a greater deviation from normal in childbirth, blood clotting You should be able to interpret information relating to examples of negative and positive feedback Exam insight: common mistake ❌ Mistake Explanation *Using generalised statements about You need to use the information provided. For example, positive feedback when explaining why [named molecule 1] causes more [named molecule 2] to be information given is an example of positive formed, which in turn causes more [named molecule 1] to be feedback.* formed. 22 More GCSE & A Level Biology resources available at stan.store/biologywitholivia Copyright © 2024 Biology with Olivia 6.4.2 Control of blood glucose concentration Describe the factors that influence blood glucose concentration Consumption of carbohydrates → glucose absorbed into blood Rate of respiration of glucose eg. increases during exercise due to muscle contraction Describe the role of the liver in glycogenesis, glycogenolysis and gluconeogenesis Glycogenesis Converts glucose → glycogen Glycogenolysis Converts glycogen → glucose Gluconeogenesis Converts amino acids and/or glycerol → glucose Explain the action of insulin in decreasing blood glucose concentration Beta cells in islets of Langerhans in pancreas detect blood glucose concentration is too high → secrete insulin: Attaches to specific receptors on cell surface membranes of target cells eg. liver / muscles 1. This causes more glucose channel proteins to join cell surface membrane ○ Increasing permeability to glucose ○ So more glucose can enter cell by facilitated diffusion 2. This also activates enzymes involved in conversion of glucose to glycogen (glycogenesis) ○ Lowering glucose concentration in cells, creating a concentration gradient ○ So glucose enters cell by facilitated diffusion 23 More GCSE & A Level Biology resources available at stan.store/biologywitholivia Copyright © 2024 Biology with Olivia Explain the action of glucagon in increasing blood glucose concentration Alpha cells in islets of Langerhans in pancreas detect blood glucose concentration is too low → secrete glucagon: Attaches to specific receptors on cell surface membranes of target cells eg. liver 1. Activates enzymes involved in hydrolysis of glycogen to glucose (glycogenolysis) 2. Activates enzymes involved in conversion of glycerol / amino acids to glucose (gluconeogenesis) This establishes a concentration gradient → glucose enters blood by facilitated diffusion Explain the role of adrenaline in increasing blood glucose concentration Fear / stress / exercise → adrenal glands secrete adrenaline: Attaches to specific receptors on cell surface membranes of target cells eg. liver Activates enzymes involved in hydrolysis of glycogen to glucose (glycogenolysis) This establishes a concentration gradient → glucose enters blood by facilitated diffusion Describe the second messenger model of adrenaline and glucagon action Adrenaline / glucagon (‘first messengers’) attach to specific receptors on cell membrane which: 1. Activates enzyme adenylate cyclase (changes shape) 2. Which converts many ATP to many cyclic AMP (cAMP) 3. cAMP acts as the second messenger → activates protein kinase enzymes 4. Protein kinases activate enzymes to break down glycogen to glucose 24 More GCSE & A Level Biology resources available at stan.store/biologywitholivia Copyright © 2024 Biology with Olivia Suggest an advantage of the second messenger model Amplifies signal from hormone As each hormone can stimulate production of many molecules of second messenger (cAMP) Which can in turn activate many enzymes for rapid increase in glucose Compare the causes of types I and II diabetes Both - higher and uncontrolled blood glucose concentration; higher peaks after meals and remains high Type I Type II Key point = β cells in islets of langerhans Key point = receptor (faulty) loses responsiveness / in pancreas produce insufficient insulin sensitivity to insulin (but insulin still produced) Normally develops in childhood due to So fewer glucose transport proteins → less uptake of an autoimmune response destroying β glucose → less conversion of glucose to glycogen cells of Islets of Langerhans Risk factor = obesity Describe how of type I diabetes can be controlled Injections of insulin (as pancreas doesn’t produce enough) Blood glucose concentration monitored with biosensors; dose of insulin matched to glucose intake Eat regularly and control carbohydrate intake eg. those that are broken down / absorbed slower ○ To avoid sudden rise in glucose Suggest why insulin can’t be taken as a tablet by mouth Insulin is a protein Would be hydrolysed by endopeptidases / exopeptidases 25 More GCSE & A Level Biology resources available at stan.store/biologywitholivia Copyright © 2024 Biology with Olivia Describe how of type II diabetes can be controlled Not normally treated with insulin injections (as pancreas still produces it) but may use drugs which target insulin receptors to increase their sensitivity ○ To increase glucose uptake by cells / tissues Reduce sugar intake (carbohydrates) / low glycaemic index → less absorbed Reduce fat intake → less glycerol converted to glucose More (regular) exercise → uses glucose / fats by increasing respiration Lose weight → increased sensitivity of receptors to insulin Describe how you can evaluate the positions of health advisers and the food industry in relation to the increased incidence of type II diabetes Consider both sides: Health advisers aim - reduce risk of type II diabetes due to health problems caused (eg. kidney failure) ○ So need to reduce obesity as it is a risk factor Food industry aim - maximise profit Exam insight: common mistakes ❌ Mistake Explanation *Mixing up glycogenesis, glycogenolysis To remember what these words mean, break them down. Lysis and gluconeogenesis.* relates to splitting, neo to new and genesis to creation. “Insulin opens existing channel proteins in Insulin causes more glucose channel proteins to join the the cell-surface membranes of target cells.” cell-surface membrane, increasing permeability to glucose. “Glucagon converts glycogen to glucose, or This suggests glucagon acts as an enzyme during these glycerol / amino acids into glucose.” reactions. Glucagon binds to receptors on the surface of cells which leads to activation of enzymes involved in this. “Insulin / glucagon / adrenaline bind to the Only enzymes have active sites. active site of the receptor.” These hormones bind to the receptor directly. “The amount / level of glucose in the blood The term amount is never accepted. is too high in people with diabetes.” You should instead use the term concentration. “Diabetics are incapable of taking any Due to insufficient insulin production or reduced responsiveness glucose into body cells.” of receptors to insulin, less glucose is taken into cells. “Type II diabetics are less responsive to This is too vague. It is the receptors on their cells that lose insulin.” responsiveness / sensitivity to insulin. 26 More GCSE & A Level Biology resources available at stan.store/biologywitholivia Copyright © 2024 Biology with Olivia Required practical 11 Production of a dilution series of a glucose solution and use of colorimetric techniques to produce a calibration curve with which to identify the concentration of glucose in an unknown ‘urine’ sample. Describe how a calibration curve could be produced for glucose 1. Use distilled water and a glucose solution of known concentration to produce a dilution series (of glucose solutions of known concentrations) 2. Heat a set volume of each solution with a set volume of Benedict’s solution 3. Measure absorbance (of light) of each solution using a colorimeter 4. Plot a graph of absorbance (y axis) against concentration of glucose solution (x axis) and draw a line / curve of best fit Note - the calibration curve will vary eg. if precipitate was removed before using the colorimeter Describe how the concentration of glucose in an unknown ‘urine’ sample can be identified using a calibration curve 1. Perform Benedict’s test on sample using same volumes of solutions used in producing calibration curve 2. Measure absorbance using a colorimeter 3. Absorbance value for ‘urine’ sample read off calibration curve to find associated glucose concentration Common questions: Give examples of variables that Volume of sample used should be controlled. (2) Volume of Benedict’s solution Temperature of water bath Time samples were heated for in water bath Explain why a high blood glucose Not all glucose reabsorbed at proximal convoluted tubule concentration can cause glucose As glucose carrier / cotransporter proteins are saturated / to be present in the urine of a working at maximum rate diabetic person. (2) 27 More GCSE & A Level Biology resources available at stan.store/biologywitholivia Copyright © 2024 Biology with Olivia 6.4.3 Control of blood water potential Describe the structure of a nephron Nephron = basic structural and functional unit of the kidney (millions in the kidney) Associated with each nephron are a network of blood vessels Summarise the role of different parts of the nephron Part of nephron Function 1. Bowman’s / renal capsule Formation of glomerular filtrate (ultrafiltration) 2. Proximal convoluted tubule Reabsorption of water and glucose (selective reabsorption) 3. Loop of Henle Maintenance of a gradient of sodium ions in the medulla 4. Distal convoluted tubule Reabsorption of water (permeability controlled by ADH) 5. Collecting duct 28 More GCSE & A Level Biology resources available at stan.store/biologywitholivia Copyright © 2024 Biology with Olivia Describe the formation of glomerular filtrate 1. High hydrostatic pressure in glomerulus ○ As diameter of afferent arteriole (in) is wider than efferent arteriole (out) 2. Small substances eg. water, glucose, ions, urea forced into glomerular filtrate, filtered by: a. Pores / fenestrations between capillary endothelial cells b. Capillary basement membrane c. Podocytes 3. Large proteins / blood cells remain in blood Describe the reabsorption of glucose by the proximal convoluted tubule 1. Na+ actively transported out of epithelial cells to capillary 2. Na+ moves by facilitated diffusion into epithelial cells down a concentration gradient, bringing glucose against its concentration gradient 3. Glucose moves into capillary by facilitated diffusion down its concentration gradient Describe the reabsorption of water by the proximal convoluted tubule Glucose etc. in capillaries lower water potential Water moves by osmosis down a water potential gradient 29 More GCSE & A Level Biology resources available at stan.store/biologywitholivia Copyright © 2024 Biology with Olivia Describe and explain how features of the cells in the PCT allow the rapid reabsorption of glucose into the blood Microvilli / folded cell-surface membrane → provides a large surface area Many channel / carrier proteins → for facilitated diffusion / co-transport Many carrier proteins → for active transport Many mitochondria → produce ATP for active transport Many ribosomes → produce carrier / channel proteins Suggest why glucose is found in the urine of an untreated diabetic person Blood glucose concentration is too high so not all glucose is reabsorbed at the PCT As glucose carrier / cotransporter proteins are saturated / working at maximum rate Explain the importance of maintaining a gradient of sodium ions in the medulla (concentration increases further down) So water potential decreases down the medulla (compared to filtrate in collecting duct) So a water potential gradient is maintained between the collecting duct and medulla To maximise reabsorption of water by osmosis from filtrate Describe the role of the loop of Henle in maintaining a gradient of sodium ions in the medulla 1. In the ascending limb: ○ Na+ actively transported out (so filtrate concentration decreases) ○ Water remains as ascending limb is impermeable to water ○ This increases concentration of Na+ in the medulla, lowering water potential 2. In the descending limb: ○ Water moves out by osmosis then reabsorbed by capillaries (so filtrate concentration increases) ○ Na+ ‘recycled’ → diffuses back in The loop of Henle acts as a countercurrent multiplier (you don’t need to know why) 30 More GCSE & A Level Biology resources available at stan.store/biologywitholivia Copyright © 2024 Biology with Olivia Suggest why animals needing to conserve water have long loops of Henle (thick medulla) More Na+ moved out → Na+ gradient is maintained for longer in medulla / higher Na+ concentration So water potential gradient is maintained for longer So more water can be reabsorbed from collecting duct by osmosis Describe the reabsorption of water by the distal convoluted tubule and collecting ducts Water moves out of distal convoluted tubule & collecting duct by osmosis down a water potential gradient Controlled by ADH which increases their permeability What is osmoregulation? Control of water potential of the blood (by negative feedback) Describe the role of the hypothalamus in osmoregulation 1. Contains osmoreceptors which detect increase OR decrease in blood water potential 2. Produces more ADH when water potential is low OR less ADH when water potential is high Describe the role of the posterior pituitary gland in osmoregulation Secretes (more / less) ADH into blood due to signals from the hypothalamus Describe the role of antidiuretic hormone (ADH) in osmoregulation 1. Attaches to receptors on collecting duct (and distal convoluted tubule) 2. Stimulating addition of channel proteins (aquaporins) into cell-surface membranes 3. So increases permeability of cells of collecting duct and DCT to water 4. So increases water reabsorption from collecting duct / DCT (back into blood) by osmosis 5. So decreases volume and increases concentration of urine produced The above applies to a decrease in water potential of the blood (eg. increased sweating, reduced water intake, increased salt intake). The body responds to an increase in water potential in the opposite way (less ADH secreted, less attaches to receptors, less addition of channel proteins, permeability remains low, less water reabsorption, higher volume & increased concentration of urine etc.). 31  ❌ More GCSE & A Level Biology resources available at stan.store/biologywitholivia Copyright © 2024 Biology with Olivia Exam insight: common mistakes Mistake Explanation “Blood / hydrostatic pressure increases in This is not wrong, but to get the mark you need to be clear that the glomerulus.” pressure is high, not just increasing. “Small molecules pass out of the You normally need to name at least two of these molecules to glomerulus into the renal capsule.” get the marks, eg. water, glucose, ions or urea. “Molecules cross the cells of the capillary Molecules don’t cross the cells themselves. They move through endothelium during glomerular filtration.” the gaps (pores / fenestrations) between cells. *Not referring to the basement membrane The basement membrane is the key filter for the molecules that when describing glomerular filtration.* pass into the renal capsule. *Using the abbreviations PCT / DCT without These are not abbreviations recognised in the specification, so writing them out in full.* they must be written in full at least once. The PCT is the proximal convoluted tubule and the DCT is the distal convoluted tubule. “The cells of the PCT have carrier / channel The key idea is that they have many of these proteins, not just proteins to adapt them for the absorption that these proteins exist. of glucose. “ *Not understanding that the filtrate in the The filtrate then moves to the distal convoluted tubule and loop of Henle will eventually form urine.* collecting duct where water moves out by osmosis back into the blood. The water, ions and urea left in the filtrate at this point move to the bladder as urine. “The pituitary or hypothalamus releases The hypothalamus produces ADH, but the posterior pituitary ADH into the blood.” releases it into the blood. Pituitary on its own is insufficient. “When water is reabsorbed, it moves into Water reabsorption refers to the movement the collecting ducts.” of water into the blood. “ADH decreases the amount of urine The term amount is never accepted. You should always use the produced.” term volume (and concentration). *Not stating the mechanism of water Water moves by osmosis, from a high to low water potential. reabsorption.* Naming osmosis is normally required to pick up the mark. Created with BioRender.com 32