AQA-Level Biology: Topic 6 Organisms respond to changes in their internal and external environments

Stimulus and Tropism Definitions

A stimulus is a change in an organism's internal or external environment; a tropism is the growth of a plant in response to a directional stimulus (positive = towards, negative = away).

Role and Effect of IAA in Shoots and Roots

Indoleacetic acid (IAA) is an auxin produced in shoot and root tips that moves via diffusion or phloem to regulate growth; high IAA concentrations stimulate cell elongation in shoots but inhibit cell elongation in roots.

Gravitropism in Flowering Plants

Cells in the shoot and root tip produce IAA, which diffuses down and moves to the lower side; in shoots this stimulates cell elongation so shoots bend away from gravity, whereas in roots it inhibits cell elongation so roots bend towards gravity.

Phototropism in Flowering Plants

Cells in the shoot and root tip produce IAA, which diffuses down and moves to the shaded side; in shoots this stimulates cell elongation so shoots bend towards light, whereas in roots it inhibits cell elongation so roots bend away from light.

Taxes vs Kinesis

Taxis is a directional movement towards or away from a directional stimulus (e.g. woodlice moving away from light); kinesis is a non-directional response where speed of movement or rate of turning changes depending on stimulus intensity (e.g. woodlice moving faster in dry air).

Protective Effect of a Simple Reflex Arc

Rapid because it involves only 3 neurones and few synapses, autonomic (does not involve conscious brain regions so does not have to be learnt), and protects from harmful stimuli to prevent tissue damage.

Required Practical 10: Choice Chamber Method

Set up compartments with different conditions (e.g. drying agent vs damp paper for humidity), control other variables (e.g. dim even light), place 12 woodlice on a mesh platform (keeping them away from drying agent), leave 15 mins to establish conditions and reduce handling stress, record numbers in each side, and repeat.

Required Practical 10: Maze Method and Statistical Test

Place a food source at one end of a T-maze, place a maggot in the stem, record turning direction, clean maze between trials to remove scents, use a new maggot each time to prevent stress/learned behaviour, repeat with food on the other side, and analyse categorical frequency data with a chi-squared test.

Pacinian Corpuscle and Generator Potential

Mechanical pressure deforms lamellae and stretch-mediated Na+ channels on the sensory neurone membrane, opening them so Na+ ions diffuse in; this causes depolarisation (a generator potential) which triggers an action potential if threshold is reached.

Rods vs Cones: Sensitivity to Light

Rods are more sensitive to light because several rods connect to a single bipolar neurone, allowing spatial summation to reach threshold; each cone connects to its own neurone so there is no spatial summation.

Rods vs Cones: Visual Acuity

Rods give lower visual acuity because several rods share one neurone and send a single set of impulses to the brain; cones give higher visual acuity because each cone connects to a single neurone and sends separate impulses to the brain.

Rods vs Cones: Colour Vision

Rods have only one type of pigment giving monochromatic vision; cones have three types (red-, green-, and blue-sensitive) with different optical pigments absorbing different wavelengths, so stimulating combinations gives colour vision.

Myogenic Heart Stimulation and Electrical Conduction

Cardiac muscle contracts without nerve impulses: the SAN (pacemaker) sends regular waves of electrical activity across atria causing simultaneous atrial contraction; non-conducting tissue prevents direct spread to ventricles; the AVN delays the impulse so atria empty fully before sending it down the bundle of His to the apex and Purkyne tissue, causing ventricles to contract from the base up.

Control of Heart Rate by Chemoreceptors and Baroreceptors

Baroreceptors (detecting pressure) and chemoreceptors (detecting CO2/pH) in the aorta and carotid arteries send impulses to the medulla (cardiac control centre); the medulla sends more frequent impulses along sympathetic neurones to increase SAN frequency and heart rate, or along parasympathetic neurones to decrease heart rate.

Establishing Resting Potential

The Na+/K+ pump actively transports 3 Na+ ions out and 2 K+ ions into the axon, creating an electrochemical gradient; the membrane is more permeable to K+ (which diffuse out) and less permeable to Na+ (closed channels), leaving the inside negatively charged relative to the outside.

Generation of an Action Potential

1. Stimulus opens Na+ channels so Na+ ions diffuse in (depolarisation); 2. If threshold is reached, more voltage-gated Na+ channels open (positive feedback); 3. At peak, voltage-gated Na+ channels close and voltage-gated K+ channels open so K+ ions diffuse out (repolarisation); 4. Slow closing of K+ channels causes hyperpolarisation; 5. Na+/K+ pump restores resting potential.

All-or-Nothing Principle

Depolarisation must exceed threshold potential to trigger an action potential; all action potentials produced have the exact same magnitude, while larger stimuli increase the frequency of action potentials.

Non-Myelinated vs Myelinated Impulse Conduction

In non-myelinated axons, action potentials travel as a continuous wave of depolarisation along the whole membrane; in myelinated axons, the myelin sheath provides electrical insulation so depolarisation occurs only at nodes of Ranvier (saltatory conduction), increasing speed.

Effects of Damage to the Myelin Sheath

Loss of saltatory conduction forces depolarisation along the whole axon, delaying impulses reaching neuromuscular junctions (slow responses), and ions may leak to adjacent neurones causing unintended muscle contractions (jerky movement).

Nature and Importance of the Refractory Period

The time during which Na+ channels are closed/inactive and no new action potential can be generated; it ensures discrete, non-overlapping impulses, limits maximum impulse frequency, and ensures impulses travel in one direction only.

Factors Affecting Speed of Nerve Conductance

Myelination (allows saltatory conduction at nodes of Ranvier), larger axon diameter (less cytoplasmic resistance to ion flow), and higher temperature (more kinetic energy for faster ion diffusion, up to the point where proteins denature).

Transmission Across a Cholinergic Synapse

1. Depolarisation opens voltage-gated Ca2+ channels so Ca2+ ions diffuse into the pre-synaptic knob; 2. Vesicles containing acetylcholine (ACh) fuse with the pre-synaptic membrane, releasing ACh by exocytosis; 3. ACh diffuses across the cleft and binds specific receptors on the post-synaptic membrane; 4. Na+ channels open so Na+ ions diffuse in, depolarising the post-synaptic membrane to threshold.

Breakdown of Acetylcholine After Transmission

Acetylcholinesterase hydrolyses ACh in the synaptic cleft and the products are reabsorbed into the pre-synaptic neurone to prevent continuous binding to receptors and overstimulation.

Unidirectional Synaptic Transmission and Summation

Impulses are unidirectional because neurotransmitter is only released from the pre-synaptic neurone and receptors are only on the post-synaptic membrane; summation builds up neurotransmitter to reach threshold via spatial summation (multiple pre-synaptic neurones) or temporal summation (one neurone firing rapidly).

Inhibitory Synapses

Inhibitory neurotransmitters open Cl- channels (Cl- diffuse in) and K+ channels (K+ diffuse out), hyperpolarising the post-synaptic membrane so more Na+ is needed to reach threshold, reducing the likelihood of an action potential.

Cholinergic Synapse vs Neuromuscular Junction

Cholinergic synapses connect neurones to neurones/effectors, can be excitatory or inhibitory, and trigger an action potential in a post-synaptic neurone; neuromuscular junctions connect motor neurones to muscle fibres, are always excitatory, and propagate an action potential along the sarcolemma down T tubules.

Effects of Drugs on Synapses

Stimulatory drugs mimic neurotransmitters, stimulate extra neurotransmitter release, or inhibit breakdown enzymes so Na+ keeps entering; inhibitory drugs block neurotransmitter receptors or prevent Ca2+ channels from opening.

Antagonistic Muscle Pairs and Skeletal Muscle Structure

Muscles work in antagonistic pairs (agonist contracts, antagonist relaxes) pulling on an incompressible skeleton; muscle fibres contain a sarcolemma folding into T tubules, sarcoplasm, multiple nuclei, many myofibrils, sarcoplasmic reticulum, and many mitochondria.

Myofibril Ultrastructure and Banding Pattern

Sarcomeres run between Z-lines with an M-line in the centre; I-bands (light) contain only thin actin filaments, A-bands (dark) contain thick myosin filaments (plus overlapping actin at edges), and the H-zone contains only myosin; during contraction, H-zones and I-bands shorten, Z-lines move closer, and the A-band stays the same width.

Sliding Filament Mechanism of Muscle Contraction

Depolarisation down T tubules releases Ca2+ from the sarcoplasmic reticulum; Ca2+ binds tropomyosin, moving it to expose myosin binding sites on actin; myosin heads (with ADP) bind actin forming actinomyosin crossbridges and change angle to pull actin (releasing ADP); new ATP binds myosin to detach it, and ATP hydrolysis (activated by Ca2+) returns myosin heads to their original position.

Muscle Relaxation and Role of Phosphocreatine

During relaxation, Ca2+ is actively transported back into the sarcoplasmic reticulum using ATP so tropomyosin blocks actin binding sites again; phosphocreatine provides inorganic phosphate (Pi) to rapidly phosphorylate ADP into ATP anaerobically during short bursts of vigorous exercise.

Slow-Twitch vs Fast-Twitch Muscle Fibres

Slow-twitch fibres are adapted for endurance/posture with aerobic respiration, slow fatigue, high myoglobin, many mitochondria, and dense capillaries; fast-twitch fibres are adapted for sprinting/brief bursts with anaerobic respiration, rapid fatigue, high glycogen, abundant glycolytic enzymes, and phosphocreatine.

Homeostasis and Negative Feedback

Homeostasis is the maintenance of a stable internal environment within restricted limits; in negative feedback, receptors detect a deviation from the optimum and effectors respond to counteract the change and return levels to normal (separate mechanisms control rises and falls for greater precision).

Importance of Stable Core Temperature, Blood pH, and Blood Glucose

Extreme temperature or pH breaks hydrogen/ionic bonds in enzymes, denaturing active sites so fewer enzyme-substrate complexes form; low blood glucose reduces respiration and ATP (causing cell death), while high blood glucose lowers blood water potential, drawing water out of tissues by osmosis and causing dehydration.

Positive Feedback

Receptors detect a change from normal and effectors respond to amplify the change, producing a greater deviation from normal (e.g. oxytocin contractions in childbirth or blood clotting); not involved in homeostasis.

Glycogenesis, Glycogenolysis, and Gluconeogenesis

Glycogenesis converts glucose into glycogen; glycogenolysis hydrolyses glycogen into glucose; gluconeogenesis converts non-carbohydrates such as amino acids and glycerol into glucose (all occurring in the liver).

Action of Insulin in Lowering Blood Glucose

Beta cells in the islets of Langerhans detect high blood glucose and secrete insulin, which binds specific receptors on liver/muscle cell membranes; this causes more glucose channel proteins to join the membrane and activates glycogenesis enzymes, increasing glucose uptake by facilitated diffusion.

Action of Glucagon and Adrenaline in Raising Blood Glucose

Alpha cells secrete glucagon when blood glucose is low, and adrenal glands secrete adrenaline during stress/exercise; both bind liver cell receptors to activate glycogenolysis (and glucagon also activates gluconeogenesis), creating a concentration gradient for glucose to enter blood via facilitated diffusion.

Second Messenger Model of Adrenaline and Glucagon

Adrenaline or glucagon (first messenger) binds a membrane receptor, activating adenylate cyclase which converts many ATP molecules into cyclic AMP (cAMP, second messenger); cAMP activates protein kinases, which activate enzymes to hydrolyse glycogen into glucose, amplifying the signal.

Type I vs Type II Diabetes: Causes and Control

Type I is caused by autoimmune destruction of beta cells so insufficient insulin is produced (controlled by insulin injections, biosensor monitoring, and regulated carbohydrate intake); Type II is caused by cell receptors losing sensitivity to insulin, often linked to obesity (controlled by diet, exercise, weight loss, and receptor-sensitising drugs).

Why Insulin Cannot Be Taken as a Tablet

Insulin is a protein, so it would be hydrolysed by endopeptidases and exopeptidases in the digestive system before being absorbed into the blood.

Required Practical 11: Glucose Calibration Curve

Make a dilution series of known glucose concentrations, heat equal volumes with Benedict's solution for the same time/temperature, measure light absorbance with a colorimeter, plot absorbance (y) against glucose concentration (x), and read off the absorbance of an unknown urine sample to find its concentration.

Parts of the Nephron and Their Functions

Bowman's capsule: ultrafiltration to form glomerular filtrate; Proximal convoluted tubule (PCT): selective reabsorption of glucose and water; Loop of Henle: maintains a Na+ gradient in the medulla; Distal convoluted tubule (DCT) and collecting duct: water reabsorption controlled by ADH.

Glomerular Filtration (Ultrafiltration)

The wider afferent arteriole and narrower efferent arteriole create high hydrostatic pressure in the glomerulus, forcing small molecules (water, glucose, ions, urea) through capillary endothelial pores, the basement membrane, and podocytes into the Bowman's capsule, while large proteins and blood cells remain in the blood.

Reabsorption of Glucose and Water in the PCT

Na+ is actively transported out of PCT epithelial cells into capillaries, allowing Na+ to diffuse back in from the filtrate via co-transporters carrying glucose against its gradient; glucose enters capillaries by facilitated diffusion, lowering capillary water potential so water follows by osmosis.

Adaptations of PCT Cells and Glucose in Diabetic Urine

PCT cells have microvilli (large surface area), many co-transporter/carrier proteins, many mitochondria (ATP for active transport), and many ribosomes; in untreated diabetes, blood glucose is so high that PCT glucose carrier proteins become saturated, leaving glucose in the urine.

Role of the Loop of Henle in Osmoregulation

In the water-impermeable ascending limb, Na+ is actively transported out into the medulla, lowering medulla water potential; in the descending limb, water leaves by osmosis into capillaries while Na+ diffuses in; this maintains a Na+ gradient down the medulla so water is reabsorbed from the collecting duct by osmosis (animals in dry habitats have longer loops of Henle).

Role of Hypothalamus, Posterior Pituitary, and ADH

When blood water potential drops, osmoreceptors in the hypothalamus detect the change and stimulate the posterior pituitary gland to secrete more ADH into the blood; ADH binds receptors on the DCT and collecting duct, inserting aquaporins into membranes to increase water permeability and osmotic reabsorption into the blood, producing a smaller volume of concentrated urine.