4.5 Homeostasis and response — coverage pack

Answer

• Homeostasis is the regulation of internal conditions to maintain optimum conditions for function.
• One of blood glucose concentration, body temperature or water level.

Method: Give both parts of the definition: regulation of internal conditions and maintenance of optimum conditions. Then select one example named in the specification.

Answer

• A receptor detects a stimulus or change.
• The coordination centre receives and processes the information.
• An effector, such as a muscle or gland, carries out an action.
• The response restores the condition towards its optimum level.

Method: Follow information through the system: detection by a receptor, processing by a coordination centre, action by an effector, then restoration towards the optimum.

Answer

• Enzymes and cells function best within narrow optimum conditions.
• Excessive temperature change can reduce the efficiency of enzyme-controlled reactions and cell functions.
• Excessive water gain or loss prevents cells from functioning efficiently.
• Receptors must detect the relevant changes.
• A coordination centre must receive and process the information.
• Muscles or glands act as effectors to produce responses that restore conditions towards optimum levels.

Method: Link stable internal conditions to effective enzyme and cell function, then apply the common control-system sequence separately to the temperature and water changes.

Answer

• Stimulus, receptor, coordinator, effector, response.

Method: Begin with the environmental change, follow its detection and processing, then finish with the structure that acts and the resulting response.

Answer

• Temperature or pain receptors in the skin detect the stimulus.
• An electrical impulse travels along a sensory neurone to the central nervous system.
• The impulse crosses synapses and passes through a relay neurone.
• A motor neurone carries an impulse to a muscle, which contracts to withdraw the hand.
• The rapid, automatic response limits damage and does not wait for conscious processing.

Method: Trace the impulse from receptor to sensory neurone, across the central relay, and out through the motor neurone. Finish by linking speed and automatic action to protection.

Answer

• One result in each condition is not enough to assess natural variation.
• Repeat each condition several times and calculate a representative value such as the mean.
• Use several participants so the conclusion is not based on one person.
• Keep the ruler, drop position, catching hand and person releasing it the same between conditions.
• Randomise or alternate the order of the music and no-music conditions to reduce practice or fatigue effects.
• Compare the processed results and identify anomalous values before deciding whether music affected reaction time.

Method: Separate reliability from validity. Repeats and more participants address random variation, while controlled variables and order effects make the comparison a fair test of background music.

Answer

• The cerebral cortex is associated with conscious thought, intelligence, memory or language.
• The cerebellum coordinates muscular activity.
• The medulla controls unconscious activities such as breathing or heart rate.

Method: Match the outer cerebral region to higher conscious functions, the cerebellum to coordination, and the medulla to automatic body functions.

Answer

• The cerebellum is most likely affected.
• The cerebellum coordinates muscular activity, matching the coordination problem.
• Understanding speech involves functions associated with the cerebral cortex, which appear to remain functional.

Method: Link the observed loss of function to the region responsible for muscular coordination, then use the retained conscious function as supporting evidence rather than as the main diagnosis.

Answer

• Loss of a particular function after localised damage can associate that function with the damaged region.
• Other damage or differences between patients can make that association uncertain.
• Electrical stimulation can produce an immediate response that links a region to a function.
• Stimulation is invasive and carries a risk of damaging delicate brain tissue.
• Magnetic resonance imaging can show which regions are active or structurally abnormal without directly stimulating them.
• Using evidence from several methods and patients makes the conclusion stronger, although the brain's complexity still limits certainty and treatment.

Method: For each method, state what observation it provides and one limitation. Conclude by explaining why converging evidence is useful but does not remove the difficulty created by a complex, delicate organ.

Answer

• The retina contains receptors sensitive to light intensity and colour.
• The optic nerve carries electrical impulses from the retina to the brain.
• The iris controls the amount of light entering the eye.

Method: Distinguish detection, transmission and light control: the retina detects, the optic nerve transmits, and the iris controls entry of light.

Answer

• For a near object, the ciliary muscles contract.
• The suspensory ligaments loosen.
• The lens becomes thicker and refracts light strongly.
• For a distant object, the ciliary muscles relax.
• The suspensory ligaments are pulled tight.
• The lens is pulled thin and refracts light only slightly.

Method: Describe the linked muscle, ligament and lens changes for the near object, then reverse each change for the distant object and compare the amount of refraction.

Answer

• The defect is myopia or short sightedness.
• A diverging spectacle or contact lens spreads the incoming rays before they enter the eye.
• The eye then focuses the rays further back, on the retina.
• Laser surgery can change the shape of the cornea so that it refracts light by the required amount.
• Alternatively, replacing the eye's lens can give the required refraction so that light focuses on the retina.

Method: Use the position of the uncorrected focus to identify myopia. For each treatment, link the change in refraction to moving the final focus onto the retina.

Answer

• The thermoregulatory centre in the brain.
• Receptors in the centre detect the temperature of the blood.
• Temperature receptors in the skin send nervous impulses to the centre.

Method: Identify the brain centre, then distinguish direct monitoring of blood temperature from information sent by receptors in the skin.

Answer

• When too high, skin blood vessels dilate.
• When too high, sweat production increases.
• When too low, skin blood vessels constrict.
• When too low, sweating stops.
• When too low, skeletal muscles contract in shivering.

Method: Group the effectors by the direction of temperature change. Give the two high-temperature responses and the three low-temperature responses specified by AQA.

Answer

• Vasodilation increases blood flow through vessels near the skin surface.
• This increases energy transfer from the warm blood through the skin to the environment.
• Evaporation of sweat transfers energy from the skin to the environment.
• Vasoconstriction reduces blood flow near the skin surface.
• This reduces energy transfer from the blood to the environment.
• Shivering increases muscle contraction and respiration, releasing energy that warms the body.

Method: Explain each mechanism using energy transfer. Increased surface blood flow and evaporation increase cooling; reduced surface blood flow conserves energy, while rapid muscle activity releases more energy by respiration.

Answer

• The gland secretes the hormone directly into the bloodstream.
• The blood carries it to a target organ where it produces an effect.

Method: State both transport stages: direct secretion into blood, then carriage in the blood to the target organ.

Answer

• Hormones are chemical substances carried in the blood, whereas nervous signals pass as electrical impulses along neurones.
• Hormonal responses are generally slower.
• Hormonal effects generally last longer.
• Hormones act on target organs, whereas nervous responses coordinate particular effectors such as muscles or glands.

Method: Compare like with like: signal and transport, speed, duration, and the structures that respond. Use relative language rather than claiming that either system always has one fixed speed.

Answer

• The pituitary responds to body conditions by releasing hormones into the blood.
• Some pituitary hormones stimulate other glands to release their own hormones, so it coordinates parts of the endocrine system.
• Any four from pancreas, thyroid, adrenal gland, ovary and testes.

Method: Use the two-step hormone pathway to justify the term master gland. Then select four distinct glands from the required diagram list, excluding the pituitary already named in the question.

Answer

• The pancreas releases insulin.
• Insulin causes glucose to move from the blood into cells.
• It also causes excess glucose to be converted to glycogen in liver and muscle cells.

Method: Name the source and hormone, then give both specified effects: uptake by cells and conversion of excess glucose to glycogen for storage.

Answer

• In Type 1 diabetes, the pancreas fails to produce sufficient insulin.
• This leads to uncontrolled high blood glucose concentration.
• Type 1 diabetes is normally treated with insulin injections.
• In Type 2 diabetes, body cells no longer respond properly to insulin made by the pancreas.
• A carbohydrate-controlled diet and exercise are common treatments for Type 2 diabetes.
• Obesity is a risk factor for Type 2 diabetes.

Method: Organise the comparison by condition. For each type, state the biological problem and treatment, then add the specified risk factor for Type 2 diabetes.

Answer

• The pancreas monitors blood glucose concentration.
• Above the optimum, the pancreas releases insulin.
• Insulin increases glucose uptake by cells and conversion of glucose to glycogen, so blood glucose falls.
• Below the optimum, the pancreas releases glucagon.
• Glucagon causes glycogen to be converted to glucose and glucose to be released into the blood, so blood glucose rises.
• Each response opposes the original change and reduces hormone release as the concentration returns towards the optimum.

Method: Describe the high and low branches separately, including stimulus, hormone and effect. Finish by showing why both are negative feedback: the response reverses the deviation.

Answer

• The kidneys filter the blood.
• Useful glucose enters the filtrate initially.
• All useful glucose is selectively reabsorbed into the blood.
• Excess water, ions and urea remain for removal in urine.

Method: Separate filtration from selective reabsorption. Filtration moves small substances out of blood, then selective reabsorption returns useful substances while wastes and excess substances form urine.

Answer

• Dialysis removes urea and helps restore suitable water and ion concentrations by exchange across a partially permeable membrane.
• Dialysis avoids transplant surgery and can be used while a patient waits for a donor.
• Dialysis must be repeated regularly and restricts the patient's time and lifestyle.
• A successful transplant can function continuously and reduce the need for repeated dialysis.
• Transplant surgery carries risks, a suitable donor may not be available, and the organ may be rejected.
• The preferred treatment depends on medical suitability, donor availability and the balance of long-term benefits and risks for the patient.

Method: Give a balanced comparison: explain the basic benefit and limitation of each treatment, then make a conditional judgement rather than claiming that one option is always best.

Answer

• The pituitary gland releases more ADH when the blood is too concentrated.
• ADH increases the permeability of kidney tubules to water.
• More water is reabsorbed from the tubules into the blood, producing a smaller volume of more concentrated urine.
• As the blood water level returns towards normal, negative feedback reduces the corrective response.
• In the liver, excess amino acids are deaminated to form ammonia.
• Toxic ammonia is immediately converted to urea for safe excretion by the kidneys.

Method: Treat the two processes separately. Trace the ADH response from concentrated blood to greater water reabsorption, then trace nitrogen from excess amino acids through deamination, ammonia and urea.

Answer

• Testosterone stimulates sperm production.
• FSH causes an egg to mature in an ovary.
• LH stimulates release of the egg in ovulation.
• Progesterone is involved in maintaining the lining of the uterus.

Method: Match each hormone to one specification role, distinguishing egg maturation by FSH from egg release by LH.

Answer

• FSH causes an egg to mature in an ovary.
• The ovary produces oestrogen.
• Oestrogen is involved in maintaining the uterus lining.
• LH stimulates the release of the mature egg from the ovary.

Method: Follow the events in time: egg maturation, ovarian hormone production and uterus-lining maintenance, then release of the egg.

Answer

• FSH causes an egg to mature and stimulates the ovary to produce oestrogen.
• Oestrogen helps maintain or rebuild the uterus lining.
• Rising oestrogen inhibits FSH, so usually only one egg continues to mature.
• High oestrogen stimulates LH release.
• The LH surge triggers ovulation.
• Progesterone maintains the uterus lining and inhibits FSH and LH after ovulation.

Method: Build a causal chain from FSH to oestrogen, then use oestrogen's two feedback effects to reach the LH surge and ovulation. Finish with progesterone's effects after ovulation.

Answer

• An oral hormonal contraceptive inhibits FSH so eggs do not mature.
• A condom forms a barrier that prevents sperm reaching an egg.
• A spermicide kills or disables sperm.

Method: Give the distinct biological action of each method: preventing egg maturation, blocking sperm movement, or disabling sperm.

Answer

• Slow-release progesterone is a hormonal method, whereas a diaphragm is a non-hormonal barrier method.
• Progesterone inhibits maturation and release of eggs.
• A diaphragm prevents sperm reaching an egg.
• An injection or implant can act for months or years, while a diaphragm must be used correctly when intercourse occurs.

Method: Compare method type, biological action and duration or pattern of use. Do not treat both methods as physical barriers.

Answer

• A slow-release progesterone injection or implant is reversible and can work for months or years without daily tablets.
• It inhibits egg maturation and release but may have hormonal side effects.
• A hormone-releasing or non-hormonal intrauterine device is another long-acting option.
• An intrauterine device may prevent implantation or release a hormone and requires fitting.
• Sterilisation is unsuitable for someone who specifically wants a reversible method.
• The choice should also consider side effects, convenience, duration, reversibility, medical advice and personal or ethical preferences.

Method: Apply both stated requirements before evaluating trade-offs. Reject permanent sterilisation, compare long-acting reversible options, and conclude with factors beyond a single effectiveness value.

Answer

• FSH stimulates eggs to mature in the ovaries.
• LH stimulates release of a mature egg in ovulation.

Method: Keep the two roles distinct: FSH matures an egg, while LH triggers its release.

Answer

• FSH and LH are given to stimulate maturation of several eggs.
• The eggs are collected from the mother.
• The eggs are fertilised by sperm in a laboratory.
• The fertilised eggs divide and develop into embryos.
• One or two embryos are inserted into the mother's uterus.

Method: Give the stages in chronological order: hormonal stimulation, egg collection, laboratory fertilisation, embryo development and transfer to the uterus.

Answer

• Both treatments can give a person the chance to have a biological child.
• FSH and LH fertility drugs may allow fertilisation to occur in the body and avoid laboratory embryo transfer.
• IVF can fertilise collected eggs in a laboratory and transfer embryos when simpler hormone treatment is insufficient.
• Success rates are not high, so treatment may fail or require repeated cycles.
• Treatment can be emotionally and physically stressful.
• Stimulation of several eggs and embryo transfer can lead to multiple births, increasing risks to the mother and babies, so benefits and risks must be considered together.

Method: Balance the chance of pregnancy against success rate, stress and multiple-birth risk. Compare what each approach involves before reaching a patient-specific judgement.

Answer

• The adrenal glands produce adrenaline.
• Adrenaline increases heart rate.
• It boosts delivery of oxygen and glucose to the brain and muscles.

Method: Name the source, then give the cardiovascular and fuel-delivery effects that prepare the body for fight or flight.

Answer

• Thyroxine is produced by the thyroid gland.
• It stimulates the basal metabolic rate.
• It is important in growth and development.
• Negative feedback keeps its concentration near the optimum by opposing deviations.

Method: State the gland and two specified roles, then link control to maintaining an appropriate hormone concentration rather than allowing continued change.

Answer

• A fall below the optimum is detected by the control system.
• Stimulation of the thyroid increases.
• The thyroid releases more thyroxine, raising its blood concentration towards the optimum.
• A rise above the optimum produces less stimulation of the thyroid.
• The thyroid then releases less thyroxine, allowing the concentration to fall towards the optimum.
• The responses oppose the original changes, which defines negative feedback.

Method: Trace both branches from deviation to corrective response. The essential conclusion is that each response reverses, rather than reinforces, the initial change.

Answer

• Phototropism is a plant growth response to light.
• Gravitropism or geotropism is a plant growth response to gravity.
• Auxin causes the unequal growth.

Method: Identify the stimulus in each term, then name auxin as the hormone that links unequal distribution to unequal growth.

Answer

• Place comparable newly germinated seedlings so one group receives light from one side.
• Use a suitable control group, such as seedlings receiving light evenly or from above.
• Keep plant type, starting size, water, temperature, light intensity and investigation time controlled where applicable.
• Use several seedlings in each condition and repeat the investigation.
• Measure growth or curvature after a fixed time using the same method for each seedling.
• Record length measurements and make careful labelled biological drawings showing the growth response.

Method: Change only the direction of light, include a comparison condition, control other growth factors, repeat with several seedlings, and collect both forms of evidence required by the practical.

Answer

• Unequal auxin distribution causes unequal growth rates in roots and shoots.
• This unequal growth produces responses to light or gravity.
• Gibberellins are important in initiating seed germination.
• Ethene controls cell division and fruit ripening.
• Detailed mechanisms of gibberellin and ethene action are not required.

Method: Compare the outcome controlled by each hormone: directional growth for auxin, germination for gibberellins, and cell division and ripening for ethene.

Answer

• Auxin is used as a weed killer, in rooting powder or to promote growth in tissue culture.
• Ethene is used to control fruit ripening during storage and transport.
• Gibberellins are used to end seed dormancy, promote flowering or increase fruit size.

Method: Choose one correct use for each hormone and keep fruit ripening assigned to ethene rather than gibberellins.

Answer

• Auxin rooting powder stimulates root formation in cuttings.
• This helps growers establish new plants from cuttings.
• Gibberellins can promote flowering or increase fruit size later in growth.
• Gibberellins can also end seed dormancy when germination is required.

Method: Link each use to a different production stage: auxin establishes roots, while gibberellins can control germination, flowering or fruit growth.

Answer

• Selective auxin weed killers kill broad-leaved weeds while leaving grass crops relatively unaffected.
• This reduces competition from weeds and can increase crop yield.
• Use of weed killers may reduce plant biodiversity and affect organisms that depend on those plants.
• Ethene allows fruit to be transported or stored before it ripens.
• Applying ethene later can coordinate ripening for sale and reduce losses from overripe fruit during transport.
• The benefits to yield, timing and waste should be weighed against environmental effects and the need to control hormone exposure carefully.

Method: Evaluate each application using a benefit and a limitation or implication. Conclude by balancing production gains against environmental and control considerations.