Biology GCSE OCR Notes – Topic 3: Organism Level Systems (Vocabulary Flashcards)
Nervous System: how it coordinates responses
The central nervous system (CNS) consists of the brain and the spinal cord.
Purpose: to make sense of surroundings and respond to survive.
General pathway:
Receptor cells convert a stimulus (e.g., bright light) into an electrical impulse.
Impulse travels along sensory neurons to the CNS.
CNS processes information and coordinates a response.
Electrical impulse travels along motor neurons to effectors.
Effectors carry out the response (e.g., muscles contract or glands secrete hormones).
Reflex arc (3.1c)
Reflex arc is a subconscious response to dangerous stimuli when a rapid response is needed and there isn’t time for brain processing.
Pathway:
1) Stimulus detected by receptors (e.g., thermoreceptors in fingertips sensing heat).
2) Impulses travel along a sensory neuron.
3) In the CNS, impulse passes to a relay neuron.
4) Impulses travel along a motor neuron.
5) Impulse reaches an effector, producing an immediate response (e.g., biceps contraction to move away from heat).
Structure of the eye (3.1d and e) – Biology only
Cornea: transparent outer part; refracts light to reach the retina.
Iris: the coloured part; controls how much light enters; pupil size changes with light.
Pupil: opening that allows light in; constricts in bright light, dilates in dim light.
Conjunctiva: thin membrane covering the sclera and lining the eyelids.
Lens: transparent, biconvex; focuses light onto the retina; attached to ciliary muscles by suspensory ligaments.
Retina: contains light receptors; rods (dim light) and cones (colour).
Fovea: region of the retina with high visual acuity.
Optic nerve: carries impulses from eye to brain.
How light control works:
In bright light: circular muscles of the iris contract; radial muscles relax; pupil becomes smaller to reduce light entry.
In dim light: circular muscles relax; radial muscles contract; pupil enlarges to let more light in.
Common defects (3.1f–h)
Colour blindness: inability to distinguish colours due to defects in retinal receptors; mostly inherited; most common form is red-green colour blindness.
Short-sightedness (myopia): see near objects clearly but not distant objects; lens focuses image in front of retina; corrected with concave lenses in glasses.
Long-sightedness (hyperopia): see distant objects clearly but not near ones; lens focuses image behind retina; corrected with convex lenses in glasses.
The brain (3.1f–h) – Biology only
Structure and functions:
Cerebrum: responsible for intelligence, vocabulary, personality, conscious thought; largest part; divided into two cerebral hemispheres; each hemisphere processes information from the opposite side of the body; outer grey matter, inner white matter.
Cerebellum: coordinates voluntary movements and helps balance.
Medulla: control centre for heart rate, blood pressure, and breathing rate.
Hypothalamus: controls temperature and water balance via hormonal system.
Pituitary gland: important gland releasing hormones (e.g., growth hormone) into the blood.
Ethical and practical limitations:
Ethical issues in brain-damage research: difficult to obtain informed consent; surgery risks; high uncertainty in case studies.
Limitations in treating brain damage: CNS cells have limited regeneration; some areas are hard or dangerous to access; drugs struggle to penetrate brain membranes.
Endocrine system (3.2a and b)
Hormones: chemical messengers secreted by endocrine glands into the bloodstream; travel to target organs.
Pituitary gland: one of the most important glands.
Endocrine vs nervous systems:
Nervous system: electrical signals; fast responses; short duration.
Endocrine system: chemical signals (hormones) in bloodstream; slower responses; longer-lasting effects.
Examples of hormones:
Adrenaline: produced by adrenal glands; targets heart, lungs; part of fight-or-flight; very fast; short duration; increases heart rate, dilates pupils, raises breathing rate, etc.
Thyroxine: produced by thyroid gland; controls metabolic rate and growth; duration is longer; slower onset compared to adrenaline.
Negative feedback (example with thyroxine):
If thyroxine levels are too low, hypothalamus releases TRH, pituitary releases TSH, thyroid produces more thyroxine.
When levels return to normal, hormone release is inhibited.
Reproduction and puberty (3.2c–f)
Hormonal control of puberty: hormones regulate puberty and the menstrual cycle.
Testosterone: produced by testes; enables sperm development and male secondary characteristics (e.g., deeper voice, more body hair).
Oestrogen: produced by ovaries; female secondary sexual characteristics (e.g., breast development, wider hips).
Menstrual cycle (3.2c–f)
Cycle length: typically 28\ ext{days}; ovulation usually around day 14 (egg release).
Hormonal sequence:
Estrogen: causes thickening of the uterine lining; levels peak around day 10 and then fall.
FSH (follicle-stimulating hormone): stimulates maturation of the egg within the ovary.
LH (luteinising hormone): stimulates release of the egg during ovulation.
Progesterone: maintains the thick lining of the uterus; inhibits LH and FSH release; rises after ovulation and peaks a few days later.
Stages within the cycle:
Follicular phase: FSH promotes egg maturation; estrogen rises.
Ovulation: LH and FSH peak; egg released around day 14.
Luteal phase: progesterone high to maintain lining; if egg not fertilised, progesterone falls, lining breaks down (period).
Menopause: usually around 50–55\ ext{years}; end of menstrual cycle.
Contraception (3.2c–f)
Hormonal methods:
Oral contraceptives (the pill): contain progesterone and oestrogen; inhibit FSH production; prevents egg maturation; >99\% effective when taken correctly; side effects such as mood swings and weight gain.
Contraceptive implants: slow release of progesterone; prevents ovulation and thickens cervical mucus; >99\% effective; no daily action required.
Non-hormonal methods:
Physical barriers (condoms): easy to use; protect against STIs; effectiveness >99\% with perfect use; risk of tearing.
Spermicides can improve effectiveness but may cause allergies.
Sterilisation (vasectomy in males, tubal ligation in females): highly reliable but may be irreversible.
Copper intrauterine device (IUD or coil): stops fertilised embryos implanting; can last up to 10\ ext{years}; over 99\% effective; small risk of ectopic pregnancy; fitted by a doctor.
Abstinence.
Fertility treatments (IVF):
Hormones (FSH and LH) stimulate egg maturation and release.
Eggs collected, fertilised in a lab, embryos implanted into the uterus.
Can be expensive and may require multiple cycles; risk of multiple pregnancies.
Plant hormones (3.2g–i) – Biology only
Plants use hormones to coordinate growth and responses to stimuli (tropisms): phototropism (light) and gravitropism/geotropism (gravity).
Hormones move from production sites to target sites to elicit responses.
Auxins:
Cause positive phototropism: shoots bend towards light due to higher auxin concentration on the shaded side, promoting growth there.
Cause negative gravitropism in shoots: higher auxin on the lower side causes cells to grow more on the lower side, bending shoot upwards.
Roots show positive gravitropism: higher auxin on the lower side inhibits growth, so roots grow downwards.
Applications:
Weed killers: synthetic auxins selectively kill broad-leaved weeds.
Rooting powders: auxin-containing powders promote rooting from cuttings.
Tissue culture: auxins used in growth media to promote root/shoot formation.
Gibberellins:
Stimulate seed germination, flowering, and leaf/fruit development.
Uses:
Ending seed dormancy; used in brewing to germinate barley seeds for malt.
Promoting flowering under non-ideal conditions; larger flowers.
Increasing fruit size by stimulating seed production of gibberellins; used to enlarge seedless fruit.
Ethene (ethylene):
Involved in cell division and fruit ripening.
Uses:
In food industry to ripen fruit after harvest; allows firm fruit during transport, then ripens to marketable state.
Regulates enzymes leading to ripening; helps reduce wastage.
Maintaining internal environments (3.3a and b) – Biology only
Homeostasis: the maintenance of a constant internal environment for optimal enzyme action and cell function.
Thermoregulation center: located in the brain; monitors blood temperature; skin receptors send impulses to regulate temperature.
Normal human body temperature: 37.5^{\circ}\mathrm{C}.
Responses to high temperature:
Sweating: sweat evaporates from skin, transferring heat away from the body.
Vasodilation: more blood flows near the skin surface, increasing heat loss.
Responses to low temperature:
Sweating stops; shivering (rapid skeletal muscle contractions) generate heat via respiration.
Hairs stand up to trap a layer of air for insulation.
Vasoconstriction: reduced blood flow near the skin surface to reduce heat loss.
Controlling blood sugar levels (3.3c–d)
The pancreas maintains blood glucose within a narrow range needed for cellular respiration.
Insulin:
Released when blood glucose is high after carbohydrate-rich meals.
Acts on target organs (muscle and liver):
Move glucose from blood into muscle cells for respiration.
Convert excess glucose into glycogen stored in the liver.
Results in lowered blood glucose concentration.
Glucagon:
Released when blood glucose is low and during strenuous activity.
Binds to liver cells, causing glycogen to be broken down into glucose and released into the bloodstream.
Negative feedback keeps blood glucose steady: if glucose rises, insulin is released; if it falls, glucagon is released.
Diabetes: disease where blood sugar control is impaired.
Type 1 diabetes: pancreas cannot produce enough insulin; high blood glucose; glucose excreted in urine; increased urination and thirst; treated with insulin injections; limit simple carbohydrates; experimental cures include pancreas/pancreatic cell transplants.
Type 2 diabetes: body cells no longer respond to insulin; linked to obesity; management includes reducing simple carbs, weight loss, exercise; medications to enhance insulin effectiveness or reduce glucose absorption.
The kidneys and osmoregulation (3.3f–j) – Biology only
Functions of the kidneys:
Filter waste products (water, ions, urea) from blood at high pressure to form urine.
Selectively reabsorb useful substances (glucose, ions, water) back into the blood.
Kidney structure:
Medulla (inner) and cortex (outer).
Ureter transports urine to the bladder; then to the urethra for excretion.
Renal artery brings blood in; renal vein takes blood away.
Nephrons: millions per kidney; made of a glomerulus (capillary bundle), a region for selective reabsorption, and a kidney tubule where water and salt balance is regulated.
Anti-diuretic hormone (ADH):
Hormone regulating water loss in urine; released when blood is too concentrated.
Acts on receptors in the distal convoluted tubule/collecting duct of the kidney.
Increased ADH raises water permeability of tubules, increasing water reabsorption into blood; urine becomes smaller in volume and more concentrated.
Negative feedback and dehydration:
If sweating increases salt loss, kidneys may retain more salt; the brain triggers thirst to encourage water intake, diluting blood salts.
Osmotic changes in body fluids:
If blood water concentration increases (higher water potential than cells), water moves into cells, causing swelling and potentially lysis.
If blood water concentration decreases (lower water potential), water leaves cells, causing shrinkage.