Control, Coordination and Reproduction
Coordination in plants: hormones and tropisms
Plants have no nerves; they coordinate by plant hormones and by changes in the water content of cells. A tropism is directional growth in response to a stimulus: phototropism (to light: shoots bend towards it), geotropism (to gravity: roots grow down, shoots up), hydrotropism (to water: roots), chemotropism (pollen tube growing towards the ovule) and thigmotropism (to touch: tendrils coil round a support). Shoots are positively phototropic and negatively geotropic, roots the reverse.
| Hormone | Where / what it does |
|---|---|
| Auxin | Made at the shoot tip; helps cells grow longer; on the shady side it makes cells elongate more, so the shoot bends towards light |
| Gibberellins | Help the growth of the stem |
| Cytokinins | Promote cell division; abundant in fruits and seeds |
| Abscisic acid | Inhibits growth; causes wilting and fall of leaves |
The sensitive plant touch-me-not (Mimosa) folds its leaves on touch without growth: cells change shape as water moves out of them, so this movement is not a tropism. Auxin, gibberellins and cytokinins promote growth; abscisic acid inhibits it.
Hormones in animals
Hormones are chemical messengers made by ductless endocrine glands and carried by blood to target organs. The amount is controlled by feedback: for example, when blood sugar rises, the pancreas secretes more insulin, and when it falls the secretion is reduced.
| Gland | Hormone | Function or deficiency effect |
|---|---|---|
| Pituitary | Growth hormone | Controls growth; deficiency causes dwarfism; called the master gland |
| Thyroid | Thyroxine | Regulates carbohydrate, protein and fat metabolism; needs iodine, deficiency gives goitre |
| Pancreas | Insulin | Lowers blood sugar; deficiency gives diabetes |
| Adrenal (above kidneys) | Adrenaline | Emergency “fight or flight”: faster heartbeat and breathing, more blood to muscles |
| Testes | Testosterone | Male changes at puberty |
| Ovaries | Oestrogen | Female changes at puberty |
Asexual reproduction
Reproduction makes new individuals and the copying of DNA gives continuity of the species; the copies are nearly identical, with small variations. In asexual reproduction a single parent produces young without gametes, so the offspring are almost clones.
Variation matters because a population of near-identical clones can be wiped out by one change in its environment, whereas the small differences from DNA copying, and the larger mixing in sexual reproduction, give some individuals a chance to survive. This idea is carried further in the chapter on heredity.
| Method | How it works | Example |
|---|---|---|
| Binary fission | Cell divides into two | Amoeba |
| Multiple fission | Many daughter cells at once | Plasmodium |
| Fragmentation | Body breaks into pieces that grow | Spirogyra |
| Regeneration | Cut parts grow into full organisms | Hydra, Planaria |
| Budding | A bud grows out and separates | Hydra, yeast |
| Spore formation | Spores in sporangia germinate | Rhizopus |
| Vegetative propagation | New plant from root, stem or leaf | Bryophyllum leaf; potato; rose cutting |
In vegetative propagation methods such as cutting, layering and grafting (jasmine, rose, banana, orange) the new plant bears flowers and fruit earlier and keeps the parent’s qualities, and it is the way to grow plants that do not form seeds. Tissue culture grows new plants from a few cells in a nutrient medium.
Sexual reproduction in flowering plants
The flower is the reproductive organ. Its parts are sepals, petals, the male stamen (anther that makes pollen, filament) and the female pistil (stigma, style, ovary with ovules). A bisexual flower has both (Hibiscus, mustard); a unisexual flower has only one (papaya, watermelon).
Pollination is the transfer of pollen from anther to stigma: self-pollination within the same flower or plant, cross-pollination between flowers of different plants by wind, water or animals. On the stigma the pollen grows a pollen tube through the style to the ovule, where the male gamete fuses with the egg: fertilisation forms the zygote, which divides to form the embryo. After this the ovule becomes the seed, the ovary becomes the fruit, and petals and other parts wither. The seed germinates into a seedling.
Human reproduction
In males the testes (in the scrotum, which keeps them cooler than the body, which sperm production needs) make sperms and testosterone; sperms travel through the vas deferens, mix with secretions of the prostate and seminal vesicles to form semen, and leave through the urethra. In females the ovaries make eggs and the hormones oestrogen and progesterone; the egg is carried into the oviduct (fallopian tube), where fertilisation takes place; the zygote divides and the resulting embryo is implanted in the uterus, whose lining is thick and rich in blood. The placenta is a disc-like tissue embedded in the uterine wall that gives the embryo nutrition and oxygen and removes its waste. Pregnancy lasts about nine months.
Puberty brings secondary sexual characters under testosterone and oestrogen. In the menstrual cycle, roughly monthly, one ovary releases an egg and the uterine lining thickens; if the egg is not fertilised the lining breaks down and leaves as blood and mucus, which is menstruation.
| Method | Type | Note |
|---|---|---|
| Condom | Barrier | Also reduces risk of sexually transmitted infections |
| Oral pills | Hormonal | Change hormonal balance, so no egg is released |
| Copper-T (IUD) | Placed in uterus | Prevents implantation |
| Vasectomy / tubectomy | Surgical | Blocks vas deferens / oviduct |
Reproductive health also includes the knowledge that sex of a child is decided by the father’s chromosomes (see the chapter on heredity), and that a balanced diet, hygiene and medical advice during puberty and pregnancy protect the health of mother and child.
Diseases such as gonorrhoea and syphilis (bacterial) and warts and HIV-AIDS (viral) spread by sexual contact; barrier methods give protection but not the other methods in the table.
Key takeaways
- The nervous system is fast: neuron = dendrite, cell body, axon; a reflex arc is receptor, sensory neuron, spinal cord, motor neuron, effector.
- Brain: cerebrum thinks, cerebellum balances, medulla controls involuntary actions such as vomiting and blood pressure.
- Plant hormones: auxin and gibberellin promote growth, cytokinin promotes division, abscisic acid inhibits growth; tropisms are directional growth.
- Thyroxine needs iodine (goitre), insulin lowers blood sugar (diabetes), adrenaline prepares for emergency, growth hormone is from the pituitary.
- Asexual methods: binary fission (Amoeba), multiple fission (Plasmodium), budding (Hydra, yeast), spores (Rhizopus), regeneration (Planaria), vegetative propagation.
- In flowers the ovule becomes the seed and ovary the fruit; in humans fertilisation occurs in the oviduct and the embryo implants in the uterus and is nourished by the placenta.
Practice questions (17)
Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.
Which part of a neuron carries the nerve impulse away from the cell body?
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Answer: C — Axon
The long axon conducts the impulse away from the cell body towards its end. Dendrites receive the impulse and bring it to the cell body, which is why they are the usual wrong choice. The synapse is only the gap between two neurons, and the nucleus is inside the cell body.Arrange the stages of a reflex arc in the correct order. P: sensory neuron. Q: motor neuron. R: relay neuron in the spinal cord. S: effector muscle. The receptor comes first.
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Answer: A — P, R, Q, S
The receptor’s signal goes by the sensory neuron to the relay neuron in the spinal cord, then by the motor neuron to the effector: P, R, Q, S. The order P, Q, R, S wrongly puts the motor neuron before the relay neuron, but the spinal cord must process the signal before it can instruct the muscle.Which part of the human brain mainly maintains posture and balance of the body?
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Answer: D — Cerebellum
The cerebellum in the hindbrain keeps posture and balance and makes voluntary movements precise. The cerebrum is for thinking and voluntary action, while the medulla controls involuntary actions such as blood pressure and vomiting.After a head injury, a person stumbles when walking and cannot touch the nose with a finger smoothly, but speaks, reasons, breathes and keeps a steady heartbeat normally. Damage to which part is the most likely cause?
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Answer: B — Cerebellum, which controls balance and precision
Poor balance and clumsy, imprecise movement with normal speech, reasoning, breathing and heartbeat point to the cerebellum. Medulla damage would disturb breathing and heartbeat, cerebrum damage would affect reasoning and speech, and spinal cord damage would cause weakness or loss of sensation rather than just clumsiness.Which plant hormone, collecting on the shaded side of a shoot tip, makes the cells there grow longer so that the shoot bends towards light?
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Answer: A — Auxin
Auxin is made at the shoot tip and moves to the shaded side, where it promotes cell elongation. Cytokinin promotes cell division and abscisic acid inhibits growth. Thyroxine is an animal hormone, not a plant one.The synthesis of which hormone requires iodine, so that a lack of iodine in the diet can cause goitre?
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Answer: C — Thyroxine
Thyroxine from the thyroid gland contains iodine, and its deficiency causes goitre. Insulin, secreted by the pancreas, is linked to blood sugar and diabetes, and the other two hormones do not need iodine.Which gland and hormone pair is NOT correctly matched?
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Answer: B — Thyroid: insulin
Insulin comes from the pancreas, not the thyroid, which makes thyroxine. The thyroid–insulin pair is therefore the mismatch. The pituitary does make growth hormone, the adrenal glands adrenaline and the pancreas insulin.Consider these statements. P: Deficiency of insulin leads to diabetes. Q: Adrenaline slows down the heartbeat. R: Deficiency of growth hormone in childhood causes dwarfism. S: Thyroxine regulates the metabolism of carbohydrates, proteins and fats. Which statements are correct?
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Answer: C — P, R and S only
P, R and S are correct. Q is wrong because adrenaline speeds up the heartbeat and breathing in an emergency. The pair P and R alone is incomplete, since S is also a true statement.A person suddenly frightened by a loud noise finds the heart beating faster and breathing quickening, with more blood sent to the skeletal muscles. Which gland releases the hormone mainly responsible?
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Answer: D — Adrenal gland, releasing adrenaline
Adrenaline from the adrenal glands prepares the body for “fight or flight”: faster heartbeat and breathing and more blood to muscles. Thyroxine controls metabolism over a long time, not an immediate emergency, and insulin and growth hormone have no such effect.Which organism is correctly paired with its method of asexual reproduction?
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Answer: A — Hydra: budding
Hydra forms buds that grow into small hydras and separate. Amoeba divides by binary fission and Plasmodium by multiple fission, so those two methods are swapped in the options, and Rhizopus reproduces by spores.A culture begins with 2 Amoebae. Each cell divides once every hour by binary fission, and no cell dies. How many Amoebae are present after 3 hours?
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Answer: B — 16
The number doubles each hour: 2 × 2 × 2 × 2 = 2 × 2³ = 16. The value 8 is 2³, which forgets the two cells that were there at the start, and 12 comes from adding 2 three times instead of doubling.After fertilisation in a flowering plant, what do the ovule and the ovary develop into respectively?
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Answer: D — Seed and fruit
The ovule becomes the seed and the ovary becomes the fruit. The pair that lists fruit first reverses the two. The zygote is the cell formed by fertilisation inside the ovule, not a final product of the ovule.Consider these statements. P: Pollination and fertilisation are the same event. Q: The pollen tube grows through the style and carries the male gamete to the ovule. R: Hibiscus has bisexual flowers. S: The ovary develops into the fruit. Which statements are correct?
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Answer: A — Q, R and S only
Q, R and S are all correct. P is wrong: pollination is the transfer of pollen to the stigma, while fertilisation is the later fusion of gametes in the ovule. The option Q and S only leaves out R, which is also true, since Hibiscus has both stamens and pistil.In a human female, where does fertilisation normally take place and where does the embryo afterwards become implanted?
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Answer: C — Fertilisation in the oviduct, implantation in the uterus
The egg meets the sperm in the oviduct, and the zygote divides while travelling to the uterus, where its thick, blood-rich lining receives the embryo. The uterus is not the site of fertilisation, which is the commonest swap, and the ovary only produces eggs.Which statement about human reproduction and contraception is NOT correct?
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Answer: D — Oral contraceptive pills work by physically blocking the oviduct
Oral pills change the hormonal balance so that no egg is released; they do not block the oviduct, which is what tubectomy does. The other three statements are correct: cooler testes, copper-T preventing implantation, and vasectomy blocking the vas deferens.Which one of these methods, besides preventing pregnancy, also gives protection against sexually transmitted infections?
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Answer: B — Condom, a barrier method
A condom physically stops contact with body fluids, so it reduces the risk of infections as well as pregnancy. Copper-T, tubectomy and hormonal pills prevent pregnancy but leave the person exposed to infection.In the menstrual cycle, what happens to the thickened lining of the uterus if the egg is not fertilised?
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Answer: A — It breaks down and leaves the body as blood and mucus
If there is no fertilisation, the lining that was prepared for the embryo is no longer needed and breaks down, passing out as menstruation. The placenta forms only when an embryo is implanted, so it is not a possible outcome here.