Biology B3 Topic 1 Notes

B3.1 -- Rhythms:

• Biological rhythms which occur over the course of a single day are called "circadian rhythms". The human brain contains its own timing mechanism -- the "biological clock" -- which controls the release of hormones at different times in the day. For example, at about 10pm each night we secrete increased amounts of a hormone called melatonin, making us feel more sleepy; levels of melatonin then drop in the daytime. As well as this, our biological clock can be affected by travelling east or west around the globe, making us less alert and less able to think clearly; this is the sensation often known as "jet lag".
• Many plants have circadian rhythms; many flowers, for example, open in the day and close at night, making sure that they are ready for pollination in the daytime and that the pollen is protected at night. In addition, the flowers only produce nectar in the daytime, when insects are likely to visit the flower. Many planets also have yearly rhythms; this is called "photoperiodism" as the plants respond to changes in day length. Many plant species die in winter, but their seeds survive to germinate in the spring; others survive year-round but grow faster when the days lengthen and grow slower or stop growing altogether when the days shorten. Flowing is also controlled by the length of the day or night over a 24-hour period in many plants.

B3.2 -- Plant defences:

• Young lupin leaves produce poisonous chemicals called alkaloids; the production of these alkaloids follows a circadian rhythm with production starting in the early morning and ceasing at night. These chemicals make the plant leaves poisonous to pests or to larger herbivores that may want to eat it; the energy and time spent produces these chemicals is thus worth it in the long run.
• Many plants also produce chemicals against pathogens; some potatoes, for example, produce chemicals against potato bight, and scientists are currently trying to produce high-yielding varieties of potato which readily produce these chemicals. Pathogens in a crop are a big concern for humans as they can destroy food supplies and thus cause famine (for example, 1845-46 in Ireland).
• In small amounts some poisonous chemicals in plants can have beneficial effects for humans. Digoxin, produced by foxgloves, can be used to improve how the heart beats in low doses; quinine, produced by the cinchona tree, can treat malaria; and aspirin, produced by the willow, can treat pain or fever. Research also suggests that alkaloids produced by potatoes could be useful in the future as treatments for cancer.
• Louis Pasteur (1822 - 1895) showed that microorganisms were responsible for food going off and for some diseases; he proposed many aspetic techniques for keeping people and food away from these microorganisms, including one now named after him: pasteurisation, where food is heated briefly before being stored.

B3.3 -- What affects the rate that microorganisms grow? (N/A: Practical)

B3.4 -- Vaccines:

• Smallpox was common in the 1700s, killing or disfiguring large…