Biology-Coordination and Control

Responding to change

• The nervous system allows humans to react to their surroundings and coordinates behaviour
• Receptors detect external stimuli
• Electrical impulses pass to the brain along neurons
• The brain coordinates responses
• The receptors are found in sense organs, the eye, ear, nose, tounge and skin
• Light stimulates receptors in the eye and electrical impulses then pass to the brain along neurons (nerve cells). Other stimuli inculde sound, chemicals, temperaure changes, touch and pain
• The brain and spinal cord form the central nervous system (CNS)
• Nerves contain neurons. Sensory neurons carry impulses from receptors to the CNS
• Motor neurons carry impulses from the CNS to effector organs which may be muscles or glands. The muscles respond by contracting. The glans respond by secreting (relasing) chemicals

Reflex Actions

• Reflex actions are rapid, automatic responses to a stimulus
• Reflexes protect us from damage
• The main steps involved in reflex actions (reflexes) are:
• A receptor detects a stimulus (e.g. a sharp pain)
• A sesnsory neuron transmit the impulse to the CNS
• A relay neuron passes the impulse on
• A motor neuron is stimulated
• The impulse passes to an effector (muscle or gland)
• Action is taken (the response)
• At the junction between two neurones is a synapse. Chemicals transmit the impulse across the gap
• The sequence from receptor to effector is a reflex arc

Hormones and the Menstrual Cycle

• The menstrual cycle takes 28 days, with ovulation about 14 days into the cycle
• The cycle is controoled by three hormones
• Follicle stimulating hormone (FSH) is made by the pitruitary gland and causes the egg to mature and oestrogen to be produced
• Oestrogen is produced by the ovaries and inhibits (stops) the futher production of FSH. It stimulates the production of LH and also stimulates the womb lining to develop to reveive the fertilised egg
• Luteninising hormone (LH) is made by the pituitary gland and stimulates the mature egg to be released from the ovary (ovulation)

The artificial control of fertility

• Contraception pills contain oestrogen and/ or progesterone to inhibit FSH
• FSH can be given to a woman to help her to produce eggs
• The contraceptive pill (oral contraceptive) may contain oestrogen and progesterone. Some pills are progesterone- only pills and have fewer side effects
• The contraceptive pill prevents the production mature eggs then FSH and LH can be given. This is known as "fertility treatment"
• The FSH causes eggs to mature. The luteinising hormone (LH) stimulates ovulation
• Advantages:
• Contraceptive pills have helped to reduce family size which has reduced poverty in some areas. It allows women to plan their pregnancies
• Fertility drugs can help infertile couples who are having IVF. IVF helps couples to have a baby
• Disadvantages: 
• The contraceptive pill can cause side effects. Some peopel object to its use for ethical or religious reasons

• IVF is an expensive process. Some people think it is unethical when older women have babies by IVF. Extra embryos produced may be stored or destroyed

Controlling conditions

• It is very important that the internal conditions of the body are kept within certain limits. The nervous system and hormones help to do this
• Water and ion content, as well as temperature and blood sugar level, are all carefully controlled
• The body carefully controls its internal environment. Internal conditions that are controlled include: 
• Water content
• Ion content
• Temperature
• Blood sugar level
• Water is leaving the body all the time as we breathe out and sweat. We lose any excess water in the urine (produced by the kidneys). We also lose ions in our sweat and in the urine
• We must keep our temperature constant, otherwise the enzymes in the body will not work properly ( or may not work at all)

Hormones and the control of plant growth

• Palnts respond to light, gravity and moisture
• Shoots grow towards light and away from gravity, this response is photoropism
• Roots grow towards gravity and water, this response is gravitropsim
• Hormones such as auxin cause these changes
• Plant hormones have uses in agriculture and horiculture
• Auxin is the hormone which controls phototropism and gravitropsim
• Unequal distribution os auxin causes unequal growth. This results in bending of the shoot or root
• Plant growth hormones can be used as weed killers and to stimulate root growth

Using hormones

• It is important to evaluate the advantages and disadvantages of using hormones to control fertility
• The incorrect use of plant hormones can damage the environment
• In women:
• Many women use the contraceptive pill to prevent unwanted pregnancies. It helps them to plan when they have a baby
• Other women use hormones to help them become pregnant
• Sometimes hormones treatment is used to help older women to have babies.This may involve using an egg donor who is given hormones to produce extra eggs
• Taking hormones for a long time can lead to side effects in some women
• In plants:
• Plant hormones can be used by farmers and gardeners
• Weedkillers are used to kill unwanted plants on lawns
• When cuttings are taken from plants, hormones are used to encourage roots to grow before the cutting is planted
• Some hormones are used to encourage fruit to ripen