1.1 The nervous system and hormones enable us to
respond to external changes. They also help us to
control conditions inside our bodies.
1.1.1 Cells called receptors detect stimuli (changes in the
environment). Receptors and the stimuli they detect
include: receptors in the eyes that are sensitive to
light; receptors in the ears that are sensitive to sound;
receptors in the ears that are sensitive to changes in
position and enable us to keep our balance; receptors
on the tongue and in the nose that are sensitive to
chemicals and enable us to taste and to smell;
receptors in the skin that are sensitive to touch,
pressure, pain and to temperature changes.
184.108.40.206 Light receptor cells, like most animal cells, have a nucleus, cytoplasm and cell membrane.
220.127.116.11.1 Information from receptors passes along cells (neurones) in nerves to the
brain. The brain coordinates the response. Reflex actions are automatic
and rapid. They often involve sensory, relay and motor neurones.
18.104.22.168.1.1 In a simple reflex action: impulses from a receptor pass along a sensory neurone to the central
nervous system; at a junction (synapse) between a sensory neurone and a relay neurone in the
central nervous system, a chemical is released that causes an impulse to be sent along a relay
neurone; a chemical is then released at the synapse between a relay neurone and motor neurone
in the central nervous system, causing impulses to be sent along a motor neurone to the organ
(the effector) that brings about the response; the effector is either a muscle or a gland, a muscle
responds by contracting and a gland responds by releasing (secreting) chemical substances.
2 B1.2.2 Control In The Human Body
2.1 Internal conditions that are controlled include: the
water content of the body – water leaves the body via
the lungs when we breathe out and via the skin when
we sweat to cool us down, and excess water is lost
via the kidneys in the urine; the ion content of the
body – ions are lost via the skin when we sweat and
excess ions are lost via the kidneys in the urine;
temperature – to maintain the temperature at which
enzymes work best blood sugar levels – to provide
the cells with a constant supply of energy.
2.1.1 Many processes within
the body are coordinated
by chemical substances
Hormones are secreted
by glands and are usually
transported to their target
organs by the
22.214.171.124 Hormones regulate the functions of
many organs and cells. For example,
the monthly release of an egg from a
woman's ovaries and the changes in the
thickness of the lining of her womb are
controlled by hormones secreted by the
pituitary gland and by the ovaries.
126.96.36.199.1 Several hormones are involved in the menstrual cycle of a
woman. Hormones are involved in promoting the release of an
egg: follicle stimulating hormone (FSH) is secreted by the
pituitary gland and causes eggs to mature in the ovaries and
stimulates the ovaries to produce hormones including
oestrogen; luteinising hormone (LH) stimulates the release of
eggs from the ovary; oestrogen is secreted by the ovaries
and inhibits the further production of FSH.
188.8.131.52.1.1 Hormones are used to control fertility. Oral contraceptives inhibit FSH
production so that no eggs mature. They may contain oestrogen and
progesteroneThe first birth-control pills contained large amounts of oestrogen
which resulted in significant side effects so birth-control pills now contain a
much lower dose of oestrogen, or are progesterone only. A 'fertility drug'
containing FSH and LH is given to a woman whose own level of FSH is too low
to stimulate eggs to mature, for example in In Vitro Fertilisation (IVF) treatment.
IVF involves giving a mother FSH and LH to stimulate the maturation of several
eggs. The eggs are collected and fertilised. At the stage when they are tiny balls
of cells, one or two embryos are inserted into the womb.
3 B1.2.3 Control In Plants
3.1 Plants are sensitive to light, moisture and gravity: their shoots grow towards light and against the force of
gravity their roots grow towards moisture and in the direction of the force of gravity.
3.1.1 Plants produce hormones to coordinate and
control growth. Auxin controls phototropism
and gravitropism (geotropism).
184.108.40.206 The responses of plant roots and
shoots to light, gravity and moisture
are the result of unequal distribution
of hormones, causing unequal
220.127.116.11.1 Plant growth
hormones are used
in agriculture and
horticulture as weed
killers and as rooting