Topic 9 Control systems summary notes (A Level Biology Edexcel B)
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Course
A Level Biology Edexcel B
Institution
PEARSON (PEARSON)
Book
Edexcel A level Biology B Student Book 2 ActiveBook
This document contains summarised notes taken for Topic 9 Control systems, using the Pearson Edexcel B biology activate textbook. Notes taken with referencing to specification.
CHAPTER 9.1 – CHEMICAL CONTROL IN MAMMALS AND PLANTS
1 – PRINCIPLES OF HOMEOSTASIS
To control processes successfully, a system of communication and regulation within the
organism is needed.
Both animals and plants rely on communication systems based on chemical signals and
animals also have a rapid response system based on electrical signals
HOMEOSTASIS - is the maintenance of a state of dynamic equilibrium
~ The internal conditions of the body must be controlled within a narrow range
~ This is dynamic equilibrium and involves matching the supply of O 2 and glucose to
the continually changing demands of the body, while removing CO2 and maintaining
an even temp and pH
DEFINITION – the maintenance of a state of dynamic equilibrium through the responses of
the body to external and internal stimuli.
MAIN HOMEOSTATIC MECHANISMS (in mammals) - systems that respond to changes in
both external and internal conditions and control the pH, temp and water potential of the
body within narrow limits
1. pH LEVELS
~ Must be maintained so that the structures of proteins remain stable
~ This allows enzymes to function at their optimum activity and the structure of
cell membranes to be maintained.
~ pH of the blood and body fluids is controlled by the transport of CO2 away from
the tissues of the body.
2. TEMPERATURE
~ Needs to be stable to maintain the optimum activity of the enzymes that
control the rate of cellular reactions.
~ A stable temperature also maintains the integrity of the membranes, so they
can control the movement of substances into and out of the cells.
~ In humans, a stable temp is 37degrees
3. WATER POTENTIAL OF THE BODY FLUIDS
~ Must remain within narrow limits to avoid osmotic effects that could damage
or destroy the cells.
NEGATIVE FEEDBACK SYSTEMS
~ Most
~ They provide a way of maintaining a condition, such as the concentration of a
substance , within a narrow range.
~ A change in conditions is registered by receptors and as a result effectors are
stimulated to restore the equilibrium.
~ If conc goes up, effectors bring it down again, vice versa.
, ~ e.g. production of hormones, temp regulation
POSITIVE FEEDBACK SYSTEM
~ Less common
~ Effectors work to increase the effect that has triggered the response
~ E.g. contractions of uterus during labour
~ The pressure of the baby’s head on the cervix causes the release of chemicals that
increase the contraction of the uterus, so the head is then pushed down even harder
2 – HORMONE PRODUCTION IN MAMMALS
~ Chemical messages can have an effect over a long period of time
~ They can reach the entire body as they are carried to their target organs in the
transport system of an animal or plant.
~ In plants, chemical control is the main system of coordination.
~ In animals, it complements and interacts with the nervous system.
CHEMICAL CONTROL IN MAMMALS
~ One of the main forms of chemical control in animals is brought about through
hormones
HORMONES – organic chemicals produced in endocrine glands and released into the blood
~ They are carried through the transport system to parts of the body where they bring
about changes, which may be widespread or highly targeted.
~ Usually proteins, peptides, or steroids
~ Once a hormone enters the blood stream, it’s carried around in the blood until it
reaches the target organ or organs.
~ The cells of the target organs have specific receptor molecules on the surface of their
membranes that bind to the hormone molecule.
~ This brings about a change in the membrane and circuits a response
WHERE ARE THE ENDOCRINE GLANDS?
~ Found around the body, often associated with other organ systems
~ Several of the glands have more than one function
e.g. Ovaries produce ova as well as hormones
Pancreas is both an exocrine gland producing digestive enzymes and hormone insulin
~ The glands all have a rich blood supply, with plenty of capillaries within the glandular
tissue itself.
3 – MODES OF HORMONE ACTION
HORMONE RELEASE SYSTEMS
~ In the endocrine system some hormones are released as a result of direct
stimulation of the endocrine gland by nerves.
~ If the gland is stimulated, hormone is released
~ If not stimulated, no hormone is released
~ The level of stimulation determines the level of response
, ~ Many hormones are released from endocrine glands in response to another
hormone or chemical in the blood.
~ When hormones are released in response to a chemical stimulus such as another
hormone or glucose, secretion is controlled by a negative feedback loop.
~ The presence of the appropriate chemical in the blood stimulates the release of the
hormone.
~ As a result of the rise in the hormone levels, the amount of stimulating chemical in
the blood drops.
~ The endocrine gland receives less stimulation and so the hormone levels drop.
~ This gives a very sensitive level of control that can be adjusted constantly to the
needs of the body.
TWO MAIN MODES OF ACTION IN HORMONES
~ Hormones attach to receptor sites and trigger the release of a second messenger
that activates specific enzymes in the cell, including adrenaline.
~ Hormones enter cells and bind directly to transcription factors, including oestrogen.
RELEASE OF A SECOND MESSENGER
~ Some hormones are not lipid soluble and cannot cross the cell membrane.
1. The hormone molecule binds to a receptor in the cell membrane.
2. This triggers a series of membrane-bound reactions that result in the formation of a
second chemical messenger inside the cell.
3. This second messenger then activates a number of different enzymes within the cell,
altering the metabolism.
4. The most common second messenger is CYCLIC AMP (cAMP), which is formed from
ATP.
5. Cyclic AMP triggers a number of responses in the cell, including increased cellular
respiration, increased contraction of muscles cells, relaxation of smooth muscle in
blood vessels…
6. This is how adrenaline, the hormone associated with the ‘fight or flight’ response,
has its effects.
THE HORMONE ENTERS THE CELL
1. A lipid soluble hormone passes through the membrane and acts as the internal
message itself.
2. Inside the cell the hormone binds to a receptor and the hormone-receptor complex
passes through the pores of the nuclear membrane into the nucleus.
3. The hormone bound to the receptor acts as a transcription factor, regulating gene
expression and switching sections of the DNA on or off.
4. This is the mode of action of the lipid-soluble steroid hormones such as oestrogen
and testosterone.
4 – CHEMICAL CONTROL SYSTEMS IN PLANTS
Chemical control in plants is brought about by plant growth substances such as auxins,
cytokinins and gibberellins.
, AUXINS
e.g. indoleacetic acid (IAA) – powerful growth stimulants that are effective in very low
concentrations.
~ Auxins are produced in young shoots and always move down the plant from the
shoots to the roots.
~ This movement involves some active transport and calcium ions.
~ Auxins are involved in apical dominance, where they supress the growth of lateral
shoots so that one main stem grows fastest.
~ In low concentrations, they promote root growth.
~ The more auxin that is transported down the stem, the more root growth occurs.
~ If the tips of the stems are removed, removing the source of auxins, the stimulation
of root growth is removed and root growth slows and stops.
~ Auxins are also involved in the tropic responses of plant shoots to unilateral light.
~ The response of a plant to auxins often depends on the concentration of the
hormone and the region of the plant.
~ Auxins affect the ability of the plant cell walls to stretch
~ E.g. IAA is made in the tip of the shoot and diffuses back towards the zone of
elongation.
1. The molecules of IAA bind to specific receptor sites on the cell surface membranes,
activating the active pumping of hydrogen ions into the cell wall spaces.
2. This changes the hydrogen ion concentration, providing the optimum pH of around 5
for enzymes that break bonds between adjacent cellulose microfibrils.
3. This allows the microfibrils to slide past each other very easily, keeping the walls very
plastic and flexible.
4. The cells absorb water by osmosis and as a result of turgor pressure, the very flexible
cell walls stretch allowing the cells to elongate and expand.
5. Eventually, as the cells mature, the IAA is destroyed by enzymes, the pH of the cell
walls rises, the enzyme is inhibited and bonds form between the cellulose
microfibrils.
6. As a result, the cell wall becomes more rigid, and the cell can no longer expand.
~ This model of the way plants grow was based on shoots kept entirely in the dark.
~ In real life plants are usually in a situation where the light on one side is stronger
than on the other.
~ The side of a shoot exposed to light contains less auxin than the side that is not
illuminated.
~ Light seems to cause the auxin to move laterally across the shoot, producing a
greater concentration on the unilluminated side.
~ This movement means the shoot tip acts as a photoreceptor.
~ More of the hormone diffuses down to the region of cell elongation on the dark side,
resulting in the shoot bending towards the light.
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