Control Systems Flashcards
What is homeostasis?
The maintenance of a state of equilibrium through the responses of the body to external and internal stimuli
How does homeostasis work with the nervous system?
Changes in the body are detected by sensors (receptors) which send messages to receptors to either cause positive or negative feedback responding to the change.
What is negative feedback?
A way of maintaining a condition within a narrow range. Restoring the equilibrium.
What is positive feedback?
Where effectors work to increase the effect that has triggered the response e.g. contraction of the uterus with oxytocin.
Where are hormones produced
Endocrine glands
What controls the pituitary gland
The hypothalamus
What are neurosecretory cells?
Nerve cells in the hypothalamus that produce secretions from the end of axons
What are the 2 parts of the pituitary gland?
Anterior and posterior
What hormones are produced by the anterior pituitary?
Thyroid stimulating hormone
Growth hormone (GH)
Adrenocorticotrophic hormone (ACTH)
Follicle stimulating hormone (FSH)
Luteinising hormone (LH)
Prolactin
What hormones are produced by the posterior pituitary?
Oxytocin
ADH (anti diuretic hormone)
What are the 2 modes of hormone action?
Release of a second messenger
The hormone enters the cell
Process of releasing a second messenger
Hormone e.g. adrenaline binds to receptor on the target cell membrane as they are not lipid soluble so can’t cross the membrane
This triggers a series of intracellular membrane bound reactions e.g. enzyme (adenyl cyclase) which converts ATP to cAMP
This stimulates the release of a second messenger e.g. cAMP
This second messenger (cAMP) activates enzymes to alter the metabolism of the cell e.g. increased respiration and muscle contraction
Process of hormone entering a cell e.g. transcription factor
Hormone e.g. oestrogen passes through membrane and binds to receptor inside the cell
Form a hormone receptor complex which passes into the nucleus and acts as a transcription factor to regulate gene expression
Binds to promoter region in DNA before the target gene
What is a tropism
Growth of a plant (towards or away) in response to a directional stimulus
What are the 3 main types of tropisms
Phototropism - light
Gravitropism - gravity
Hydrotropism - water
How do plants grow?
Cell division occurs in meristem tissues (roots and shoots) which is influenced by plant hormones e.g. auxins, cytokinins or gibberellins
Auxins
Auxins (IAA) are produced in young shoots
They move down to the roots
Movement involves active transport and calcium ions
What are auxins involved in?
Apical dominance
Promoting root growth
Tropic response of plant shoots to unilateral light
How do auxins work?
Cells in shoot tip produce IAA
IAA diffuses back down to the zone of elongation
IAA binds to specific receptor sites on cell membrane which activates the pumping of H+ ions into the cell wall spaces
This changes pH to 5 which is optimum for the enzymes which break bonds between cellulose myofibrils
This means cell can absorb more water by osmosis causing the cell wall to stretch which means the cell can elongate and expand
Why do shoots grow towards the light?
When a shoot is exposed to light the auxins diffuse to the shaded side
Greater concentration of auxins in the zone of elongation on the shaded side
This stimulates these cells to grow so the shoot grows towards the light
Once the shoot is towards the light the transport becomes asymmetric (same on both sides) so the shoot continues to grow towards the light
Some functions of gibberellins
Growth regulators
Stimulate elongation of growing cells
Promote growth of fruit
Break dormancy of seeds
Stimulate formation of enzymes in germination
Stimulate bolting (period of sudden growth and flowering)
Role of gibberellins in seed germination
Seed absorbs water and swells (embryo activated)
Embryo secretes gibberellins that diffuses to aleurone layer
Gibberellin stimulates aleurone layer to produce amylase that diffuses into endosperm and breaks down food stores to provide embryo with materials for respiration and growth
Enzymes produced in response to gibberellins digest endosperm.
Products released from endosperm are used by embryo to make new cells and germinate
Functions of cytokinins
Promote cell division in the apical meristem and cambium
Promote lateral bud development
Work with ethene in abscission (natural removal of) of leaves, flowers and fruits
Auxins on apical dominance
- More auxin = more abscisic acid - stops lateral bud growth
- More auxin = inhibits cytokinins - reduces bud growth