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
Hormones on leaves falling
- Cytokinins = more nutrients to leaf
Autumn = less cytokinins = leaves die and fall - Auxins = keeps leaves on trees
Autumn = less auxins = leaves drop
What is photomorphogenesis
The control of plant development by levels and type of light
What is phytochrome
A blue green pigment inside plants which exists in two interconvertible forms:
Pr (P660) - biologically inactive and absorbs red light
Pfr (P730) - biologically active and absorbs far red light (unstable)
Conversion of phytochromes
When one form of the pigment absorbs light it is converted reversibly to the other form
When Pr absorbs red light it is converted into Pfr
When Pfr absorbs far red light it is converted back to Pr
Conversion is only quick in high light intensities (seconds)
Phytochrome conversion differences in day and night
During the day levels of Pfr rise as there is more red light than far red light so conversion from Pr to Pfr happens more rapidly in the daytime
During the night levels of Pr rise as red light wavelengths are not available so Pfr slowly converts back to Pr as it is unstable
What is photoperiodism
The reaction of plants to day lengths
How do phytochromes affect flowering
Phytochromes enable plants to respond to environmental cues
In long day plants, Pfr stimulates flowering
In short day plants, Pfr inhibits flowering
Day neutral plants have different flowering triggers
Method overview of the gibberellins practical
- Embryo is soaked in gibberellic acid
- Embryos are placed onto starch agar plates and incubated
- Agar plate is flooded with iodine (will stain blue / black)
- The areas where starch has been digested will not stain
- This means the size of the clear area around the embryo indicates the amount of amylase produced by the seed
What is etiolation
When plants grown in the dark grow rapidly using up food reserves in an attempt to reach the light. The plants therefore end up tall and thin with fragile stems, small internodes and small, pale yellowish leaves as no chlorophyll is formed.
Etoliation seems to be a survival mechanism
What are typical characteristics of etiolation in an emerging seedling
Rapid stem lengthening but little thickening (to make it tall to reach light)
Relatively little root growth (only enough to act as an anchor and obtain water)
No leaf growth (leaves remain furled so no energy is wasted producing leaf tissue that is useless underground)
No chlorophyll (seedling is white or pale yellow so no energy is wasted making chlorophyll that is useless in the dark)
What changes to the plant take place once the tip of a new shoot breaks through the soil surface into the light
Elongation of the stem slows down
Stem straightens
Cotyledons (monocots) / first leaves open
Chlorophyll forms and seedlings begin to photosynthesise
How do phytochromes affect the growth in seedlings
In the seed there is Pr but no Pfr so internodes grow but leaves and chlorophyll don’t. Once plant is exposed to light Pr rapidly converts to Pfr which inhibits lengthening of internodes. Pfr stimulates leaf development and production of chlorophyll.
How does Pfr act as a transcription factor?
It’s involved in switching genes on and off in the nuclei of plant cells
How did scientists investigate if Pfr acts as a transcription factor
They linked genes for production of phytochrome with genes for green fluorescent protein resulting in fluorescent phytochrome
Seedlings in dark = fluorescence linked to Pr was spread evenly through cytoplasm
Seedlings exposed to light = Pr was converted to Pfr and moved into nucleus and appeared as specks linked to chromosomes
What is the current model for Pfr as a transcription factor
When Pr is converted to Pfr in presence of light, it moves into nucleus through pores in nuclear membrane
In nucleus, it binds to a nuclear protein known as PIF3
PIF3 is a transcription factor
PIF3 only binds to Pfr not Pr
PIF3 only activates gene transcription and formation of mRNA when it is bound to Pfr
