Homeostatis Topic 9 Flashcards
Define homeostasis
The maintenance of a state of dynamic
equilibrium through the responses of the
body to external and internal stimuli
Why is it important to control pH in the body ?
-vital for managing homeostasis
-if pH deviates too far from the optimum pH, enzymes can denature , stopping metabolic reactions
-too high or too low pH can disrupt hydrogen bonds that maintain a proteins tertiary structure , dentature
Why is it important to control water potential (osmoregulation)
Water potential affects osmosis
• If water potential outside cells too high (hypotonic), water enters cells by osmosis, causing lysis
• If water potential outside cells too low (hypertonic), water leaves cells by osmosis ,causing crenation
Why’s it important to control temp in the body
-maintained by homeostasis
-too low or too high temps will effect the KE of mols , effecting the frequency of successful collisions and formation of enzyme sub complexes, effecting reactions rates
-High Temperatures: Excessive heat can cause enzymes to denature, as H bonds mainting their tertiary structure break, loss of function , CAUSING SHAPE OF AS TO CHANGE
Homeostasis involves detection of changes in the body
Effectors reverse these changes thru + or - feedback
how does + feedback work?
x raises to a higher level, receptor detects change, communication of hormones , effector carries out the response to bring about the corrective change, fall to normal lvl
how does - feedback work?
x raises to a lower lvl, receptor detects change, communication of hormones, effector carries out response to bring corrective change , rise to normal lvl
What does the pituitary gland do
Releases hormones that effect many other endocrine glands of the body
Contains anterior and posterior lobes
Why are hormones so specific
They only effect target organs and cells which contain complimentary receptors on their plasma membrane
What is a mode of hormone action
Hormones (e.g. adrenaline the first messenger ) bind to a receptor on the target cell membrane. triggers a series of membrane-bound reactions which stimulates the release of a second messenger e.g. cAMP. 2nd messenger activates specific enzymes to alter the metabolism of the cell e.g. CAMP increases cellular respiration,
What’s a mode of hormone action 2
Hormones (e.g. ostrogen) pass through the cell membrane and bind to a receptor inside the cell.
Forms hormone-receptor complex which passes into the nucleus and acts as a transcription factor to regulate gene expression.
So for the mode of hormone action they can either
Release a second messenger
Or enter the target cell
Why is a second messenger used in one mode of hormone action
Some hormones can’t cross over the membrane as they are water soluble and hydrophilic
How do auxins cause plants to grow towards the light (gcse)
if shoots are grown with light coming from one direction, auxins move to the shaded side of the root, making cells on the shaded side elongate more causing the shoots to grow towards the light.
Auxins are growth stimulants they have several effects including:
- Cell elongation
-suppression of lateral buds
-promoting root growth
-tropic responses to unilateral light
What’s an auxin example
IAA
Describe the structure of a cellulose microfibril
Cellulose contains beta glucose
Glycosidic bonds formed in condensation reactions when joining glucose together
1-4 glycosidic bonds
Microfibrils are composed of many cellulose molecules
Cellulose chains held by hydrogen bonds
How do auxins eg IAA work
IAA made in the tip of shoot and diffuses down towards zone of elongation
IAA binds to receptors on cell membranes and activates the pumping of H+
ions from the cytoplasm into cell wall spaces which lowers the pH of the walls. Low pH makes cell walls flexible - this makes cell walls stretch to accommodate more water- thus enabling the expansion and growth of cells.
When the shoot tip is illuminated on all sides
the IAA auxins are distributed evenly and move down the shoot tip thus causing elongation of cells equally across the zone of elongation.
When the shoot is only illuminated from one side
the IAA auxins move towards the shaded part of the shoot thus causing elongation of the shaded side only , causing which results in bending of the shoot towards the light. The shaded side grows at a faster rate
What is photomorphogenesis
The process by which plant development is controlled by light
Many seeds will only germinate when..
Exposed to light , red light has been shown to be most effective at stimulating germination , far red light inhibits germination,
What’s phytocrome
A plant pigment that exists as 2 interconvertible forms
Pr = biologically inactive form , absorbs red light
Pfr = Biologically active form , Absorbs far red light
When phytocrome absorbs one of the 2 respective types of light
It’s converted into the other form at a rate dependent on light intensity
In long day plants …
In short day plants…
pfr- stimulates flowering
pfr - inhibits flowering
The length of time it takes for one form to be
converted into the other depends on
Light intensity
during day light more phytocrome exists as..
pfr as pr is converted into pfr when it absorbs red light
in long day plants pfr stimulate
flowering, during shorter periods of darkness little pfr is converted into pr so high lvls of pfr remain and stimulate flowering
in short day plants the pfr inhibit
flowering, during long periods of darkness the lvls of pfr fall as its converted into pr so theres no flowering
how do day neutral plants differ
their lvls of pr and pfr are similar , they have other flowering triggers
What’s photomorphogensis
What does etiolated mean
The process by which plant development is controlled by light
A plant grown in the absence of light or in very low light conditions
What are characteristics of etiolated plants
-Long spindly stems because the stems grow longer than usual as the plant stretches to find light
-pale colour because chlorophyll production has been suppressed
Why are the early stages of Plant growth etiolated
Seeds germinate under the ground in thr dark , when the seedling breaks thru the soil etoilation is reversed bc pfr acts as a transcription factor
How do plants recover from, etoilation
De etiolation takes place which occurs when the plants r exposed to light and r starting to grow above ground, pr converted to pfr
Pfr is the biologically active form that initiates the plants recovery
Elongation of stem slows
Pale colour turns green as chlorophyll forms
How does pfr act as transcription factors
Pr is exposed to red light converted into pfr, once activated pfr moves into the nucleus, in the nucl pfr interacts with specific proteins such as phytochrome interacting factors , these pifs are transcription factors that regulate the expression of genes involved in growth and developmeth (pale to geeen leaves )
What do giberellins do
Stimulate elongation of cell internodes
Stimulates growth of fruits
Stimulates germination
How is germination stimulated
-seed absorbs water activating the embryo, which secretes giberellins , gib diffuse to the aleurone layer , aleurone layer produces amylase , amylase diffuses to endosperm layer and breaks starch into glucose
What do cytokinins do
Promote cell division
Promote lateral bud development
Cyotkinins work synergistically with
Ethene in the shedding of leaves
What works antagonistically (against each other)
Giberellins and cytokinins in apical domimance
What’s apical dominance
The main shoots of a plant grow more than lateral buds
IAA in the stem of the plant is involved in phototropism.
(i) Give three similarities between IAA and animal hormones.
both chemicals ;
produced in cells / eq ;
idea that they move away from site of production
(b) IAA is known to bind to transcription factors.
Suggest how IAA can stimulate cells to synthesise proteins.
idea that IAA enters the cell ;
2. reference to moves in nucleus , binds to receptors
3. activates transcription factors
4. reference to switching on gene ;
5. Transcription factor binds to promoter region
6. allows formation of (m)RNA / eq ;
7. idea of translation produces protein ;
how do auxins and cytokinins control the growth of lateral buds
-work antagonistically
-auxins inhibit lateral shoot development
-cytokinins stimulate lateral shoot development
-auxin action is more dominant over cytokinin action
