F215:04:02 How plant respond to the environment Flashcards
Why are plants cell often limited in ability to divide and expand (unlike animal cells)?
As they have cells walls
It means they can only expand where there are meristems
What is are meristems?
immature cells that are still capable of dividing
What are the 4 types of meristem cells?
Apical
Lateral Bud
Lateral Intercalary
Where are apical meristems found?
at the tips or spices of roots and shoots and are responsible for the roots and shoots getting longer
What are apical meristems responsible for?
the roots and shoots getting longer
Where are lateral bud meristems found?
found in the buds
allow side buds to grow
What are lateral bud meristems responsible for?
The growth of side buds
Where are lateral meristems found?
in a cylinder near the outside of roots and shoots
make them wider
What are lateral meristems responsible for?
making roots and shoots longer
Where are intercalary meristems found?
between the nodes (where the leaves and buds branch off the stem)
Growth between the nodes is responsible for the shoots getting longer
What are intercalary meritstems responsible for?
the shoots getting longer
In a shoot, where does cell division and cell elongation occur?
division: closest to the apex
elongation: just behind the apex
Where are auxins produced?
At the tip of the apex
What is the role of auxins?
They cause cells to elongate, and therefore the shoots to grow
How do auxins travel?
Either by diffusion or active transport
What is the extent of which the cells elongate proportional to?
The concentration of auxins
How do auxins encourage cell elongation?
They increase the stretchiness of the cell wall by promoting the active transport of hydrogen ions, by an ATPase enzyme on the plasma membrane into the cell wall
The resulting low pH provides optimum conditions for expansins to work
What are expansins?
wall loosening enzymes
What do expansins do?
Break bonds within the cellulose so the walls become less rigid and can expand as the cell takes in water
What happens in a phototropic response?
a shoot bends towards the light source
How do shoots bend towards a light source?
the cells on the shaded side elongate faster than those on the illuminated side
As light causes auxins to be transported to the shaded side where they promote elongation
What is thought to cause a redistribution of auxin?
Althought there is still some uncertainty, two enzymes have been identified (phototropin 1 and 2) whos activity is promoted by blue light, so there is a lot of phototropin 1 activity on a light side and progressively less heading to the dark side
This gradient is thought to be the cause
What do cytokinins do?
they stop the leaves of deciduous trees from senescing
What does senescing mean?
ageing
turning brown and dying
How do cytokinins stop the leaves of deciduous trees from senescing?
by making sure the leaf acts as a sink for phloem transport
Why is it important that cytokinins make sure leaves act as a sink for phloem transport?
ensures the plant gains a good supply of nutrients
What is abscission?
When leaves start to shred
What happens if cytokinin levels drop?
the supply of nutrients dwindle and senecence begins
This is usually followed by leaves beginning to shed
Despite the fact auxin inhibits abscission by acting on cells in the abscission zone, why is it not entirely effective?
- Leaf senescence causes auxin production at the tip of the leaf to drop
- This causes cells in the abscission zone more sensitive to another growth substance called ethene
- If auxin concentration drops, causes an increase in ethene production as well
- These increases cause an increase in production of the enzyme cellulase which digests the walls of the cells in the abscission zone and eventually separates the petiole from the stem
Tropic responses are described as growth responses
Do they actually involve growth?
‘growth’ is limited to getting larger, and is irreversible as it is an increase in cell division whereas tropic responses are not limited to just getting larger
Roots are negatively phototropic. However, auxins still control their response and are still thought to travel to the more shaded side of the root.
Suggest how auxin in roots may be responsible for negative phototropisms
Roots grow in response to low levels of auxins, but high levels of auxins inhibit growth
Auxin migrates away from the light side of the root to the dark side of the root but promotes growth in the light side
So the root bends away from the light and into the soil where it is dark
What effect do auxins have on cells?
They cause cell elongation by increasing the stretchiness of the cell wall by promoting H+ active transport by the ATPase enzyme on the plasma membrane into the cell wall
This lowered pH provides optimum conditions for wall loosening enzymes to work which break the bonds with cellulose (and at the same time the increased H+ ions disrupt the hydrogen bonds within cellulose)
So the walls become less rigid and can expand as the cell takes in water
Explain how auxin is involved in shoot growth
Auxins stimulate shoot growth by causing cell elongation
The extent to which cells elongate and the shoot elongates is proportional to the concentration of auxins
Explain how auxin is involved in leaf abscission
Usually auxin inhibits abscission by acting on cells in the abscission zone
However:
- Leaf senescence causes auxin production at the tip of the leaf to drop
- This causes cells in the abscission zone more sensitive to another growth substance called ethene
- If auxin concentration drops, causes an increase in ethene production as well
- These increases cause an increase in production of the enzyme cellulase which digests the walls of the cells in the abscission zone and eventually separates the petiole from the stem
Explain how positive phototropisms in plant shoots and negative phototropism in plant roots would be selected for
Plants whose shoots grow towards the light would have the advantage over others as they can photosynthesise more
Plants whose roots are negatively phototropic have the advantage as they would grow down and gain more water and stabilise the plant more
Allows them to survive longer and more likely to reproduce
After many generations, the whole populations of plants would respond to light through phototropisms
