Topic 9 Flashcards
define transpiration [1]
loss of water/evaporation through stomata/leaves
Describe the processes that allow water to be moved from the plants roots to their leaves [5]
Explain the process of water uptake and transport by plants [8]
- water transported in the xylem
- transpiration is the loss of water vapour from the leaves of plants
- loss of water creates low pressure in leaves/tension
- creates tension/transpiration pull when water is evaporated FROM THE CELL WALLS in the mesophyll
- cohesion between water molecules due to polarity and H bonds allows for them to form 1 continuous column of water
- water transported under tension
- transpiration stream is the flow of water in the xylem from roots to leaves
extra marking points for the second question:
- passive process
- water absorbed in root hair cells
- via osmosis
- solute concentration in cell higher than outside cell
- due to active transport of mineral ions
- xylem wall reinforced with lignin to prevent it collapsing from tension
- water movement in xylem due to TRANSPIRATION PULL
- flow/stream of water FROM ROOTS TO LEAVES
Evaporation in leaves
- light energy absorbed by the leaves is converted into heat
- heat evaporates the water in the spongy mesophyll layer > it leaves the cell through the stomata
- this creates a negative pressure gradient in the leaf
- negative pressure gradient creates tension force on the leaf cell walls -> drawing water from the xylem
- water drawn from xylem due to ADHESION BETWEEN WATER AND LEAF CELL WALLS
How stomata control transpiration
- when plant is water-stressed, dehydrated mesophyll cells release abscisic acid
- this causes the efflux of K+ ions from the guard cells
- the guard cells lose turgor as water potential decreases-> become flaccid
- when flaccid, they occlude the opening of the stomata
Structure of the xylem vessel
- It is made up of hollow dead cells, with no cytoplasm, organelles or nucleus
- the walls are reinforced with lignin -> either spiral lignin or annular, walls have thickened cellulose
- unidirectional movement of water, passive
- cell walls have many pores (pits) allowing the water to be transferred between cells
2 types of xylem vessels
Tracheids:
- tapered ends
- exchange water solely through pits
- slower rate of water transfer
- in all plants
Vessel elements:
- ends are fused to form a continuous tube
- allows for faster water transfer
- certain vascular plants only
Advantages of vascularisation in plants [3]
- provides structural support
- could put leaves higher in the air to get more sunlight
- could more efficiently translocate sugars from leaves to roots for storage
- transport of water supply from roots to other tissues
Order of the layers of the roots from outside to inside
epidermis -> root hair cell -> cortex -> endodermis -> pericycle -> vascular bundle
How are minerals absorbed in plants
- can be absorbed through facilitated diffusion
- diffusion is movement from area of high [] to low []
- absorbed in ionic form
- phosphate/nitrate/potassium
- can also be absorbed via active transport
- as mineral ion [] is higher inside than outside
- active transport requires ATP
- occurs through pump/carrier proteins
- H+ ions pumped via proton pumps from the roots allowing mineral ions to move into the cell
- root hair cells maximise SA:V ratio to maximise transport
- fungal hyphae help absorb mineral ions
2 water pathways in the root
Symplastic:
- through the cytoplasm of cells
- moves continuously -> cells connected via plasmodesmata
Apoplastic:
- through the cell walls
- then enters the cytoplasm of the endodermis -> it cannot cross casparian strip in this pathway otherwise
4 main points for xylem vessel drawings
- continuous tube for vessel elements
- the fused ends should be seen as indents
- pits between the vessels
- spiral lignin
Xerophyte adaptations
- rolled leaves -> reduce the exposure to wind
- thick waxy cuticle on leaves -> prevent loss of water through evapotrans.
- CAM physiology: open stomata at night
- reduced leaves: reduce water loss through evapotrans
- stomata in pits, surrounded by hairs: reduces water loss as water vapour trapped in the hairs
- low growth: less exposed to wind, more likely to be shaded, reduced water loss
Halophyte adaptation
- leaf dropping: concentrating all the salt in a few leaves then dropping them
- filtering water while absorbing it
- secreting salt from leaves
- cellular sequesteration: they can sequester toxic ions or salts in cell wall or vacuoles
- may flower at specific times to minimise salt exposrue
Factors affecting transpiration in plants
Temperature:
- as temperature increases, rate increases
- more water evaporates as there is more heat
LI:
- as LI increases, rate of photosynthesis increases
- stomata open more frequently for gas excahnge, hence water is more easily lost through the stomata
- guard cells open/close stomata to allow for gas exchange
- abscisic acid released to close guard cells
relative humidity:
- as humidity increases, rate decreases
- the [] gradient isn’t as steep
wind:
- as wind speed increases, the rate increases
- the water molecules around the leaf are carried away by the wind -> the [] grdient is maintained
Define translocation
the active movement of organic compounds from sources to sinks
Describe the phloem structure
Sieve element cells:
- no nucleus, few organelles -> so the sap can flow through the vessel
- thickened and rigid cell walls -> withstand high hydrostatic pressure
- sieve end plates, perforated -> allow the sap to flow between cells
Companion cells:
- lots of mitochondria for the active transport
- infolding of the plasma membrane -> facilitate material exchange
- transport protein in PM to move materials in and out of the sieve element cell
The 2 types of vessels are connected via plasmodesmata
symplastic exchange of metabolites
Describe the differences in the root and stem structures for monocot and dicot plants
Monocot:
- root: large stele. the vessels form a radiating circle around the central pith. phloem on the outer edge
- stem: the whole vascular bundles are scattered across the stem. phloem positioned outside
Dicot:
- root: small stele. xylem may form an X with the phloem around it
- stem: pith in the centre, vascular bundle forms a ring around it with the phloem outside. separated by cambium
General structural differences between monocot and dicot
- Seeds: monocot has 1 cotyledon/seed leaf while dicot has 2
- Veins: Monocot has parallel, dicot has net-like
- roots: monocot has fibrous, dicot has tap roots
- flower petals: monocots have petals in threes, dicots have them in fours or fives
How the phloem is loaded apoplastically
- companion cells actively pump H+ ions out of the cell
- H+ ion concentration outside builds up, creating [] gradient
- diffuses back into the companion cell through a co-transport protein requiring sucrose movement
- sucrose loaded into the companion cells
- sucrose transferred from the companion cells to the phloem through symplastic transport (via plasmodesmata)
- the high [] of sugars in the phloem causes water from the xylem to also diffuse into the phloem
Factors affecting translocation rate
- rate of photosynthesis
- rate of cellular respiration (impacted by any factor that may stress the plant)
- rate of transpiration -> impacts the amount of water that enters the phloem
- the diameter of the phloem -> impacts the hydrostatic pressure
Define meristem
tissue in plants that consists of undifferentiated cells capable of indeterminate growth
Types of meristems
- apical meristems: found at the tips of shoots -> enable primary growth (plant lengthening)
- lateral meristems: found at the cambium -> responsible for plant widening (thickening of the bark/bark production)
Apical dominance
- The growth of plants is controlled by the release of hormones from the shoot apex
- Main group of plant hormones involved in root and shoot growth -> indole-3-acetic acid auxins
- auxins released by shoot apical meristem -> promotes cell elongation and division in the shoot apex. also inhibits cell division in the axillary buds -> apical dominance
- ensures that plants energy is used to grow vertically -> reach more sunlight
- once the distance between the terminal bud and axiallry bud is large enough -> the auxins do not inhibit the growth of axillary buds as much
How auxins control the growth of plants
ALWAYS mention auxin efflux pumps:
- set up concentration gradients within the plants -> control the distribution of auxins
- can change position in plant due to membrane fluidity
- hence control the direction in which plant grows by controlling auxin concentrations
- in roots -> auxins inhibit cell division and elongation, in shoots, opposite
In shoots:
- auxins promote cell division
- auxins activate a proton pump which causes the secretion of H+ ions into the plant cell walls
- this lowers the pH and reduces the rigidity of the cell wall by breaking bonds between cellulose fibres
- auxins also upregulate the expression of genes producing expansins -> increase cell wall elasticity
- all this causes the cell to elongate/expand
