Transport in plants Flashcards
(48 cards)
1
Q
why do plants need transport systems?
A
- they are large so have a small SA:V ratio - can’t rely on diffusion alone for transport of molecules up and down the plant
- high metabolic demands in parts of plant that don’t photosynthesise eg. mineral ions absorbed in the root needed in all cells
2
Q
dicotyledenous plant
A
- two cotyledons
3
Q
structure of phloem
A
- sieve tube elements - most of living organelles lost to make room for phloem sap - separated by sieve plates - allow contents to pass between cells
- companion cell (have nucleus and organelles) attached to each sieve tube element by channels called plasmodestmata - ATP and proteins can move into sieve tube element cells
- contain fibres (long and narrow) and sclereids which have thickened cell walls
4
Q
structure of xylem tissue
A
- made of dead cells - living contents die and end walls between cells fuse together
- walls made of lignin arranged in spirals or rings - impermeable to prevent substances passing through walls, help maintain structure and allow flexibility under transpiration pull
- regions of cell wall free of lignin - pits allow water and dissolved substances to pass between vessels
5
Q
functions of thick-walled xylem parenchyma
A
- stores food
- contains tannin deposits (bitter tasting chemical that protects plants against herbivores)
6
Q
xylem function
A
- carries water and mineral ions from the roots of the plant through the stem to the leaves
- support
7
Q
phloem function
A
transports organic molecules such as sugars produced by photosynthesis in the leaves - up or down the plant
8
Q
vascular bundle in the root
A
- vascular bundle in centre - xylem in middle (cross shape) - strong - prevents plant being pulled out of soil
- phloem surrounding xylem
- epidermis on outside of root
- cortex - thick layer of cells containing parenchyma cells
9
Q
vascular bundle in stem
A
- vascular bundles arranged in a ring on edge of stem - helps withstand bending from wind
- centre of stem contains parenchyma cells
- around edge is epidermis and cortex
- phloem on outside, xylem on inside
10
Q
vascular bundle in leaf
A
- xylem at upper part, phloem at lower part
- photosynthesis takes place in palisade mesophyll - upper part of leaf
- the midrib of a leaf carries vascular tissue and supports structure
11
Q
adaptations of root hair cells
A
- large SA:V ratio - each has many microscopic hairs
- surface consists of only cell wall and cell membrane - thin
- soil contains solutes - maintains water conc. gradient for osmosis
- very small to penetrate between soil particles
12
Q
how does water move from soil to root hair cells to xylem?
A
- soil has higher water pot. than cytoplasm of root hair cell - water moves in by osmosis
- water then moves through root to xylem by symplast or apoplast pathway
13
Q
symplast pathway
A
- water moves from cytoplasm of one cell to cytoplasm of another through plasmodesmata linking cells
- driven by water potential gradient between root hair cells and the next cell - water continually moving into root hair cells so high water pot. - osmosis
- slow - obstructed by organelles
14
Q
apoplast pathway
A
- water moves through cell walls and spaces between cells
- cell walls have relatively open structure so water can move easily between cellulose fibres
- as water moves into xylem, more water is pulled along through cell walls due to cohesion
- continuous flow through cell wall - less resistance than symplast
15
Q
casparian strip
A
- made of waterproof, waxy material - suberin
- surrounds each endodermal cell
- water can go no further through apoplast pathway
- instead water moves through selectively permeable cell membrane and into cytoplasm (symplast), excludes any potentially toxic solutes in soil
16
Q
what is root pressure?
A
- solute conc in endodermal cells is low compared to xylem, they then increase the conc further by actively transporting mineral ions into the xylem
- creates water pot. grad. so water moves into xylem by osmosis through symplast pathway
- the bottom of xylem has a higher hydrostatic pressure than the top of xylem, bc water is lost by transpiration, causing water to move up the xylem
- once in xylem, water returns to apoplast pathway to move up the plant
- only gives water a small push upwards
17
Q
evidence of root pressure
A
- inhibit mitochondria and therefore respiration using cyanide - root pressure stops
- exclude oxygen and therefore stop aerobic respiration - root pressure stops
- root pressure increases with rise in temperature
- levels of oxygen or respiratory substrate falls - root pressure falls
18
Q
function of waxy cuticle
A
reduce water loss from surface of leaf by evaporation
19
Q
how does CO2 get into leaf?
A
diffuses from external air through stomata
20
Q
transpiration stream
A
- movement of water into the roots by osmosis and transported up xylem in apoplast pathway
- at the leaves it moves by osmosis through membranes and by diffusion from xylem to the cells where is evaporates from the mesophyll cell walls into the air spaces - lowers water pot., so water into cell by osmosis along apoplast and symplast pathways
- water vapour moves into external air through stomata down a conc grad
21
Q
adhesion
A
water forms hydrogen bonds with carbohydrates in xylem vessel walls
22
Q
capillary action
A
- water can move up xylem against the force of gravity by cohesion and adhesion
23
Q
transpiration pull
A
- when water is removed from top of xylem vessels due to transpiration, more water moves up xylem vessels by capillary action to take its place
24
Q
cohesion-tension theory
A
- whole process of how water moves into roots, up stem, out of leaves
- root pressure, tension, adhesion, cohesion, capillary action, transpiration pull
25
evidence for cohesion-tension theory
- if plant stem is cut, air sucked into xylem - under tension
- diameter of tree trunk reduces when transpiration is at maximum - transpiration pull creates tension in xylem
26
how do you calculate rate of water uptake from a potometer?
- measure how far air bubble move in certain time
- mm/min
27
weaknesses of potometer
- only measures water uptake - not all used it transpiration - some used in photosynthesis
28
precautions of potometer
- air taken up into xylem when stem is cut so cut underwater
- set up potometer under water to prevent air gaps
- smear vaseline around connection between stem and tube
- allow plant to adapt to surroundings for 10 mins before starting experiment
29
mass potometer
- place plant of scales and see how much mass is lost
- prevent water evaporating from soil by covering soil with plastic wrap
- directly measures rate of transpiration, not water uptake
- much less disruptive to plant - don't cut stem
30
how do guard cells open?
- open stomata in light - solutes actively transported into guard cells through trasnport proteins lowering water pot. of interior, water moves in by osmosis causing them to become turgid and open
- close stomata in dark - reduce water loss
- cell wall of inner side is thicker and less flexible than rest of cell - prevents them expanding evenly and creates curved shape allowing stomata to open
31
what happens to guard cells in a drought?
- hormonal signal sent to leaves from roots causing guard cells to lose turgidity and stomata to close
- reduces water loss
32
effect of light intensity on rate of transpiration
- graph is diagonal line up then flat
- light causes stomata to open and allow water vapour to diffuse out of leaf
- at high light intensities rate of transpiration no longer increases because almost all stomata are open
33
effect of humidity on transpiration
high humidity - smaller conc. grad. between inside of leaf and outside, slower transpiration rate
34
effect of temperature on transpiration
- at high temps water molecules have more kinetic energy and diffuse out of leaf faster
- at high temps humidity of outside air decreases - high conc grad
35
effect of air movement on transpiration
- water vapour can build up around external surface of leaf - reduces conc grad and rate of transpiration
- aire movement removes water vapour from external surface of leaf
36
effect of water availability on transpiration
- drought conditions - hormonal response causes stomata to close - reducing transpiration
37
adaptations of xerophytes - spines
- spines replace leaves reducing SA:V ratio and reducing water loss - photosynthesis takes place in stem, trap moist air reducing transpiration eg. cacti
38
adaptations of xerophytes - thick waxy cuticle
- waterproof layer reduces water loss by evaporation
39
adaptations of xerophytes - sunken stomata
- traps layer of moist air around stomata reducing transpiration rate
- cacti only open stomata at night to absorb CO2 when conditions are cool -reduces water loss - and use it in day during photosynthesis
40
adaptations of xerophytes - shallow roots/deep roots
- shallow - absorb water from a rain shower before it evaporates
- deep - access water from lower levels of soil
41
marram grass adaptations
- stomata on inside of leaves trapping water on inside rather than being blown away
- sunken stomata
- fine hairs projecting inwards - moist air is trapped around stomata
- thick waxy cuticle
- long roots to find water deep into sand
- shallow roots to retain water
42
hydrophyte adaptations
- thin or no waxy cuticle - don't need to reduce water loss
- always-open stomata on upper surfaces - maximises gas exchange
- reduced structure - water supports leaves so no need for supporting structures
- wide, flat leaves - captures as much light as possible
- small roots - water diffuses directly into stem so less need for uptake by roots
- large SA of stems and roots underwater - maximises area for oxygen to diffuse in
- air sacs - float
- aerenchyma - specialised parenchyma with air spaces - buoyancy, low resistance pathway for movement of substances
43
why is glucose converted into sucrose in leaves?
less reactive than glucose
44
sources
where assimilates are produced
- green leaves or stems, storage organs, food stores in seeds when they germinate
45
sinks
regions where assimilates are required
- roots (active transport so high respiration rate), meristem actively dividing, storage organs
46
phloem loading
- apoplast route - active process
- sucrose from source travels to companion cell
1. protein on cell membrane of companion cell uses ATP to actively pump H+ out cytoplasm into spaces of cell wall (ATP produced by lots of mitochondria in companion cell)
- this creates conc. grad.
2. H+ co-transports sucrose back into companion cell through co-transport protein
- foldings on cell membrane creates large SA for more proteins involved
3. sucrose diffuses through plasmodesmata from companion cells into sieve tube element cells
4. now low water pot. in sieve tube elements so water moves into them by osmosis from xylem vessels - hydrostatic pressure
5. phloem sap moves up or down towards the sink to area of lower pressure - mass flow
47
phloem unloading
- sucrose diffuses out phloem sieve tube element at sink and is converted into glucose for respiration or is converted into starch or diffuses into another cell to maintain the conc grad
- increases water pot. in sieve tube element - water moves out to surrounding cells by osmosis, some goes back to xylem and joins transpiration stream
48
evidence of translocation
- rate of flow of sucrose in phloem is much faster than could take place by diffusion alone
- if companion cell mitochondria is inhibited by cyanide, translocation stops - requires ATP
- aphids - show there is a pressure from the phloem that forces sap out
