adaptations for transport - plants Flashcards
1
Q
define symplast
A
the pathway by which water and dissolved mineral ions diffuse through the cytoplasm and plasmodesmata
2
Q
define apoplast
A
the pathway by which water and dissolved mineral move through the cell walls and inter-cellular spaces by cohesion and adhesion
3
Q
define vacuolar
A
the pathway by which water and dissolved mineral ions move through vacuoles
4
Q
define casparian strip
A
an impermeable barrier in the endodermis formed from suberin which blocks the apoplast pathway
5
Q
define transpiration
A
the evaporation of water inside the leaves, through the stomata to the atmosphere
6
Q
define cohesion-tension
A
the process by which water molecules are moved up through the xylem by cohesion and adhesion
7
Q
define root pressure
A
hydrostatic pressure in the root due to active transport of ions and water following by osmosis which forces water upwards
8
Q
define translocation
A
the transport of soluble organic materials produced by photosynthesis e.g sucrose and amino acids, in the phloem
9
Q
define mass flow
A
passive flow of sugars from area of high concentration in the leaf to areas of low concentration
10
Q
define mesophyte
A
plants that grow in the temperate regions
11
Q
define hydrophyte
A
plants adapted to grow either fully or partially submerged in water
12
Q
define xerophyte
A
plants adapted to grow in very dry environments
13
Q
describe root pressure
A
when water moves from the endodermal cells of the root into the xylem by osmosis - this creates hydrostatic pressure and forces water a small distance up the xylem
14
Q
With reference to the role of the casparian strip, explain how cyanide would result in the reduction in root pressure
A
- casparian strip stops apoplast pathway
- movement of ions into xylem requires active transport
- cyanide is a respiratory inhibitor so prevents cells respiring so stops ATP synthesis
- lowers water potential gradient reduces root pressure
15
Q
why does the plant need to control entry of mineral ions into the xylem?
A
mineral ions may be toxic, so this can be harmful to the plants
16
Q
how does the plant ensure toxic ions cannot enter its cells?
A
there are no carrier proteins specific to these ions on the membranes so these ions cannot enter by facilitated diffusion
17
Q
why do plants need to absorb nitrated from the soil?
A
required for synthesis of amino acids, proteins, DNA, RNA, ATP, nucleotides and nitrogenous bases
18
Q
name some other mineral ions absorbed by plants and state their uses
A
potassium - stomatal opening
phosphate - phospholipids, ATP and nucleotides
19
Q
give TWO features of root hair cells that are adaptations for uptake of water and mineral ions
A
- large surface area
- many mitochondria for active transport
20
Q
what are the 3 transport routes of water across the root?
A
- symplast
- apoplast
- vacuolar
21
Q
what are the xylem and phloem an example of?
A
vascular tissue
22
Q
what is the arrangement of vascular tissue in the leaf, root and stem?
A
LEAF - arranged as a vein or midrib
STEM - vascular bundle arranged around the periphery of the stem
ROOT - vascular tissue arranged in the centre of the root and together with the endodermis and pericycle in the stele ; no vascular bundles in the root
23
Q
what type of movement takes place in the phloem?
A
bi-directional movement
24
Q
describe the process of transpiration
A
- water is absorbed by the root hair cells
- water moves through the root tissue, into the xylem and is transported up the xylem in the plant stem to the leaf
- water is transported by osmosis from the xylem in the leaf to the cells of the spongy mesophyll, where it evaporates from the surface of the cells into the air spaces
- water vapour then diffuses from the air spaces out of the leaf through the stomata down a water potential gradient
25
describe the movement of water through the plant
from the root to the xylem
from xylem to palisade mesophyll cell and spongy mesophyll (by osmosis)
from spongy mesophyll out to air through stomata (by diffusion)
26
compare cohesion and adhesion
COHESION - water molecules are attracted to each other by hydrogen bonds
ADHESION - water molecules are attracted to the hydrophilic lining of the lignified xylem vessel walls
27
how does water travel up the xylem to the leaves of the plant
the cohesion-tension theory
28
what is the cohesion-tension theory?
as water vapour diffuses out of the stomata of the leaf by transpiration, water molecules are drawn up from behind to replace those lost. Water molecules are drawn across the leaf and up the xylem. This is possible because of cohesion between water molecules due to hydrogen bonds, and adhesion between water molecules and the xylem vessel walls. This upward movement of water creates tension on the xylem vessel walls.
29
what are the 2 other processes that help water move up the xylem by a small amount?
describe them
1. CAPILLARITY - forces of adhesion and cohesion allow water molecules to rise up narrow tubes for a short distance (useful in small plants)
2. ROOT PRESSURE - water moves from the endodermal cells of the root and into the xylem by osmosis, this generates hydrostatic pressure and forces water a small distance up the xylem (useful for small plants)
30
why is capillarity not useful in large trees?
the xylem's diameter would be too large to overcome the force of gravity
31
what factors affect transpiration rate?
- temperature
- wind speed
- humidity
- light intensity
32
how does temperature affect transpiration rate?
a rise in temperature increases the kinetic energy of the water molecules and increases the rate of evaporation and diffusion of water vapour into the atmosphere.
the water potential of the atmosphere is also lower in higher temperatures - this increases the water potential gradient
33
how does wind speed affect transpiration rate?
- still air results in a layer of water vapour around the stomata of a leaf - diffusion shell
- this reduces the water potential gradient between the inside and outside of the leaf.
- air movement blows away the diffusion shell and increases the rate of transpiration from the leaf
34
how does humidity affect the transpiration rate?
- the water potential gradient between the inside and the outside of the leaf decreases when there is more water vapour in the atmosphere
- however, there is usually a steep water potential gradient between the inside of the leaf and the atmosphere as the leaf is saturated with water vapour
35
how does light intensity affect the transpiration rate?
light causes stomata to open to allow gas exchange for photosynthesis
36
what is a potometer?
a piece of apparatus which measures the rate of uptake of water by the shoot which indicates the rate of transpiration
37
why is the rate of uptake only an estimate of transpiration rate?
some of the water is used up in photosynthesis
38
describe how to set up an apparatus for potometer
1. cut a leafy shoot underwater to prevent the entry of air bubbles into the xylem vessels as these would break hydrogen bonds, affecting cohesion
2. completely fill the apparatus with water to avoid introducing air bubbles into the glassware
3. fit leafy shoot and seal all joints with vaseline so apparatus is airtight
4. pat the leaves dry otherwise the water potential gradient will be reduced and this could affect the results
5. introduce one air bubble into the capillary tube - open the tap
6. measure the distance the air bubble moves along the scale in a specific time
39
how could you calculate the volume of water in the capillary tube?
h = distance moved by the bubble
r = internal radius of capillary tube
volume = πr^2h
40
how could you improve the reliability of the data collected?
repeat the experiment
41
what is the function of the xylem tissue?
to transport water and mineral ions
42
what is the xylem tissue made up of?
4 different types of cell
43
what are the 4 different types of cell that the xylem tissue is made up of?
- vessels
- tracheids
- fibres
- xylem paranchyma
44
describe vessels
the main cells that conduct water
45
describe tracheids
conduct water but less efficient than vessels
46
describe fibres
no role in transport (support only)
47
describe xylem parenchyma
living tissue
48
what are the adaptations of the vessels in the xylem tissue?
- continuous columns of dead cells arranged end to end, with completely dissolved cross-walls to form long, hollow tubes that transport water from roots to leaves
- the walls are thickened with lignin. lignin is impermeable to water and strengthens and supports the plant (stops vessels collapsing under pressure)
49
what is the adaptation for tracheids?
- similar to vessels but are more elongated with tapering ends
- the cells walls are also thickened with lignin for the same reason
50
why are the cell walls' of vessels and tracheids thickened with lignin?
to stop the vessels or tracheids from collapsing under the pressure of water
51
what do both vessels and tracheids have?
both have PITS in their side walls
- in vessels, these allow movement of water between adjacent vessels
- in tracheids, they are involved in movement of water to nearby living tissue
52
why is it important that vessels walls are impermeable to water and solutes?
so water keeps moving upwards to the leaves in one unbroken stream
53
lignin is hydrophilic. why is this important for xylem function?
water molecules are attatched to the lignified walls of the xylem walls, allowing adhesion.
54
what is the function of the phloem?
transports sucrose and amino acids
55
what is the phloem composed of?
4 different types of cell
56
what are the 4 different types of cells that make up the phloem?
- sieve tubes
- companion cells
- phloem fibres
- phloem parenchyma
57
describe sieve tubes
transport sucrose and amino acids up/down plant stem
58
describe companion cells
connect to sieve tubes via plasmodesmata
59
describe phloem fibres
these are for support
60
describe the phloem parenchyma
living tissue
61
what are the adaptations for the sieve tubes
- the phloem tissue contains sieve tubes to transport organic compounds (e.g sucrose and sieve tubes)
- the sieve tubes are formed from cells called sieve elements placed end to end
- the thin cellulose walls at the ends of these cells is perforated to form sieve plates that allow the cytoplasm from one cell to run into an adjacent cell
62
why are companion cells are important?
- do not transport organic materials but they have a nucleus and they provide ATP for the active transport of sugars into/out of the sieve tubes.
- this is important since sieve tubes cells lose their nucleus and other organelles as they mature (e.g mitochondria)
63
give 3 ways in which sieve tube elements are adapted to their function
- few organelles (no nucleus, few mitochonria) - more space for transport of solutes
- they have companion cells (with organelles for metabolism e.g ATP synthesised by mitochondria)
- end walls perforated (sieve plates) to allow passage of dissolved solutes (sucrose, amino acids)
64
what is translocation
the transport of soluble organic materials produced by photosynthesis (e.g sucrose and amino acids) in the phloem
65
how is the movement of these molecules described, and why?
BI-DIRECTIONAL
- may be downwards (e.g from the leaf to the root)
- or upwards (e.g from the leaves to fruits and shoot tips)
66
what is the source?
the region where the products of the photosynthesis are produced and exported
67
what is the liquid inside the phloem called?
sap
68
what is the sink?
the region where the products of photosynthesis are stored or used for growth
69
what are the source and sink regions of plants?
SOURCE : leaf
SINK : root, shoot tips, flower, fruit, seeds
70
what is the mass flow hypothesis?
the main theory to explain translocation
- sugars flow passively from areas of high concentration in the leaf (source) to areas of low concentration e.g growing tissue (sink) down a pressure gradient
71
how does mass flow work?
- sucrose made in photosynthesis is loaded by active transport into the sieve tubes, using ATP
- water enters the sieve tubes along a water potential gradient by osmosis
- the pressure in the sieve tubes increases and the sucrose moves down a pressure gradient through the phloem towards the sink
- sucrose is unloaded by active transport into the cells at the sinks
- water moves by osmosis out of the phloem as the sucrose is removed and the pressure in the phloem tissue becomes lower at the sink
72
what are some arguments against mass flow?
- no explanation of sieve plates which seem to act as barriers to flow
- sucrose and amino acids have been observed moving at different rates and in different directions
- sieve tubes have a high rate of ATP consumption, and translocation is slowed or stopped if respiratory inhibitors such as cyanide are added
- the companion cells are found all the way along the sieve tubes (not just in the sources and sinks) and contain numerous mitochondria for production of ATP. If companion cells purely load and unload photosynthates from the sieve tubes they would not be needed anywhere but sources and sinks
- an active process within the sieve tubes may be involved
73
what are the alternative theories for the explanation of mass flow?
- streaming in the cytoplasm of sieve tubes could be responsible for bi-directional movements
- protein filaments have been observed passing through the sieve pores, suggesting different solutes are transported by different filaments
74
what is evidence for translocation in the phloem?
- ringing experiments
- aphid experiments
- radioisotope labelling
75
describe ringing experiment as evidence for translocation in plants
removal of a ring of outer bark tissue from a woody stem removes the phloem.
analysis of the phloem contents just above and below the rings shows that organic compounds cannot be transported past the region where the bark has been removed
BULGE SEEN DUE TO ACCUMULATED PHLOEM SAP THAT CANNOT MOVE DOWN ANY FURTHER
- the results show that the phloem is responsible for the transport of sucrose and amino acids
- transport downwards was prevented
76
describe aphid experiments as evidence for translocation in plants
- aphids are small insects that can be used to collect the contents of individual phloem sieve tube cells
- aphids (such as greenfly) have specialised mouthparts called STYLETS which they use to penetrate sieve tubes in order to feed on the sugar sap inside
- if the aphids are anaesthetised with carbon dioxide, the stylet can be cut off and left in the stem
- this means that pure phloem sap can be collected through the stylet for analysis
- this technique is more accurate than a human with a syringe/needle as the aphid's enzymes ensure that the stylet doesn't get blocked
77
describe radioisotope as evidence for translocation in phloem
- an experiment using a plant supplied with radioactive 14 CO2
- the radioactive 14 CO2 is supplied to one leaf, and the plant will fix the radioactive carbon during photosynthesis to form photosynthates
- after 20 minutes the whole plant is places on photographic film ; dark areas on the film show areas containing radioactivity
78
what do the results of radioisotope labelling show?
- the results show that the radioactive carbon is fixed into the sugar at the source
- it is then translocated to sink parts of the plant
- this technique shows that the sugar is transported bidirectionally since the radioactivity is seen in the aerial parts of the plants as well as the roots
79
what are mesophytes?
plants that grow in temperate regions and most crop plants
- they grow best in well drained soils and moderately dry air
80
give 3 types of plant behaviours which allow mesophytes to survive at unfavourable times
1. DECIDUOUS TREES shed leaves in autumn to survive unfavourable conditions over winter. New leaves then grow in spring
2. BULBS and CORMS are produced by non woody plants to survive underground over winter
3. ANNUAL PLANTS produce seeds and die in the same year, seeds survive winter frost and germinate the next spring when conditions are more favourable
81
what are hydrophytes?
water plants that lived submerged or partially submerged in water
82
give an example of a hydrophyte
water lily
83
what are some features of a hydrophyte, and describe how they help a hydrophyte to be adapted to its environment.
- stomata on the upper epidermis ; to allow gas exchange with the air above
- large air spaces ; to provide bouyancy for the leaves and act as a reservoir of oxygen and carbon dioxide
- thin or no waxy cuticle ; no need to reduce water loss as they live in or on water
- poorly developed xylem tissue ; no need to transport large quantities of water as plant is aquatic
- little lignin ; water is a supportive medium and so little lignin is required to support the xylem tissue
84
what are xerophytes?
plants that are adapted to conditions of low water availability
- they live in hot and dry desert conditions OR cold regions where soil is frozen for much of the year, OR exposed windy locations
85
give 2 examples of xerophytes
marram grass
sand dunes
86
what are some features of a xerophyte, and describe how they help a xerophyte to be adapted to its environment.
- SUNKEN STOMATA ; water vapour is trapped in the pits. this decreases the water potential gradient between the inside and outside of the leaf. therefore less water is lost by transpiration
- HAIRS ON LEAF SURFACE ; water vapour is trapped between the hairs. this decreases the water potential gradient between the inside and outside of the leaf. therefore less water is lost by transpiration
- THICK CUTICLE ; reduces water loss from epidermis
- ROLLED LEAVES ; stomata are less exposed to the atmosphere. water vapour is also trapped and so the water potential gradient between the inside and outside of the leaf decreases. less water is therefore lost by transpiration
- REDUCED LEAF SIZE / SPINES : reduces surface area from which transpiration can occur
87
with reference to the polar nature of water molecules, explain how the loss of water from leaves enables water to be transported upwards through xylem vessels (4 MARKS)
- cohesion between water molecules
- evaporation of water from leaves places tension on water column in xylem
- pulls water up the xylem transpiration stream
- adhesion of water molecules to xylem
88
explain how a feature of the cells found in the endodermis means that oxygen is needed for phosphate ions to pass into the xylem
- the casparian strip blocks the apoplast pathway and forces into the symplast route
- therefore, active transport is needed to move ions into endodermal cells
89
explain why a group of xylem vessels form a tissue rather than an organ
only one type of cell aggregated to perform the same function
90
xylem vessels and phloem sieve tubes are strengthened by different chemicals found in their cell walls. name these chemicals.
XYLEM - lignin
PHLOEM - cellulose
91
give two structural features of guard cells
- chloroplast
- unevenly thickened walls
