Chapter 9 - Transport in Plants Flashcards
(91 cards)
1
Q
What is the primary function of the xylem?
A
To transport water and minerals, and support the plant structurally
2
Q
What is the direction of flow in the xylem?
A
From roots to shoots and leaves (up the plant)
3
Q
What is the xylem mainly made up of?
A
Dead cells
4
Q
What is the function of lignin in the xylem?
A
- Supports and strengthens the xylem
- Impermeable so prevents incorrect water loss
5
Q
What is the function of xylem parenchyma?
A
- Stores food
- Contains tannin deposits; tannin is a bitter chemical which protects the plant from herbivores
6
Q
Where is xylem parenchyma found?
A
It is packed around xylem vessels
7
Q
What occurs at the unlignified bordered pits in the xylem?
A
Water leaves the xylem tube it’s currently in, either entering a new xylem tube, or entering cells
8
Q
What is the main function of the phloem?
A
To transport food in the form of organic solvents from the leaves (chloroplasts) to the rest of the plant- it supplies cells with the necessary sugars and amino acids
9
Q
What is the direction of flow in the phloem?
A
Both up and down the plant
10
Q
What are the transporting vessels of the phloem called?
A
Sieve tubes
11
Q
What are companion cells?
A
Cells that form with sieve tubes, and are believed to function as a life support system for the sieve tubes.
They contain both mitochondria and a nucleus
12
Q
What two cell types make up the phloem?
A
Sieve tube elements and companion cells
13
Q
What is a difference in the cells of the xylem and phloem?
A
Xylem cells are mostly dead, phloem cells are living
14
Q
What are sieve plates?
A
They are the original cell walls of the sieve tube elements, which have holes in to allow the contents of the phloem to pass through
15
Q
What are 2 roles of turgor (water) pressure?
A
- Hydrostatic skeleton (maintains structure and supports stem and leaves)
- Drives root expansion
16
Q
Give 2 other roles of water in plants (other than turgor pressure)
A
- Transport medium for mineral ions and other important substances
- Water is a raw material required for photosynthesis
17
Q
What are root hairs?
A
Exchange surface of plants in roots, where water is drawn into the plant
18
Q
How are root hairs well adapted as exchange surfaces?
A
- Microscopic size allows them to penetrate easily through soil
- Large SA:V ratio
- Thin surface layer = small diffusion distance
19
Q
How do root hair cells maintain a water potential gradient?
A
There is a high concentration of solutes in the cytoplasm of the root hair cells, which lowers the water potential, meaning there is a water potential gradient, so water enter the cells via osmosis
20
Q
What is the transport system called in plants?
A
Vascular tissue
21
Q
What are vascular bundles?
A
The phloem and the xylem together
22
Q
What is the structure of the vascular tissue in the stems?
A
Passionfruit looking one.
Seeds are the vascular bundles- inside is xylem, outside is phloem
23
Q
What is the structure of the vascular tissue in the roots?
A
x or + in a circle in the middle
The x inside is the xylem, the circles around the x are the phloem
24
Q
In roots, why is the vascular bundle in the middle?
A
To help withstand tugging strain as a result of the leaves and stem moving in the wind
25
What is the structure of the vascular tissue in the leaf?
Rain drop looking one.
| Circle in middle with top one being xylem and bottom phloem
26
What are the two different pathways of water from roots to xylem?
Symplast and apoplast
27
What is the symplast pathway?
Where water travels through the continuous cytoplasm of cells.
It travels via osmosis, as the root hair cell has a higher water potential than the next cell, and as the water flows into this cell, it then has a higher water potential than the next cell and so on
28
What is the plasmodesmata?
Fine strand of cytoplasm that connects the cytoplasms of different cells, meaning a water can travel through a continuous cytoplasm in symplast
29
What is the apoplast pathway?
Where water travels through the cell walls and intercellular spaces.
The cohesive forces between water molecules means water is being continuously pulled up into the xylem
30
What eventually happens to the apoplast pathway (in roots) and why is this significant?
It cannot go any further due to the impermeable nature of the Casparian strip, so is forced into the cytoplasm to join the symplast pathway.
This is significant, as it means the water had to pass through a selectively permeable membrane, meaning potentially-toxic solutes in the soil water cannot reach the living tissue
31
What is root pressure?
The movement of water up the xylem generated from the pumping of minerals into the xylem (this is independent from any forces of transpiration)
32
When water reaches the endodermal cells surrounding the xylem, what happens?
The water potential of water in xylem cells is lower than in the cytoplasm of endodermal cells, so water enters the xylem via osmosis
33
How is active transport believed to be involved in the process of water transport/root pressure?
Endodermal cells are believed to actively move mineral ions into the xylem by active transport. This creates a steeper water potential gradient, meaning the water can enter efficiently via osmosis
34
What is evidence for the role of active transport in root pressure?
- Cyanide affects the production of ATP; when cyanide is applied to root cells (so there is no energy supply), root pressure disappears
- Root pressure increases and decreases with increases and decreases in temperature, indicating chemical reactions being involved
35
What are stomata opened and closed by?
Guard cells
36
What 2 things happen when stomata are open?
- Oxygen can enter and CO2 can leave
| - Water is lost via transpiration
37
Why do some stomata have to be open at night?
To take in oxygen for cellular respiration
38
Summarise the transpiration stream
Water enter through roots
Transported along apoplast or symplast pathway to xylem
Transported up xylem until it reaches the leaves
It then leaves through the stomata
39
Why is it important that the xylem tubes are narrow enough?
So the water can flow in a continuous stream
40
What is cohesion-tension theory?
The model in which water moves from the soil, up the xylem and across the leaves in a continuous stream
41
What is capillary action and what causes it in plants?
Capillary action is where water can rise up a narrow tube against the force of gravity.
This occurs as a result of the combined effects of the adhesion and cohesion of water moving up the xylem
42
What causes adhesion in water moving up the xylem?
Water molecules forming hydrogen bonds with the carbohydrates in the walls of the xylem vessels
43
What happens when the water reaches the leaves?
It either goes along the apoplast pathway, in which it will diffuse into the air spaces,
or along the symplast pathway, in which it travels through the mesophyll cells, until it evaporates from the freely permeable cell wall of the mesophyll cells into the air spaces.
It then leaves into the external air down a concentration gradient through the stomata
44
What causes cohesion in water moving up the xylem?
Water molecules forming hydrogen bonds between one another, meaning they tend to stick together
45
What is transpiration pull?
The continuous stream of water drawn from the roots to replace the water lost by evaporation
46
Where are stomata located?
In the lower epidermis of leaves (underside)
47
How do stomata open?
When guard cells have low water potential, so water enters the cell and makes it a turgid cell. Due to their inner cell wall being thicker and more rigid, the guard cells bend away from each other when they fill with water and this opens a pore - the gates open.
48
What happens to stomata when water becomes scarce?
Hormonal signals can trigger turgor loss from the guard cells, which close the stomatal pores and so conserve water
49
4 factors that affect transpiration
- Light (lack of=no photosynthesis=no need for co2=stomata close)
- Relative humidity of air (less humid=higher concentration gradient=more transpiration)
- Temperature (more kinetic energy = faster evaporation from mesophyll + increases volume of water external air can hold)
- Soil-water levels (more water = more transpiration
50
Where is the Casparian strip found?
Runs around endodermal cells, forming a waterproof layer
51
What are xerophytes?
Plants that have developed a wide range of adaptions that enable them to live in areas of very low water availability
52
How does an especially thick waxy cuticle help conserve water?
It helps minimise the volume of water lost via transpiration through the cuticle
53
How do sunken stomata help conserve water?
Many xerophytes have their stomata located in pits, which reduce air movement, producing a microclimate of still, humid air, which reduces the water potential gradient, so reduces the rate of transpiration
54
How does a reduced number of stomata help conserve water?
Less stomata = less water loss via transpiration
55
How do reduced leaf areas help conserve water?
Less leaf area = lower SA:V ratio = lower rate of transpiration
56
How does hairy and/or curled leaves help conserve water?
It creates a microclimate of still, humid air, which reduces the water potential gradient, so reduces the rate of transpiration
57
What are succulents?
Plants that store water in specialised parenchyma tissue- the water is then used in times of drought
58
How does leaf loss help conserve water?
Some plants lose their leaves when water is not available, so as to stop transpiration from occurring
59
Examples of root adaptations in xerophytes
- Long tap roots that can penetrate deep into soil, accessing deep underground water
- Shallow roots with a large surface area that can absorb water from a rain shower before it evaporates
60
What are hydrophytes?
Plants that have developed a wide range of adaptions that enable them to live in areas of very high water presence
61
What is the major problem that hydrophytes must prevent?
Water-logging- air spaces filling up with water, rather than air
62
How does very thin/no waxy cuticle help hydrophytes?
It means water can be easing lost through the cuticles via transpiration, helping prevent waterlogging
63
How do many always open stomata help hydrophytes?
It maximises gaseous exchanges, and the do not need to worry about turgor loss, as they live in an abundance of water
64
Where are stomata found on water lillies?
On the upper side, as the bottom is in the water
65
Why do hydrophytes have a reduced structure?
As water supports the leaves and flowers, so there is no need for strong supporting structures
66
How do wide, flat leaves help hydrophytes?
They spread across the greatest surface area, so as to capture as much light as possible
67
Why do hydrophytes have small roots?
As water can diffuse directly into the stem and leaf tissue
68
Why do hydrophytes have air sacs?
To allow them to float on the surface of water
69
What is aerenchyma?
Specialised parenchyma that forms in hydrophytes, which makes leaves and stems more buoyant, and forms a low-resistance internal pathway for the movement of substances (e.g. water).
70
What are anoxic conditions?
Extreme low oxygen conditions
71
What is translocation?
The transport of organic compounds in the phloem from sources to sinks
72
What are assimilates?
The products of photosynthesis that are transported
73
What is the main assimilate transported around the plant?
Sucrose
74
What happens to glucose before being translocated and why?
It is converted into sucrose before being transported, as this is because sucrose is less likely to be used in metabolism than glucose and so more likely to reach its destination
75
What are the main sources of assimilates in plants?
- Green leaves and green stems
- Storage organs
- Food stores in seeds when they germinate
76
What are the main sinks in a plant?
- Roots that are growing/actively absorbing mineral ions
- Meristems that are actively dividing
- Any parts of a plant that need energy to grow (such as developing seeds)
77
What is phloem loading?
When the soluble plants from photosynthesis are actively moved into the phloem from the sources
78
What are the two main ways in which plants load assimilates into the phloem?
- The symplast route
| - The apoplast route
79
How does the symplast route work phloem loading?
Sucrose moves through the cytoplasm of mesophyll cells and into the sieve tubes via diffusion
80
Where are proton pumps found, and what do they do?
They are found in companion cells; they pump H+ ions into the outside cell area, creating a concentration gradient of H+ cells from outside to inside the companion cells
81
Where are co-transporters found, and what do they do?
They are found in companion cells; the H+ ions pumped out by the proton pumps want to re-enter the companion cell, and they do this through the co-transporter. However to enter through the co-transporter, they must also take with them sucrose, therefore sucrose also enters into the companion cells
82
What happens when sucrose enters the companion cells during the apoplast route?
The sucrose will enter the sieve tube elements by diffusion through the plasmodesmata
83
What happens when sucrose enters the sieve tube elements?
The water potential in the phloem decreases, causing water to enter via osmosis.
This leads to a build up of turgor pressure, and causes mass flow to occur
84
How does mass flow work in the phloem?
Assimilates flow from the area of higher turgor pressure (the sources) down the concentration gradient to the areas of lower turgor pressure (the sinks)
85
What type of process is phloem loading and why?
It is an active process, as ATP is used to pump the H+ ions out of companion cells during the apoplast route
86
How do the assimilates enter the sinks?
The assimilates will enter via diffusion from the phloem into the sinks
87
How is the concentration gradient maintained from phloem to sink and why?
- Sucrose either rapidly moves onto other cells by diffusion, or is converted into another substance (e.g. glucose or starch)
- This is to make sure the sinks continue to receive sucrose
88
What happens to water in the phloem after the assimilates enter the sink, and why is this important?
Assimilates entering the sink will make the water potential in the phloem higher, which will cause water to often follow the assimilates into the sink via osmosis, or sometimes be drawn into the transpiration stream in the xylem
This is important because it maintains a low turgor pressure at the sinks
89
Proof of the involvement of active transport in translocation
- Microscopy allows us to see adaptions in the companion cells for active transport
- If mitochondria are poisoned by cyanide, translocation stops
- The flow of the phloem is 10,000 faster than lone diffusion would be, indicating active transport is driving the mass flow
90
How is phloem and xylem transport similar?
- Both involve mass flow
| - Solutes carried in solution in both
91
How can some parts of a plant act as both a source and a sink (with an example)?
Certain parts can store and then release carbohydrates when needed- an example is a leaf, which can act as a sink or source at different times of the year
