3.1.3 Transport in Plants Flashcards
(60 cards)
1
Q
Why do plants need a transport system?
A
- Larger plants have a smaller SA:V ration
- Minimally active so respiration rate and oxygen demand is low
- Diffusion is sufficient to meet oxygen demand
- Can only absorb water and minerals at roots but can only produce sugars at the leaves
- Need transport system to move water, minerals and sugars to all cells
2
Q
What are dicotyledonous plants?
A
Plants that have two seed leaves
3
Q
What is the structure and function of the vascular tissue in the roots of dicotyledonous plants?
A
- Found at the centre
- Xylem vessels are arranged in an X-shape
- Surrounded by phloem tissues which are found between the arms of the X
- Provides strength to tolerate pulling forces
- Endodermis sits around the vascular bundles
- Within this there is the pericycle
4
Q
What is the endodermis?
A
A sheath of cells which sits around the vascular bundles and supplies the xylem with water
5
Q
What is the pericycle?
A
A layer of meristem cells next to the endodermis
6
Q
What is the structure and function of the vascular tissue in the stem of dicotyledonous plants?
A
- Near the outer edge of the stem
- Bundles are separate and discrete in non-woody plants but a continuous ring in woody plants
- Provides strength and flexibility to tolerate bending forces
- Xylem tissue is at the inside of each vascular bundle
- Phloem tissue on the outside
- Separated by the cambium
7
Q
What is the cambium?
A
A layer of meristem cells involved in the production of new xylem and phloem tissue
8
Q
What is the structure and function of the vascular tissue in the leaves of dicotyledonous plants?
A
- Vascular bundles form the midrib and veins of a leaf
- Branching network of veins which get smaller as they spread
- Phloem is beneath the xylem in each vein
9
Q
What are the features of a xylem vessel?
A
- Made from dead cells which align to form a continuous column
- Lignified cell walls add strength to prevent the vessel collapsing under pressure and are impermeable to water
- Lignin is deposited in spiral, annular or reticulate plants, allowing flexibility
- No end plates and no cell contents, allowing for mass flow of water and solutes
- Tubes are narrow to prevent water column breaking
- Parenchyma cells acts as packing tissue
- Sclerenchyma cells provide strength and support
10
Q
What are bordered pits?
A
- Non-lignified sections of the xylem
- Enable the lateral movement of water
11
Q
What are the features of the sieve tube elements in a phloem vessel?
A
- Elongated sieve tube elements line up end to end to form sieve tubes
- No nucleus and limited cytoplasm to maximise space for the mass flow of sap
- Sieve plates are perforated cross-walls allowing the movement of sap from one element to the next
- These are found at the end of the sieve tube elements
- Have thin walls and 5 or 6 sides (angular in transverse)
12
Q
What are the features of the companion cells in a phloem vessel?
A
- Found between the sieve tubes
- Small with a large nucleus, dense cytoplasm and lots of mitochondria
- Carry out metabolic processes needed to load assimilates
13
Q
What is the apoplast pathway?
A
- Water passes through the spaces in cell walls and between the cell
- Doesn’t pass through any plasma membrane
- Water moves by mass flow instead of osmosis and dissolved mineral ions/salts can be carried
14
Q
What is the symplast pathway?
A
- Water enters the cell’s cytoplasm through the plasma membrane
- Passes through the plasmodesmata of one cell into the next
15
Q
What are the plasmodesmata?
A
Gaps in the cell wall containing cytoplasm that connects two cells
16
Q
What is the vacuolar pathway?
A
- Water can enter and pass through vacuoles as well as the cytoplasm
17
Q
How does water move?
A
- Moves from an area of higher water potential to an area of lower water potential
- Down a water-potential gradient
- Pure water has a potential of 0
- Plant cells have a more negative potential due to the cytoplasm containing mineral ions/sugars
- When plant cells are touching, water molecules pass from one cell to another
18
Q
How does water move in a hypotonic solution?
A
- In pure water
- Plant cell takes up water by osmosis as water moves into cell
- Cell becomes turgid
- Cellulose cell wall prevents it from bursting
- Water starts to exert pressure on cell wall (pressure potential), reducing influx of water
19
Q
How does water move in a hypertonic solution?
A
- In a salt solution
- Plant cell loses water by osmosis and water molecules move out
- Cytoplasm and vacuole shrink
- Cell is no longer turgid
- Continued water loss leads to plasmolysis, which is when the plasma membrane loses contact with the cell wall
20
Q
What is transpiration?
A
The loss of water vapour from the upper parts of the plant (particularly the leaves)
21
Q
What is the process of transpiration?
A
- Water enters leaf via the xylem and moves by osmosis into the cells of the spongy mesophyll
- Water evaporates from the cell walls of the spongy mesophyll
- Water vapour diffuses out of the leaf through the stomata, relying on the water vapour potential gradient
22
Q
How does the waxy cuticle impact transpiration?
A
- Limits water loss from the upper leaf surface
- Most vapour is therefore lost through the stomata
23
Q
What is water used for in the plant?
A
- Growth
- Cell elongation
- Photosynthesis
- Temperature control
24
Q
What are the factors that affect transpiration?
A
- Light intensity
- Temperature
- Relative humidity
- Air movement/wind
- Water availability
25
How does light intensity affect transpiration?
- Higher light intensity increases transpiration rate
- More stomata are open to allow for gas exchange
26
How does temperature affect transpiration?
- Higher temperature increases transpiration rate
- Increases the rate of evaporation so water vapour potential in the leaf rises
- Increases the rate of diffusion through the stomata as molecules have more kinetic energy
- Decreases relative water vapour potential in the air
27
How does relative humidity affect transpiration?
- Higher humidity decreases transpiration rate
- Smaller water vapour potential gradient between the air spaces in the leaf and the air outside
28
How does air movement/wind affect transpiration?
- More air movement will increase transpiration rate
- Maintains a high water vapour potential gradient
29
How does water availability affect transpiration?
- Low water availability will decrease transpiration rate
- Little water in the soil will prevent plant from replacing water that is lost
- Stomata will close and leaves will wilt
30
How is transpiration measured?
- Using a potometer
- Rate of water uptake measured as water vapour lost by the leaves is replaced by water from the capillary tube
31
What are the key procedures to follow when using a potometer?
- Set up underwater to prevent air bubbles
- Use a healthy shoot and dry the leaves
- Cut stem underwater and at an angle to provide a large SA
32
33
What are the mechanisms of the transpiration stream?
- Water uptake
- Water movement across the root
- Water movement up the stem
- Water movement in the leaf
34
How is water taken in at the root?
- Root epidermis contains root hair cells
- These increase SA of the root through long extensions to maximise absorption from soil
- Minerals taken up by active transport and diffusion, which lowers water potential of root hair cells
- Water uptake is therefore done by osmosis
35
How does water move across the root?
- Move across root cortex to endodermis through the apoplast pathway
- This pathway eventually becomes blocked by the Casparian strip
- Water is forced into the symplast pathway (moving through plasma membranes)
- Transported proteins actively pump minerals ions from the cytoplasm into the cortex cells of the medulla and xylem
- This makes their water potential more negative so water moves into the medulla and xylem by osmosis
- Casparian strip prevents it moving back
36
What is the Casparian strip?
- An impermeable barrier
- Made of suberin
37
How does water move up the stem?
- By mass flow (the flow of water and mineral ions in the same direction)
- Achieved by root pressure, transpiration pull, capillary action
38
What is root pressure in the transpiration stream?
- Endodermis moves minerals into the medulla and xylem by active transport
- This draws water into the medulla by osmosis
- Pressure builds in the root as water volume increases
- Drives water above it into the xylem
39
What is the cohesion-tension theory in the transpiration stream?
- Forces of cohesion mean that water molecules are attracted to each other
- Forms a long column
- Water molecules lost by evaporation are replaced by water from the xylem
- Whole column is pulled up creating tension
- Relies on the plant maintaining an unbroken column of water throughout the xylem
- Bordered pits maintain water column if it is broken in one vessel
40
What is capillary action in the transpiration stream?
- Adhesion forces attract water molecules to the side of the xylem vessels
- Xylem vessels are narrow so adhesion forces can pull water up the sides of the vessel
41
How does water move in the leaf?
- Diffuses out of the sub-stomatal air space, which lowers the water potential
- Water moves from the mesophyll cells into the air spaces
- Water therefore moves into the mesophyll cells from other cells and the xylem to maintain the transpiration stream
42
What is a xerophyte?
A plant adapted to living in dry conditions
43
How is maram grass adapted?
- Leaf is rolled longitudinally to trap air inside which becomes humid and reduces water loss
- Thick waxy cuticle on upper epidermis to reduce evaporation
- Stomate are in pits on the lower epidermis, which is folded and covered by hairs to reduce air movement and water loss
- Spongy mesophyll is dense with little air space to create less SA for evaporation
44
How are cacti adapted?
- Leaves are spines to reduce SA and water loss by transpiration
- Green stem for photosynthesis
- Widespread roots to maximise any rain
45
What are other features of xerophytes?
- Closing stomata in low water availability to reduce water loss
- Low water potential inside lead cells by having a high salt concentration, which reduces the water potential gradient and reduces evaporation
- Long tap root that reaches water deep underground
46
What is a hydrophyte?
A plant adapted to living in water or where the ground is very wet
47
What a hydathodes?
- Structures at the tips or margins of leaves which release water droplets that may evaporate from the leaf surface
- Water cannot evaporate into water or air with an extremely high humidity, which would stop the transpiration stream
48
How is a water lily adapted?
- Multiple large air spaces in the leaf to keep the leaves afloat, ensuring they are in the air and can absorb sunlight
- Wide and flat leaves to provide a large SA
- Stomata are constantly open and on the upper epidermis so they are exposed to the air to maximise gas exchange
- Stem has many large air spaces so oxygen can diffuse quickly to the roots and to help with buoyancy
49
What is translocation?
The transport of assimilates through a plant in the phloem
50
What are the mechanisms of translocation?
- Active loading
- Sucrose movement
51
What is active loading?
- Loading sucrose into the sieve tube involves using energy from ATP in companion cells for cotransport
- H+ ions are actively transported out of the companion cells to increase concentration outside the cell, creating a concentration gradient
- H+ ions diffuse back into companion cells thorough cotransporter proteins
- These only allow the movement of H+ ions if they are bringing sucrose molecules
- As sucrose concentration increases in the companion cell, sucrose can diffuse into the sieve tube through the plasmodesmata
52
How is sucrose moved?
- By mass flow
- Sap is a solution of sucrose, amino acids and other assimilates
- It can flow up or down the plant
- Water enters the tube at the source, which increases pressure
- It leaves at the sink, which reduces pressure
- Sap always flows from the source to the sink as the difference in hydrostatic pressure creates a pressure gradient
53
What is a source?
A part of the plant that loads materials into the transport system
54
What happens at the source?
- Sucrose entering the sieve tube element makes water potential more negative
- Water molecules move into the sieve tube element by osmosis, which increases hydrostatic pressure
55
What are examples of sources?
- Leaves as sugars made in photosynthesis are converted to sucrose and are loaded into the phloem sieve tubes
- This happens in late spring, summer and early autumn when the leaves are green
- In early spring, the roots as starch is converted to sucrose and moved to other parts of the plant for growth
56
What is a sink?
A part of the plant where materials are removed from the transport system
57
What happens at the sink?
- Sucrose diffuses out of the sieve tube through the plasmodesmata or is removed by active transport
- This makes the water potential in the sieve tube elements less negative so water moves out of the sieve tube
- Reducing the hydrostatic pressure in the phloem at the sink
58
What are examples of sinks?
- In a meristem, sucrose is used for respiration and growth
- In a root, sucrose is converted to starch for storage
59
What is callose?
- Large polysaccharide deposited in sieve tubes at the end of the growing season
- Dep0sited around the sieve pates
- Blocks the flow in the sieve tubes
- Prevents the spread of pathogens
60
What is tylose?
- Balloon-like swelling of parenchyma cells that blocks the xylem
- Water cannot be carried
- Prevents the spread of pathogens
- Contains chemicals which are toxic to pathogens
