Transport In Plants Flashcards
What affects a plants need for specialised exchange surfaces and transport system?
Size
What does every cell in a multicellular plant need?
Oxygen, water, nutrients and minerals
How can these demands be met?
Plants aren’t very active therefore need less oxygen so can be filled by diffusion
Water high needed absorbed through soil
Minerals needed absorbed through soil
Sugars-photosynthesis
A transport system of a plant needs to move?
Water and minerals from soil up to the leaves
Sugars from the leaves to the rest of the plant
What does the transport consist of
Specialised vascular tissue
What travels in the xylem tissue and what direction?
Water and soluble ions travel upwards in xylem tissue
What travels in phloem and what direction?
Assimilates such as sugars travel up or down
How are phloem and xylem different to transport system of an animal?
Xylem and pholem have no pump and respiratory gases aren’t carried by these tissues
What are dicotyledonous plants?
Plants with two seed leaves and a branching pattern of veins in the leaf.
Distribution of vascular tissue
Distributed throughout plant
Where are xylem and pholem found together
In the vascular bundles
What do vascular bundles contain?
Xylem and pholem
May contain other tissues such as collenchyma and sclerenchyma
What do the other tissues in vascular tissue give it?
Strength and help to support the plant
What’s found at the centre of a young root?
Vascular bundle
What is the structure of this vascular bundles?
Central core of xylem often found in shape of X
Phloem found between arms of X-shaped xylem tissue
What does this arrangement give?
Strength to withstand the pulling forces to which roots are exposed
What around the vascular bundle and what’s its key role
Endodermis (special sheath),inside I’d a meristem cells called the pericycle
Getting water into xylem vessels
Where are the vascular bundles found?
Near the outer edge of the stem
What is the difference in non-woody and woody vascular bundles?
In non-woody plants bundles are separate and discrete.
In woody plants bundles are separate in young stems but become a continuous ring in older stems.
What does this arrangement provide?
Strength and flexibility to withstand bending forces to which stem and branches are exposed
Where is the cambium found and what is it?
In between xylem and phloem inside a vascular bundle
Cambium is a layer of meristem that divide to produce new xylem and phloem.
What does the vascular bundle form of a leaf
The vascular bundles form midrib and veins of a leaf.
Dicotyledonous leaf has a branching network of veins that get smaller as they spread away from the midrib within each vein the xylem is located on top of the phloem.
How can you carry a dissection out on a plant?
Stain the plant
Most easily demonstrated on leaf stalk of celery but can be carried out on Busy Lizzie (Impatiens).
Thin sections can be cut and viewed at low power.
Allow leafy stem to take up water by transpiration
The stem can be longitudally or transversely and examined with a hand lens or microscope
What is the function of xylem?
To transport water and mineral ions from roots up the leaves and other parts of the plant
What is the structure of xylem?
Vessels to carry water and dissolved mineral ions
Fibres to help support the plant
Living parenchyma cells which act as packing tissue to separate and support the vessels
Xylem vessels form with no centre contents how?
As xylem vessels develop, lignin impregnates walls of cells making walls waterproof. This kills cells. Ends of walls and contents of cells decay, leaving long column if dead cells with no contents.
What does the lignin do?
Strengthens vessel walls
prevents the vessel from collapsing
Keeps vessel open even at times when water may be in short supply
How do patterns form in the cell wall?
Lignin thickening forms patterns in cell wall
May be spiral, annular (rings) or reticulate (network of broken rings)
What does the lignin thickening do?
Prevents vessels from being too rigid and allows some flexibility of stem or branch
What is lignification in some places?
Not complete,
leaving gaps in cell wall
Gaps form pits or bordered pits
Bordered pits in two adjacent vessels aligned allow water to leave one vessel and pass into the next vessel
Allows water to leave xylem and pass into living parts of the plant
What adaptations does xylem have to it’s function?
Made of dead cells aligned end to end to form continuous column
Tubes are narrow so water column doesn’t break easily and capillary action can be effective
Bordered pits in lignified walls allow water to move sideways from one vessel to another
Lignin deposited in walls in spiral, annular or reticule patterns allow xylem to stretch as the plant grows and enables stem/ branch to bend.
Why isn’t The flow of water impeded in a xylem vessel
No cross-walls
No cell contents, nucleus or cytoplasm
Lignin thickening prevents walls from collapsing
What is the function of phloem?
Tissue used to transport assimilated around the plant. The sucrose is dissolved in water to form sap.
What is assimilates
Mainly sucrose and amino acids
What is the structure of phloem?
Phloem tissue consists of sieve tubes-made up of sieve tube elements and companion cells
What are sieve tubes structure like?
Elongated sieve tube elements lined up end to end to form sieve tubes.
No nucleus, Very little cytoplasm leaving space for mass flow of sap to occur
At ends of sieve tubes elements are sieve plates. Sieve played allows movement of Sap from one element to the next. Sieve tubes have very thin walls, when seen in transverse section usually 5-6 sided
Companion cell structure?
Between sieve tubes are small cells each with large nucleus and dense cytoplasm (companion cells).
Have numerous Mitochondria to produce ATP needed for active processes
Companion cells carry out metabolic processes needed to load assimilated actively into sieve tubes.
What is plasmodesmata?
Cell junctions at which cytoplasm of one cell is connected to that of another through gap in cell wall.
Apoplast pathway?
Water passes through spaces in cell walls and between cells
Doesn’t pass through plasma membrane into cells
Means water moves by mass flow rather than osmosis.
Dissolved minerals and salts can be carried with the water
Symplast pathway
Water enters cell cytoplasm through plasma membrane
Can pass through plasmodesmata from one cell to the next
Vacuolar pathway?
Similar to symplast pathway
Water not confined to cytoplasm of cell
Able to enter and pass through the vacuoles
Water potential
Measure of tendency of water molecules to move from one place to another
Explain diffusion using water potential
Water always move from a region of higher water potential to a region of lower water potential
What is the water potential of pure water?
0
What is in a plant cell that reduces the water potential and why?
Mineral ions and sugars (solutes)
Fewer free water molecules available than in pure water as result water potential in plant cells is always negative
What happens if you place a plant in pure water and why?
it will take up water by osmosis because water potential in the cell is more negative than water potential out of the water. Water molecules will move down the water potential of water won’t continue until bursts, because cell has strong cellulose cell wall, (makes cell turgid), water inside exert pressure potential on the cell wall. As pressure builds up, it reduces the reflux of water.
What happens when you put a plant in a salt solution with a very negative water potential?
It will lose water by osmosis
Why does this happen?
The water potential in the cell is less negative than the water potential of the solution so water moves out of the cell.
What happens as the water loss continues?
The cytoplasm and vacuole shrink.
Eventually, the cytoplasm is no longer pushed against the cell wall and the cell is no longer turgid. If water continues to leave the cell then plasma membrane will have contact with the wall (plasmolysis) the tissue is now flaccid.
How does water move between cells?
When plant cells are touching each other, water molecules can pass from one cell to another. The water molecules will move from the cell with less negative water potential to the more negative water potential (osmosis).
What is transpiration?
The loss of water from the upper parts of the plant-particularly the leaves.
What could some water evaporate through and how is this limited?
Some water may evaporate through the upper leaf surface
Loss limited by waxy leaves.
How does most of the water vapour leave?
Through the stomata which is open to allow gaseous exchange for photosythesis.
What stops so much water being lost at night?
Since photosynthesis occurs only when there is sufficient light, the majority of water vapour is lost during the day.
What is the typical pathway taken by most water leaving the leaf?
1) water enters the leaf through xylem and moves by osmosis into cell of spongy mesophyll. May pass along cell walls via apoplast pathway.
2) water evaporates from cell wall of spongy mesophyll
3) water vapour diffuses out of leaf through open stomata
water vapour diffuses out of leaf through open stomata relies on what?
Difference in concentration of water vapour molecules in the leaf compared to outside leaf (water vapour potential gradient)
Must be less negative water vapour potential inside the leaf than outside.
What does the movement of water vapour lost do?
Transports useful mineral ions up the plant
Maintains cell turgidity
Supplies water for growth, cell elongation and photosynthesis
Supplies water that, as it evaporates can keep the plant cool on a hot day.
What factors affect transpiration?
Light intensity, temperature, relative humidity, air movement (wind) and water availability.
What affect does light intensity
In light, stomata open allow gaseous exchange for photosynthesis. Higher light intensity increases the transpiration rate.
How does temperature affect the rate of transpiration?
Higher temperature increases rate of transpiration it will:
Increase rate of evaporation from cell surface so water vapour potential in leaf rises
Increase rate of diffusion through stomata because water molecules have more kinetic energy
Decrease relative water vapour potential in air, allowing rapider diffusion of molecules out of the leaf
What is the effect of relative humidity on the rate of transpiration?
Higher realities humidity in air will decrease rate of water loss because there will be smaller water vapour potential gradient between air spaces in leaf and air outside.
What is the effect of air movement (wind) on rate of transpiration?
Air moving outside leaf will carry away water vapour that has just diffused out of the leaf. This will maintain a high water vapour potential gradient.
What is the effect of water availability on transpiration rate?
If there is little water in the soil, then the plant can’t replace water that is lost. If insufficient water in the soil, the stomata close and the leaves wilt.
What can a potometer be used for?
To estimate the rate of transpiration
What does a potometer actually measure?
The rate of water uptake by a leafy shoot
How does this give a good estimate for how much water is lost by transpiration?
Assuming cell is turgid, more than 95% of the water taken up is lost by transpiration
How is using a potometer?
Straightforward once set up
Water vapour lost by leaves is replaced by the water in the capillary tube. The movement of the meniscus at the end of the water column can be measured.
How do you study the effects of different environments factors in the rate of transpiration using a potometer?
You can place while apparatus under different sets of conditions
Remember to vary only one factor at a time in order to determine the effect on the transpiration.
What precautions should be taken to ensure the results are valid?
1) set it up to make sure no air bubbles inside appartus
2) ensure shoot is healthy
3) cut the stem under water to prevent air entering xylem
4) cut stem at an angle to provide a large surface area in contact with the air
5) dry the leaves
Measuring volume and transpiration rate
Measuring rate of water taken up by shoot involves calculating volume of cylinder (length of capillary tube). Volume of cylinder given in formula
V=(Pi)(r2)l
Transpiration rate is volume calculate divide by time taken
Rate=volume/time
What is the transpiration stream?
Movement of water from soil through plant to air surrounding leaves
Main driving force is water potential gradient between soil and air in leaf air spaces
Water uptake and movement across the root?
The epidermis of a root contains root hair cells (cells with long extension (root hair)) increase surface are of root
Cells absorb minerals ions and water from soil
Water moves across root cortex down water potential to endodermis of vascular bundle
Water may travel through apoplast pathway as far as endodermis
Must enter symplast pathway as apoplast pathway blocked by casparian strip
What is epidermis?
The outermost layer of cells
What is the movement of water across the root driven by?
An active process that occurs at the endodermis.
What is endodermis?
A layer of cells surrounding medulla and xylem
Also known as starch sheath as contains granules of starch-sign that energy is being used
What does the caparian strip block
The apoplast pathway between the cortex and the medulla.
What does endodermis ensure water and dissolved mineral ions( most nitrates) do?
Pass into cell cytoplasm through the plasma membrane
What does the plasma membrane contain?
Transporter proteins which are actively pump minerals ions from cytoplasm of cortex cells into the medulla and xylem
What does endodermis do to the water potential to the medulla and xylem and what does it cause?
More negative
Causes water to move from cortex cells into medulla and xylem by osmosis
What can’t water do once it has entered the medulla?
Pass back into cortex as the apoplast pathway of endodermis cells is blocked by casparian strip
How does water move up the xylem?
Mass flow
A flow of water minerals ions in the same direction
There are processes that help to move water up the stem
What does root pressure do?
Action of endodermis moving minerals into medulla and xylem by active transport draws water into medulla by osmosis.
Pressure in root medulla builds up and forces water into xylem pushing water up xylem
Root pressure can push water a few metres up a stem but can’t account for water getting to top of talk trees
Why do plants need a transport system?
Living things need to take substances from and return wasted to their environment,
What must happen to the water evaporated from the leave?
It must be replaced by water coming from the xylem.
What attracts water molecules to each other?
Forces of cohesion
What are these cohesion forces strong enough to do?
Hold the molecules together in a long chain or column.
What happens as molecules are lost at the top of the column?
The whole column is pulled up as one chain.
What does the pull from above create?
Tension in the column of water. This is why xylem vessels must be strengthened by lignin
Lignin prevents the vessel collapsing under tension
What is the mechanism that involves cohesion between water molecules and tension in the column of water called?
Cohesion-tension theory.
What cohesion-tension theory?
A mechanism that involves cohesion between water molecules and tension in the column of water.
What does the cohesion-tension theory rely upon?
The plant maintaining an unbroken column of water all the way up the xylem,
If water column is broken in one xylem vessel the water column can still be maintained through another vessel via the bordered pits.
What are the forces that hold water to the side of the xylem vessel?
Same forces that hold water molecules together
Called adhesion
What effect does xylem being very narrow have on adhesion?
These forces of attraction can pull the water up the sides of the vessel
What width is the xylem vessels?
Very narrow
How does water leave the leaf?
Most as vapour through the stomata
Tiny amount leaves through the waxy cuticle
How does the water escape the stomata?
Water evaporates from cells lining cavity just above guard cells
Lowers water potential in cells causing water to enter them by osmosis from neighbouring cells
Water drawn from xylem in leaf by osmosis
Water may reach cells by apoplast pathway from xylem.
What must plants on land be adapted to be?
Reduce loss of water
Replace lost water
What structural and behavioural adaptations do plants have to reduce water loss?
Waxy cuticle on leaf will reduce water loss due to evaporation through epidermis
Stomata often found under-surface of leaves not on top surface reduces evaporation due to direct heating from sun
Stomata close at night when no light for photosynthesis
Deciduous plants lose leaves in winter when ground frozen (water less available), when temperature may be too low for photosynthesis.
Where does the marram grass specialise in living?
Sand dunes
What conditions like in the sand dunes for a plant?
Harsh
Any water in sand dunes drains away quickly
Sand may be salty
Leaves may be exposed to very windy conditions
Adaptations of marram grass are:
Leaf rolled longitudally (air trapped inside, air becomes humid reduces water loss from leaf). Leaf can roll tightly
Adaptations cactuses have to overcome arid conditions?
Succulents
Leaves reduced to spines
Green stem
Root widespread
What does the adaptation succulent mean?
Store water in stems become fleshy and swollen. Stem often robbed/ fluted so can expand when water is available
What does adaptation leaves reduced to spines mean?
Reduces surface area of leaves.
When total leaf surface area is reduced, less water is lost by transpiration.
What does the adaptation green stem mean?
Allows photosynthesis
What does widespread root adaptation mean?
Take advantage of any rain that does fall.
What are some other xerophytes features?
Closing stomata when water availability is low
Some plants have low water potential inside leaf cells.
Very long tap root
What does the adaptation closing stomata when water availability is low mean?
Reduce water loss so reduce need to take up water
How is the adaptation some plants have low water potential achieved?
Maintaining high salt concentration in the cell
Low water potential of water from cell surface as water potential gradient between cells and leaf air spaces reduced.
What does the adaptation a very long tap root mean?
Can reach water deeper underground.
Hydrophytes are:
Plants that live in water
Family nympheales
Easy access to water
Faced with other issues such as getting oxygen to submerged tissues but keeping afloat
Need leaves in sunlight for photosynthesis
What are some adaptations of a water lily?
Many large air spaces in leaf
Stomata on upper epidermis
Leaf stem has many large air spaces
What is the meaning of the adaptation Many large air spaces in leaf?
Keeps leaves afloat so they are in air and can absorb sunlight
What is the meaning of the adaptation the stomata are on the upper epidermis?
exposed to air to allow gaseous exchange
What is the meaning of leaf stem has many large air spaces?
Helps with buoyancy
Allows oxygen to diffuse quickly to roots for aerobic respiration
What is transpiration?
Loss of water from surfaces of leaf
When will water vapour not evaporate off the surface of a leaves into air or water?
When air or water has high humidity.
What happens if water can’t leave a plant?
Transpiration stream stops and plant can’t transport mineral ions up to leaves
Where is a hydathrode?
At tips or margins of their leaves
What is the function of a hydathode?
Can release water droplets which may evaporate from leaf surface.
What is the movement of assimilates through out the plant called?
Translocation
What is translocation?
Transport of assimilates throughout a plant.
What does transpiration occur?
In the phloem.
What are assimilates
Substances made by only the plant using substances absorbed from the environment
What examples of assimilates are there?
Sugars (mainly transported as sucrose)
Amino acids
What is the part of the plant that loads assimilates into phloem sieve tubes called?
Source
What is the source?
Part of plant that loads assimilates into the phloem sieve tubes
What is the part of the plant that removes assimilates from the phloem sieve tubes?
Sink
What is the sink?
Part of plant that removes assimilates from phloem sieve tubes
How is sucrose loaded into sieve tubes?
An active process
What does the loading of Sucrose into sieve tube require therefore?
Energy from ATP into companion cells,
What is this energy used for?
Used to actively transport hydrogen ions (H+) out of companion cells
What happens because hydrogen ions have left companion cells?
Increases concentration outside the cells and decreases their concentration inside the companion cells
As a result concentration gradient is created.
What happens to the hydrogen ions after they have been actively transported out of the companion cells?
They diffuse back into companion cells through co-transporter proteins
What do co-transporter proteins do?
Only allow movement of hydrogen ions if accompanied by sucrose molecules
What are the two names of only allowing diffusion if accompanied by something else?
Co-transport
Secondary active transport (results from active transport of H+ out of cell and moves sucrose against concentration gradient
Concentration of sucrose in companion cell increases can diffuse through plasmodesmata into sieve tube
Where does the movement of sucrose occur?
Along the phloem
What causes the movement of sucrose?
Mass flow
What flows along the phloem in the movement of sucrose?
Solution of sucrose, amino acids and other assimilates (sap)
What way can sucrose move in the movement of sucrose?
Either up or down as the plant requires
What is the mass flow caused by?
Difference of hydrostatic pressure between two ends of the tube which produces a pressure gradient
Why does the sap flow from source to sink?
Water enters tube at sink Increasing pressure Leaves tube at sink Reducing pressure Sap flow from source to sink
What is the first step in the movement of sap in phloem?
Sucrose is actively loaded into sieve tube element and reduces the water potential
What is the second step of the movement of sap in the phloem?
Water follows osmosis
Increases hydrostatic pressure into sieve tube element
What is the third step of the movement of sap in the phloem
Sap moves down sieve tubes
From higher hydrostatic pressure at source to lower hydrostatic pressure at sink
What is step 4 of the movement of sap in the phloem?
Sucrose removed from sieve tube by surrounding cells
Increases water potential in sieve tube
What is the fifth and last stage of the movement of sap in the phloem?
Waves moves out of sieve tube
Reduces hydrostatic pressure
What does sucrose entering the sieve tube element make water potential inside the sieve tube?
More negative (lower)
What is the result of a more negative water potential inside the sieve tube?
Water molecules move into sieve-tube element by osmosis from surrounding tissues
Increases hydrostatic pressure in sieve tube at the source
What is a source?
Any part of the plant that loads sucrose into the sieve tube.
What could the source be?
In early spring, could be roots
Leaf
Why could the source be roots in early spring?
Energy had been stored as starch converted to sucrose and moved to other parts of the plant to enable growth
When can the leaves be a source?
Late spring, summer and early autumn
How can the leave be a source during this time?
Sugars made in photosynthesis converted to sucrose and loaded into phloem sieve tubes
How does the root and meristems become a sink during the time when the leaf is a source?
Sucrose transported to other areas that may be growing (meristems) or to areas for storage (roots)
What is a sink?
Anywhere that removes sucrose from phloem sieve tubes
What can this sucrose be used for?
Respiration
Growth in meristem
Converted into starch for storage in root
What can sucrose do when it is used in a cell?
Diffuse out of sieve tube via plasmodesmata
May be removed by active transport
What does the removal of the sucrose from the sap do?
Make the water potential less negative
Water moves out of sieve tube into surrounding cells
Reduces hydrostatic pressure in phloem at sink
What does water entering the sieve tube at the source do?
Increase the hydrostatic pressure
What does water leaving sieve tube at sink do?
Reduces hydrostatic pressure
What do these two things mean?
Pressure gradient is set up along sieve tube
Sap flowing from higher pressure to lower pressure
Could be either depending on where sucrose is produced and where it is needed
Is it possible for sap to flowing in opposite directions in different sieve tubes at a time?
Yes
Why is this mass flow?
Sap in one tube all moving in same direction
