9.1 Xylem Transport Flashcards
What is transpiration?
Transpiration is the loss of water vapour from the stems and leaves of plants
- What role does light energy play in transpiration?
Light energy converts water in the leaves to vapour, which evaporates from the leaf via stomata
- What does the evaporation of water cause? (transpiration)
New water is absorbed from the soil by the roots, creating a difference in pressure between the leaves (low) and roots (high)
- What is the transpiration stream and what role does it play? transpiration
Water will flow, via the xylem, along the pressure gradient to replace the water lost from leaves (transpiration stream)
What are stomata and what are their role?
Stomata are pores on the underside of the leaf which facilitate gas exchange (needed for photosynthesis)
What requires stomata to be open and how does this affect transpiration?
As photosynthetic gas exchange requires stomata to be open, transpiration will be affected by the level of photosynthesis
What is transpiration a consequence of?
Hence, transpiration is an inevitable consequence of gas exchange in the leaf
How is water lost from the plant?
Water is lost from the leaves of the plant when it is converted into vapour (evaporation) and diffuses from the stomata
- Where is the first place that transpiration occurs and what happens?
Some of the light energy absorbed by leaves is converted into heat, which evaporates water within the spongy mesophyll
- Where does the water vapour go? What is created?
evaporation
This vapour diffuses out of the leaf via stomata, creating a negative pressure gradient within the leaf
- What does this negative pressure create? What is its use?
evaporation
This negative pressure creates a tension force in leaf cell walls which draws water from the xylem (transpiration pull)
- What causes the tension?
evaporation
The water is pulled from the xylem under tension due to the adhesive attraction between water and the leaf cell walls
How is water loss regulated?
The amount of water lost from the leaves (transpiration rate) is regulated by the opening and closing of stomata
What roles do guard cells play in the regulation of water loss? (specifically decreasing)
Guard cells flank the stomata and can obstruct the opening by becoming increasingly flaccid in response to cellular signals
What is released when plants wilt?
When a plant begins to wilt from water stress, dehydrated mesophyll cells release the plant hormone abscisic acid (ABA)
What is the role of abscisic acid?
Abscisic acid triggers the efflux of potassium from guard cells, decreasing water pressure within the cells (lose turgor)
What does loss of turgor do to the stomatal pore?
A loss of turgor makes the stomatal pore close, as the guard cells become flaccid and block the opening
When will transpiration rates be higher?
Transpiration rates will be higher when stomatal pores are open than when they are closed
What will affect levels of transpiration? (direct link, not just a factor)
Stomatal pores are responsible for gas exchange in the leaf and hence levels of photosynthesis will affect transpiration
What factors will affect transpiration?
humidity
temperature
light intensity
wind
What is the transpiration stream?
The flow of water through the xylem from the roots to the leaf, against gravity, is called the transpiration stream
What two key properties of water help it travel through the xylem?
Water rises through xylem vessels due to two key properties of water – cohesion and adhesion
What is cohesion?
Cohesion is the force of attraction between two particles of the same substance (e.g. between two water molecules)
How do water molecules exhibit cohesion?
Water molecules are polar and can form a type of intermolecular association called a hydrogen bond
How does cohesion help the water travel up the xylem?
This cohesive property causes water molecules to be dragged up the xylem towards the leaves in a continuous stream
What is adhesion?
Adhesion is the force of attraction between two particles of different substances (e.g. water molecule and xylem wall)
What is water attracted to in the xylem (adhesion) and why?
The xylem wall is also polar (specifically lignin) and hence can form intermolecular associations with water molecules
How does adhesion help water travel up the xylem?
As water molecules move up the xylem via capillary action, they pull inward on the xylem walls to generate further tension
What is the xylem?
The xylem is a specialised structure that functions to facilitate the movement of water throughout the plant
What are 4 key structural properties of the xylem?
- composed of dead cells
- dead cells are hollow
- cells wall contains pits
- walls have thickened cellulose and are reinforced by lignin
Why is it important the xylem is composed of dead and HOLLOW cells?
It is a tube composed of dead cells that are hollow (no protoplasm) to allow for the free movement of water
In which direction does water move during transpiration and to which structural property is this related to?
Because the cells are dead, the movement of water is an entirely passive process and occurs in one direction only
Why is it important that the xylem has pits?
The cell wall contains numerous pores (called pits), which enables water to be transferred between cells
Why is it important that the xylem has thickened cellulose walls and is reinforced by lignin?
Walls have thickened cellulose and are reinforced by lignin, so as to provide strength as water is transported under tension
What is the xylem composed of?
- tracheids (all plants)
- vessel elements (angiosperms)
What are tracheids?
Tracheids are tapered cells that exchange water solely via pits,
How do tracheids affect the rate of water transport?
lead to a slower rate of water transfer (water only transported through pits)
What are vessel elements? (structure)
In vessel elements, the end walls have become fused to form a continuous tube
How do vessel elements affect the rate of water transfer?
result in a faster rate of water transfer
What are all xylem vessels reinforced by?
lignin
In what 2 ways can lignin be deposited?
- in annular vessels
2. in spiral vessels
How is lignin deposited in annular vessels?
In annular vessels, the lignin forms a pattern of circular rings at equal distances from each other
How is lignin deposited in spiral vessels?
In spiral vessels, the lignin is present in the form of a helix or coil
How do plants take up water and minerals?
Plants take up water and mineral ions from the soil via their roots
What do plants need to ensure their roots have to optimise mineral and water uptake?
maximal surface area to optimise this uptake
In what two ways (simple) do plants maximise the SA of their roots?
Some plants have a fibrous, highly branching root system
Other plants have a main tap root with lateral branches
How does a fibrous, highly branching root system help increase SA?
increases the surface area available for absorption
How does a main tap root with lateral branches help uptake water and minerals?
can penetrate the soil to access deeper reservoirs of water
How may the epidermis of roots be adapted to increase SA?
The epidermis of roots may have cellular extensions called root hairs, which further increase the surface area for absorption
- Where do materials move once absorbed by the root epidermis?
Materials absorbed by the root epidermis diffuse across the cortex towards a central stele, where the xylem is located
- What blocks the passage of water in the roots?
The stele is surrounded by an endodermis layer that is impermeable to the passive flow of water and ions (Casparian strip)
- How does water and minerals pass through the Casparian strip?
Water and minerals are pumped across this barrier by specialised cells, allowing the rate of uptake to be controlled
What does fertile soil type usually contain?
Fertile soil typically contains negatively charged clay particles to which positively charged mineral ions (cations) may attach
What 4 main minerals need to be absorbed by the plant?
Minerals that need to be taken up from the soil include Mg2+ (for chlorophyll), nitrates (for amino acids), Na+, K+ and PO43–
In what two ways may mineral ions be transported into the roots?
Mineral ions may passively diffuse into the roots, but will more commonly be actively uploaded by indirect active transport
- What do root cells contain which is the main component of active transport? (indirect)
AT
Root cells contain proton pumps that actively expel H+ ions (stored in the vacuole of root cells) into the surrounding soil
- How do the H+ ions affect the positively charged mineral ions?
AT
The H+ ions displace the positively charged mineral ions from the clay, allowing them to diffuse into the root along a gradient
- How do the H+ ions affect the negatively charged mineral ions?
AT
Negatively charged mineral ions (anions) may bind to the H+ ions and be reabsorbed along with the proton
How is water taken into the root? What process?
Water will follow the mineral ions into the root via osmosis – moving towards the region with a higher solute concentration
What will regulate the rate of water uptake?
The rate of water uptake will be regulated by specialised water channels (aquaporins) on the root cell membrane
In what two ways can water move towards the xylem?
Once inside the root, water will move towards the xylem either via the cytoplasm (symplastic) or via the cell wall (apoplastic)
How does water move in the symplastic pathway?
In the symplastic pathway, water moves continuously through the cytoplasm of cells (connected via plasmodesmata)
How does water move in the apoplastic pathway?
In the apoplastic pathway, water cannot cross the Casparian strip and is transferred to the cytoplasm of the endodermis
What are desert plants called?xe
xerophytes
What are plants that grow in high salinity called?
halophytes
Why must xerophytes have adaptations for water conservation?
Xerophytes will have high rates of transpiration due to the high temperatures and low humidity of desert environments
Why must halophytes have adaptations for water conservation?
Halophytes will lose water as the high intake of salt from the surrounding soils will draw water from plant tissue via osmosis
What are xerophytes? (definition)
Xerophytes are plants that can tolerate dry conditions (such as deserts) due to the presence of a number of adaptations
What 6 (simple) adaptations can xerophytes have?
- reduced leaves
- rolled leaves
- thick, waxy cuticle
- stomata in pits
- low growth
- CAM Physiology
How do reduced leaves help reduce water loss?
reducing the total number and size of leaves will reduce the surface area available for water loss
How do rolled leaves help reduce water loss?
reducing the total number and size of leaves will reduce the surface area available for water loss
How does a thick waxy cuticle help reduce water loss?
having leaves covered by a thickened cuticle prevents water loss from the leaf surface
How do stomata in pits help reduce water loss?
having stomata in pits, surrounded by hairs, traps water vapour and hence reduces transpiration
How does low growth help reduce water loss?
low growing plants are less exposed to wind and more likely to be shaded, reducing water loss
How does CAM physiology help reduce water loss?
plants with CAM physiology open their stomata at night, reducing water loss via evaporation
What are halophytes?
Halophytes are plants that can tolerate salty conditions (such as marshlands) due to the presence of a number of adaptations:
What 5 (simple) adaptations can halophytes have?
- cellular sequestration
- tissue partitioning
- root level exclusion
- salt excretion
- altered flowering schedule
How does cellular sequestriation help reduce water loss?
halophytes can sequester toxic ions and salts within the cell wall or vacuoles
How does tissue partitioning help reduce water loss?
plants may concentrate salts in particular leaves, which then drop off (abscission)
How does root level exclusion help reduce water loss?
plant roots may be structured to exclude ~95% of the salt in soil solutions
How does salt excretion help reduce water loss?
certain parts of the plant (e.g. stem) may contain salt glands which actively eliminate salt
How does an altered flowering schedule help reduce water loss?
halophytes may flower at specific times (e.g. rainy seasons) to minimise salt exposure
What apparatus can be used to model the movement of water up the xylem?
These include capillary tubing, filter or blotting paper and porous pots
How does water flow up capillary tubing?
Water has the capacity to flow along narrow spaces in opposition to external forces like gravity (capillary action)
How is water able to move against gravity?
This is due to a combination of surface tension (cohesive forces) and adhesion with the walls of the tube surface
how does the diameter of a tube affect capillary action?
The thinner the tube or the less dense the fluid, the higher the liquid will rise (xylem vessels are thin: 20 – 200 µm)
What does filter paper do in relation to water?
Filter paper (or blotting paper) will absorb water due to both adhesive and cohesive properties
What will happen when filter paper is placed perpendicularly to a water source?
When placed perpendicular to a water source, the water will hence rise up along the length of the paper
What can the movement of water up filter paper be compared to?
This is comparable to the movement of water up a xylem (the paper and the xylem wall are both composed of cellulose)
What are porous pots?
Porous pots are semi-permeable containers that allow for the free passage of certain small materials through pores
What is the loss of water from a porous pot similar to?
The loss of water from the pot is similar to the evaporative water loss that occurs in the leaves of plants
What can be done to a porous pot to represent the movement of water up the xylem?
If the porous pot is attached by an airtight seal to a tube, the water loss creates a negative pressure that draws more liquid
- What is a potometer?
A potometer is a device that is used to estimate transpiration rates by measuring the rate of water loss / uptake
- How can a potometer be used to measure transpiration?
When a plant is affixed to the potometer, transpiration can be indirectly identified by the movement of water towards the plant
- How can the water change in a potometer be assessed?
This water movement can be assessed as a change in meniscus level or by the movement of an air bubble towards the plant
- How can the initial starting point of the meniscus/air bubble in a potometer be altered?
The initial starting position of the meniscus or air bubble can be adjusted by introducing additional water from a reservoir
What is important to remember when using a potometer to measure transpiration?
When measuring transpiration rates with a potometer, it is important to remember that not all water is lost to transpiration
In what ways can water be lost apart from transpiration?
A small amount of water (~2%) is used in photosynthesis and to maintain the viable turgidity of plant cells
What variables can affect transpiration?
temperature
humidity
light intensity
wind exposure
In what way does temperature affect transpiration?
Increasing the ambient temperature is predicted to cause an increase in the rate of transpiration
How does temperature change transpiration rates?
Higher temperatures lead to an increase in the rate of water vaporisation within the mesophyll, leading to more evaporation
How can the effect of temperature variation on transpiration be tested?
use of a potometer
The effect of temperature variation can be tested experimentally by using heaters or submerging in heated water baths
In what way does humidity affect transpiration?
Increasing the humidity is predicted to cause a decrease in the rate of transpiration
How does humidity change transpiration rates?
Humidity is the amount of water vapour in the air – less vapour will diffuse from the leaf if there is more vapour in the air
How can the effect of humidity on transpiration be tested?
use of a potometer
The effect of humidity can be tested experimentally by encasing the plant in a plastic bag with variable levels of vapour
In what way does light intensity affect transpiration?
Increasing the light intensity to which a plant is exposed is predicted to cause an increase in the rate of transpiration
How does light intensity change transpiration rates?
Increasing light exposure will cause more stomata to open in order to facilitate photosynthetic gas exchange
How can the effect of light intensity on transpiration be tested?
use of a potometer
The effect of light intensity can be tested experimentally by placing the plant at variable distances from a lamp
In what way does wind exposure affect transpiration?
Increasing the level of wind exposure is predicted to cause an increase in the rate of transpiration
How does wind exposure change transpiration rates?
Wind / air circulation will function to remove water vapour from near the leaf, effectively reducing proximal humidity
How can the effect of wind exposure on transpiration be tested?
use of a potometer
The effect of wind can be tested experimentally by using fans to circulate the air around a plant
