9.1 Transport in the xylem of plants Flashcards
What is transpiration?
Transpiration is the loss of water vapour from the stems and leaves of plants
What are stomata?
Stomata are pores on the underside of the leaf which facilitate gas exchange
What does light energy convert in transpiration?
Water in the leaves into vapour
What happens to the vapour after it has been converted from water?
It evaporates from the leaf via the stomata
Where is new water absorbed?
From the soil by the roots
What does the new water absorbed create?
A difference in pressure between the leaves and the roots
During transpiration, what has low pressure?
The leaves
During transpiration, what has high pressure?
The roots
What requires the stomata to be open?
Photosynthetic gas exchange
What affects transpiration?
The levels of photosynthesis
What is transpiration a consequence of?
Gas exchange in the leaf
Why does new water need to be absorbed during transpiration?
To replace the lost water from transpiration
When is water lost from the leaves?
When it is converted into vapour and diffuses out the stomata
What happens to some of the light energy absorbed by leaves?
It is converted into heat
What does the heat that was converted by light energy do?
Evaporates water within the spongy mesophyll
When the vapour diffuses out of the leaf what does it create?
A negative pressure gradient within the leaf
What does the negative pressure gradient within the leaf create?
A tension force in leaf cells which draws water from the xylem
How is water pulled from the xylem under tension?
By the adhesive attraction between water and the leaf cell walls
What is the transpiration rate?
The amount of water lost from the leaves
What regulates the transpiration rate?
The opening and closing of the stomata
How do guard cells cause the stomata to open?
By being on either side of the stomata and by becoming increasingly flaccid in response to cellular signals
What do dehydrated mesophyll cells release?
Plant hormone abscisic acid (ABA)
When do mesophyll cells release ABA?
When a plant begins to wilt from water stress
What does abscisic acid trigger?
The efflux of potassium from guard cells
What does the efflux of potassium from guard cells trigger?
Decease in water pressure within the cells (lose turgor)
What does lose turgor mean?
Decrease in water pressure within the cell
What does a loss of turgor cause?
The stomata to close as the guard cells become flaccid and block the opening
When will transpiration rates be highest?
When stomatal pores are open than when they are closed
Apart from photosynthesis what are other factors that affect transpiration rates?
Humidity
Temperature
Light intensity
Wind
What are stomatal pores responsible for?
Gas exchange in the leaf
When a stoma is open it is…
Turgid
When a stoma is closed it is…
Flaccid
What is a transpiration stream?
The flow of water through the xylem from the roots to the leaf against gravity
What are the two key properties responsible for helping water ride through the xylem?
Cohesion and adhesion
What is cohesion?
Cohesion is the force of attraction between two particles of the same substance
Are water molecules polar or non polar?
Polar
Why can water molecules form intermolecular accossiations?
Because water molecules are polar
What is the type of bond water molecules form?
Hydrogen bond
What does the cohesive property cause?
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?
Why can the xylem wall form polar intermolecular associations with water molecules?
Because the xylem wall is polar
What are an example of the different substances in adhesion?
Xylem wall and water molecule
What are an example of the same substances in cohesion?
Water molecules
How do water molecules move up the xylem by?
Capillary action
What does water moving up via capillary action cause?
Water molecules pull inwards on the xylem wall which creates further tension
What is the xylem? (function)
A specialised structure that functions to facilitate the movement of water throughout the plant
What is the xylem?
(Structurally)
It is a tube composed of dead hollow cells to allow the free movement of water
Why is the movement of water in the xylem passive?
Because the cells are dead
How many directions does the movement of water flow in the xylem?
One direction only
What is the structure of the wall of the xylem?
The walls have thickened cellulose and reinforced by lignin
Why is the xylem wall thickened by cellulose and reinforced by lignin?
To provide strength as water is transported under tension
What does the cell wall of the xylem contain?
Numerous pores called pits
What do pits enable?
It enables water to be transferred between cells
What can xylems be composed of?
Tracheids or vessel elements
What are all vascular plants composed of?
Tracheids
What are only certain vascular plants composed of?
Vessel elements
What are tracheids?
Tracheids are tapered cells that exchange water solely via pits
What do tracheids result in?
A slower rate of water transfer
In vessel elements, what happens to the end walls?
The end walls have become fused to form a continuous tube
What do vessel elements result in?
A faster rate of water transfer
What are the two types of xylem vessels?
Annular vessels and spiral vessels
How is lignin deposited in annular vessels?
In a pattern of circular rings that are equally distanced from each other
How is lignin deposited in spiral vessels?
In the form of a helix or a coil
What does the xylem conduct?
Water and minerals
How do plants take up water and mineral ions via?
Via their roots
What do roots need to optimise their uptake of water and mineral ions?
A maximal surface area
What does a fibrous, highly branching root system do?
Increases the surface area available for absorption
What does a main tap root with lateral branches do?
Penetrate the soil to access deeper reservoirs of water
What will the epidermis of roots have?
Root hairs
What do root hairs do?
Further increase the surface area for absorption
What surrounds the steele?
An endodermis layer that is impermeable to the passive flow of water and ions
What pumps water and minerals across the barrier?
Specialised cells
What do the specialised cells allow?
The rate of uptake to be controlled
Where do materials absorbed by the root epidermis diffuse to?
Across the cortex towards a central stele
What is found in the central stele?
The xylem
What does fertile soil typically contain?
Negatively charged clay particles
What can attach to negatively charged clay particles?
Positively charged mineral ions
What are five examples of minerals that need to be uptaken from the soil?
Mg2+
nitrates
Na+
K+
PO43–
How are mineral ions diffused into the roots?
Passively
How are minerals transported up the xylem?
Actively by indirect active transport
What do root cells contain?
Proton pumps
What do the proton pumps in root cells do?
Actively expel H+ ions into the surrounding soil
What do the H+ ions displace?
They displace the positively charged mineral ions from the clay
What does displacing the positively charged mineral ions from the clay do?
Allows them to diffuse into the root along a gradient
What may negatively charged mineral ions bind to and what happens?
H+ ions and they are then reabsorbed with the proton
What follows mineral ions into the root?
Water
How does water follow mineral ions into the root?
Via osmosis
Why dpes water move into the root via osmosis?
Its moving towards the region with a higher solute concentration
What regulates the rate of water uptake?
Specialised water channels called aquaporins on the root cell membrane
Once water is inside the root how will water move towards the xylem?
Via the cytoplasm or via the cell wall
If the water moves via the cytoplasm what is it called?
Symplastic
If the water moves via the cell wall what is it called?
Apoplastic
What happens in the symplastic pathway?
Water moves continuously through the cytoplasm of cells
What happens in the apoplastic pathway?
Water cannot cross the casparian strip and is transferred to the cytoplasm of the endodermis
How are the cytoplasm of cells connected?
Via plasmodesmata
When drawing the structure of the xylem, how should vessel elements be drawn?
As a continuous tube
When drawing the structure of the xylem, how should the remnants of the fused end wall be represented as?
Indents
When drawing the structure of the xylem, what should the xylem wall contain?
Gaps
When drawing the structure of the xylem, how should lignin be represented?
As either a spiral or rings
What are xerophytes?
Desert plants
What are halophytes?
Plants that grow in high salinity
Why will xerophytes have high rates of transpiration?
Due to high temperatures and low humidity of the desert environments
Why will halophytes lose water?
Due to the high intake of salt from the surrounding soils draw water from the plant tissue
What are the six adaptations xerophytes have?
Reduced Leaves
Rolled Leaves
Thick waxy cuticle
Stomata in pits
Low growth
CAM physiology
How does reduced leaves help xerophytes?
Reducing the number and size of the leaves reduces the surface area for water loss
How does rolled leaves help xerophytes?
Rolling up leaves reduces the exposure stomata have to the air thus reducing evaporative water loss
How does thick waxy cuticles help xerophytes?
Having leaves covered by a thickened cuticle prevents water loss from the leaf surface
How does stomata in pits help xerophytes?
Having stomata in pits and surrounded by hair traps water vapour and reduces transpiration
How does low growth help xerophytes?
Low growing plants are less exposed to wind and are more likely to be shaded which reduces water loss
How does CAM physiology help xerophytes?
Plants with CAM physiology open their stomata at night, reducing water loss via evaporation
What are the five adaptations halophytes will have?
Cellular sequestration
Tissue Partitioning
Root level exclusion
Salt excretion
Altered flowering schedule
How does cellular sequestration help halophytes?
Halophytes can sequester toxic ions and salts within the cell wall or vacuoles
How does tissue partitioning help halophytes?
Plants may concentrate cells in particular leaves which then drop off
How does root level exclusion help halophytes?
Plant roots may be structured to exclude 95% if the salt in the soil solutions
How does salt excretion help halophytes?
Certain parts of the plant may contain salt glands which actively eliminate salt
How does altered flowering schedule help halophytes?
Halophytes may flower at specific times to minimise salt exposure
What are the three apparatus used to model the movement of water up the length of the xylem?
Capillary tubing
Filter paper
Porous pots
Why does water have to capacity to flow along narrow spaces?
Due to a combination of surface tension and adhesion with the walls
What does capillary tubing show?
The thinner the tube or the less dense the fluid, the higher the liquid will rise
How does filter paper work?
Filter paper absorbs water due to both adhesive and cohesive properties
What happens when filter paper is placed perpendicular to the water source?
The water will rise up along the length of the paper
What does water rising along the length of the filter paper compare to?
The movement of water up a xylem
What is filter paper made of?
Cellulose
What are porous pots?
Porous pots are semi permeable containers that allow the free passage of certain materials through pores
What is the loss of water from porous pots similar to?
It is similar to the evaporative water loss that occurs in the leaves of the plants
What happens if the porous pot is attached by an airtight seal to a tube?
The water loss creates a negative pressure which brings in more liquid
What is a potometer?
A potometer is a device that is used to estimate transpiration rates by measuring the rate of water gain/loss
What is shown when a plant is affixed to a potometer?
Transpiration is shown by the movement of water towards the plant
How can water movement in a potometer be assessed?
By a change in meniscus level or by the movement of an air bubble towards the plant
What can alter the initial starting point of the meniscus or air bubble in a potometer?
Adding additional water
What is important to remember when measuring transpiration rates with a potometer?
Not all water is lost to transpiration
If not all water is lost to transpiration where else is it lost?
To photosynthesis and to maintain the viable turgidity of plant cells
What can potometers be used to test?
Variables that may affect the rate of transpiration in plants
What are the variables potometers can measure?
Temperature
Humidity
Light intensity
Wind exposure
What does increasing the temperature do to the rate of transpiration?
Increases the rate of transpiration
How can you test temperature using a potometer?
With a heater or a heated water bath
How do higher temperatures lead to more evaporation?
It increases the rate of water vaporisation within the mesophyll
What does increasing the humidity do to the rate of transpiration?
Decreases the rate of transpiration
What is humidity?
Humidity is the amount of water vapour in the air
How can you test humidity using a potometer?
By encasing the plant in a plastic bag with variable levels of vapour
How does humidity affect the amount of vapour diffused out of a leaf?
Less vapour will diffuse from the leaf if there is more vapour in the air
How does increasing light intensity affect the rate of transpiration?
It causes an increase in the rate of transpiration
What does increasing light exposure do?
Increasing light exposure causes more stomata open in order to facilitate photosynthetic gas exchange
How can you test light exposure using a potometer?
By placing a plant at variable distances from a lamp
How does increasing the level of wind exposure affect the rate of transpiration?
It increases the rate of transpiration
How does increasing wind exposure reduce proximal humidity?
Wind removes water vapour from near the lead reducing proximal humidity
How can you test wind exposure with a potometer?
By using fans
