Chapter 8 - Transport in Plants and Transpiration Flashcards
Draw a cross-section of a plant root
Textbook page 135
Epidermis
(Root hair extensions should be present on the epidermal layer)
Cortex
Endodermis
Central stele (vascular cylinder) composed of vascular tissue (xylem and phloem)
Describe the structure of the endodermal layer in a plant root
Single layer of cells immediately outside the stele
Cells have a waterproof layer of suberin imbedded in their cell walls known as the Casparian strip
The layer of cells between the epidermis and endodermis is the …
Cortex
Cells in the cortex of a plant root are …
Undifferentiated
Have small air spaces between them
Are rich in starch grains
The stele is composed of …
Mainly xylem tissue
Smaller amounts of phloem tissue
Cambium tissue (meristematic region)
What are the two transport systems in flowering plants?
Xylem
Phloem
The xylem is differentiated into a number of cell types but the main type involved in water and ion transport is the …
Xylem vessel
How are xylem vessels specialised for the transport of water and inorganic mineral ions?
- No end walls
- No cell contents
- Dead when fully formed
- Have an impermeable secondary cell wall composed of lignin inside the primary cellulose cell wall
- Have pores
Why are mature xylem vessels dead?
Lignin is impermeable to water, so mature xylem vessels are dead.
What are the two types of xylem?
Protoxylem
Metaxylem
Why do xylem vessels have no end walls or cell contents?
No end walls = continuous tubes.
No cell contents = empty tube.
Therefore, water movement through xylem vessels requires less pressure than through living cells, where movement would be slowed down by cell contents.
As the xylem vessels form, their end walls …
Break down
Why do xylem vessels have a large vessel lumen?
Large empty lumen allows flow of large volumes of water
What is protoxylem and where is it found?
Protoxylem is first-formed xylem
Found in young growing regions (more specifically, the region of elongation) behind root and shoot tips
What are the four different types of patterns of lignification?
Spiral
Annular
Reticulate
Pitted
What is metaxylem and where is it found?
Mature (older) xylem
Found in more mature parts of the plant
What are the major differences between protoxylem and metaxylem?
- In metaxylem, the mature xylem vessels are dead.
- In metaxylem there is greater deposition of lignin (as the protoxylem cell walls are less thick, and due to the different patterns of lignification).
- Metaxylem is older compared to the younger protoxylem.
- Protoxylem cells are typically smaller.
What two patterns of lignification are present in protoxylem vessels?
Spiral
Annular
What two patterns of lignification are present in metaxylem vessels?
Reticulate
Pitted
Describe the pattern of thickening in spiral vessels of protoxylem
Cell wall thickening in the form of a continuous spiral.
Describe the pattern of thickening in annular vessels of protoxylem
Cell wall thickening in the form of discrete loops.
Why does lignin laid down in protoxylem vessels form annular and spiral patterns?
Lignin laid down in these patterns does not restrict the elongation of the xylem vessels along with other tissues as growth of root tips takes place
In the xylem found in the mature parts of the plant, known as metaxylem, there is greater …
Deposition of lignin
Describe the pattern of thickening in reticulate vessels of metaxylem
Reticulate vessels are thickened by interconnecting bars of lignin
Describe the pattern of thickening in pitted vessels of metaxylem
Pitted vessels are uniformly thickened, except at pores seen as pits that allow rapid movement of water and ions between adjacent vessels and surrounding cells.
What are the two properties of lignin?
- It provides great strength that prevents the vessels from collapsing when under pressure exerted by the transpiration stream ‘sucking’ water up the plant. This strength is also important in providing structural support for the plant.
- It is waterproof, which prevents the leakage of water.
A column of xylem vessels produces …
A long continuous tube up the plant
Draw all four patterns of lignification in xylem vessels
Textbook page 136
What is the function of the small pits present in pitted xylem vessels of metaxylem?
The small pits (pores) allow the movement of water and inorganic mineral ions between adjacent vessels and surrounding cells.
Knowledge check 16
List the features of xylem that make it well adapted to the transport of water.
Xylem vessels lack cross walls, lack any cytoplasm (to impede the flow of water), are lignified (to keep them open and prevent them collapsing) and have pores (to allow water to leave laterally).
Where are new xylem cells produced?
New xylem cells are produced in a meristematic region (the Cambium) between the xylem and the phloem in roots.
Where is protoxylem and metaxylem found in the xylem tissue of the stele of a plant root?
In roots the protoxylem is found on the outer edge of the stele with the metaxylem found innermost.
Reference the photograph of a section through a root vascular cylinder, textbook page 137
Draw a longitudinal and transverse section of metaxylem vessels
Textbook page 136
Longitudinal section
- Thick lignified walls
- Pits
- Cellulose cell wall of adjacent cells
- Remains of vessel cells’ end walls
- Xylem vessels
Transverse section
- Pit
- Vessel lumen
What are the two main cell types in phloem tissue?
Sieve tube elements
Phloem companion cells
The two transport systems in flowering plants (xylem and phloem) are contained within the plant’s …
Vascular tissues
Sieve tube elements are aligned end to end to form a continuous row of cells called …
The sieve tube
What is a sieve tube?
A continuous row of sieve tube cells (elements) aligned end to end.
What are the transporting cells in phloem tissue?
Phloem sieve tube cells (elements)
What are sieve plates?
Sieve plates are end walls in a sieve tube that consist of a thin cellulose cell wall that is perforated to form sieve pores.
How do xylem vessels and sieve tubes differ structurally?
Sieve tubes have end walls called sieve plates
Describe the ultrastructure of phloem sieve tube cells (elements)
Living cells with cell contents
No nuclei
Reduced volume of cytoplasm which is displaced to the side walls
Very few organelles are present
Microtubules extend between sieve elements and pass through the sieve pores
What is the role of microtubules present in sieve tube elements?
Microtubules extend between sieve elements and pass through the sieve pores. It is though that these are involved in the process of translocation.
Each sieve tube cell (element) is closely associated with one or more …
Phloem companion cells
Describe the ultrastructure of phloem companion cells
Dense cytoplasm rich in mitochondria and other organelles.
Have a high metabolic rate.
Companion cells are linked to the sieve tube elements by plasmodesmata.
Prominent nucleus.
What is the role of the plasmodesmata between each phloem sieve tube and its associated companion cell?
Plasmodesmata aid transport of sucrose into and out of sieve tube element
What is the function of the companion cells?
They act as supporting cells, carrying out many metabolic activities for the highly specialised sieve tube elements, allowing them to be specialised for the transport of organic solutes through the plant.
Why are the thin cell walls of phloem sieve tubes not lignified?
As the cells are not under tension
Why do sieve tube elements have their cytoplasm displaced to the side walls, few organelles and no nucleus?
Allows the movement of phloem sap through the centre of the cell (element)
Knowledge check 17
Explain the role of companion cells in translocation
Companion cells contain numerous mitochondria that produce the ATP used to carry out active processes such as loading sucrose etc. into the sieve tubes.
Draw a cross section of a plant stem
Textbook page 138
Epidermis Cortex Vascular bundles - Xylem (innermost) - Phloem (outermost)
Describe the distribution of vascular tissue in a plant stem
The vascular tissue is arranged as vascular bundles around the outside of the stem
Describe the distribution of vascular tissue in a plant root
Vascular tissue forms a central stele (vascular cylinder) in the root
In a plant stem, vascular tissue is arranged as vascular bundles around the outside of the stem. Why is this?
Provides greater support necessary in stems to support branches and leaves.
Where is protoxylem and metaxylem found in a plant stem?
In the vascular bundles, the xylem is found innermost (and the phloem is found outermost). Within the xylem, the protoxylem is usually in the section of xylem closer to the centre of the stem, with the metaxylem in the section of xylem closer to the outer edge of the stem.
Draw a cross section of a plant leaf showing the major tissue layers present
Textbook page 138
- Upper epidermis
- Midrib (xylem above phloem)
- Veins (xylem above phloem)
- Mesophyll
- Lower epidermis
As smaller branches (stems) continually branch they eventually form …
Leaves
As smaller branches (stems) continually branch they eventually form leaves. A vascular bundle continues into the leaf as the …
Midrib
As smaller branches (stems) continually branch they eventually form leaves. A vascular bundle continues into the leaf as the midrib, which branches to form smaller …
Veins that are distributed throughout the leaf
As smaller branches (stems) continually branch they eventually form leaves. A vascular bundle continues into the leaf as the midrib, which branches to form smaller veins that are distributed throughout the leaf. The leaf veins are typically found in …
The spongy mesophyll just below the palisade layer.
What is transpiration?
The evaporation of water from the cell surface membranes of the spongy mesophyll cells (and palisade mesophyll cells to some degree) into the intercellular air spaces and the subsequent diffusion of water vapour down the water potential gradient through the stomata (or through the waxy cuticle) and into the atmosphere.
It is convenient to consider the movement of water and ions through a plant as having three distinct phases:
- The transport of water (and ions) into and across the root.
- The transport of water up the root and stem in the xylem.
- The transport of water through the leaf and the evaporation of water from the leaf.
Knowledge check 15 Describe the distribution of: a) xylem b) phloem in both roots and stems
a) Xylem is the star-shaped tissue located centrally in the root and on the inner sides of the vascular bundles in the stem.
b) Phloem is found between the arms of the xylem in the root and on the outer sides of the vascular bundles in the stem.
What are the two types of transpiration?
Cuticular transpiration
Stomatal transpiration
What is the effective exchange surface in root hair cells?
The cell surface membrane
Water enters the root hair cells by …
Osmosis
Inorganic mineral ions enter the root hair cell by …
Active transport or facilitated diffusion (depending on the concentration gradient of the ion involved)
The cell surface membranes of root hair cells are …
Thin
Why does water enter the root hair cells by osmosis?
The water in the soil has a higher water potential compared to the root hair cell (which contains sugars and other compounds).
Once into the root hair cell, the water (and ions) move across the cells of the cortex and into the xylem in the stele. There are two main pathways involved:
- The apoplast pathway
* The symplast pathway
What is the apoplast pathway?
Water (and ions) move along the cellulose microfibrils of the cell walls.
What is the symplast pathway?
Water (and ions) move by osmosis from cell to cell through plasmodesmata.
Why and how does the apoplast pathway occur in the root?
The apoplast pathway - involves water (and ions) moving along the cellulose microfibrils of the cell walls.
As the water moves through the wall, the cohesive properties of the water (aided by hydrogen bonding) help pull the water column along.
Parallel arrangement of the microfibrils in the cellulose cell wall allows water to pass easily between the different layers, rather than through them.
The general mesh-like arrangement of the walls further aids movement.
Turn to textbook page 139 to see a diagram explaining this.
Most water tends to move through the cortex by the …
Apoplast pathway
Why does most water tend to move by the apoplast pathway?
Due to the limited resistance to water movement
Why and how does the symplast pathway occur?
Symplast pathway - Water moves by osmosis from cell to cell through plasmodesmata.
The movement of water across the root creates the water potential gradient necessary for this to take place.
As a root hair takes in water, it’s water potential becomes less negative and is higher than the adjacent cell in the root cortex. Water moves from the root hair cell into the cortex cell by osmosis and so on across the cells of the cortex.
All water moving into the stele is transported by the …
Symplast pathway
The apoplast and symplast pathways occur in …
The root and leaf
What is the role of the Casparian strip in the endodermis?
- It has a waterproof layer formed of Suberin (the Casparian strip) embedded in the cellulose cell wall that encircles each cell.
- The positioning of the Casparian strip prevents the movement of water through the endodermis via the apoplast pathway.
- At that point, water being transported by the apoplast pathway moves into the protoplast to join the water transported by the symplast pathway.
- Consequently, all water moving into the stele is transported by the symplast pathway.
- This ensures that water transport at this point is under metabolic control.
What benefit to the plant does all the water moving into the stele by the symplast pathway have?
Water (and ion) transport at this point is under metabolic control.
What is the role of the endodermis?
Ensuring the symplast pathway into the stele
Why does water move from the endodermis into the stele?
- The endodermal cells pump ions into the xylem cells, a process that involves energy expenditure.
- This creates a water potential gradient that draws water from the endodermis into the xylem.
- Additionally, the cohesive pull of water by the transpiration stream in the xylem helps move water from the endodermis into the xylem.
The movement of water osmotically into the base of the xylem tissue in the root creates …
A root pressure force
The movement of water osmotically into the base of the xylem tissue creates a root pressure force which …
Helps to move water up the plant.
The movement of water from root hairs to xylem tissue in roots occurs by which two pathways?
Apoplast pathway
Symplast pathway
What forces move water up the root and stem in the xylem?
- Cohesion-tension theory
- Root pressure (mass flow)
- Adhesion (capillarity)
What effect does the negative pressure produced in the leaf (caused by the evaporation of water) have on the transpirational stream?
Pulls the water column up through the xylem as a mass flow movement
What is some of the evidence for the cohesion-tension theory?
- If the water column in the xylem is broken and an air gap appears, water below the gap cannot be pulled up (in reality each column of xylem vessels can be treated as a single column of water, so a water column can be disrupted in one xylem column but continue in others).
- During the day, when transpiration is normally at its greatest, there is much more tension, or negative pressure, in the xylem. This negative pressure tends to pull the walls of the xylem vessels in and can reduce the diameter of a tree trunk.
What makes xylem vessels highly adapted for water transport by the cohesion-tension method?
Missing end walls and absence of cell contents
What is the cohesion-tension theory?
- As water evaporates out of the stomata in the leaf, it creates a negative pressure that pulls the water column up through the xylem as a mass flow movement.
- This process requires the water column to form a continuous unbroken pathway through the xylem (the transpiration stream).
- Water molecules form hydrogen bonds between one another, a feature known as cohesion, and this tends to stick the water molecules together.
- It is the cohesive properties of water that allows the water to be ‘sucked’ up the xylem in a continuous column, as the water at the leading edge of the column evaporates out of the leaf.
What is the transpiration stream?
The continuous unbroken flow of water from the roots to leaves through the xylem.
What is the driving force for the cohesion-tension theory?
The evaporation (transpiration) of water out of the leaves, resulting in the transpirational pull.
What is the transpirational pull?
Negative pressure (tension) caused by the evaporation (transpiration) of water out of leaves.
What process in the xylem is the most important for the movement of water up through the xylem in roots and stems?
Cohesion-tension theory
If a drinking straw or a capillary tube is placed in water, the water will move up the tube to some extent. Why?
This is due to the adhesive forces between the substances in the tube and the water being stronger than the cohesive forces between the water molecules themselves.
What is adhesion?
The attraction of unlike materials.
If a drinking straw or capillary tube is placed in water, the water will move up the tube to some extent. This is due to the adhesive forces between the substances in the straw, or capillary tube, and the water being stronger than the cohesive forces between the water molecules themselves. What is this phenomenon known as?
Capillarity
What is the role of capillarity (adhesion) in water transport in the xylem?
It may not have a significant role in moving the water up the xylem but the adhesive forces between the water column and the xylem walls may reduce the forces necessary for the transpiration pull.
What is capillarity?
The tendency of a liquid in a capillary tube to rise due to the adhesive forces between the capillary tube and the liquid molecules being greater than the cohesive forces between the liquid molecules themselves.
As the rate of transpiration increases in a plant (during the day), the tree trunk diameter …
Decreases
Where does water enter the leaf?
The midrib (vascular bundle)
How does water in the leaf move from the xylem to the spongy mesophyll cells?
The apoplast and symplast pathways
Give two examples of places in the leaf where the apoplast and symplast pathways occur
Water transport across the leaf (from the xylem to spongy mesophyll cells)
Water transport across the root (from the epidermis to the stele)
What are the two types of factors which affect the rate of transpiration?
Internal and external
Internal factors are sometimes referred to as …
Leaf characteristics
Name some of the internal factors that affect the rate of transpiration.
- Stomatal density
- Leaf surface area
- Cuticle thickness
External factors are sometimes referred to as …
Environmental factors
Name some of the external factors that affect the rate of transpiration.
- Light intensity
- Wind speed
- Temperature
- Humidity
- Soil water availability
Describe some of the internal factors that affect the rate of transpiration.
- Stomatal density
- Number of stomata per unit area of leaf.
- The more stomata per unit area in a leaf, the more evaporation (and transpiration). - Leaf surface area
- The greater the leaf surface area, the more evaporation of water (and transpiration).
- This is because there is a positive correlation between leaf area and stomatal number. - Cuticle thickness
- Thicker cuticles tend to lose less water by evaporation than thinner cuticles.
- Reduces cuticular transpiration
Describe some of the external factors that affect the rate of transpiration
- Light intensity
- Light has little effect directly on evaporation rate.
- However, the rate of evaporation and transpiration will be greater during the day (in light) compared to during the night (in darkness), as the stomata of most plants are closed when it is dark. - Wind speed
- Increasing wind speed increases the rate of evaporation as the wind removes diffusion shells by blowing humid air away from the leaf.
- This maintains a steep water potential gradient between the inside and outside of the leaf.
- This allows water to evaporate rapidly from spongy mesophyll cells into the air spaces of the spongy mesophyll, water vapour which then diffuses out of the stomata. - Temperature
- The higher the temperature, the faster the rate of evaporation of water from the spongy mesophyll cells (and the faster diffusion of water vapour through the stomata). - Humidity
- Increasing humidity decreases the rate of evaporation and transpiration.
- Humid air decreases the water potential gradient between the inside of leaves and the surrounding atmosphere.
- Sub-stomatal air spaces become more humid due to the build-up of water vapour, reducing evaporation from the spongy mesophyll cells. - Soil water availability
- Evaporation and transpiration rates are affected by the availability of water to the plant.
- If water is in short supply, plants are unable to replace water lost in transpiration with water from the soil, so the stomata close, reducing transpirational loss.
- During very dry summers, evaporation and transpiration rates can be lower than expected due to lack of water in the soil.
- If a plant (leaf) is dehydrated, the stomata will close automatically. This is a defensive mechanism to conserve water in times of significant water stress.
If a plant (leaf) is dehydrated, the stomata will …
Automatically close
If a plant (leaf) is dehydrated, the stomata will automatically close. Why?
This is a defensive mechanism to conserve water during times of significant water stress.
What is cuticular transpiration?
The loss of water by evaporation through the waxy cuticle.
What is translocation?
The bidirectional movement (transport) of organic solutes in the phloem.
What type of vascular tissue is involved in the transport of organic solutes?
Phloem
Where do organic substances synthesised in the leaf get transported to?
To growing regions of the plant (for example, carbohydrate (mainly sucrose) which is converted into glucose for energy, and amino acids for growth)
And to the roots (for storage in the form of starch)
What is the main substance transported in the phloem?
The disaccharide sucrose
What are the two key features associated with translocation?
- The process is energy requiring
2. Two-way transport exists
What does ‘two-way transport’ mean in relation to translocation?
Translocation can move sucrose both up and down sieve tubes from source to sink.
What is some of the evidence in support of energy expenditure in the process of translocation?
Evidence for energy expenditure comes from a number of sources:
- Companion cells have high rates of metabolic activity
- They are intimately associated with sieve tube elements and their energy output is linked to processes involved in the uptake of sucrose from adjacent (photosynthesising) cells and the subsequent loading of sucrose into the sieve tube elements via plasmodesmata, before being transported around the plant.
- Companion cells have high rates of metabolic activity
- Metabolic inhibitors, such as cyanide, that stop respiration in plant cells also disrupt the process of translocation.
- The rate of flow (1 mh-1) is higher than can be accounted for by diffusion.
What happens to sucrose transported to growing regions of the plant?
It is converted into glucose for use in respiration
What happens to sucrose transported to plant roots?
It is built up into starch for storage
What are some of the organic solutes/substances that are transported in the phloem?
Carbohydrate (mainly in the form of sucrose)
Amino acids
How is radioactively-labelled sucrose produced?
Using carbon dioxide (CO2) containing Carbon-14 isotopes
What is some of the evidence in support of two-way movement in the process of translocation?
The use of radioactively-labelled sucrose (sucrose produced using 14CO2) shows that the sucrose can move both up and down the stem.
Describe simply the two-way movement of sucrose (organic solutes) in the phloem
Sucrose can move up in one sieve tube and down in an adjacent sieve tube.
Movement occurs from source to sink.
What forces cause the movement of organic solutes in the phloem (translocation) to occur?
Although translocation is an example of a mass flow system, the localised build-up of sucrose (‘source’) helps create a hydrostatic gradient between some parts of the plant and the ‘sink’ where sucrose levels are lower.
Translocation is an example of …
A mass flow system
What is a ‘source’ in relation to translocation in the phloem?
The organ where sugar is produced in photosynthesis or by the breakdown of starch.
Give an example of a ‘source’ in relation to translocation in the phloem
Leaves
What is a ‘sink’ in relation to translocation in the phloem?
The organ that consumes or stores carbohydrate.
Give examples of a ‘sink’ in relation to translocation in the phloem
Growing shoot tip regions Roots Root storage organs Developing buds Flowers Fruit
In most plants a conflict exists between having a large exchange surface for gas exchange and reducing water loss by transpiration. Why?
As the same exchange surface (the cell surface membrane of the spongy mesophyll cells) is responsible for both gas exchange and the evaporation of water in transpiration.
Rapid photosynthesis = …
Rapid growth
What are mesophytes?
Typical plants which only require a moderate amount of water.
What are some mesophytic adaptations for water conversation?
- Waterproofed cuticle
- Stomata (that can be opened and closed)
- Exchange surfaces that are protected within the leaf from excessive rates of evaporation
What is the role of mesophytic adaptations for water conservation?
These features reduce excessive transpiration but do not overly restrict the rate of gas exchange, which is necessary for rapid photosynthesis and hence rapid growth.
What are xerophytes?
Plants adapted to live in arid conditions. They are very highly adapted to reduce water loss by transpiration.
Give some examples of xerophytes
Cacti
Marram grass
Jade
Pine
What are some xerophytic structural adaptations?
- Leaf curvature
- Reduced leaf surface area
- Cuticular thickening
- Leaf hairs
- Sunken stomata
- Succulent tissue
- Deep roots
What are diffusion shells?
Layers or zones of humid air
Sunken stomata (a xerophytic structural adaptation) is often accompanied by …
A reduction in the number of stomata in the leaf
Explain why leaf curvature is an effective structural adaptation of xerophytes
- Some xerophytes fold their leaves so that the ‘lower’ epidermis is enclosed and protected within the leaf.
- This is particularly effective as the stomata are confined to the lower epidermis (as in many species) and it is this layer that lies within the fold.
- This folding creates a layer of humid air within the leaf, significantly reducing the water potential gradient between the inside and outside of the leaf.
- This limits the rate of evaporation and consequently transpiration.
Give an example of a xerophyte which has folded (curved) leaves
Marram grass
Why does the lower epidermis lie within the fold of a xerophytic curved leaf?
As most stomata are situated on the lower epidermal layer.
Explain why reducing the surface area of the leaf is an effective structural adaptation of xerophytes
- Many cacti have their leaves reduced to spines or needles.
- This reduction in leaf surface area reduces the area across which transpiration can take place.
- The needles also prevent the plant being grazed by herbivores - particularly important as many cacti have succulent (juicy) stems.
- In these plants the ‘leaves’ no longer have a photosynthesising role, the stem carries out this role instead.
Explain why cuticular thickening is an effective structural adaptation of xerophytes
A thick cuticle makes this waterproofed layer even more efficient in reducing evaporation.
Explain why leaf hairs are an effective structural adaptation of xerophytes
- Many leaves have a layer of hairs, often confined to the lower epidermis.
- These hairs restrict air flow over the leaf surface and help trap a layer of humid air.
- This serves to reduce the water potential gradient between the inside and outside of the leaf.
- Most of these hairs are very small and therefore cannot be seen by the naked eye.
Why can leaf hairs not be seen by the naked eye?
They are very small
Explain why sunken stomata are an effective structural adaptation of xerophytes
- Stomata sunk in pits or grooves is another important method of reducing transpiration losses by creating a layer of humid air (diffusion shell) around the stomata.
- This reduces the water potential gradient between the inside and outside of the leaf.
- This adaptation is often accompanied by a reduction in the number of stomata in the leaf.
Give two examples of xerophytes which have succulent leaves/stems
Jade = Succulent leaves Cacti = Succulent stem
Explain why succulent tissue is an effective structural adaptation of xerophytes
Many xerophytes have succulent (juicy) leaves that store large quantities of water which can be used in periods of drought, for example, cacti and jade.
Explain why deep roots are an effective structural adaptation of xerophytes
- Roots that penetrate deep into the soil to reach water well below the ground are common in xerophytes.
- Another adaptation is very shallow roots.
- Many desert plant have very shallow roots that cover a wide area around the plant but lie just below the surface.
- This ensures that the infrequent rain water that does fall can be quickly absorbed before it gets a chance to evaporate from the top layers of the soil.
Where can marram grass be found?
In sand dune systems adjacent to beaches
Draw a cross-section through a marram grass leaf
Textbook page 145
Labels
- Vascular bundle
- Stomata are present in sunken pits
- Lower epidermis surrounded by humid air due to folding of leaf
- Hairs to reduce movement of air and water vapour creating diffusion shells
- Thick waxy cuticle
- Upper epidermis
Why does marram grass, which can grow on sand dunes immediately adjacent to beaches, require xerophytic adaptations?
While there is abundant rainfall, the inability of the sand to retain water means that plants able to colonise these habitats have many xerophytic adaptations.
What are some of the xerophytic adaptations in:
a) Cacti
b) Jade
c) Marram grass
d) pine trees
a) Cacti leaves are reduced to sharp spines, which both reduce water loss, due to the reduction in leaf area, and help protect the succulent stem.
b) Succulent leaves and a very thick cuticle.
c) Leaf curvature (folding)
Sunken stomata
Leaf hairs
Thick waxy cuticle on upper epidermis
d) Needle-shaped leaves
Stomata sunken in pits
Where do pine trees typically occur?
Northern boreal forests
Why do pine trees show xerophytic structural adaptations?
Occur in northern boreal forests.
Although rain/snow may be common the frozen ground reduces water availability.
Give an example of a hydrophyte
Water lilies
What are hydrophytes?
Aquatic flowering plants adapted to growing on or in water
What are some typical structural adaptations of hydrophytic plants?
- Stomata being restricted to the upper leaf surface - to prevent them being submerged in water and ensuring that gas exchange with the atmosphere can take place.
- The presence of large air spaces (aerenchyma) that enables the plant (leaf) to float.
Knowledge check 18
Explain why a lack of oxygen reduces ion uptake in the roots.
Lack of oxygen reduces the rate of aerobic respiration, which produces ATP used in the active uptake of ions.
Knowledge check 19
Explain why a respiratory poison would reduce the effect of the root pressure
Root pressure results from the active secretion of ions into the xylem. A respiratory poison prevents the production of ATP used in active transport.
Knowledge check 20
With reference to the water potential gradient, explain how sunken stomata reduce transpiration.
Water vapour diffusing out of the stomata is trapped and the region immediately outside becomes saturated. The water potential gradient between the inside of the leaf and the outside is reduced.
Knowledge check 21
Experiments with labelled sucrose show that radioactivity is detected in very young leaves but not in mature leaves. Explain why.
Ver young leaves are removing sucrose from the phloem (they are a ‘sink’) while mature leaves are releasing sucrose since they are photosynthesising at a high rate (they are a ‘source’).
The transport of water is a (blank) process, whereas the transport of organic solutes is an (blank) process.
Passive
Active
What are non-woody plants referred to as?
Herbaceous plants
What are herbaceous plants?
Non-woody plants
The cross-sections of plant roots and stems covered in this section describe the pattern in (blank) plants.
Herbaceous (non-woody)
Give two examples of woody plants
Trees
Shrubs
What is wood formed of?
Xylem tissue
Woody plants in cross-section are composed of …
Almost all xylem, with very small amounts of phloem and other tissue.
Plants continually lose water by …
Evaporation
Why does water move from the endodermis into the xylem vessels in the root?
- The endodermal cells pump ions into the xylem cells, a process that involves energy expenditure. This creates a water potential gradient that draws water from the endodermis into the xylem by osmosis.
- Additionally, the cohesive pull of water by the transpiration stream in the xylem helps move water from the endodermis into the xylem.
What is the role of water in the plant?
Photosynthesis
Provision of turgor (pressure)
Transpiration
What is the role of sieve plates in phloem sieve tube elements?
Allows the movement of phloem sap between sieve tube elements to occur easier
Transpiration represents a loss of water to the plant. This is only a problem if …
The loss is excessive.
Or
If soil water is unavailable, as in a drought.
What is the role of transpiration?
Transpiration is the major force drawing water (containing dissolved ions) up the plant and it therefore results in a continuous supply of ions (and water) for use in the leaf
In all cases of water movement in a plant, water moves …
Down a water potential gradient, from a region of high water potential to a region of lower (more negative) water potential.
In the leaf, stomata are the main route of …
Transpiration
In the leaf, the cuticle is a minor alternative route of …
Transpiration
Give an example of a plant organ which can be a source or sink depending on the season
Potato tuber
A potato tuber is a sink during the summer as it builds up stores of carbohydrate but is a source in the spring when starch is broken down to supply the energy for the growth of shoots.
Translocation
Source and sink are dependent on …
The season
