9.1 Transport in the xylem of plants Flashcards
What is transpiration?
The loss of water vapour through the leaves, stems, and other above-ground parts of the plant.
How do plants absorb water and what is this used for?
-Plants absorb water through their roots and transport the water through the stems to the leaves.
-This provides all parts of the plant with the water needed for cellular support and metabolism.
-However, over 95% of the water absorbed will be released into the atmosphere by transpiration.
Describe photosynthesis in plants
-The process of photosynthesis requires carbon dioxide and water to produce carbohydrates for structure and energy storage.
-Oxygen gas is also produced as a byproduct of carbohydrate synthesis.
-For most plant species, photosynthesis can occur only during the day when light is available.
-Leaves are the main photosynthetic organ for most plants, though some have adaptations like photosynthetic stems.
Describe cellular respiration in plants
-In cellular respiration, the energy that was stored in carbohydrates during photosynthesis is released.
-Cellular respiration takes place in all living plant tissues both day and night, providing energy to sustain cell functions.
Diagram showing the substrates and products of photosynthesis
Diagram showing the substrates and products of cellular respiration
What do plants need for the processes of photosynthesis and cellular respiration and how do they obtain it?
-As plants carry out these processes, they must have a continual supply of substrates.
-During the day, the rate of photosynthesis in the leaves is generally much greater than the rate of respiration, so the leaves have a large net requirement of carbon dioxide.
-Plant leaves obtain carbon dioxide from the air.
How is the stomata an adaptation for gas exchange?
Stomata are tiny pores, usually located on the underside of the leaf, whose opening and closing is controlled by two guard cells.
Diagram of a tomato plant stomata
How is the spongy mesophyll adapted for gas echange?
The lower tissue layer of the leaf known as spongy mesophyll, which provides the large surface area and moist surface necessary for gases to be exchanged.
Diagram of a cross section of a leaf, showing large surface area for gas exchange in the spongy mesophyll
Describe the diffusion of carbon dioxide in plants
-Diffusion is a passive and natural process.
-As the plant cells use up dissolved carbon dioxide in photosynthesis, the CO 2 concentration drops.
-Carbon dioxide from the air spaces between cells will dissolve and diffuse into the cell, moving from higher to lower concentration.
-When the concentration of carbon dioxide drops in the air within the leaf, there will be a net movement of carbon dioxide molecules into the leaf through the stomata, again by diffusion.
Describe diffusion of oxygen in plants
-Oxygen diffuses out of the leaf cells, into the internal air spaces, and out into the atmosphere through the (open) stomata.
-It is important to lower the concentration of oxygen gas during photosynthesis, because it is a competitive inhibitor of a key enzyme, rubisco.
Describe diffusion of water vapor in plants
-Water vapour (H 2 O (g)) is also a gas, and will necessarily diffuse from the highly humid air spaces in the leaf to the areas of lower concentration in the atmosphere outside the leaf.
-Thus, the loss of water vapour (transpiration) is the inevitable consequence of gas exchange in the leaf.
-It is not possible to pick and choose which gases will diffuse; when conditions allow gas exchange, it will occur with all gases present.
Key facts about transpiration
Transpiration is the loss of water vapour from the above-ground parts of plants, primarily the leaves. The large majority of water absorbed in the roots is lost in transpiration. Transpiration:
-Occurs mainly through open stomata
-Is an inevitable consequence of gas exchange in the leaf
-Has a cooling effect on the leaf
-Exerts a pull to move water from the roots into the leaves.
How can plants limit the water lost in transpiration?
By closing the stomata.
How much water is lost through the stomata and where else is water lost?
Over 90% of water loss is through stomata, though some is lost directly across the outer epidermal cells.
What is a downside of closing the stomata to limit the water lost in transpiration?
Closing the stomata also means that carbon dioxide cannot move into the leaf, and photosynthesis cannot occur.
When are stomata usually open in plants and why?
-For most plants, stomata are open during the day when light is also present to allow photosynthesis to proceed.
-Additionally, the evaporation of water from the leaf cells helps cool leaves that might overheat in direct sunlight, through evaporative cooling.
When are stomata closed and why?
Stomata are usually closed at night to conserve water, though some plants have CAM metabolism to store carbon dioxide at night so that stomata may be closed during the day.
What does a plant have to do when stomata are open?
When stomata are open, the water that is lost must constantly be replaced or the plant will dry out.
How does transpiration affect the movement of water in plants?
Transpiration itself exerts a pulling force that helps to move the water from the roots all the way up to the leaves.
A plant has open stomata.
Under which conditions would the rate of transpiration be greatest?
Dry and hot with wind
Why is transpiration inevitable in typical leaves?
When stomata are open to allow carbon dioxide to diffuse into the leaf, water (and oxygen) will diffuse out; all gases must diffuse down their concentration gradients.
Water lost by evaporation from the leaves is replaced by ___
Transport of water from the roots to the leaves.
What does the xylem transport and where does this go to?
The xylem in vascular plants transports water and dissolved minerals from the roots to all other parts of the plant.
Describe the structure of xylem
Xylem forms long continuous tubes, like extremely narrow drinking straws, that run from the roots through the stems of plants.
What is the state of xylem at maturity?
Unlike most tissues, xylem is dead at maturity.
Why is the xylem a tube shape?
-The cell membrane and internal structures break down to create hollow xylem vessels that conduct water efficiently.
-The horizontal cell walls also are partially or completely broken down, leaving a continuous tube.
Describe the presence of lignin in xylem vessels
-The walls of the xylem vessels are strengthened with lignin, a complex polymer that binds with cellulose to provide great strength and rigidity to the cell walls.
-The woody tissue made from lignified xylem can support plants many metres tall.
-Lignin also allows the xylem vessels to withstand the forces involved in transpiration without collapsing.
-Lignin can be deposited throughout the cell walls or as rings or spirals inside the xylem vessels.
Light micrograph of xylem vessels with various forms of lignified support (red) in longitudinal section of a stem (left) and diagram showing similar structures (right).
The structure of xylem supports its function of ___
Transporting water throughout the plant.
Drawing xylem vessels in exam
You should be able to draw and label the structure of primary xylem vessels in sections of stems based on microscope images.
Diagram showing the structure of primary xylem vessel
What type of process is transpiration?
Although transpiration is capable of moving water against the force of gravity, it is a passive process.
Describe how water evaporates from mesophyll cell walls
-The cellulose in mesophyll cell walls is hydrophilic and water adheres to it, creating a film of water on the surface of the cells.
-When water vapour diffuses out of the stomata, the internal air spaces of the leaf become less humid.
-Water then evaporates from the moist mesophyll cell walls into the air spaces.
What happens when a water molecule evaporates from the mesophyll cell wall and why?
-When a water molecule evaporates from the cell wall, it exerts a pulling force, or suction, on water molecules within the cell.
-This is because water is cohesive; the attraction between the slightly negative oxygen atom in one water molecule and the slightly positive hydrogen atoms in a different water molecule creates hydrogen bonds between water molecules.
What is the transpiration pull?
-The tension caused by the pull of evaporating water molecules draws water from the xylem into the leaf cells.
-The transpiration pull begins with water molecules evaporating from the cell wall into the air spaces of the leaf and is transmitted through the column of water in each xylem vessel.
-Transpiration pull is similar to the way suction (low pressure) draws a column of liquid up a drinking straw.
Diagram showing how transpiration pull extends from the water vapour in the leaf through the column of water in xylem vessels
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Describe cohesion in xylem vessels
-Water molecules form weak hydrogen bonds with each other due to their polarity.
-This allows transpiration pull to extend, unbroken, through long columns of water in xylem vessels.
Describe adhesion in xylem vessels
-The polarity of water also interacts with the hydrophilic cellulose in the cell walls of the leaf.
-This helps create the pull that draws water out of the xylem and into the leaf cells.
What is the pathway most water follows from roots to the outside atmosphere?
Root xylem → stem xylem → leaf xylem → mesophyll cell wall → leaf air spaces → water vapour lost to the atmosphere.
How are water and dissolved minerals typically taken into a plant?
They are absorbed from the soil through the roots.
Why do plants need mineral ions like nitrate and potassium?
For growth and metabolism
Give some examples of mineral ions that plants need
Nitrate and potassium
Describe the presence of mineral ions in soil
These ions are found dissolved in relatively low concentrations in the water found in the soil.
What is the purpose of root hairs?
Some root cells have extensions called root hairs that greatly increase the surface area available for absorption.
Describe the plasma membrane of root hairs
The plasma membrane of root hairs has many protein pumps that actively transport mineral ions from the surrounding water into the cytoplasm of the cell against the concentration gradient.
What does active transport across the plasma membrane of root hairs require?
This process requires energy stored in ATP molecules.
Due to the high demand for ATP, root hairs have ___
A high rate of cellular respiration, many mitochondria, and a high demand for oxygen gas.
How does oxygen diffuse into root cells?
The oxygen is dissolved from air pockets in the soil into the surrounding water and from there, diffuses into the root cells.
Diagram showing active transport of mineral ions in root hairs is followed by the passive absorption of water by osmosis
What is the effect of the high concentration of mineral ions in the cytoplasm? (of…)
The high concentration of mineral ions in the cytoplasm, which can reach 100× the concentration outside the cell, allows water to move into the plant cells through osmosis.
How are the dissolved minerals and water moved into the vessel and carried to all parts of the plant?
By transpiration pull
Diagram showing water uptake through osmosis in root hair
The movement of water is always ___
Passive
Give an overview of how water is drawn into the roots
-To draw water into the roots, plants create a concentration gradient by actively transporting ions into the root.
-Water follows passively by osmosis through protein channels called aquaporins in the cell membrane.
___ is the main driver for moving water through the xylem
Transpiration pull
What factors (other than transpiration pull) affect water movement in the plant?
-The concentration gradient between roots and soil, pressure from the cell walls, gravity, and more.
-Water potential, which is a measure of water’s tendency to move in a particular direction, brings all of these effects together.
What is water potential?
A measure of water’s tendency to move in a particular direction.
What is the water potential of pure water?
0.0 MPa (megapascals, a unit of pressure), with all other values being negative.
Water potential
-Water potential is a measure of water’s tendency to move in a particular direction.
-Pure water has a water potential of 0.0 MPa (megapascals, a unit of pressure), with all other values being negative.
-Water always moves toward the value that is more negative
Diagram summarizing water transport in vascular plants
Summary of osmosis from the soil
-Water moves from the soil into the root cells (especially root hairs) through osmosis .
-Water can move by osmosis due to a high solute concentration inside the cytoplasm, established by active transport of mineral ions .
Water moves through the ___ from the ___ towards the ___.
Root cells into the xylem vessels
Roots
Leaves (and other aerial tissues)
Summary of transpiration pull in xylem vessels
-Water moves in a continuous column through the xylem vessels by transpiration pull .
-Cohesion of water molecules due to hydrogen bonding transmits the pulling force from one water molecule to the next.
-Xylem vessels can withstand the suction caused by transpiration pull without collapsing because lignin deposits in the cell wall provide extra strength.
Water moves from the xylem into the ___
Leaf cells
Why is the adhesion of water to cellulose in the cell walls important?
-Water is adhesive to the hydrophilic cellulose in the cell walls, so the cell surfaces are moist.
-The water adhering to cellulose also allows carbon dioxide to dissolve and diffuse into the cytoplasm, and excess oxygen gas to diffuse out.
Water evaporates from the ___ into the ___
Mesophyll cell walls
Air spaces inside the leaf
Describe how water vapor diffuses out of the leaf
-Water vapour diffuses out of the leaf into the atmosphere primarily through open stomata.
-The loss of water vapour is a necessary consequence of gas exchange for photosynthesis.
What are xerophytes?
Plants that have adapted to live in conditions where liquid water is difficult to obtain, such as deserts, areas with seasonal drought, or areas like the Arctic when the soil water is frozen.
How do the adaptations of xerophytes help them?
-Their adaptations help to conserve the little water plants acquire.
-Some xerophytes have special tissues to store water; others have adaptations to reduce water loss through transpiration.
Examples of xerophytic adaptations
-Thick waxy cuticle on leaf and/or stem
-Fewer stomata
-Stomata in sunken pits
-Fine hairs along underside of leaf
-CAM physiology (crassulacean acid metabolism)
-Reduced air spaces in leaf mesophyll
-Few/small leaves, or photosynthesis moved into stem
-Curled or rolled leaves
-Water storage tissue
-Deep, highly branched roots
What is the advantage of a thick waxy cuticle on the lead and/or stem for xerophytes in dry conditions?
Reduces non-stomatal transpiration rate because the cuticle is hydrophobic and creates a barrier to prevent water loss.
What is the advantage of fewer stomata for xerophytes in dry conditions?
Reduces transpiration rate by having fewer openings in the leaf.
What is the advantage of stomata in sunken pits for xerophytes in dry conditions?
Reduces transpiration rate by allowing moisture (humidity) to build up near stomata.
What is the advantage of fine hairs along the underside of the leaf for xerophytes in dry conditions?
Reduces transpiration rate by retaining a layer of moisture near the stomata.
What is the advantage of reduced air spaces in leaf mesophyl for xerophytes in dry conditions?
Reduces transpiration rate due to reduced surface area for evaporation.
What is the advantage of CAM physiology (crassulacean acid metabolism) for xerophytes in dry conditions?
-Reduces transpiration rate enormously because stomata close during the day.
-Stomata open at night to collect and store carbon dioxide, when darkness and cooler temperatures reduce evaporation.
-During the day, pre-collected carbon dioxide allows photosynthesis to occur without water loss.
What is the advantage of few/small leaves, or photosynthesis moved into stem for xerophytes in dry conditions?
Reduces transpiration rate because there is reduced surface area for light to strike and water to evaporate.
What is the advantage of curled or rolled leaves for xerophytes in dry conditions?
Reduces transpiration rate because there is reduced surface area for water loss and there can be production of humid areas by the stomata.
What is the advantage of water storage tissue for xerophytes in dry conditions?
-Increased water storage when water is available.
-Succulent plants have tissues in stems or leaves adapted to store large amounts of water; other plants store water in tubers.
What is the advantage of deep, highly-branched roots for xerophytes in dry conditions?
-Increased ability to take up water because deep roots may reach a lower water table beyond the dry soil.
-Branched roots provide increased surface area for water absorption.
Describe the adaptations of the saguaro cactus (Carnegiea gigantea) (italics)
-It has a thick waxy cuticle on the stem.
-It has one tap root plus a highly branched surface root system to collect water during infrequent rains, which it then stores in the stem for later use.
-It has a very reduced surface area with photosynthesis moved into the stem.
-Leaves are reduced to spines that protect the stem from herbivores and intense sunlight.
-The saguaro also uses CAM photosynthesis, so its stomata are closed during the day.
Picture of a saguaro cactus
A saguaro cactus is a ___.
Xerophyte
Where does marram grass (Ammophila arenaria) grow?
On sand dunes
What are the adaptations of marram grass (Ammophila arenaria)?
It has a thick outer cuticle, rolled shape, few stomata, and hair-like structures to reduce transpiration.
Marram grass is a ___
Xerophyte
Micrograph of a cross section of a marram leaf
What are halophytes?
Plants that have adapted to grow in areas with high salinity, such as along an ocean shoreline or in certain swamps and marshes.
Why do halophytes need certain adaptaions?
-Plants usually establish a higher concentration of ions in the roots than in the surrounding area to draw water in by osmosis.
-Areas of high salinity make this difficult since the ion concentration outside the plant is very high.
Give an overview of the adaptations of halophytes
Some halophytes have adaptations to excrete excess salt, some establish high concentrations of other solutes, and some have xerophytic adaptations to conserve fresh water in the plant.
What is the difference between halophytes and xerophytes
Halophytes are plants that have adaptations to survive in environments of high salinity, while xerophytes are plants that have adaptations to survive in dry conditions.
Examples of adaptaions of halophytes
-Salt storage in vacuoles
-High concentration of organic solutes
-Salt storage glands in leaf
-Leaf abscission for some leaves
-Selectively permeable membrane in root cells
-Xerophytic adaptations
What is the advantage of salt storage in vacuoles for halophytes in saline environments?
Compartmentalises salt in vacuoles, thus protecting cellular organelles and enzymes from damage by high salt concentration.
What is the advantage of a high concentration of organic solutes for halophytes in saline environments?
Increases osmolarity by having a high concentration of sugars and other solutes, thus water can still enter by osmosis.
What is the advantage of salt storage glands in leaves for halophytes in saline environments?
Accumulates salt in a limited area by filling the salt glands until they release salt crystals onto the leaf surface where they will fall off or be dissolved in rain.
What is the advantage of leaf abscission for some leaves for halophytes in saline environments?
Removes salt by breaking off leaves with toxic levels of salt and letting them fall from the plant.
What is the advantage of a selectively permeable membrane in root cells for halophytes in saline environments?
Excludes salt by having no ion channels to allow passage of Na + and Cl - , and/or has active transport pumps to remove the ions.
What is the advantage of xerophytic adaptations for halophytes in saline environments?
Conserves water by having few stomata, water storage tissue, thick cuticle and other adaptations.
Describe the adaptations of mangrove trees
-Mangrove trees are halophytes that have a variety of strategies for dealing with high salt concentrations.
-Grey mangrove leaves have salt glands that excrete salt crystals.
-Red mangroves have root cell membranes that mostly exclude salt ions, and they also store salt in vacuoles that keep the cell turgid while protecting the cellular mechanisms from salt damage.
Picture of mangrove trees
Picture of a mangrove leaf with excreted salt crystals
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As a biologist, it is your job to look for plants that have evolved structures with a selective advantage in dry, hot conditions.
Which adaptations are you most likely to find?
-Small, thick leaves with stomata on the lower surface
-CAM plants that grow slowly
-A thick cuticle on fleshy leaves
What type of plant has a waxy cuticle, reduced leaves and a low number of stomata?
Xerophyte
Factors affecting rate of transpiration- practical
What is an inevitable consequence of gas exchange for photosynthesis?
Transpiration (the loss of water from the leaves and stems of plants)
Why might some plants have a high or low rate of transpiration?
Some plants use a high rate of transpiration to cool the leaves; others limit transpiration to conserve water.
What are the different TYPES of factors that affect the rate of transipration
Features of the plant itself (internal factors) and environmental conditions (external factors).
Examples of some internal factors that affect the rate of transpiration
-Root to shoot ratio
-Surface area of leaves
-Number of stomata per unit leaf area
-Leaf structure, for example, the presence of hair or thick waxy cuticle
Examples of some external (environmental) factors that affect the rate of transpiration
-Light
-Wind
-Temperature
-Humidity
-Water availability.
What is the effect of light on the transpiration of plants?
As light intensity increases, the rate of transpiration increases.
Explain the effect of light on the transpiration of plants
-Stomata are closed in the dark, but as light intensity increases, stomata open and allow water vapor to escape from the air spaces of the leaves.
-Therefore, bright sunlight increases the rate of transpiration.
-Photons also provide energy for evaporation.
What is the effect of wind on transpiration in plants?
As wind velocity increases, the rate of transpiration increases.
Explain the effect of wind on transpiration in plants
-In low wind conditions, the air underneath leaf becomes increasingly humid.
-This reduces the water vapour concentration gradient from the leaf’s air spaces to the outside air, and so reduces the rate of transpiration.
-As wind speed increases, the humid air is blown away more quickly and is replaced by drier air, which increases the rate of transpiration due to the increased concentration gradient for water vapour.
-However, if the wind speed reaches a critical level, the stomata may close to reduce the rate of transpiration.
What is the effect of temperature on transpiration in plants?
As temperature increases, the rate of transpiration increases.
Explain the effect of temperature on transpiration in plants
-Higher temperatures provide more energy for evaporation of water from the cell walls and decrease the humidity of the external atmosphere.
-However, if the temperature gets too high for enzymes to function efficiently, the stomata may close and the transpiration rate may fall.
What is the effect of humidity on the transpiration of plants?
As humidity increases, the rate of transpiration decreases.
Explain the effect of humidity on the transpiration of plants
-Humidity refers to the percentage of water vapour present in the atmosphere.
-When the air surrounding a leaf is dry (low humidity), the concentration gradient for diffusion of water vapour from the air spaces within the leaf to the outside is steep and transpiration occurs quickly.
Which set of conditions would cause the highest rate of transpiration in a well-watered plant?
Hot, dry air in bright sunlight
A particular plant will close its stomata at times of water stress when transpiration rates are very high.
Which conditions would be most likely to trigger stomatal closure to conserve water?
Very dry with strong winds
