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.
Diagram showing active transport of mineral ions in root hairs is followed by the passive absorption of water by osmosis
Diagram showing water uptake through osmosis in root hair
Diagram summarizing water transport in vascular plants
Picture of a saguaro cactus
Micrograph of a cross section of a marram leaf
Picture of mangrove trees
Picture of a mangrove leaf with excreted salt crystals
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