Transport in plants and Transpiration Flashcards
Describe a plant root
The outer layer of the root (leaves and stem) is the epidermis. Vascular tissue is concentrated in a central stele (vascular cylinder) that contains the xylem and phloem. The xylem is responsible for the transportation of water and ions while the phloem transports organic molecules e.g sugars and amino acids.
Describe the xylem
It is differentiated into a number of cell types but the main type involved in water and ion movement is the xylem vessel, which is highly specialised for transport.
They have no end walls and no cell contents, meaning they are dead when fully formed. The fact that they have no cell contents allow them to be hollow- more movement of H20.
Column of vessels produce a long continuous tube up the plant. Secondary cell walls are specially thickened with an impermeable substance called lignin.
There are 2 types of xylem- protoxylem and metaxylem.
Protoxylem
Protoxylem is the early first formed xylem consisting of the spiral vessel and annular vessel. It is found in short lived structures such as growing tip shoots and stems.
It does not restrict the elongation of xylem vessels but allows xylem to stretch for growth.
Metaxylem
Metaxylem is the mature form, consisting of reticulated and pitted vessels and is found in plant structures. The pits allow movement of water between adjacent vessels and surrounding cells.
State the properties of lignin
It provides great strength that prevents vessels from collapsing when under pressure exerted by the transpiration stream ‘sucking’ H20 up the plant- provides structural support for the plant.
The lignin is also waterproof, preventing leakage of water.
Phloem
The cells primarily concerned with transport in the phloem are the sieve tube elements. These are aligned from end to end and form a continuous row called the sieve tube. They do have end walls, prefronted with sieve pores to form sieve plates. Sieve tube elements are living cells with cell components. It has no nuclei and the reduced volume of cytoplasm is displaced to side walls and there are few organelles.
Sieve tube elements have microtubules that extend between sieve elements and pass through sieve pores- involved in translocation of solutes.
Each sieve tube element is closely associated (linked by plasmodesmata) with one or more companion cell.
Companion cells have dense cytoplasm rich in mitochondria and other cell organelles and have a high metabolic rate.
Companion cells 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 substances in phloem.
Structure of plant stems
Vascular tissues are arranged in vascular bundles around the outside of the stem. One advantage of this is the provision of greater support necessary in stems to support branches and leaves. Vascular bundles continue from branches/stems into leaves as the midrib, which branches to form smaller veins that are distributed through the leaf. Leaf veins are found in spongy mesophyll just below the palisade layer.
Definition of transpiration
The evaporation of H20 from mesophyll surface and subsequent diffusion of H20 vapour through the stomata and into the atmosphere.
The transport of water and ions across the root.
Root hair cells have a large S.A and thin exchange surface, facilitating the uptake of water into the root. The water enters the root by osmosis from a less negative to a more negative water potential due to the sugars in the cell.
Minerals move into the cell by active transport, creating a large conc gradient and allowing H20 to move into root.
Once the water is in the cell it moves across the cells of the cortex by the apoplast and symplast pathway, and then into the xylem in the stele.
Apoplast pathway
Water moves across the cellulose microfibrils of the cell wall facilitated by the parallel arrangements of microfibrils that allow water to pass easily between different layers rather than through them. This is further aided by the mesh like arrangement of walls. The cohesive properties of water (aided by hydrogen bonds) help pull water column along. Most of the water moves by this method.
Advantage of the apoplast pathway
There is no resistance to water movement as there are no barriers.
Symplast pathway
Water moves by osmosis from cell to cell (through plasmodesmata) across the cortex. The movement of H20 across the root creates a water potential gradient. As water enters the W.P in cell will be less negative and higher than adjacent cells in the root cortex. The water moves through the cortex cells by osmosis and the movement of water is under metabolic control.
Advantage of symplast pathway
The movement of water is under metabolic control.
How do ions enter the root cell?
Ions enter by diffusion or by active transport depending on the conc gradient of the particular ion concerned. Active transport causes the build-up of ions in the plant cells, requiring them to be moved against the conc gradient.
Transport of water and ions into the xylem (root pressure theory)
The endodermis is a layer of cells directly outside the stele. There is a waterproof layer called the casparian strip, which is embedded into the cell wall that encircles each cell. The casparian strip prevents the movement of H20 via the apoplast pathway (prevents leakage of water and ions) so the H20 travelling by the apoplast pathway joins the water travelling by the symplast pathway- ensures that water transport is under metabolic control.
The endodermal cells pump ions into xylem cells by energy expenditure, creating a water potential gradient that draws water from the endodermis down into the xylem.
The osmotic movement of water into the xylem tissue base creates a root pressure, a force that helps water move up the plant. Root pressure is caused by the accumulation of water in the xylem.
Transport of water up the root and stem in the xylem (cohesion tension theory)
As water evaporates out of the stomata in the leaf, it creates a negative pressure. This pressure pulls water up through the xylem as a mass flow movement. This process requires water to form a continuous unbroken pathway through the xylem called the transpiration stream. Water molecules create hydrogen bonds between one another, a feature known as cohesion, which tends to stick water molecules together. The cohesive properties allows water to be sucked up in a continuous column through the xylem as the water leading the edge of the column evaporates through the leaf, creating tension. This cohesion tension results in a transpirational pull.
Evidence for the cohesion tension theory
If water column in the xylem is broken down and an air gap appears, water below the air gap cannot be pulled up. A water column could be disrupted in one xylem column but continue in others.
During the day, when transpiration is normally at its greatest, much more tension or negative pressure is present in the xylem. This negative pressure tends to pull the walls of xylem vessels in and can reduce the diameter of the tree trunk.
Changing the diameter of tree trunk is more obvious than in herbaceous plants due to the fact that the tree trunk is almost all xylem, so much of it is involved in water uptake.
The missing end walls and absence of cell contents (making it a hollow tube) in the xylem vessels make them highly adapted for water transport by the cohesion tension method.
Adhesive properties are also important. Adhesion is the attraction of unlike materials (e.g water and xylem walls). This reduces the forces necessary for transpiration pull.
The transport of water through the leaf and evaporation of water from the leaf
Water enters the leaf in the midrib (vascular bundle). In most leaves, midrib splits into a number of veins that distribute water around the leaf. Water passes from the vein to the surrounding cells, where some is used in photosynthesis or in providing turgor, but most is lost in transpiration.
Transpiration loses water due to water vapour evaporating from the cell surface membrane of spongy mesophyll and diffusing down the conc gradient out of the stomata. This process helps set up a conc gradient that is responsible for the transpiration pull.
Internal factors that affect rate of transpiration
Stomatal density- this is the number of stomata per unit area of leaf. More stomata = more evaporation.
Leaf surface area- greater S.A = more evaporation as more stomata present.
Cuticle thickness- thicker cuticle = less water lost by evaporation.
External factors that affect the rate of transpiration
Light intensity- rate of evaporation is greater during the day as stomata is closed during the night.
Windspeed- increasing windspeed increases evaporation as the wind removes hydrogen shells by blowing humid air away from the leaf, creating a steeper water potential gradient.
Temperature- increasing temp = faster evaporation due to more kinetic energy.
Humidity- increasing humidity = decrease in transpiration as it decreases W.P gradient between inside of leaf and surrounding atmosphere.
Soil water availability- dry soil = less evaporation as if a plant is dehydrated, stomata automatically close as a defence mechanism to conserve H20.
What is translocation?
The movement of organic substances in the phloem from leaves in growing regions of the plant (e.g carbs for energy and amino acids for growth) and the roots (for storage).
The main substance transported through the phloem is the disaccharide sucrose.
What are the 2 features associated with translocation?
The process is energy requiring and 2 way transport exists (translocation can move sucrose both up and down sieve tubes).
Evidence for energy expenditure
Companion cells have high metabolic activity. They are intimately associated with sieve tube elements and their energy output is linked to the processes that take place in the sieve tubes.
Companion cells in phloem in leaf veins are involved in the uptake of sucrose into the sieve tube elements via plasmodesmata before being transported around the plant.
Metabolic inhibitors such as cyanide, that stop respiration in plant cells, also disrupt the process of traslocation,
Evidence for two-way transport
The use of radioactively labelled sucrose (sucrose producig 14CO2) shows that sucrose can move up and down the stem. A leaf on a branch halfway down the stem may send sucrose both up the growing shoot tip regions and also down in an adjacent tube. Although translocation is another example of the mass flow system, the localised build-up of sucrose (source) helps create a hydrostatic gradient between some parts of the plant (e.g leaves) and then sink where sucrose levels are lower, for example roots where it is built up into starch for storage or growing regions where it is converted into glucose for respiration.
