Transport In Plants Flashcards
Why would single-called algae not need a specialised transport system
It has a very large surface area to volume ratio. Thus can rely on diffusion for the transport of molecules.
Why would multicellular organisms need a specialised transport system
They have a low surface area to volume ratio. Thus cannot rely on diffusion alone for the transport of molecules.
Chlorophyll allows multicellular plants to carry out photosynthesis. This produces oxygen and the carbohydrate glucose. The glucose and oxygen is used for aerobic respiration. However, many parts of the plant cannot carry out photosynthesis and undergo high rate of metabolic reactions (cells in root tissue absorb mineral ions by active transport). Therefore, due to these high metabolic reactions sugar must be transported to these tissues.
Mineral ions transported from the roots to other parts of the plant. (Nitrate ion to make amino acids)
Plants transport hormones from where they are synthesised to their target tissue
Describe what is meant by monocotyledonous and dicotyledonous herbaceous plant
Some plants have one cotyledon. These are called monocotyledonous plants.
Some plants have one cotyledon. These are called monocotyledonous plants.
Describe the process of a cotyledon forming
Seeds contain an embryonic leaf called a cotyledon. When the seed germinates, the cotyledon unfurls allowing the seedling to carry out photosynthesis.
What are some example of dicotyledonous plants
Herbaceous dicotyledonous plant (Geranium) - short lived ,fast growing and no woody stem
Woody dicotyledonous plant- (Oak tree , shrubs) - long-lived and woody stem
Explain the function of the xylem
Xylem vessel:
Carries water and mineral ions from the roots of the plant up the stem to the leaves
Xylem fibre:
Xylem fibres are not used to transport water. They instead provide mechanical support the plant
Explain the function of the phloem
In the leaves, the plant carries out photosynthesis which produces a sugar glucose. The glucose is used to form other compounds e.g different sugar or amino acids.These are called assimilates.
Transport organic molecules (assimilates) such a glucose produced by photosynthesis in the leaves.
What are the xylem and phloem vessel grouped together in
Vascular bundles
Describe the cross section of a root
Root hair cells grow from a layer of external tissue called the epidermis. We then have a thick layer of cells called the cortex. This contains parenchyma cells which are found extensively in plants.
In the centre of the root there are vascular bundles (also also called a stele) which are surrounded by a layer of cells called he endodermis.
In the vascular bundle the xylem vessel is in the centre with he phloem vessel around the xylem.
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Explain the adaptations of the xylem
Xylem vessels are mechanically strong therefore when they are grouped up in the centre of the root this helps the root from being pulled out of the soil.
If a xylem vessel is blocked or damaged then water can move through the pits to different vessels. The pits allow water to move out of the xylem (cells in leaves)
The spiral arrangement of lignin help to support the structure of the xylem vessel. When water is pulled up the xylem vessel this causes the pressure in the vessel to fall slightly. The lignin in the vessel wall helps to prevent the vessels from collapsing.
Xylem contain parenchyma cells. These act as a store of starch. They also contain tannins which are bitter compounds that deter herbivores from eating the plant.
Describe the cross section of a plant stem
Vascular bundles are arranged in rings around the edge of the stem. The centre of the plant stem is called the pith which consist of parenchyma cells.
Around the edge we have the epidermis and the cortex. Within the vascular bundle the phloem vessel are located around the edge of the stem. The xylem vessel are found closer to the centre. As the vascular bundles are around the edge of the steam this helps the stem to withstand bending due to the wind.
Describe the cross section of a leaf
We can find the main vascular bundle in the centre. This part is called the midrib. IT provides both transport and support to the leaf.
The leaf is also supported by smaller vascular bundles connected to the main one.
In the leaf, the xylem is at the upper part of the vascular bundle and the phloem is at the lower part.
Photosynthesis mainly takes place in the palisade mesophyll which is in the upper half of the leaf.
Explain the structure of the xylem
Xylem consist of two different main types of tissue:
Xylem vessels-
Xylem vessel start as a series of plant cells running up the stem to the leaves. At a certain point, the carbohydrate lignin forms within the cell walls.Lignin is impermeable and prevents substances from passing through the cell wall. The living content of the cell wall die. The end walls between the cells break down.
Region of the cell wall remain free of lignin. These are called pits. These allow water and dissolved substances to pass between vessels.
The final xylem vessel consist of non-living, hollow tubes.
Lignin can be arranged in spirals or rings. The lignin can be continuously apart form the pits. This lignin helps the structure of the xylem vesel.
Xylem fibres:
Xylem fibres are formed from long narrow cells. Very large amounts of lignin form in these cells.
Similarly in xylem vessels, the interior content of the cells die. Both xylem vessel and fibres are non-living tissue.
Xylem contain parenchyma cells which act as a store of starch.
parenchyma cells contain tannins which are bitter compounds
Describe the structure of the phloem
The fluid moving in the phloem is called phloem sap. Phloem is a living tissue.
phloem consists of two different types of tissue:
sieve tube element-
Consists of a long line of cells arranged end to end. Inside these cells almost all of the organelles have been lost including the nucleus and vacuole.
The end walls of these cells have been modified to contain large pores (sieve plates) . The sieve tube element cells have lost most of their organelles. this means that they cannot produce a large amount of essential molecules (ATP).
Companion cells:
Next to the sieve tube element cells are companion cells. These companion cells contain a nucleus as well as a large amount of mitochondria. Microscopic channels link the companion cells to the sieve tube element cells. These channels are called plasmodesmata. Molecules such as ATP and protein can move through the plasmodesmata into the sieve tube element cells.The role of the companion cells is to provide essential molecules to the sieve tube element cells.
Unlike xylem, phloem tubes do not contain lignin in the cell walls. However, phloem contains two types of tissues which provide support. These are called fibres and sclereids. Both of these have thickened cell walls containing lignin. Fibres are long and narrow whereas sclereids have a variety of shapes.
Describe the adaption of the phloem
Consists of a long line of cells arranged end to end. Inside these cells almost all of the organelles have been lost including the nucleus and vacuole. This leaves the interior of these cells almost entirely free to transport phloem sap.
The end walls of these cells have been modified to contain large pores (sieve plates) .Sieve plates allow the phloem sap to move between the cells. The sieve tube element cells have lost most of their organelles. this means that they cannot produce a large amount of essential molecules (ATP).
Next to the sieve tube element cells are companion cells. These companion cells contain a nucleus as well as a large amount of mitochondria. Microscopic channels link the companion cells to the sieve tube element cells. These channels are called plasmodesmata. Molecules such as ATP and protein can move through the plasmodesmata into the sieve tube element cells.The role of the companion cells is to provide essential molecules to the sieve tube element cells.
Unlike xylem, phloem tubes do not contain lignin in the cell walls. However, phloem contains two types of tissues which provide support. These are called fibres and sclereids. Both of these have thickened cell walls containing lignin. Fibres are long and narrow whereas sclereids have a variety of shapes.
Describe how root hairs cells are adapted for the absorption of water and minerals
Root are coved with very fine root hairs. Root hairs grow from cells in the epidermis of the root (outer layer). Water moves into the root hairs by osmosis. Root hair cells are adapted so osmosis takes place rapidly. The densely packed root hairs massively increase the surface area to volume ratio of the root. The surface of the root hair consists only of the cell wall and the cell membrane. This makes the surface extremely thin, increasing the rate of osmosis.
Water in the soil contain dissolved mineral ions (Mg^2+ which plants use to make chlorophyll). The concentration of these mineral ions are lower in the soil compared to inside the root hair cell. Therefore, root hair cells use active transport to move these mineral ions into the cell. The root hair cell contain other dissolved compounds such as sugars. Therefore, the water potential inside the root hair cell is lower than in the soil. Thus water moves into the root hair cell by osmosis down the water potential gradient.
Describe the how water passes through the root to the xylem
Now the water moves from the root hair cells through the root cortex to the xylem. Water moves through the cortex by two pathways:
In the symplast pathway, water moves from the cytoplasm of one cell to the cytoplasm of an adjacent cell. To do this, the water moves through the plasmodesmata linking the cell (microscopic channels through the cell wall connecting the cytoplasm of cells). The symplast pathway is driven by the water potential gradient between the root hair cells and xylem. Water continually moves into the root hairs cells by osmosis from the soil. This makes the water potential of the root hairs cells greater than the cortex cells. In the xylem the water potential is relatively low. So water moves by osmosis across the cortex down the water potential gradient . The symplast pathway is relatively slow. This is because the pathway for water in the cytoplasm is obstructed by the organelles.
Water also moves through the apoplast pathway. In this case water moves through the cell wall and the spaces between the cells. The cellulose cell walls have relatively open structure allowing water to move easily between the cellulose fibres. Water molecules are attracted to each other through cohesion. This is due to the fact that hydrogen bonds form between water molecules. As water moves into he xylem and is carried away more water moves along the apoplast pathway due to cohesion. The apoplast pathway offers much less resistance to water flow than the symplast pathway.
Describe what happen before the water enters the xylem
Before the water passes into the xylem it must pass through a layer of cells called the endodermis. The cells in the endodermis has band of waterproof material called suberin which runs around the cell wall.
This is called the casparian strip. Because of the casparian strip water can no longer move through the apoplast pathway. Instead , the water passes through the cell membrane and into the cytoplasm becoming part of the symplast pathway. By forcing all water through the cytoplasm this allows the cell membrane to control which substances can enter the xylem. Cells in the endodermis use active transport to pump mineral ions into the xylem. This lowers the water potential of the xylem triggering water to move into the xylem vessels by osmosis. This is called root pressure.
Root pressure is an active process and requires respiration. If we inhibit respiration by using a metabolic poison then root pressure stops. Root pressure can also stop if we prevent aerobic respiration by excluding oxygen.
Describe how transpiration takes place in plants
Water passes into the leaf through the xylem vessels in the vascular bundle. The surface of the leaf is covered in a waterproof layer called the waxy cuticle. The job of the waxy cuticle is to reduce water loss from the surface of the leaf by evaporation. Therefore, water is an essential reactant for photosynthesis.
However, photosynthesis also requires the gas carbon dioxide which diffuses into the leaf from the external air. Photosynthesis produces the gas oxygen which diffuses out of the leaf. On the lower surface of the leaf, we find thousands of tiny pores called the stomata. When a plant photosynthesises, these stomata are open allowing carbon dioxide to diffuse into the leaf and oxygen to diffuse out.
The surface of the cells in the leaf are covered in a thin layer of water. This water evaporates from the surface of the cells. Therefore, the internal leaf spaces contain a high concentration of water vapour. The level of water in the external air is generally low so when the stomata are open the water vapour diffuses out of the leaf to the external air. This evaporation of water followed by the diffusion of water vapour is called transpiration.
Describe the cohesion-tension theory of water molecules
The continuous evaporation of the surface the water potential of the cells in the leaf decreases. This causes water to move by osmosis from adjacent cells like this. This now lowers the water potential of these cells causing water to move into them. At some point, this reaches the xylem with water passing out of the xylem to adjacent cells like this. During transpiration water is continuously pulled out of the xylem vessel. This effect is called tension.The movement of water from the roots,up the xylem and out of the leaf is called the transpiration stream.
Water molecules form hydrogen bonds with each other. This attraction is called cohesion. Water can also form hydrogen bonds to molecules in the xylem vessel walls. This attraction is called adhesion. (On effect of cohesion and adhesion is that water can move up very thin tubes against the force of gravity- this is called capillary action)
When water is removed from the top of the xylem vessel due to transpiration more water moves up the xylem vessel by capillary action to take its place. This is process is called transpiration pull.
The combined effect of transpiration pull coupled with cohesion and adhesion is that water is drawn into the roots, moves up the stem and passes out of the leaves. This process is the cohesion-tension theory.
Describe the cohesion-tension theory of water molecules
If a plant stem is cut air is sucked into the xylem suggesting that the xylem vessel are under tension. However, the air prevents cohesion between water molecules so water movement stops.
If we measure the diameter of the tree trunk. We can see that this reduces when transpiration is at its maximum. This supports the idea that transpiration pull generates a negative pressure or tension in the xylem.
Describe how to use a bubble potometer to measure the rate of water uptake in a plant
A bubble potometer consist of a fine capillary tube which is filled with water.The tube is connected to a plant that has been cut at the stem. The tube is connected to a syringe filled with water. Finally, we use a needle to place a small air bubble at the end of the capillary tube. As water evaporates from the leaves of the plant, water is drawn into the stem. This causes the air bubble to move towards the plant. By measuring how far the air bubble moves in a given time. We can calculate the rate of water uptake in the plant. We can then see the rate of water uptake changes if we change the conditions. (Different light intensities by increasing distance of plant to a light source or wind conditions fan compared to still air) In between experiments, we can reset the position of the air bubble by adding more water from the syringe.
The potometer only measures water uptake into the plan. Not all the water takes part in transpiration. (Some water taken in may be a reactant for photosynthesis. However, the vast majority of water taken in by the plant will take part in transpiration).
When setting up a potometer when taking our cutting from the parent plant air will be sucked into the xylem vessels. These air gaps would prevent water from being taken up the stem. Therefore, we must cut the stem of our plant into water (cut off the last 1cm). Water will flow into the xylem and we will avoid any air gaps.
We then place the potometer under the water and insert the cut end, again avoiding any air gaps.
Avoid damaging the plant and avoid getting water onto the underside of the leaves where most of the stomata are found
Ensure that the potometer is fully sealed. Thus we must smear some petroleum jelly around the connection between the stem and the tube.
We also need to allow the plant to adapt to its surroundings for 10 minutes before starting the experiment. (Acclimatise)
Describe how to use a mass potometer to measure the rate of water uptake in a plant
We place the plant on a pot ontop of a balance. As the plant loses water through transpiration, the total mass decreases. With this potometer we prevent evaporation of water from the soil. Otherwise, this would contribute to mass loss and give a false reading for transpiration. We do this by covering the soil with plastic wrap.
The mass potometer directly measures the rate of transpiration rather than the rate of water uptake.
This is much less destructive to the plant as it does not involve cutting the stem.
Describe the role of stomata in controlling the rate of transpiration in the plants
Each stoma is surrounded by two guard cells. The shape of the guard cell determine whether the stoma is open or closed.
There are two key features of guard cells:
The cellulose cell wall on the inner side of the guard cell is thicker than the rest of the cell.
Some of the cellulose microfibrils in the cell wall are arranged in ring shapes.
During photosynthesis plants harness light energy to produce the sugar glucose. For photosynthesis to take place carbon dioxide must diffuse into the leaf through the stomata. So in light conditions the stomata open allowing carbon dioxide to diffuse into the leaf.
Light condition triggers solutes such as potassium ions to be transferred into the guard cells. This lowers the potential of the interior of the guard cells. Water now moves into the guard cells by osmosis causing the guard cells to swell (become turgid). However, as water enters the rings of cellulose prevent the guard cells from expanding widthwise. Instead, they expand lengthwise. The thickened cell walls prevent the guard cell from expanding evenly forcing the guard cell to develop a curved shape to to allow the stoma to open between the guard cells.
When stoma are open water vapour diffuses out of the lead. This loss of water could cause the plant to dry out.
At night time, when the plant no longer carries out photosynthesis the stomata close to reduce water loss.
During a drought the level of water in the soil can fall. Under these conditions, the roots send a hormonal signal to the leaves. This hormone triggers the guard cell to lose their turgidity causing the stomata to close. By closing their stomata under drought conditions again this reduces water loss from the plant. Closing their stomata prevents the plant from carrying out photosynthesis. However, this is relatively a small price to pay to prevent the plant from dying from water loss.
Describe the factors that affect the rate of transpiration in plants
Light intensity:
For transpiration to take place it must take place in light conditions. Stomata open in light conditions to allow carbon dioxide to diffuse into the leaf to take part in photosynthesis. As we increase the light intensity , the rate of transpiration increases. This is because increasing light intensity increases the number of open stomata allowing more water vapour to diffuse out of the leaf. However, at high light intensities the rate of transpiration no longer increases as all stomata will be open.
Relative humidity:
water vapour diffuses out of the leaf down the concentration gradient. This is because the concentration of water vapour outside the leaf is generally lower than the inside. The relative humidity tell us the concentration of water vapour in the air as a percentage of the maximum possible. (100% relative humidity = concentration of water vapour is as high as it could be) If the relative humidity outside the leaf increase then that means there is a smaller concentration gradient between the inside of the leaf and the outside. So increasing the relative humidity outside the leaf reduces the rate of transpiration.
The rate of transpiration is increased by temperature. Firstly, at higher temperatures, water molecules have more kinetic energy. This means that there is a greater rate of evaporation of water from the internal surfaces of the leaf. At higher temperatures the relative humidity of the external air decreases Due to these two effects, the concentration gradient of water vapour between the inside of the leaf and the external air increases at higher temperatures. This increases the rate of transpiration.
Movement of air:
When water vapour moves out of the stomata during transpiration that water vapour can build up around the external surface of the leaf. The effect of this is to reduce the concentration gradient for water vapour between the inside of the leaf and the outside. This reduces the rate of transpiration. Air movement such as wind removes the water vapour as it diffuses out of the leaf. Because air movement increase the concentration gradient of water vapour the effect of this is to increase the rate of transpiration. The rate of transpiration falls on still days, when there is little air movement.
Water availability in soil:
During drought condition the roots of a plant produces a hormone.The hormone triggers a stomata to close in order to reduce the rate of transpiration to reduce water loss by the plant.
Describe how plants are adapted to living in condition where water is scarce: Cacti
In may species of cacti, leaves have been replaced with fibrous spines with photosynthesis taking place in the stem of the cactus.
In these cacti, the stomata are found on the surface of the stem. Replacing the leaves with spines reduces the surface area to volume ratio of the cactus reducing water loss.
The spines also trap moist air near the cactus, reducing the rate of transpiration as well as providing some shade for the stem from the heat of the sun.
Cacti also have a thick waxy cuticle to reduce evaporation of water.
In cacti the stomata are sunken into pits. This traps a layer of moist air around the stomata reducing water loss by transpiration. Cacti only open their stomata at night to absorb carbon dioxide when condition are relatively cool. The carbon dioxide is then used during the day for photosynthesis. By only opening stomata at night, cacti reduce water loss during the heat of the day time.
Cacti often have extensive shallow roots. This allows the cacti absorb water after a rain shower before the water evaporates.
Cacti can also have very deep roots to access water from lower levels of the soil
Cacti can store water in their stem (these plants are called succulents)
Describe how plants are adapted to living in condition where water is scarce: Marram grass
Leaves of marram grass can roll into a tube with the stomata on the inside. Moist air is trapped within the tube, rather than being blown away by the wind. The stomata in marram grass are found in sunken pits with fine hairs projecting inwards towards the centre. These ensure that the moist air is trapped around the stomata. This reduces the concentration gradient for water vapour between the air and the internal spaces between the lead cells. This reduces the rate of diffusion of water vapour out of the stomata.
Marram grass also have a very thick waxy cuticle to reduce evaporation from the surface.
The roots of marram grass are very long and extend deep into the sands to find water.
marram grass also have extensive roots which are closer to the surface. These roots help the sand to retain water.
Describe translocation of solutes in the phloem
Plants produce the monosaccharide glucose during photosynthesis in the leaves. All parts of the plant require glucose for respiration. The glucose produced in the leaves is first converted to the disaccharide sucrose.
Sucrose is less reactive than glucose and is less likely to react with other molecules. Molecules such as sucrose which are made as a result of photosynthesis are called assimilates. These are transported around the plant in the phloem. This process is called translocation.
Assimilates are transported from sources where assimilates are produced (e.g storage organic such as tubers which can release their carbohydrate stores when they are needed) to sinks where assimilates are required (roots which carry out active transport and have a high rate of respiration. Storage organs can also act as sinks when refilling their carbohydrate store. Other sinks include growing regions such as shoots which contain dividing meristem tissues.
The source is connected by the phloem to a sink (e.g photosynthesising leaf to a root). At the source the sucrose is loaded into the phloem by an active process.
A protein on the cell membrane of the companion cell uses ATP to pump hydrogen ions out of the cytoplasm and into the spaces of the cell wall. This process is active transport and creates a concentration gradient for hydrogen ions with more hydrogen ions on the outside of the cell membrane
The hydrogen ions can now flow through a cotransporter protein down the concentration gradient back into the cell. This inward flow of hydrogen ion is coupled to an inward flow of sucrose into the companion cell. Companion cells have a large number of mitochondria which provide the ATP needed for the active transport of hydrogen ions.
Folding on the cell membrane also increases the surface are for the proteins involved. Because of this transport process the concentration of sucrose into the companion cells is high. The sucrose can now diffusion into the plasmodesmata from the companion cells into the sieve tube element cells. This means that we now have a high concentration of sucrose in the sieve tube element cells.
The effect of this is to lower the water potential inside the sieve tube element. Water now moves into the sieve tube element by osmosis from nearby tissues including the xylem vessels. This now increases the hydrostatic pressure inside the sieve tube element. As a result the phloem sap now moves up or down the sieve tube element towards the sink. This bulk movement of phloem sap is called mass flow.
At the sink the sucrose moves out of the sieve tube element and is converted to glucose for use in respiration. Or in the case of storage organs, the sucrose is converted to starch. As the sucrose leaves, this increases the water potential in the sieve tube element causing water to move out of the sieve tube element by osmosis,
Some of this water will go back into the xylem and join the transpiration stream.
What evidence supports the active model of movement in the phloem
The rate of flow of sucrose in the phloem is much faster than could take place by diffusion alone. If we inhibit the companion cell mitochondria, then translocation stops.
