Plant Biology

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Starting from the cuticle downwards, name the structures in an angiosperm leaf
Cuticle, upper epidermis, palisade mesophyll, spongy mesophyll (which includes the vascular bundle made from phloem and xylem), lower epidermis
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What does the lower epidermis contain?
Stomata
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In the vascular bundle, where are the phloem and the xylem positioned?
The xylem is on the top and the phloem is on the bottom
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What is the word equation for photosynthesis?
Carbon Dioxide + Water ----> Glucose and Oxygen
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What are the 2 external adaptations a leaf has for gas exchange?
It is flat and think so provides a short distance for gases to diffuse, the leaf also has a large surface area to volume ratio.
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What are the internal adaptations for light absorption and gas exchange?
Waxy cuticle reduces water loss by evaporation, the waxy cuticle and upper epidermis are transparent to allow light to enter, palisade mesophyll cellls are tightly packed together to ensure maximum light absorption by the leaves
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What are the internal adaptations for light absorption and gas exchange? pt. 2
Spongy mesophyll cells are irregularly shaped which creates air spaces between cells which allow gases to diffuse rapidly through the leaf, the leaf has pores called stomata which allow gases in and out of the leaf, guard cells and vascular bundle
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What 3 cells posses chloroplasts?
Palisade mesophyll cells, spongy mesophyll cells and guard cells
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Why are guard cells unusual (structure)?
They have unevenly thickened walls i.e. the inner wall is thicker than the outer wall
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What are the functions of stomata?
They open to allow CO2 and O2 to enter and leave the leaf, they close to prevent excessive loss of water from the plant
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Why are stomata only open during the day? (Its advantage)
Carbon dioxide can enter for photosynthesis
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Why do stomata close at night?
To reduce transpiration and water loss
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Explain why stomata open during the light and close in the dark using the malate theory.
The chloroplasts in the guard cells photosynthesise producing ATP, the ATP provides energy for active transport of K+ ions into the guard cells, stored starch is converted to malate, The K+ ions and the malate lower the water potential of the
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Explain why stomata open during the light and close in the dark using the malate theory. pt. 2
guard cells to below that of the surrounding cells, therefore water enters by osmosis and turgor increases, The guard cells curve/bend due to the inner walls being thicker than the outer walls. The outer walls are pushed outwards pulling the
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Explain why stomata open during the light and close in the dark using the malate theory. pt. 3
inner walls away from each other, this creates a pore known as the stoma, at night the reverse process occurs and the pore closes
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Explain why light affects the opening and closing of stomata
As light increases, the rate of photosynthesis increases and more ATP is made, increase in active transport of K+ ions into the guard cells, so they open wider
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Explain why temperature affects the opening and closing of stomata
As temp. increases or decreases towards the optimum for the plants's enzymes, the rate of photosynthesis increases etc.
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Explain why humidity affects the opening and closing of stomata
As humidity decreases i.e. water availability, the stomata open less wide to conserve water
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How do respiratory poisons effect stomatal opening and closing?
Cyanide stops respiration and the production of ATP, ATP isn't made so K+ ions cannot be actively transported into the guard cells and the stomatal pores will not open
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Describe the shape of the xylem and phloem in the roots
The xylem is central and star shaped, the phloem lies between the arms of the star
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What is the area between the epidermis and the vascular tissue called?
The cortex
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What is the layer that surround the vascular tissue in the roots?
The endodermis
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Describe a transverse section of a leaf
Xylem and phloem are found in vascular bundles in a ring with the xylem towards the centre and the phloem towards the outside
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Why are the vascular bundles arranged in a cylinder around the outside of the stem?
They help to add strength to the stem against external forces such as wind
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What are the two main functions of the xylem?
Transport of water and dissolved minerals , to provide strength and mechanical support
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What are the main cell types in xylem?
Vessels and tracheids
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Where do xylem vessels only occur?
Angiosperms (flowering plants)
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How are xylem vessels made?
Waterproof lignin builds up in vessel cell's cell walls, the contents of the cells die leaving an empty space, the end walls of the cells break down leaving a long holllow tube like a straw, through which water climbs straight up the plant
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Why are xylem easy to identify in microscope sections?
They stain red
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Where do xylem tracheids occur?
In ferns, conifers and angiosperms
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How are xylem tracheids similar to xylem vessels?
They are both elongated cells joined end to end and their walls are also impregnated with lignin
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How are xylem tracheids different to xylem vessels?
Their horizontal walls have not broken down completely, they have gaps known as pits through which water passes
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Why are tracheids not as efficient as vessels?
They are spindle-shaped so water takes a twisting rather than a straight up path
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Why must the roots absorb water from the soil?
To make up for the water lost by transpiration
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What are the cells specialised for the uptake of water known as?
Root hair cells
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What are the adaptations of the root hair cells for water absorption?
They have a large surface area, they have thin cell walls, they have many mitochondria for active transport of mineral ions
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How does osmosis take place from the soil to the root hair cells?
Soil water is a very dilute solution of mineral salt, the vacuole and cytoplasm of the RHC have a very concentrated solution of solutes maintained by the active transport of ions
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When water moves across the plasma membrane and enters the cytoplasm, what pathway does it take to cross the cortex?
The symplast pathway
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When water enters the cell wall of a root hair cell, what pathway does it take to cross the cortex?
the apoplast route
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What is the symplast pathway?
When water travels to the xylem through the cytoplasm and plasmodesmata which join adjacent cells
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What is the apoplast pathway?
When water travels to the xylem via the cell walls due to cellulose fibres being separated by spaces. 90% of water travels via this method
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What is the vacuolar pathway?
When water moves through the cytoplasm and vacuoles of cells
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Why is water moving in the apoplast pathway diverted into the cytoplasm at some point?
It is moving through non-living parts of the cells (cell walls) so if the plant is to have some control over the absorption of water and mineral ions it needs to be diverted into living parts of the cell
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When does this change happen?
When water reaches the endodermis
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Why can't water enter the xylem from the apoplast (cell walls)?
The lignin makes the xylem walls waterproof
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What are the only two ways in which water can enter the xylem?
From the symplast or vacuolar pathway
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What is the vascular tissue at the centre of the root surrounded by?
A region called the pericycle
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What is the pericycle surrounded by?
A single layer of cells, the endodermis
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What are cells of the endodermis impregnated with and what does this form?
A waxy material called suberin, this forms a waterproof band known as the Casparian strip
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What is the significance of the Casparian strip?
As suberin is water proof, the Casparian strip prevents water moving any further in the apoplast, it is forced into the cytoplasm i.e. the symplast route
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Name 3 mineral ions absorbed at the roots
Nitrates, phosphates and potassium ion
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Describe the 2 main methods of mineral ion uptake pt.1
Soil is a very dilute solution with very low conc. of minerals, minerals are present in higher conc. in the root cells than in the soil, they are absorbed into the cytoplasm by active transport (against a concentration gradient)
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Describe the 2 main methods of mineral ion uptake pt. 2
However mineral ions can also move along the apoplast pathway dissolved in water along cell walls. When they reach the Casparian strip, the minerals enter the cytoplasm by active transport and then diffuse or are actively transported into the xylem
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What drives the movement of water across the cortex and into the xylem? pt. 1
The vacuole and cytoplasm of the RHC contain a concentrated solution of solutes and have a lower water potential than the soil water. This lower water potential is maintained by active transport of ions into the RHC. Water then passes in by osmosis
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What drives the movement of water across the cortex and into the xylem? pt. 2
Water moves from the RHC to the xylem across the cortex by osmosis, for this to be efficient, the water potential of the xylem must be much lower than that of other cells in the cortex. The water potential of the endodermis cells is increased by
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What drives the movement of water across the cortex and into the xylem? pt. 3
water being forced out of the apoplast due to the Casparian strip. The water potential of the xylem is decreased by active transport of mineral salts, mainly sodium ions, from the endodermis and pericycle into the xylem
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What is transpiration?
Transpiration is the loss of water vapour from the stomata of leaves down a diffusion gradient
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What are the factors which affect transpiration?
Genetic factors such as those controlling the number, distribution and size of stomata, environmental factors like temperature, humidity and air movement, light intensity
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How does temperature affect transpiration?
As temperature increases, the kinetic energy of water molecules increases, this increases the rate of evaporation from the mesophyll cells and if stomata are open, speeds the rate of diffusion into the atmosphere
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How does temperature affect transpiration externally?
The higher temperature also causes water molecules to diffuse away from the leaf more quickly and reduces humidity.
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What is the consequence of this?
The water potential of the atmosphere around the leaf decreases, the size of the water potential gradient between the inside and outside of the leaf increases so rate of transpiration increases
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How does humidity affect transpiration? pt. 1
The air inside the lead is saturated with water vapour so its relative humidity is 100%. The humidity of the atmosphere surrounding the leaf varies but is never greater than 100%
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How does humidity affect transpiration? pt. 2
There is a water potential gradient therefore between the leaf and the atmosphere and when the stomata are open, water vapour diffuses out of the leaf down the gradient
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How does humidity affect transpiration? pt. 3
The higher the humidity, the higher the water potential and the smaller the gradient. (and vice versa) As humidity increases, the rate of transpiration decreases
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What does transpiration in still air result in?
The accumulation of a layer of saturated air at the surface of the leaves. The water vapour gradually diffuses away leaving concentric rings of decreasing humidity. These are known as "diffusion shells"
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How does air movement affect transpiration?
Movement of surrounding air blows away the layer of humid air at the leaf surface. The water potential gradient between the inside of the leaf and the outside increases and water diffuses out more quickly. Faster air = faster rate
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How does light intensity affect transpiration?
As light intensity increases the stomata open wider (due to rate of photosynthesis increasing etc etc) so the rate of transpiration increases
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What are the benefits of transpiration?
It allows water and mineral ions to reach the leaves from the roots and water is delivered to the leaf cells for photosynthesis
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What are the 3 main methods for water moving up the xylem?
Cohesion-tension, adhesion and root pressure
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How does cohesion work?
As water molecules leave the xylem cells in the leaf, water is pulled up the xylem due to cohesive forces between the water molecules.
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How does adhesion work?
The charges on the water molecules also cause attraction to the hydrophilic lining of the xylem vessels, this is adheison
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What is root pressure?
Water entering the xylem generates an upwards push, the root pressure, on water already in the xylem.
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How can we measure the rate of transpiration?
It can be measured using a potometer
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What does a potometer actually measure?
It measures the rate of water uptake by the cut stem
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What is the measurement used for the rate of water uptake?
mm cubed per second
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What are the measurements that need to be made to work out rate of transpiration?
Distance traveled by bubble, time taken, diameter of capillary tube
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Why must the stalk be cut underwater, assemble the apparatus underwater and seal the joints of potometer with vaseline?
To stop air bubbles entering the xylem/apparatus which will prevent the movement of water
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After the apparatus is set up, then what must you do? pt1
Remove capillary tube from the water and let air bubble in, place the end back under water, when the air bubble reaches the scale record how far it travels in a given time
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After the apparatus is set up, then what must you do? pt2
Repeat that to calculate a mean, record the diameter of the inside of the tube, remove the leaves, place on graph paper, draw around the edge and calculate the total SA of leaves
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After the apparatus is set up, then what must you do? pt3
Express the final rate of transpiration as the volume of water lost per mm cubed per second
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What is the equation for the rate of transpiration?
(Pi)R squared * h (h is the distance the air bubble moved)
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What are mesophytes?
Plants that live in conditions of adequate water supply
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What are xerophytes?
Plants that live in conditions where water is scarce
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What are hydrophytes?
Plants that live partly or wholly in water
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What is an adaptation of some mesophytes?
They shed their leaves before winter to reduce water loss by transpiration
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What are the adaptations of xerophytes and why?
The leaves are smaller (there is a reduced SA so less water vapour is lost), The stomata are sunken i.e. found in pits (Water vapour is trapped in the pits, humidity increases, reduces the gradient)
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What are the adaptations of xerophytes and why? pt. 2
Stomata are surrounded by hairs (The hairs trap water vapour, again this reduces WP gradient), They have a thick cuticle (Waterproofs the leaves and therefore reduces water loss), they have fewer stomata (Less openings)
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What are the adaptations of xerophytes and why? pt. 3
They have long deep roots (long roots can reach deeper into the soil so have a larger SA to absorb more water
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What is a popular example of a xerophyte and what other feature does it have?
Marram grass, the leaves are rolled up
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What is an example of a hydrophyte?
Water lilies
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What is the greatest problem for hydrophytes?
Obtaining oxygen for respiration
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What are some common features for hydrophytes?
The presence of aeration tissue which provides large air spaces called lacunae, this allows rapid diffusion of oxygen throughout the plant, this also increases buoyancy so it can obtain as much light as possible
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What are some common features for hydrophytes? pt 2
Absence of supporting tissue, water provides support so xylem are poorly developed because water transport is less important
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What are some common features for hydrophytes? pt 3
Non functioning or absent stomata - Where leaves are submerged in water, stomata are absent, where leaves float e.g. water lilies, only the upper surface possess them
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After glucose is made during photosynthesis what is the disaccharide it is converted into for transport?
Sucrose
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What is the transport of sucrose and amino acids called?
Translocation
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What happens to the sucrose and amino acids?
They are translocated in the phloem, away from the leaves (the source) to all other parts of the plant (the sinks) where they are used for growth or storage
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What type of cells make up the phloem?
Sieve tubes and companion cells
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What are the sieve tubes?
They compromise end-to-end cells called sieve tube elements, the end walls are perforated in areas called sieve plates. These sieve plates allow bi-directional flow through the plant
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Why do sieve tube elements lose their nucleus, most organelles and have only a thin layer of cytoplasm?
To ensure there is no obstruction to the flow of sucrose and amino acids
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What are companion cells?
Companion cells are biochemically very active, as indicated by the large nucleus and many mitocondria/ This means they make large amounts of ATP
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How are companion cells connected to sieve tube elements?
By plasmodesmata, they allow the transport of ATP, sucrose and amino acids between the 2 cells
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What experiments were conducted to prove translocation?
Ringing experiments, radioactive tracers and autoradiography and aphid experiments
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What is the ringing experiment?
Cylinders of outer bark tissue were removed, this removed the phloem. After leaving the plant for some time it could be seen that above the ring there was a lot of sucrose built up causing the bark to swell
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How are radioactive tracers used?
If a plant photosynthesises in the presence of a radioactive isotope such as 14C in carbon dioxide, any sugars made will be labelled with 14C. The stem is cut to create a transverse section and placed on a photographic film, it will cause it to fog
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What does the position of the fogging relate to ?
The position of the phloem
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What are the aphid experiments?
The aphids have a hollow needle-like mouth called a stylet. It can be inserted into the sieve tube and the contents move into the stylet due to pressure. The ahpid can them be removed leaving the stylet inserted into the sieve tube.
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What are the aphid experiments? pt 2
The sap continues to exude from the stylet due to it being under pressure and it can be collected for analysis. The analysis shows that the fluid leaving the phloem contains sucrose
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What is the most popular theory about translocation?
The Mass Flow Hypothesis
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Who was it proposed by and when?
Ernst Munch 1937
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What does the hypothesis suggest?
That there is a passive flow of sugars and amino acids from the source where they are in high conc to the sinks where they are in low conc
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Explain the Mass Flow Hypothesis
Glucose is produced by photosynthesis in the mesophyll cells of leaves, some glucose is used in respiration but the rest is converted into sucrose. The sucrose is loaded into the companion cells of the phloem by active transport. ATP is needed.
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Explain the Mass Flow Hypothesis pt 2
The sucrose then diffuses through the plasmodesmata into the sieve tube elements. As a result the water potential of the ST elements decreases due to the increase in sucrose.
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Explain the Mass Flow Hypothesis pt 3
Water moves into the phloem from the nearby xylem by osmosis which increases the hydrostatic pressure of the ST elements.
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Explain the Mass Flow Hypothesis pt 4
The hydrostatic pressure moves the sucrose and other substances through the sieve tube cells, towards a sink. At a sink e.g. the root, sucrose is unloaded from the sieve tube elements into nearby cells. The sucrose can be converted back into glucose
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Explain the Mass Flow Hypothesis pt 5
to be used for respiration, used to make cellulose cell walls or stored as starch. As a result the conc. of sucrose and amino acids decreases. This increases the water potential of the sieve tubes and water moves out by osmosis into the xylem.
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Explain the Mass Flow Hypothesis pt 6
This lowers the hydrostatic pressure of the sieve tube elements of the sink. A pressure gradient is established
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What are some aspects of translocation not explained by the Mass Flow Theory?
Mass flow is meant to be a passive process but the rate of phloem transport is about 10,000 times faster than if substances were moving by diffusion alone
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What are some aspects of translocation not explained by the Mass Flow Theory? pt 2
Sucrose and amino acids move at different rates and in different directions in the same tissue, this would not be possible if movement was due to diffusion alone.
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What are some aspects of translocation not explained by the Mass Flow Theory? pt 3
The phloem has a high oxygen consumption and translocation is slowed or stopped at low temps or if respiratory poisons are applied. This suggests that it is an active process.
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What are some aspects of translocation not explained by the Mass Flow Theory? pt 4
The companion cells are biochemically very active but the mass flow theory does not state a role for them or the sieve plates also
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What other theories have been placed forward instead of the mass flow hypothesis?
Translocation is an active process due to the transport of sucrose and amino acids being inhibited at low temps or with cynaide
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What other theories have been placed forward instead of the mass flow hypothesis? pt 2
Protein filaments pass through sieve pores from one sieve tub element to the next. the bi-directional flow of sucrose and amino acids could be due to these protein filaments, different solutes are carried along different protein filaments via the sam
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What other theories have been placed forward instead of the mass flow hypothesis? pt 3
Cytoplasmic streaming through plasmodesmata might also be responsible for movement in of sucrose and amino acids in different directions as long as there was some mechanism to transport solutes through the sieve plates
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Card 2

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What does the lower epidermis contain?

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Card 3

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In the vascular bundle, where are the phloem and the xylem positioned?

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Card 4

Front

What is the word equation for photosynthesis?

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Card 5

Front

What are the 2 external adaptations a leaf has for gas exchange?

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