AS Biology OCR Unit F211: Cells, Exchange & Transport ... · PDF file... the structure and...
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1.2.3 Andy Todd 1 thebiotutor AS Biology OCR Unit F211: Cells, Exchange & Transport Module 2.3 Transport in Plants Notes & Questions
Transcript of AS Biology OCR Unit F211: Cells, Exchange & Transport ... · PDF file... the structure and...
1.2.3
Andy Todd 1
thebiotutor
AS Biology OCR
Unit F211: Cells, Exchange & Transport
Module 2.3 Transport in Plants
Notes & Questions
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Explain the need for transport systems in multicellular plants in terms of size and surface area to volume ratio.
Like all multicellular organisms;
o Demands on nutrients are large
o Diffusion distance is great
o Some areas are sources and some are sinks
Plants require
o Water, oxygen and nutrients by every cell
Plants need to get rid of
o Metabolic wastes, such as oxygen and carbon dioxide from every cell
Dilemma!
o Leaves are the sites of photosynthesis where glucose is synthesised.
o Roots are surrounded by water and dissolved minerals
o leaves cannot obtain water from the air and roots cannot absorb sugars from the soil
Photosynthesis
o Carbon dioxide + Water Glucose + Oxygen
Found (Air) (Soil) (Leaves) (Air)
Needed (leaves) (All cells)
Describe, with the aid of diagrams and photographs, the structure and function of xylem vessels, sieve tube elements and companion cells.
The plant has a transport system like that of animals except it does not have a pump nor does it carry gases.
The plant uses specialised tissues in its transport system known as vascular tissues
Vascular tissue is made of two main tissues
o Xylem Tissue – carries water and dissolved substances up the plant
o Phloem Tissue – carries assimilates up and down the plant
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Xylem tissue
Xylem
• Water
• Dissolved minerals
Roots
Rest of the plant
Structure of the Xylem
o Xylem vessel elements are made from meristem (unspecilaised) tissue within plants. The xylem vessels are long narrow plant cells.
o As the meristem cells mature the walls become impregnated with Lignin.
lignin is a strong woody insoluble polymer. It adds strength to the cell walls and waterproofs them.
o As a result the cells DIE.
o The end plates and cell contents decay and leave behind the lignin side walls
o The lignin strengthens the xylem vessel preventing it from collapsing in on itself when water is in short supply
o Lignin thickening can form in patterns;
Spiral
Annular (rings)
Reticulate (broken rings)
o These patterns prevent the vessels from
being too rigid and allows flexibility.
o Xylem vessels have pores along the length
of their side walls. These are known as
bordered pits
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Adaptations of the Xylem
o Xylem tissue can carry water and dissolved minerals from the roots to the top of a plant because;
o It is made from dead cells aligned end-to-end to form a continuous column
o Narrow to prevent the column of water breaking from capillary action, adhesion of water molecules to the sides of the xylem vessels
o Bordered pits to allow water to enter and leave xylem vessels and allows water to transfer between xylem vessels
o Lignified to provide strength to prevent xylem vessels collapsing under tension
o Lignin deposited in spiral, annular and reticulated patterns to allow the vessel to flex and prevent the xylem breaking
o Water is not impeded because;
There are NO end walls
There are NO cell contents
There is NO nucleus or cytoplasm
The lignin strengthens the walls so they stay open.
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Phloem Tissue
o Phloem tissue transports sugars to all parts of the plant both up and down.
o Made of two types of tissues
Sieve tube elements
Companion cells
Phloem
• Sugars
Leaves
Rest of the plant
Rest of the plant
Structure of Phloem
Sieve tube elements
o Not really considered true cells as they contain very little cytoplasm and no nucleus – allowing easy flow of assimilates
o Like the xylem vessel the sieve elements line up end-to-end forming a tube
o Sucrose and other assimilates are transported dissolved in water to form SAP.
o Unlike xylem vessels the phloem cells have end walls. These end walls are perforated by many pores to allow the sap to flow.
Hence the name, sieve plate.
o The sieve tubes have thin walls and are generally 5 or 6 sided.
Companion cells
o Alongside the sieve tube vessels are small cells with large nuclei and dense cytoplasm.
o They have many mitochondria to produce ATP needed for active processes.
o They carry out many metabolic processes needed by the sieve tube elements. This includes using the ATP to load the sieve tube elements with sucrose.
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o The companion cells and sieve tube elements are linked through holes called Plasmodesmata
o Plasmodesmata are gaps which allow communication and flow of minerals between cells
Describe, with the aid of diagrams and photographs, the distribution of xylem and phloem tissue in roots, stems and leaves of dicotyledonous plants.
Vascular Tissue
o Vascular tissues of the xylem and phloem are found together in Vascular bundles.
o Vascular bundles have different arrangements between different plant tissues and different groups of plants
Dicotyledonous plants – Flowering plants
o These are the only types of plants that you need to be aware of when describing the distribution of vascular tissue.
o The examiners outline this specifically as monocotyledonous plants have a very different distribution.
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Monocotyledonous plants – Grasses, Cereals
o You do not need to be aware of the distribution of vascular tissue in monocotyledonous plants
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Plant Roots
o There is a large central core of xylem often in an X-shape.
o The phloem is found in between the arms of the xylem.
o This arrangement provides strength to withstand pulling forces to which the roots are exposed.
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Plant Stems
o Vascular bundles in young dicotyledon stems are separate but become continuous in older stems.
o This means that there is a complete ring of vascular tissue just under the bark.
o This arrangements provides strength and flexibility to withstand the bending forces to which branches and stems are exposed (i.e. Wind).
Cortex
Medulla
Phloem
Separate Vascular tissues
Xylem
Cambium
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Leaves
Xylem
Phloem
Define the term transpiration.
The loss of water vapour by evapouration from the aerial parts of plants (mainly leaves)
Transpiration involves 3 processes
o Osmosis from the xylem to the mesophyll cells
o Evaporation from the moist surface of the mesophyll cells into the inter-cellular spaces
o Diffusion of water vapour from the intercellular spaces out through the stomata.
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Explain why transpiration is a consequence of gaseous exchange.
Plants lose water through their stomata, as outlined previously.
Stomata are open during the day because the plant needs to take in carbon dioxide from the atmosphere and get rid of oxygen to the atmosphere from photosynthesis.
Gas exchange is necessary for photosynthesis, which provides the plant with glucose for energy and allows the plants to build themselves by creating cellulose, fatty acids and amino acids.
For gas exchange to occur stomata must be open, therefore allowing transpiration to occur.
Leaves have many stomata for gas exchange
Mesophyll cells have a moist surface which means a high water potential compared to a low water potential of the dry air (water potential gradient)
Why is it a problem?
o If water loss is greater than water uptake then a plant can suffer from water stress.
o Cells lose their turgidity and may even undergo plasmolysis.
o Non-woody parts wilt and eventually die.
It’s not all bad.
o Transpiration from the leaves creates a tension on the water column in the xylems.
o This water column will not want to break so water is drawn up the xylem from the roots.
o This results in more water entering at the roots and bringing dissolved minerals into the plant roots along with the water.
o Water loss by evapouration from the leaves can have a cooling effect on the leaves, supporting optimum temperatures for photosynthetic enzymes.
Describe the factors that affect transpiration rate.
Plant factors which affect transpiration rate
o Number of leaves
More leaves = more transpiration
o Number, size and position of stomata
Many stomata = more transpiration
large stomata = more transpiration
Stomata on upper epidermis = more transpiration
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o Waxy cuticle
Thinner waxy cuticles = more transpiration
Environmental factors which affect transpiration rate
o Amount of light
More light = stomata open = more transpiration
o Relative humidity
Greater the humidity gradient = more transpiration
o Temperature
Warm air can hold more water = greater gradient
Increased evaporation from mesophyll cells
Water vapour has more kinetic energy = more movement through the stomata
o Wind
More wind = less barrier = more transpiration
o Water availability at roots
More water available = more water up the xylem
Describe, with the aid of diagrams, how a potometer is used to estimate transpiration rates.
water loss by transpiration
reservoir syringe
apparatus filled with water
capillary tube
meniscus
graduated scale
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A photometer actually measures
o Water uptake of the plant
o This can be used as an estimate of water loss by the plant.
o Assumption that water uptake = water loss
o Assumed that the detached branch is representative of the plant as a whole
o Problems with this,
Some water may be used for turgidity of the plant cells
Some water will be used in photosynthesis
leafy shoot
air bubble
scale
screw clip
waterreservoir
How to setup a photometer
1 cut (healthy) shoot under water (to stop air entering xylem vessels); 2 cut shoot at a slant (to increase surface area); 3 check apparatus is full of water / is air bubble free / no air locks; 4 insert shoot into apparatus under water / AW; 5 remove potometer from water and ensure, airtight / watertight, joints around shoot; 6 dry leaves / AW;
7 keep, condition(s) / named condition(s), constant; 8 allow time for shoot to acclimatise / AW; 9 shut screw clip; 10 keep ruler fixed and record position of air bubble on scale; 11 start timing and, measure / calculate, distance moved per unit time
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Explain, in terms of water potential, the movement of water between plant cells, and between plant cells and their environment.
Water Potential
o ‘A measure of the tendency of water molecules to diffuse from one area to another’
o Water always moves from an area of high water concentration (dilute) to an area of low water concentration (concentrated).
o Water exerts a pressure (Osmotic Pressure)
Main points to consider;
o Pure water has a water potential of zero
o Cells have -ve water potential because they contain dissolved salts and sugars
o Water molecules move from less -ve regions (fewer dissolved sugars and salts) to more -ve regions (more dissolved sugars and salts)
Water entering a plant cell
o Water will enter the plant cell if water potential is lower inside the cell compared to outside the cell
o Water will enter the cell but will not burst due to the strong cellulose cell wall.
o The water inside the cell exerts a pressure pushing the cell membrane against the cell wall.
o As the pressure builds up it reduces the influx of water entering the cell.
o The cell is referred to as being Turgid
Water Leaving a plant cell
o Water will leave the plant cell if water potential is lower outside the cell
o Water will leave the cell and the cell will lose its turgidity.
o If water loss continues the vacuole and cytoplasm shrink.
o Eventually the water will not have enough pressure to push the cell membrane against the cell wall.
o If water loss continues from the cell the cell membrane loses contact with the cell wall - a condition known as Plasmolysis and the cell is said to be Plasmolysed.
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Describe, with the aid of diagrams, the pathway by which water is transported from the root cortex to the air surrounding the leaves, with reference to the Casparian strip, apoplast pathway, symplast pathway, xylem and the stomata.
Water moves between cells from cells with a higher water potential to cells with a lower water potential.
This water potential gradient is usually setup across the plant from root to leaf.
Transpiration creates low water potential at the leaves.
Water entering the plant at the root creates a high water potential at the root.
Water therefore streams from root to leaf via the xylem.
There are 3 possible pathways that water molecules can take between cells;
o Apoplast pathway
Water passes through the water-filled spaces between cellulose molecules of the cell walls
As the water does not move in and out of the cells it does not cross any plasma membranes and as a result the water can carry dissolved mineral ions and salts easily.
o Symplast pathway
Water moves from cell to cell through gaps in the cell walls called plasmodesma(ta).
Plasmodesmata are gaps in the cell wall filled with thin strands of cytoplasm.
Once inside the cytoplasm water can move through the continuous cytoplasm from cell to cell
o Vacuolar pathway
Similar to the symplast pathway except water is not confined to the cytoplasm of the cell.
The water is able to enter and pass through the vacuoles as well.
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Adaptations of roots
o Roots contain specialised cells called root hair cells on the outermost layer of the root tissue
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Movement of water across the root (soil to Xylem)
o Root hair cells – water into root
Increase surface area in contact with the soil
Absorb minerals by active transport using ATP
Absorbed minerals reduce water potential in the root = water enters the root across the plasma membrane via osmosis.
o Endodermis – water crossing root cortex and into xylem
This is an active process involving endodermis tissue
Endodermis tissue is a layer of cells that encircle the xylem and contain granules of starch (Starch = fuel).
The endodermis has a water proof strip called the casparian strip.
The endodermis cells move minerals from the root cortex to the xylem by active transport (hence the starch).
This decreases water potential in the xylem and so water follows.
Just outside the endodermis in the cortex the water potential drops due to water leaving to the xylem.
This water drop combined with water entering the root results in a steep water potential gradient across the cortex.
Water therefore flows across the cortex via the symplast pathway and the apoplast pathway.
The water meet before passing through the endodermis
o The Casparian Strip
Blocks the apoplast pathway ensuring water with dissolved nitrates ions pass into the cell cytoplasm through the cell membranes
To support this there are transporter proteins in the cell membrane
Fast transfer of the nitrates from the cortex to the xylem vessel.
Water potential in the xylem is lowered so water follows and leaves the cortex
Water cannot flow back from the xylem to the cortex because the apoplast pathway is blocked due to the casparian strip.
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Three factors help move water up the stem
Root pressure
o The endodermis driving minerals into the xylem lowers the water potential and as a result water follows into the xylem
o This pushes water already in the xylem further up the stem
o Root pressure can move water a few metres up a plants stem.
o However it cannot account for water reaching the tallest branches of tall trees.
Transpiration pull
o Water is lost through evaporation at the leaves. This water must be replaced by water coming up the xylem.
o Water molecules are attracted to one another by a force called Cohesion. These forces are strong enough to hold a long chain or column of water together.
o As water is lost through the leaves it puts tension on the water column pulling it further up the xylem through its reluctance to break (Cohesive forces).
o This is called the cohesion-tension theory
o Cohesion-tension theory relies of the water column remaining unbroken.
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o If the water column breaks, it can be maintained by water from neighbouring vessels flowing through the bordered pits to complete the column.
o The tension can put the xylem vessel under huge pressure and so the xylem is dependent on the strength provided by the lignin to stop it from collapsing.
Capillary action
o The same forces of cohesion holding the water molecules together attract the water molecules to the sides of the xylem. This is known as Adhesion.
o As the xylem vessel is very narrow (only as a wide as a cell) the force of adhesion can pull water up the sides of the xylem vessel.
Explain the mechanism by which water is transported from the root cortex to the air surrounding the leaves, with reference to adhesion, cohesion and the transpiration stream.
In the leaves
o Transpiration from the leaves sets up a water potential gradient
o Most negative water potential, at the leaf / in the atmosphere;
o Loss of water from spongy mesophyll;
o From the cell walls;
o More water is drawn from, cytoplasm / cell / AW;
o Water from xylem / xylem vessels replaces water lost from the cells;
o Water moves down a, water potential / Ψ, gradient;
In the stem
o This places water in xylem under, tension / hydrostatic pressure gradient
o Cohesion of water molecules;
o By hydrogen / H, bonds;
o Creates continuous water columns / AW;
o Adhesion of water molecules to the sides of the xylem help the water columns climb the xylem vessels
In the root
o Water enters the root from the soil
o Via, symplast / apoplast / vacuoles / description /
o This generates a root pressure, forcing more water into the xylem vessels at the roots;
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Describe, with the aid of diagrams and photographs, how the leaves of some xerophytes are adapted to reduce water loss by transpiration.
Xerophytes are plants that are adapted to living in arid conditions
They have a number of adaptations that enable them to limit water loss from their leaves.
Adaptations
o Small leaves (needles)
Reduces surface area for stomata and therefore reduces transpiration
o Densely packed spongy mesophyll
Fewer air spaces results in less surface for evapouration to occur
o Thick waxy cuticle
Reduces the amount of evapouration through the upper and lower epidermis
o Closing stomata when water is limited
Reduces the number of open stomata and therefore transpiration
o Hairs
Builds humidity shields
Preventswind blowing moist air away from stomata
Decreasing the water potential gradient between the air spaces inside the spongy mesophyll layer and the air outside the leaf
o Pitted stomata
Same effect as hairs
o Rolled leaves
Same effect as hairs
o High salt concentration in the leaves reducing water potential
Lowers the water potential inside the leaf therefore reducing the water potential gradient between inside the leaf and the air outside.
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Marram Grass (Ammophilia spp)
o Specialised to living in sand dunes where;
o Water drains away quickly from the sand
o Sand can be salty
o Strong winds
Marram Grass
•
Adaptations of Marram Grass (Ammophilia)
o Rolled leaves
o Hairs
o Pitted stomata
o Waxy cuticle
o Trapped air
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Explain translocation as an energy-requiring process transporting assimilates, especially sucrose, between sources (e.g. leaves) and sinks (e.g. roots, meristem).
Source
site where, sucrose or sugars or assimilates are loaded into phloem
ACCEPT where, sucrose / sugars / assimilates, produced/created or converted from stored products
DO NOT ACCEPT glucose / substance throughout
DO NOT ACCEPT terms ‘loading’ and ‘unloading’ in wrong context
A source can be either a leaf during mid to late spring and summer and a root during winter and early spring.
Sink site where, sucrose or sugars or assimilates are unloaded or removed from phloem
ACCEPT where, sucrose or sugars or assimilates, stored or used in metabolic processes
DO NOT ACCEPT ‘required’ or ‘needed’ instead of ‘used’
A Sink can be either a leaf or growing tip during early spring and a root during late spring and summer.
Additional points
High hydrostatic pressure makes it a source and low hydrostatic pressure a sink;
When loading it is a source and when unloading a sink;
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Describe, with the aid of diagrams, the mechanism of transport in phloem involving active loading at the source and removal at the sink, and the evidence for and against this mechanism.
Glucose is added to fructose to create Sucrose.
Features of the Phlooem that enable active loading and mass flow.
o sieve elements are stacked end to end and have sieve plates perforated / sieve pores, for ease of flow
o companion cells are metabolically active and so have many mitochondria to produce ATP
o Proton pumps in the membranes of companion cells pump protons out of the companion cells
o Protons diffuse back into the companion cells through co-transporter proteins bringing sucrose into the companion cells as well.
o There are plasmodesmata between the companion cells and sieve tube elements which allows the sucrose to pass into the sieve tube element.
o sieve element has few organelles allowing ease of flow for more sucrose.
Active Loading
o Sucrose is made in leaf cells (source)
o Companion cells pump protons (H+) out of the companion cell into the surrounding cells that have sucrose.
o The protons (H+) are pumped using proton pumps.
o This pumping requires ATP, formed from the many mitochondria in the companion cells cytoplasm
o This builds a high concentration of H+ in the surrounding cells
o The protons move into the companion cell by diffusion down their concentration gradient.
o The protons move back through a co-transporter protein bringing sucrose with them into the companion cell.
o The sucrose is now in high concentrations in the companion cells and moves through the plasmadesmata into the sieve tube elements.
This is called active loading as the loading of sucrose into the phloem uses ATP, and therefore it is an active process.
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Mass Flow
At source o The sucrose can then diffuse down the concentration gradient into the sieve
tube element via the plasmodesmata that connects the companion cell with the sieve tube element.
o This lowers the water potential of the sieve element o Water enters the sieve tube element from xylem vessels by osmosis. o This increase in water molecules, increases the hydrostatic pressure at this
point on the sieve tube element. o Sucrose and other assimilates will move from areas of high hydrostatic
pressure (source) to areas of lower hydrostatic pressure (sink).
At sink o sucrose will be unloaded from the phloem into tissue. o It is likely that the sucrose moves out by diffusion and is then converted into
another substance (starch) to maintain a concentration gradient. o Water will move out of the sieve tube element into the xylem vessel by
osmosis.
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Evidence for translocation being an active process
o Rate of flow in the phloem is about 10,000 times faster than it would be if it was due only to diffusion, therefore it is an active process
o The pH of the phloem sap is around 8 (it is alkaline due to loss of hydrogen ions).
o An electrical potential difference across the cell surface (negative inside due presumably to the loss of positively charged ions).
o Research using carbon dioxide containing a radioactive label, C14, has revealed the following evidence about the mechanism of translocation:
Labelled carbon can be observed in the phloem soon after being supplied to a well-lit plant;
The rate of movement of sugars in the phloem is many times faster than could be achieved by diffusion alone.
An insect such as an aphid feeds by inserting its proboscis (mouth parts) into the phloem;
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o Additional piece of evidence are;
ATP is involved or respiration is involved or there are many mitochondria in companion cells
It is temperature dependent
loading is against a concentration gradient
Ringing a tree
o When the bark is removed from a tree, the phloem is also removed. If a complete ring of bark is removed, the tree trunk can be seen to swell above the cut.
sugars cannot pass the cut
decrease water potential;
water moves into cells;
damage triggers increased cell division;
to produce cells to store sugars;
cut causes, gall / infection;
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1. The figure below is an electron micrograph of xylem tissue in the stem of a plant.
spiral band
pit
(i) State one function of xylem tissue.
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[1]
(ii) The spiral band in the xylem vessel shown in the figure above contains a substance called lignin.
State the function of this spiral band of lignin and explain why it is important that the xylem vessel becomes lignified in this way.
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[3]
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(iii) Explain the function of the pits seen in the figure above.
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[2]
[Total 6 marks]
2. Translocation is the movement of the products of photosynthesis within a plant.
Translocation occurs in the phloem and involves sources and sinks.
Using the outline below, draw in the position of the phloem in the root of a dicotyledonous plant.
root
[Total 1 mark]
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3. The diagram below is a vertical section through part of a leaf of a dicotyledonous plant.
E
F
G
H
I
Reproduced by kind permission of D.G. Mackean
Complete the table below to identify xylem and phloem from the tissues labelled E to I.
tissue letter
xylem
phloem
[Total 2 marks]
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4. Transpiration may be defined as the loss of water vapour by diffusion from a plant to its environment.
The diagram below shows apparatus that can be used to estimate transpiration rates of a leafy shoot.
water loss by transpiration
reservoir syringe
apparatus filled with water
capillary tube
meniscus
graduated scale
(i) State the name of the apparatus shown in the diagram.
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[1]
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(ii) A student was told that any results gained by using the apparatus shown in the diagram above are not measures of the actual transpiration rate, but only give values from which transpiration can be estimated.
With reference to the definition of transpiration and the apparatus in the diagram above, explain why the results gained by using the apparatus are not measures of the actual transpiration rate.
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[3]
(iii) Describe the precautions you would take when setting up and using the apparatus shown in the diagram above in order to get valid readings from which the transpiration rate can be estimated.
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[4]
[Total 8 marks]
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5. (a) Complete the following paragraph about the loss of water from plants.
The loss of water from the aerial parts of a plant is known as
.......................................... .
The majority of water is lost from the leaves. Water is transported up the stem in
the .......................................... and passes into the mesophyll cells of the leaf by
.......................................... . Water evaporates from the surface of these cells.
From the air spaces in the leaf, the water vapour diffuses out of the leaf through
the ...........................................
[4]
(b) (i) Explain why water loss from the leaves of a plant is unavoidable.
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[2]
(ii) Name the type of plant adapted to reduce water loss from its leaves.
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[1]
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(iii) State and explain two adaptations of leaves that reduce evaporation.
In your answer, you should use appropriate technical terms, spelt correctly.
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[5]
[Total 12 marks]
6. Explain how transpiration results in the movement of water up a plant stem.
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[Total 4 marks]
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7. Carbohydrate moves from regions of plants called sources to regions called sinks.
Explain how, at different times, the same plant root may be a source or a sink.
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[Total 2 marks]
8. (a) From the list below, circle the carbohydrate that is transported in phloem.
auxin fructose glucose glycine glycogen starch sucrose
[1]
(b) Phloem is responsible for the transport of carbohydrate in plants. The diagram below shows the structure of the cells in phloem.
P
Q
A-level Biology, page 362 Fig. 31.10A, by W D Phillips and T J Chilton,
published by Oxford University Press, 1989. (ISBN 0 19 914089 8)
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(i) Name the cells P and Q in the diagram.
P .............................................................................................................
Q ............................................................................................................
[2]
(ii) Outline how P and Q are involved in the transport of carbohydrate in phloem.
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[3]
[Total 6 marks]
