nutrition and transport in plants
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Nutrition in Plants
Structure of the dicotyledonous leaf
Part Name Feature(s) Function(s)
A Cuticle Waxy andwaterproof
prevent excessive evaporation ofwater from the leafs inner cells.
Transparent Allows sunlight to reach inner cellsfor photosynthesis
B Palisademesophyll
cell
Has the mostchloroplasts
within cell
has highest amount of chlorophyllpresent to trap maximum amount
of sunlight in a given timemaximum photosyntheticactivity in a given time
Cells arecloselypacked inverticalalignment
to trap maximum amount ofsunlight in a given time
maximum photosyntheticactivity in a given time
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A
B
C
D E
F
G
H
I
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Part Name Feature(s) Function(s)
C Spongymesophyll
cell
Containchloroplasts
Chlorophyll traps light forphotosynthesis
Film of water
on outer cellwall
To dissolve gases, e.g. carbon
dioxide in intercellular air spaceswill dissolve into film of waterbefore diffusing into the cells.
Cells arelooselyarrangedwith airspaces
Allows the air space to beinterconnected for rapid diffusionof gaseous carbon dioxide to thecells
D Stomata(plural:stoma)
Openingsurrounded by apair of guardcells
Opening through which gases andwater vapour diffuse into and outof the leaf.
Usually closes to: Reduce amount of water
vapour lost through excessivetranspiration due to: Hot andlow humidity (e.g. dry) weather,windy conditions etc.
Ensure a high concentration ofcarbon dioxide in air spaces formaximum photosyntheticactivity in leaf during daybreak.
E Guardcells
Containchloroplasts
Inner cell wallis thicker thanouter cell wall
Controls the size of the stoma.
* A pair is required to form anopening between them.
Contains the least number ofchloroplasts among thephotosynthetic cells.
Uneven thickness of the cell wallresults in uneven expansion of theguard cell, therefore guard cellscurve.
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Parts Name Feature(s) Function(s)
F Air space Gases and
water vapourpresent
All air spacesare linkedcontinuouslybetweenspongymesophyllcells in leaf
Facilitates the diffusion of gasesrapidly to spongy mesophyll cells.
Usually has a higher concentration
of oxygen and water vapour thanthe surrounding air outside leaf,thus maintaining a concentrationgradient for diffusion of oxygenand water vapour out of the leafthrough the stomata.
G Phloem
Companioncells havelarge numberofmitochondria.
Translocation of sucrose andamino acids from leaves to thecells of the plant
mitochondria release energyneeded for active transport ofsucrose and amino acids in sievetube elements.
Holes insieve plate
allow rapid or easy flow of sucroseand amino acids through sieve
tube elements
H Xylem Has onlycell
wall Cytoplasm
absent
Lignindeposited onwalls
Long and narrow lumenWater and dissolved mineral saltscarried up xylem in an unbrokencontinuous stream.
Lignin deposited on xylem wallsIncreases its mechanical strength
to support the plant stem upright
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Parts Name Feature(s) Function(s)
I Epidermis(epidermal
cell) Thickcellulose cell
wall Does not
containchloroplasts
Cover the entire surface of plants,e.g. leaves, to protect inner cellsfrom physical damage or bacterialentry.
Contains guard cells at intervals
Note: In land plants, lowerepidermis has more guard cellsand thinner cuticle on its surfacethan the upper epidermis.
Action of Guard Cells
Stomata closed stomata open
During the day, the chloroplasts trap sunlight to produce sugar Water potential in the cell sap of the central vacuole is hence
lowered Water molecules enter the guard cells from the surrounding
epidermal cells by osmosis
Guard cells become more turgid As inner cell wall of guard cell is thicker than the outer cell wall,
the inner cell wall expands less
guard cell curve more, widening the size of the stomatabetween the guard cells
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Thicker inner cellwall
Thinner outercell wall
chloroplast
stomata
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Photosynthesis Importance of photosynthesis
Source of energy
Sunlight (ultimate source of energy) is converted to chemicalenergy in carbohydrate (e.g. starch stored in plants)
animals obtain these energy directly or indirectly by eating theplants containing the sugar or starch.
Maintain the oxygen and carbon dioxide level in the atmosphere.
Equations
Word equation:
Carbon dioxide + water glucose + oxygen
Chemical equation:
6CO2 + 6H2O C6H12O6 + 6O2
Laboratory investigations
Usually to prove that photosynthesis has occurred, one would carryout the starch test on the leaf.
However, one must first make sure there is no starch in the leaf at thestart of the experiment, i.e. must destarch the plant. To remove thestarch from the plant, simply place the plant in darkness (e.g. in adark cupboard) for a few days.
During destarching of the plant,
the starch in the leaf cells are converted back to glucose, where it isused by the leaf cells as respiratory substrate to release energyneeded for cellular activities.
Some of the starch are also converted to sucrose and translocated toother parts of the leaf via the phloem.
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Light trapped bychlorophyll
Light trapped bychlorophyll
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To prove that a leaf has photosynthesized
(1) Destarch the plant by placing the plant in a dark cupboard forat least one day.
(2) Do starch test on a leaf to confirm absence of starch.
(3) Place the plant under the sunlight for a few hours.
(4) Remove a leaf from the plant.
(5) Boil the leaf for at least fifteen minutes to kill the leaf cellsand stop all chemical activities within.
(6) Immerse the leaf in a boiling tube of hot ethanol until the leaf
has become white.Ethanol would dissolve the chlorophyll
(7) Rinse the white leaf in the hot water to soften it.
(8) Place the treated leaf onto a white tile.
(9) Add a few drops of iodine solution.
(10) The iodine solution would turn blue-black, showing thepresence of starch in the leaf.
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hot water
leafethanol
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To investigate if carbon dioxide is needed for photosynthesis
Results: leaf from test plant remains yellow while leaf fromcontrol plant turns blue-black
To investigate if chlorophyll is needed for photosynthesisand to investigate if light is needed for photosynthesis
Results: Disc 1 remains yellow (due to absence of chlorophyll)Disc 2 turns blue-blackDisc 3 remains yellow (due to absence of light andchlorophyll)
Disc 4 remains yellow (due to absence of light)
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sodium hydroxide (toabsorb carbondioxide)
water
Test plant Control plant
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black paper
green area
white area
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Rate of PhotosynthesisTo measure the rate of photosynthesis, one usually measures thenumber of bubbles released in one minute or one hour by a water plant,e.g. Hydrilla.
Factors affecting rate of photosynthesis
Primary Factor Reason
Temperature, rateEnzyme activity requires a suitabletemperature
Light intensity, rate
Light energy trapped by chlorophyllis converted into chemical energy inglucose.
Carbon dioxide
concentration, rate
Carbon dioxide is a raw material
required for the process ofphotosynthesis.
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oxygen
Hydrilla
support
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Limiting factor
Any factor that directly affects a process if the quantity of thatfactor is changed
At low light intensity, as light intensity increases, rate ofphotosynthesis increases
Light intensity is a limiting factor
At high light intensity, after a specific light intensity (i.e. runits), although light intensity increases, rate ofphotosynthesis remains constant
Light intensity is no longer a limiting factor
Some other factor, e.g. carbon dioxide concentration, is
limiting the photosynthetic process.
Fate of Products of Photosynthesis Glucose
1 Cellular respiration
Plant cells uses glucose as a raw material forrespiration to release energy.
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rate ofphotosynthesis
light intensityr
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Examples of uses of energy in plant cellsinclude:
For growth For active transport of dissolved mineral salts
from surrounding soil solution into cytoplasmof root hair cells.
2
Formation ofproteins
Nitrate is absorbed from soil by active transportand diffusion into root hair cells. In the leafcells, nitrate ions react with glucose, formed inphotosynthesis, to form amino acids, which inturn form proteins.
3Translocation
Glucose is converted to sucrose andtransported in the phloem to other parts of theplant, i.e. roots, fruits.
4 Formation of starch
Excess glucose is stored as starch in the leafcells.
Oxygen
1 Cellular respiration
Plant cells uses oxygen as a raw material foraerobic respiration to release energy.
2
Excretion
Excess oxygen diffuses out of the leaf throughthe stoma, as a waste product, to thesurrounding air (most likely during high lightintensity)
Minerals ion essential for plantsMineral ion Effect on plants
MagnesiumRequired for the synthesis of chlorophyll.Lack of magnesium would result in small yellowleaves.
Nitrate
Required for synthesis of amino acids andenzymes.Lack of nitrate would result in poor growth, i.e.leaves are small and few.
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Transport in Plants
Vascular bundles in dicotyledonous plantsDicotyledonous plants are a group of flowering plants whose seeds have
a pair of cotyledons (food reserves for growth of embryo plant). Themajor veins in the leaves of such plants are usually branched.The vascular bundle consists of conducting tissues (xylem and phloem)as well as supportive and protective tissue.
In roots
In stems
In leaves
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xylem
phloem
phloem
xylem
xylem
phloem
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Pathway of foodGlucose synthesized in photosynthesis is converted to sucrose andamino acids in leaf cells.
The sucrose and amino acids are then transported from the leaves to the
others parts of the plants, e.g. flowers, fruits and roots, through thephloem.
The movement of sucrose and amino acids through the phloem istermed as translocation.
Pathway of water
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inner cells of stem
outer cells of stem
mouth parts of aphid
xylem
phloem
root hair cell
vacuolecytoplasmcell wallwater
soil particle
air pocket
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Water potential in the surrounding soil solution is higher than that ofthe cell sap of the root hair cells. Water molecules move into the roothair cells by osmosis.
Water potential in root hair cells is now higher than the inner root cells,hence water molecules move into the inner cells by osmosis.
Water molecules move from one inner cell into the next inner cell byosmosis until they reach the root xylem.
Water molecules are drawn up the root xylem, through the stem xylemand finally to the leaf xylem by transpiration pull.
Water molecules are drawn from the leaf xylem into the surroundingspongy mesophyll cells by osmosis.
Some of the water in turn moves into the palisade mesophyll cells byosmosis and is used by the photosynthetic cells as a raw material forphotosynthesis.
Water then moves out of the spongy mesophyll cells onto its outer cellwall to form a film of water which evaporates into the air spaces.
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epidermal cell
palisade mesophyllcell
spongy mesophyllcell
stomata
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The water vapour molecules diffuse out of the leaf, down a diffusiongradient, through the stomata to the outside atmosphere.
Effects of TranspirationTranspiration is the loss of water vapour from the stoma of the leaves ofa plant.
Transpiration pull
Such loss results in the water being continually drawn from the leafxylem to the spongy mesophyll cells.
Hence, it ultimately generates the transpiration pull a force whichdraws water and mineral ions in an unbroken column continuously,up the stem xylem from the root xylem to the leaf xylem.
The unbroken column of water and mineral ions in the xylem isknown as transpiration stream.
Cooling effect
Water needs latent heat in order to evaporate.
Hence, the evaporation of water from the film of water on the outercell wall of the spongy mesophyll cells results in the loss of excessheat from the plant.
This lowers the temperature of the plant leaves and reduces risk ofenzymes in the leaf cells being denatured by the high heat, e.g.photosynthetic and respiratory enzymes.
Wilting
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Occurs during excessive transpiration, i.e. rate of water absorption isless than rate of transpiration and rate of photosynthesis.
Leaf cells lose turgidity and become flaccid.
As a result, the leaves would fold and droop
reducing the surface area of the leave exposed to sunlight.
Prolonged period of wilting can result in slowing down of cellularactivities in the plant and hence ultimate death of the plant.
Advantage of wilting
Reduces further loss of water vapour from the leaves.
The folding of leaves causes still air to be trapped in the
immediate region outside the stomata the diffusion gradient between air space and air outside leaf isreduced
rate of transpiration decreases
Disadvantages of wilting
Rate of photosynthesis decreases
strength of transpiration pull is reduced
rate in which water is brought up to the leaves decreaseswater is a raw material
guard cells becomes flaccid and loses curvature
Width of stoma decreases, i.e. close
less carbon dioxide diffusing from atmospheric air into airspace
carbon dioxide concentration decreases
carbon dioxide is a raw material
leaves fold up, reducing surface area exposed to sunlight
reducing rate of sunlight being trapped by chlorophyll
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Rate of transpirationUse of potometer
A potometer measures the rate of absorption of water,
*Assumption: rate of water absorption is proportional to the rate ofwater loss by transpiration
As water is absorbed by the shoot, the bubble in the capillary tubewith the ruler is drawn towards the shoot.
Rate of water absorption = distance moved by bubble per unit time.
The syringe would bring the bubble back to the starting point for thenext measurement.
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syringe
capillary tube with rulerwater
shoot