Nutrition and Transport in Plants

8/7/2019 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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4

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.

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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

Nutrition and Transport in Plants

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