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1 TRANSPIRATION

= Inescapable consequence of gaseous exchange in the leaves of plants

Why Plants Transpire
1) CO2 + H20 = glucose + 02 (Photosynthesis in leaves)
- C02 is absorbed through STOMATA (because waxy cuticle = waterproof)
 Gas exchange needs large area of moist surface = provided by MESOPHYLL – spongy
mesophyll in lower part of leaf + network of air spaces = increase SA of moist cell
walls exposed to air
 BUT: if CO2 is allowed in, H20 evaporates OUT from moist cell walls (unless
air spaces are fully saturated)
 Cell walls = moist to maintain humidity gradient = H20 diffuses from
them to outside the leaf
2) Transpiration has some COOLING EFFECTS and is essential for MINERAL UPTAKE &
TRANSPORT

Measuring Transpiration Rates
-

Rate of H20 uptake measured by using a POTOMETER

METHOD:
1) Cut fresh shoot under H20 = stop air entering xylem vessels & at a slant to increase SA
2) Check apparatus is full of water/is air bubble free/no air locks
3) Transfer shoot to apparatus under H20 = avoid air bubbles water uptake
4) Remove potometer from water & ensure it is airtight/watertight at the joints around shoot
5) Dry leaves
6) Keep conditions constant
7) Shut screw clip
8) Keep ruler fixed & record position of air bubble on scale
9) Start timing & measure distance moved per unit time
10) Repeat measurements = reliable results

Factors Affecting Transpiration Rates
- Temperature


HEAT LAMP to vary the temp & an INFRARED THERMOMETER to measure leaf temp
- As temp increases, the rate of transpiration increases
 WHY? Heat is needed for evaporation of H20 from surface of mesophyll cells
 Higher temps also increase rate of diffusion through air spaces & lower relative
humidity of air outside the leaf

- Humidity




TRANSPARENT PLASTIC BAG to enclose leafy shoot
MIST SPRAYER to raise humidity inside bag & DESICCANT BAGS containing silica gel to lower it
ELECTRONIC HYGROMETER to measure relative humidity
- As atmospheric humidity increases, the rate of transpiration decreases

 WHY? H20 diffuses out of leaf when there is a CG between the humid air spaces
inside leaf & air outside – When atmospheric humidity increases, this reduces the CG




- Wind Speed
ELECTRIC FAN to generate air movement (vary velocity by changing rate of rotation)
ANEMOMETER to measure speed of the air moving across plant leaves
- Too low/high wind speeds decrease rate of transpiration
 WHY? In still air, humidity builds up around leaf, reducing CG of H20 vapour
 In high velocities, stomata close

Models of H20 Transport in Xylem
1) Water has adhesive properties:
 Beaker + H20 & capillary tube = H20 adheres to glass so rises up capillary tube
 Beaker + MERCURY & capillary tube = does not adhere to glass thus doesn’t rise
2) Water is drawn through capillaries in cell walls:
 Strip of paper (e
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chromatography) + H20 in test tube
- Paper is made of cellulose cell walls so H20 rises up through it against gravity in pores in
paper
3) Evaporation of H20 can cause tension:
 POROUS POT = similar to leaf cell walls as H20 adheres to it & there are many narrow pores
running through
1
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More H20 is drawn into pot to replace losses as it rises up the tube

H20 Uptake


ROOT HAIRS = found in zone behind APICAL MERISTEM
- Epidermal cells in this zone differentiate into RHCs = increase SA for absorption



FUNCTIONS OF ROOTS
1) Allow plant to take up H20 & minerals from soil
2) Anchorage & support & storage

-

Uptake of MINERAL IONS: PASSIVE TRANSPORT

= FACILITATED DIFFUSION using channel or carrier proteins
BUT, as soils are usually mineral poor:

-

Uptake of MINERAL IONS: ACTIVE TRANSPORT followed by Facilitated Diffusion

Soils contain clay particles* which have a –ve charge = prevents easy uptake of CATIONS
SO WHAT HAVE PLANTS DONE?
Some plants = developed a relationship with a fungus – grow on surface of roots & hyphae grow out
into soil (& form a network called mycelium) & absorb mineral ions = supplied to roots
PUMP PROTEINS FOR ACTIVE TRANSPORT – in root cell plasma membranes
1) H+ pump proteins actively move H+ out into soil water
2) H+ allow anions E
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Cl- to travel through co-transport carrier proteins into cell
3) H+ displace cations (e
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K+) from clay particles*
= Free to move down their CG by FD

BUT, in really mineral poor soils:

-

Uptake of MINERAL IONS: ONLY ACTIVE TRANSPORT

1) Free cations in the soil water can be taken up by active transport pump proteins (separate
PPs for each ion)
2) = Allows roots to build up concentrations of minerals in cells

-

Uptake of WATER

1) Uptake of mineral ions reduces water potential of root hair cells
2) H20 enters cells by osmosis down H20 potential gradient
3) H20 moves from root hair cells to xylem by:
- Symplast pathway (cytoplasm)
- Apoplast pathway (cell walls)
- Vacuolar pathway (vacuole & cytoplasm)
Symplast pathway
1) H20 enters root hair cells by OSMOSIS & passes between cells through PLASMODESMATA
2) H20 moves down WATER POTENTIAL GRADIENT through adjacent cells to cells of
ENDODERMIS
3) H20 enters XYLEM from cells of ENDODERMIS
Apoplast pathway
1) H20 enters cell wall of root cell & is drawn through adjacent cell walls by CAPILLARY ACTION
2) Movement along this pathway = blocked by CASPARIAN STRIP in the ENDODERMIS
3) H20 enters cells of endodermis & enters xylem by SYMPLASTIC PATHWAY
= Plant can control H20 & mineral uptake
Vacuolar pathway
= Same as Symplast pathway but H20 passes through vacuoles too

-

Crossing the root: the CASPARIAN STRIP

= In all cell walls of ENDODERMAL DELLS surrounding the CENTRE OF ROOT
= Impermeable & include waxy band of chemical SUBERIN
- This chemical forces H20 SYMPLASTIC PATHWAY
–

A few don’t contain suberin = PASSAGE CELLS & allow Apoplast pathway to continue to root
core

Vascular Tissue in Plants
TISSUE
Epidermis
Cortex
Vascular
Bundle
Xylem
Phloem
Cambium
Pith

STRUCTURE
Surface of stem, multiple layers, often
with a waxy cuticle
Cylinder of tissue around outer edge of
stem, often contains cells with secondary
thickening in cell walls
Contains xylem, phloem & cambium

FUNCTION
Reduces H20 loss

Longitudinal tubes

H20 & minerals from roots to
leaves
Transports sap around plant
Produces secondary xylem &
phloem through cell division
Can degenerate = hollow stem

Lateral meristem
Parenchyma (tissue with thin walled
cells)

Additional support

- Xylem & phloem in stems
 Between phloem & epidermis = Cshaped regions of tough lignified cells
(fibres) – provide support

- Xylem & phloem in roots

The Xylem – Mechanism of transpiration: MASS FLOW
= Tissue in plants that provides support & transports H20


Transpiration stream = flow of H20 in the xylem from the roots to leaves, to replace H20
losses from transpiration
How this works:
1) Tension causes H20 to move up to the leaves – generated in the leaves by transpiration & are
due to ADHESIVE property of H20
 H20 adheres to cellulose in plant cell walls (H-bonding)
2) When H20 evaporates from mesophyll cell walls in the leaf, more H20 is drawn through
cellulose-lined pores in leaf cell walls from the nearest xylem vessels to replace it = tension
3) Tension can be transmitted from 1 H20 molecule to the next because of cohesive property of
H20
 Tension generated in leaves is transmitted all the way down columns of H20 in xylem
vessels to roots
 Cohesion also means that the water column is continuous (no H-bonds in other liquids
= cavitation)

- Xylem tissue


VESSELS = short & wide lumen, blunt ended
- Walls impregnated with LIGNIN
- Xylem = dead cells & end walls degenerate to create a single tube with PITS



SCLERENCHYMA (FIBRES)
- Thickened cellulose cell walls = resist compression forces



LIGNIN
= Branched organic polymer – impermeable to H20
- Allows stem to flex in the wind by dissipating shear forces
- Prevents xylem collapsing due to low hydrostatic pressure
- Improves efficiency of H20 transport by minimising losses



PITS
- Regions which lack lignin
 Contain exposed, unthickened cellulose cell wall
- Allow H20 to leave xylem vessels

- Formation of Xylem
1) Primary xylem – walls of vessels are thickened in a helical/annular pattern
 Allows vessel to elongate as the root/shoot grows
2) Plant grows = more lignin deposited in cell walls & cell dies (lack of H20)
3) Walls at either end of the cell break down = hollow tube

Lumen = filled with sap as
cytoplasm & nuclei break
down

No PMs in mature
xylem vessels = H20
can move in & out
freely

Adaptations of Xerophytes (in deserts)
XEROPHYTES =:
1) Plants adapted to live in DRY conditions
2) Adaptations to reduce rate of TRANSPIRATION incl
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2 TRANSPORT IN PHLOEM OF PLANTS
The Function of Phloem
= Transport organic molecules, e
...
sugars & amino acids, from one part of the plant to another

- Phloem tissue


SIEVE TUBES
- Link together = phloem tube
- Cell wall resists high pressures inside sieve tube
- Lumen has no organelles that would obstruct flow of sap
- Sieve plate = cross wall that strengthens the sieve tube with pores that allow sap to pass
through in either direction
- Plasmodesmata = narrow cytoplasmic connections with adjacent companion cell
Movement of organic compounds takes place here


SOURCES
- Stems & leaves where PS is occurring
- Storage organs where stores are being utilised
Where sugars & amino acids are loaded into phloem sieve tubes by ACTIVE TRANSPORT


SINKS
- Roots, storage organs & growing fruits (i
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anywhere that needs sucrose but where they
cannot be produced)
Where sugars & other organic compounds are unloaded from phloem sieve tubes

- Transport in the phloem
Sap moves by MASS FLOW in phloem (same way as H20 in xylem) … HOW?
-

Incompressibility of H20 allows transport along HYDROSTATIC PRESSURE GRADIENTS =
pressure in a liquid

1) High concentration of solutes e
...
sugars in phloem sieve tubes at the source = H20 uptake by
osmosis & high hydrostatic pressure because of H20 POTENTIAL GRADIENT
2) Low concentration of solutes in phloem sieve tubes at the sink = exit of H20 by osmosis & low
hydrostatic pressure
= Therefore a PRESSURE GRADIENT that makes sap inside phloem sieve tubes flow from
sources to sinks

-

LOADING PHLOEM SIEVE TUBES (AT THE SOURCE)

Main sugar carried by phloem sieve tubes = SUCROSE (PS produced triose sugars which can be
converted to sucrose)
1) Transport from source cells to companion cells occurs by the apoplast or symplast pathways
2) Companion cells then actively load sucrose into the phloem, which requires AT
3) H+ pumped out of companion cells into adjacent cells e
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source cells, which creates a PROTON
GRADIENT
4) H+ re-enter cell down the CG through co-transporter proteins with sucrose
 This is an example of facilitated diffusion using symport co-transport proteins
5) Sucrose diffuses from the companion cell into the phloem through plasmodesmata

-

UNLOADING PHLOEM SIEVE TUBES (AT THE SINK)

Unloading in actively growing regions of the plant (e
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leaves, shoots) = PASSIVE PROCESS…
1) At active sink regions, sucrose is immediately used for RESPIRATION or converted into starch
etc
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3 GROWTH IN PLANTS
Indeterminate Growth in Plants

 Animals: go through a period of DETERMINATE growth – it occurs during an
embryonic/juvenile stage & then stops
 A fixed number of parts develop
 Plants: show INDETERMINATE growth = can continue to grow throughout their lifespan
 Many plant cells (incl
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of extra branches, leaves & flowers
 PRIMARY GROWTH:
= Growth at apical meristems in the vertical plane
- Causes:
 Limited mitosis in the meristem under control of CYTOKININS* (hormone produced
at root)
 Extensive cell elongation under control of AUXIN

Growth of the Shoot
-

Shoot = stem + leaves – at tip of root there is the SHOOT APICAL MERISTEM

Cells in SAM carry out mitosis & cell division repeatedly = generate new cells needed for extension of
stem & development of leaves:
1) As cells divide, 1 remains in meristem (so it can continue to go through the cell cycle & produce
more cells), whilst the other is pushed out
2) Cells on the edge stop dividing & undergo rapid growth & differentiation = become stem/leaf
tissue
3) Leaves = start as small bumps at side of apical dome
 Bumps = LEAF PRIMORDIA & through cell division and growth they develop into
mature leaves

-

PROTODERM creates epidermis
PROCAMBIUM creates vascular
tissues
GROUND MERISTEM creates pith

- Plant growth – lateral meristem (developed in DICOTYLEDENOUS
plants)
1) As plants grow taller, they need to support extra mass
2) Lateral meristem growth results in extra (secondary) xylem growth in a ring inside the
cambium
3) Secondary phloem also grows

- Axillary buds & apical dominance
Axillary buds = shoots that form at the junction/node of the stem & base of leaf
 At high concentrations, auxin can inhibit growth
1) As the SAM grows & forms leaves, regions of meristem = left behind at node
 Growth at these nodes is inhibited by auxin produced by SAM = apical dominance
2) The further a node is from the SAM, the lower the conc
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Have to cope with whatever climatic conditions they are in as they CANNOT MOVE
2
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B
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g
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4 REPRODUCTION IN PLANTS
Flowering

 Vegetative phase = when a plant germinates & a young plant is formed that grows roots, stems
& leaves = vegetative structures
 Reproductive phase = when a trigger causes the plant to produce flowers
This change happens when meristems in the shoot start to produce parts of flowers
instead of leaves

- Purpose of flowering
= To allow for pollination, fertilisation & subsequent seed dispersal
POLLINATION
= Transfer of pollen from the ANTHER of 1 flower to the STIGMA of another
SEXUAL REPRODUCTION of flowering plants depends on this
1) Abiotic pollination – accounts for 10% of pollination in Angiosperms
= Without involvement of other organisms e
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wind, water
2) Biotic pollination – accounts for 90% of pollination in Angiosperms
= With assistance of pollinators e
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insects, birds, or mammals (rodents/bats)
-

MUTUALISM = close association between 2 organisms where both organisms benefit from
the relationship
 E
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POLLINATORS gain food in form of nectar & PLANT gains a means to
transfer pollen to another plant

FERTILISATION
1) Pollen grain reaches STIGMA
2) From each pollen grain on the stigma a POLLEN TUBE grows down the STYLE to the OVARY
3) Pollen tube carries male gametes to fertilise the ovary (located in ovule)
4) DOUBLE FERTILISATION occurs as there are 2 sperm nuclei
1 sperm fertilises the central cell
1 sperm fertilises the egg cell
SEED DISPERSAL
= Travelling of seeds long distances from the parent plant, even though they cannot move themselves
- Method of this depends on the structure of the fruit
 E
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dry & explosive, fleshy & attractive for animals to eat, feathery or winged
to catch the wind, covered in hooks that catch onto the coats of animals
WHY? – Reduces competition between offspring & parent and helps to spread the species

Germination
= Early growth of a seed & involves growth of the embryo root & shoot
Before germination, seeds are dormant with a metabolic rate of nearly 0 (allows time for seed
dispersal)
 Ideal conditions for germination:
- Water (taken in through micropyle & used to activate the seed) = metabolic process restart
- Oxygen – for aerobic cell respiration

-

Ideal temperatures = warmth required as germination involves enzyme-catalysed
metabolic reactions
pH for enzyme activity
LIGHT (amount differs)

 Specialised conditions for germination (some seeds)
- Being digested & passed
- Removal of inhibitor hormones of germination by ‘washing’ e
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beans
- Fire (induced by smoke)

- Stages of germination of starchy seeds
1) H20 absorbed through the micropyle & activates cells
2) Synthesis of gibberellins (plant growth hormones)
3) Gibberellins cause synthesis of amylase (but also stimulates mitosis & cell division in the
embryo)
4) Amylase hydrolyses stored starch to make maltose (water needed for this)
5) Maltose absorbed by plumule & radicle
6) Further hydrolysis breaks down maltose into glucose
 Why glucose? – Whereas starch is insoluble & immobile, glucose can be
transported from the food reserves to where they are needed in germinating
seed
- = Used for aerobic cell respiration in the growing tissues – embryo root & shoot need
sugars for growth, together with AAs etc
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