Transport in Plants

Chapter 9

why do multicellular plants need transport systems?

Metabolic demands - need oxygen and glucose transported to places that don’t photosynthesise, hormones and mineral ions absorbed by the roots need to be transported to all cells. Size - larger plants have a greater distance for substances to travel, requiring efficient transport systems to ensure all cells receive necessary nutrients and water. SA:V - leaves have high SA:V for gas exchange and light absorption but stems and trunks have a relatively small SA:V so diffusion is inefficient

herbaceous dicotyledonous plant

have soft tissues, short life cycle, leaves and stem die at end of growing season, soft green stem

perennial plants

plants that live a long time and reproduce year after year

woody dicot

also called arborescent, hard lignified tissues, long life cycle

dicot

vascular plants that reproduce by the use of flowers which have 2 seed leaves (two cotyledons)

plasmodesmata

channels through the cellulose cell wall linking the cytoplasm of adjacent cells allowing for the transport substances/signalling

parenchyma cells

main function is to store food (can also stores water), play a role in photosynthesis and tissue repair (because they can divide), thin cell wall, large vacuole

sieve tube element structure and function

what is it - single cell of phloem, has sieve plates at the end of each cell/sieve tube element. function - main transporting vessel of phloem, transport of nutrients, particularly assimilates. structure - elongated form, connected to a companion cell, no nucleus, not liginifed, sieve plates in areas between the cells

companion cells structure and function

what is it - attached to side of sieve tube element. function - supply ATP, facilitate loading/unloading sugars in/out of the sieve tubes, provide metabolic support to sieve tube elements as they have lost most their normal cell functions. structure - numerous mitochondria, dense cytoplasm, many plasmodesmata connecting companion cell to sieve tube element

xylem vessel structure and function

what is it - type of tissue found in xylem (NOT THE XYLEM ITSELF). function - transport water and mineral ions, provide support. structure - long hollow structures made by several columns of cells fusing together end to end creating a continuous tube for transport

difference between xylem, xylem fibres, and xylem vessels

xylem is the non-living tissue in a plant that transports water, mineral ions and provides support. xylem fibres are dead sclerenchyma cells that provide structural support to the xylem and are NOT involved in direct water transport. xylem vessels are the primary water-conducting cells forming long, continuous tubes for water transport forming xylem.

transport of water into the plant

soil water has a low solute concentration therefore a high water potential. the root hair cell has a high solute concentration therefore a low water potential. so water moves into the root hair cells by osmosis down a water potential gradient.

differences between epidermal cell and endodermal cell

	Epidermal Cell	Endodermal Cell
Location	Outermost layer	Innermost layer of cortex (in roots)
Primary Function	Protection, gas exchange	Selective barrier for water and nutrient transport
Structure	Thin-walled, rectangular or irregular	Thickened cell walls with Casparian strip
Distribution	Throughout the plant	Primarily in roots

transport of water across the root

once water is in the root hair cell it moves across the root to the xylem in the symplast pathway or the apoplast pathway. until it reaches the endodermis where the casparian strip forces water into the cytoplasm to join the water in the symplast pathway. water from the apoplast pathway must pass through the selectively permeable cell-surface membranes, excluding potentially toxic solutes reaching living tissues as the cell-surface membranes do not have carrier proteins to admit them

the symplast pathway

water moves through the continuous cytoplasm of root cortex cells that are connected by plasmodesmata via osmosis. the root hair cell has a high water potential than the neighbouring cell (as a result of water moving into the root hair cell from the soil making its cytoplasm more dilute). so water moves into the next cell by osmosis and this process is continued cell to cell across the root until the xylem is reached. as water leaves the root hair cell by osmosis, its water potential decreases, maintaining a steep water potential gradient between the root hair cell and soil (ensure as much water as possible moves from soil into cell).

the apoplast pathway

water moves through the cell walls and intercellular spaces of root cortex cells. water fills the space between the loose open fibres of the cellulose cell wall and moves from cell wall to cell wall via intracellular spaces. as water moves into the xylem, more water is pulled through the apoplast due to cohesive forces between the water molecules. this creates tension meaning there is a continuous flow of water through the open structure of the cellulose cell wall with little to no resistance.

transport of water into the xylem

endodermal cells move mineral ions into the xylem by active transport, increasing the solute concentration of the xylem and decreasing the water potential of the xylem. this increases the rate of water moving into the xylem by osmosis due to a steep water potential gradient

difference between endodermis and endodermal cell

endodermis is a tissue with a single layer of cells that surrounds vascular tissue (xylem/phloem) formed of many endodermal cells

the casparian strip

band of waxy material (called suberin) that runs around each of the endodermal cells forming a waterproof layer that forces water in the apoplast pathway to join the water in the symplast as water is forced into the cytoplasm

transport of water up the xylem

the active pumping of minerals into the xylem (active transport) to produce movement of water by osmosis results in root pressure which is independent of transpiration effects. root pressure contributes to water being forced up the xylem

vaculor pathway

water transported vacuole to vacuole

evidence for the role of active transport in root pressure

cyanide affect mitochondria and prevent ATP production so when applied root pressure disappears as there is no energy supply for active transport. root pressure increase and decreases with changes in temperature suggesting chemical reactions like aerobic respiration to produced ATP are involved. low oxygen levels cause root pressure to fall. guttation (xylem sap forced out of cut stems) occurs at night suggesting changes in pressure still occur at night.

differences between mass flow and osmosis

	Mass Flow	Osmosis
Driving Force	Pressure gradients	Concentration gradients (solute)
What is moved	Fluids (liquids or gases)	Water molecules
Required membrane	No specific membrane required	Requires a semi-permeable membrane
Energy Requirement	Can be active or passive	Passive process
Examples	Blood circulation, phloem transport	Water uptake by plant roots, cell sw

main sources of assminilates in plants

green leaves, green stems, tubers that are unloading their stores at a begining of a growth period, food stores in seeds when they germinatem

main sinks in a plant

roots that are actively growing or absorbing mineral ions, meristems that are dividing, any part of the plant that are laying down food stores like developing seeds, fruits, storage organs

translocation

energy-requiring process transporting assimilates, especially sucrose. substances are transported up or down the plant from sources to sinks

phloem loading

assimilates are moved into the phloem from the sources by an active process, done through the symplast route - passive - or apoplast route - active.

apoplast route - phloem loading

sucrose from source travels through cell walls to companion cells and sieve elements by diffusion down a concentration gradient. hydrogen ions are actively pumped out of the companion cell into the surrounding tissues using ATP. the hydrogen ions return via a co-transporter in the cell-surface membrane with sucrose. increases sucrose concentration in companion cells then moves into sieve tube element through plasmodesmata. due to this water also moves in to sieve tube element by osmosis from xylem/companion cell. leads to build up of turgor pressure. water and assimilates move up or down by mass flow to lower pressure (sink)

phloem unloading

diffusion of sucrose from phloem to surrounding cells. leads to rise in water potential of phloem. water moves out into surrounding cells by osmosis to lower water potential.

evidence for translocation

if mitochondria of companion cell poisoned translocation stops. aphids feed from phloem, pressure and flow rate is lower closer to the sink and concentration of sucrose is higher at the source, measured contents of sap ahids ate. bark ringing. tracing of radioactively labelled carbon can be traced when its been converted to sucrose and where phloem is transporting sugars to and how quickly

xerophyte

plants that live in dry habitats which have adaptations enabling them to live in places with low water availability. examples: cacti and marram grass

hydrophytes

plants that live submerged/on the surface of/the edges of bodies of water with special adaptations to cope with growing in water or in permanently saturated soil. example: water lilies

adaptations of xerophytes

Thick waxy cuticle. Sunken stomata. Reduced leaves. Hairy leaves. Curled leaves. Succulents. Leaf loss. Root adaptations (long tap roots that grow deep/ widespread shallow roots). Avoiding the problem (lose leaves/ become dormant/ die completely and leave behind seeds/ survive as storage organs)

adaptations of hydrophytes

Very thin/no waxy cuticle. Stomata usually always open. Reduced structure. Wide flat leaves. Small roots. Large SA of stems and roots underwater. Air sacs. Aerenchyma.

Aerenchyma

specialised parenchyma tissue forms in leaves and roots, has many large air spaces formed by apoptosis (programmed cell death), makes leaves and stem more bouyant, forms low resistance pathway for movement of oxygen to tissues below the water, this helps cope with extreme low oxygen conditions

why do hydrophytes have very thin/no waxy cuticle

do not need to conserve water so water loss by transpiration is not an issue.

why are Stomata in hydrophytes usually always open

to maximise gas exchange, there is no risk of turgor loss due to abundance of water, guard cells are usually inactive, plants with floating leaves like water lilies their stomata needs to be on the upper surface of the leaf so they are in contact with air.

why do hydrophytes have reduced structures

water supports leaves and flowers so no need for strong supporting structures like bark.

why do hydrophytes have wide flat leaves

to capture as much light as possible

why do hydrophytes have small roots

water can diffuse directly into stem and leaves so there is less need for uptake of water by roots

why do hydrophytes have large SA of stems and roots

to maximises the area for photosynthesis and oxygen to diffuse into submerged plants.

why do hydrophytes have air sacs

enable the leaves/flowers to float to the surface of the water

why do xerophytes have a thick waxy cuticle

minimise water loss

why do xerophytes have sunken stomata

somata located in pits, reducing air movement, producing a microclimate of still humid air, reducing water potential gradient, reduces transpiration. examples: cacti, marram grass

why do xerophytes have a reduced number of stomata

this reduces water loss by transpiration but also gas exchange capabilities

why do xerophytes have reduced numbers of leaves

water loss is reduced by reduced leaf area, conifers leaves are thin needles with small SA:V minimising water loss

why do xerophytes have hairy leaves

create a microclimate of still humid air, reduces water potential gradient, minimise water loss by transpiration from the surface of leaves, marram grass have micro hairs in sunken stomatal pits

why do xerophytes have curly leaves

reduces water loss by transpiration, curled leaves confine stomata in a microenvironment of still humid air, reduce water potential gradient, reduce diffusion of water vapour from stomata, marram grass does this

why are some xerophytes succulants

succlant plants store water in specialised parenchyma tissue in stems and roots, water is stored in plentiful supply and used in times of drought, example: cacti

why do xerophytes lose their leaves

prevent water loss through leaves by losing leaves when water is not available

what root adaptations do xerophytes have

help them get as much water from the soil as possible, long tap roots grow deep into the ground to access water several meters below the surface, widespread shallow roots have large SA:V so absorb available water before a rain shower evaporates, marram grass has long vertical roots penetrating meters of sand

how do xerophytes avoid problems with water loss

plants lose leaves and become dormant, die completely and leave behind seeds to germinate and grow rapidly when rain falls again, survive as storage organs like bulbs and daffodils or tubers and potatoes, few plants can withstand complete dehydration and recover (has been linked to disaccharide trehalose)

environmental factors that affect transpiration rate

light intensity, relative humidity, temperature, air movement, soil water availability

light intensity effect on transpiration rate

light = stomata open, increase rate of water diffusing out, increase evaporation, increase transpiration rate. dark = stomata close, decrease rate of water diffusing out, decrease evaporation, decrease transpiration rate.

relative humidity effect on transpiration rate

high = reduced water potential gradient, decrease transpiration rate. low = increase water potential gradient, increase transpiration rate

temperature effect on transpiration rate

high = high kinetic energy, increase evaporation, increase concentration of water vapour in air spaces, increase water potential gradient, increase transpiration rate. low = low kinetic energy, decrease evaporation, decrease concentration of water vapour in air spaces, decrease water potential gradient, decrease transpiration rate

air movement effect on transpiration rate

each leaf has layer of accumulated water vapour. high wind = moves vapour waya, increase water potential gradient, increase transpiration rate. low wind = vapour remains, keeps constant water potential gradient, decreasing transpiration

soil water availability effect on transpiration rate

dry soil = plant is under water stress, stomata close to prevent water loss, decrease rate of transpiration. moist soil = plant easily absorb water, stomata remain open, increase transpiration rate

cohesion-tension theory

1) water vapour diffuses out stomata 2) water evaporates from mesophyll cell walls into air spaces 3) lowers water potential of mesophyll cells 4) water moves in from adjacent cell by osmosis through apoplast/symplast pathways 5) water potential is lower in mesophyll cells than xylem so water moves out xylem into mesophyll by osmosis 6) water molecules move up xylem by cohesion tension 7) the transpiration pull creates tension in xylem helping pull water from soil into roots 😎 water moves into xylem by apoplast/symplast pathway

how is cohesion tension theory different from transpiration stream

The transpiration stream is the actual flow of water through a plant, driven by the evaporation of water from leaves (transpiration). The cohesion-tension theory explains the mechanism of how that stream is maintained

define cohesion tension

water moving from the soil in a continuous stream up xylem and across the leaf due to adhesion (water molecules form hydrogen bonds with carbohydrates in the xylem walls) and cohesion (water molecules form hydrogen bond with each other)

transport of water to the air surrounding the leaves (transpiration stream)

1) water evaporates from mesophyll cell walls into air spaces 2) air space becomes very saturated with water 3) water diffuses out of the leaf down a diffusion/water potential gradient

the role of the stomata

open to allow gas exchange and close to prevent water loss, day = CO2 moves in and O2 and water move out, night = O2 move in as needed for aerobic respiration and none produced by photosynthesis due to no light

how do stomata open

guard cells pump in solutes by active transport (like potassium ions), increasing the guard cells turgor (swell), the inner wall of the guard cell is less flexible and thicker so the cells become bean shaped and open the pore

how and why do stomata close

when water is scarce, hormonal signals sent to the leaves trigger turgor loss from guard cells, closing the pore

practical to estimate transpiration rates

1) Cut a shoot underwater to prevent air from entering the xylem 2) Place the shoot in the tube 3) Set up the apparatus making sure its airtight, using vaseline to seal any gaps 4) Dry the leaves of the shoot 5) Remove the capillary tube from the beaker of water to allow a single air bubble to form and place the tube back into the water 6) Set up the environmental factor you are investigating, allow the plant to adapt to the new environment for 5 min 7) Record the starting location of the air bubble 8) Leave for a set period of time 9) Record the end location of the air bubble 10) Change the factor being investigated 11) Reset the bubble by opening the tap below the reservoir 12) Repeat the experiment 13) The further the bubble travels in the same time period, the faster transpiration is occurring and vice versa.

rate of water uptake equation

distance moved by air bubble / time taken for air bubble to move that distance (units are cm/s or cm s^-1)

why did bright light change the rate of water uptake - potometer

more light, more photosynthesis, more stomata open to collect carbon dioxide, also losing water through evaporation, increases transpiration

why did high wind change the rate of water uptake - potometer

wind displaces water from leaves, higher diffusion gradient between inside and outside of plant, increase uptake of water

why did adding vaseline change the rate of water uptake - potometer

vaseline is waterproof, prevent water loss from stomata, less transpiration

why are potometers not a true measure of transpiration rate

measure water uptake not transpiration directly and do not account for the water that is used for photosynthesis

variable of potometer experiment

independent - conditions (wind, temp, light). dependent - time to move set distance. controlled - plant species, age, number of leaves

parenchyma cells

just normal plant cells? unspecialised so can preform many functions

cross section of a dicot stem

cross section of a dicot root

cross section of a dicot leaf