3.1.3. Transport in plants

why do plants need a transport system

• large plants have a small SA:volume- diffusion too slow for movement of substances throughout the plant

• metabolic demands of tissues that don’t photosynthesise

• plants can grow to very tall

what is water transported through the plant in and which way

through the xylem, one direction- upwards

how does water enter the roots

osmosis through root hairs on root hair cells

how do minerals enter the plant

taken up by active transport by root hairs on root hair cells, decreases the water potential to draw more water in

what does the xylem carry

water and minerals

where does sugar come from in plants

made by photosynthesis in the leaves or stored in roots as starch

what does the phloem carry

assimilates, mainly sugar in the form of sucrose

which direction does the phloem transport sugars

both directions

what is the transport sugar in plants and why

sucrose- less reactive than glucose

how does the transport system in plants compare to animals and why

much slower transport system for their sugar as less metabolically active

what are the gases needed in plants

CO2 for photosynthesis and O2 for aerobic respiration

how do the gases needed (CO₂ and O₂) enter the plant

rely on diffusion, don’t use a transport system

what are the large gas exchange surfaces in plants

spongy mesophyll cells in leaves, root hair cells

define transpiration stream

the uninterrupted flow of water taken up by the roots, through the xylem and out of the stomata in the leaves due to the pull of transpiration

define transpiration

the loss of water vapour from aerial parts of a plant mainly through stomata

what is the summary of movement of water through plants

transpiration from leaves, movement of water across leaf cells, movement of water through xylem, movement of water across root cells, uptake from soil into root hair cells

what does transpiration rely on

the water potential gradient between the substomatal airspace and the atmosphere

what are the factors that effect rate of transpiration

temperature, wind speed, humidity and light intensity

what does the graph for the effect of temperature on rate of transpiration look like

increasing and then a hump and decreases a bit

how does temperature effect rate of transpiration

as temperature increases kinetic energy of water molecules at stomatal opening increases, increases rate of movement of water vapour away from stomata, increases water potential gradient, increases rate of transpiration, at high temperature stomata close so rate of transpiration reduces

what does the graph for the effect of wind speed on rate of transpiration look like

increasing and then plateaus

how does wind speed effect the rate of transpiration

air current moves water molecules away from stomatal opening, reduces water potential in air surrounding stomata, increases water potential gradient, increases rate of transpiration, when all stomata are open a plateau is reached (massively increased wind may cause stomata to close so the rate of transpiration may decrease)

what does the graph for the effect of humidity on rate of transpiration look like

linear and decreasing

how does humidity effect rate of transpiration

increasing humidity surrounding stomata increases water potential of air outside, decreases water potential gradient, decreases rate of transpiration

what is the effect of light intensity on rate of transpiration

stomata close in the dark, as light intensity increases, stomata opening increases, rate of transpiration increases, when all stomata are open, light intensity has no further effect on rate of transpiration

define stomatal density

the number of stomata in a unit area

how do you find the stomatal density on a leaf

• take an epidermal impression by coating an area of the lower leaf surface with a thin layer of nail varnish

• wait until it dries and then peel it off using Sellotape

• stick Sellotape on a slide

• count number of stomata in field of view

• repeat 3 times for different parts of epidermis, calculate mean

• use stage micrometre to calculate area of field of view

what is the name of broad leaved plants

dicotyledon leaves

what are dicotyledons

broad leaved plants

what sort of structure is a leaf

an organ

what is the name of the flat parts of the leaf either side of the central vein

lamina/blade

what is the name of the middle stem of a leaf

central vein

what is the name of the parts in a leaf that come off the central vein

lateral veins

what is the name of the bit that attaches a leaf to the stem

petiole

what make up the phloem

sieve tube elements, companion cells

what is the name for the xylem and phloem

vascular bundle

what is the name for the pockets of air around the spongy mesophyll cells

substomatal air space

what cells surround the substomatal air spaces

spongy mesophyll cells

how does transpiration occur

when the water potential of the atmosphere is less than the water potential of the sub-stomatal airspace, water molecules diffuse out when the stomata are open

when does movement of water vapour in the substomatal air space occur

when the water potential of the sub-stomatal airspace is less than the water potential of the cell walls of the spongy mesophyll cells: water vapour evaporates from the walls of the spongy mesophyll cells and diffuses down the water potential gradient into the sub-stomatal airspace

what are the routes for water across leaf cells called

the apoplast pathway, the symplast pathway

how does the apoplast pathway work with the structures of the cell wall

• through the cell walls and extracellular space

• cellulose microfibrils of the cell wall have channels between them through which water can flow easily

how does the apoplast pathway work for the movement of water across a leaf

as water evaporates from the cell walls into the substomatal airspace from one cell, it creates a tension which pulls water from spaces in the walls of adjacent cells, this pull occurs because of cohesive forces between the water molecules due to hydrogen bonds

which pathway is more common for water movement and why

apoplast pathway- 90%, it is the path of least resistance

what is the method of movement in the apoplast pathway

cohesion tension

how does the symplast pathway work

water moves through the cytoplasm’s of adjacent cells connected by plasmodesmata

why does less water travel through the symplast pathway

there is resistance to flow due to the presence of organelles

what is the process of movement in the symplast pathway for the movement of water across a leaf

• cytoplasm of one cell loses water to the sub-stomatal airspace

• the water potential of the cell becomes lower

• the cell pulls in more water from its neighbour with a higher water potential

• the water potential of this cell becomes lower

• the process is repeated for cells across the leaf in the direction of the xylem to the stomata

• a water potential gradient is established

what is the method of movement for the symplast pathway

water potential gradient

in a leaf cross-section is the xylem or phloem on top

xylem is above phloem (xp)

where is the xylem found in the stem

within the vascular tissue

what structures are in a vascular bundle in the stem

xylem, cambium, phloem, sclerenchyma

in the stem what is the structure of the vascular bundle

xylem on inside, then cambium, then phloem, then sclerenchyma

what is sclerenchyma

dead fibres which give mechanical support to the vascular bundles

what are the dead fibres which give mechanical support to the vascular bundles called

sclerenchyma

what is cambium

meristematic tissue that divides laterally to form the xylem and phloem

what is meristematic tissue that divides laterally to form the xylem and phloem called

cambium

what is the outer layer in the cross-section of a stem called

cortex

what is the cortex

outer layer of the stem, made from collenchyma cells, have thickened cellulose cell walls

what is the inner layer in the cross-section of the stem called

pith

what is the pith

inside layer in the stem, made from parenchyma cells- unspecialised packing tissue

what packs around xylem vessels in the stem

parenchyma cells- unspecialised packing tissue (forms the pith)

how can xylem vessels be identified

thickened walls, no cell content, large lumen

in an image how can you identify companion cells from sieve tube elements

companion cells are darker

what is the structure of the xylem

• xylem tissue is made of vessels packed between parenchyma cells

• dead cells lined end to end whose horizontal walls have been broken down

• lined with rings/spirals of lignin

• large bordered pits on the side of some vessels to allow sideways movement of water for living cells in the stem

why have the horizontal walls in xylem been broken down

so water can travel in a continuous uninterrupted column

what is the function of lignin in the xylem

give mechanical strength to the vessel, allow flexibility of movement, keep the vessels open and stops them collapsing especially when the transpiration pull exert negative pressure on the walls

what are the mechanisms of movement of water in xylem

cohesion tension, adhesion- capillarity, root pressure

how does cohesion tension allow for movement of water in the xylem

as water molecules move up the xylem they attract neighbouring water molecules by cohesion tension due to intermolecular hydrogen bonds between the polar water molecules, water is pulled up huge heights due to the collectively strong cohesive forces giving rise to a negative hydrostatic pressure in the xylem vessels

what is evidence for the cohesion tension theory of the mechanism of movement of water up the xylem

changes in diameter of trees- during the day negative pressure in xylem vessels causes tension and narrowing of the vessels and shrinkage of the trunk diameter, at night water remains in the trunk when transpiration is low, tension is low and the trunk swells

how does adhesion- capillarity allow for the movement of water up the xylem

water molecules are attracted to the walls of the xylem vessels due to adhesive forces, this however can create a frictional drag

how does root pressure allow for the movement of water up the xylem

minerals are pumped into the xylem tissue at the roots by active transport using ATP, this lowers the water potential of the xylem at the roots, water diffuses in due to the water potential gradient, this is called root pressure, it facilitates the movement of water into the roots from the soil

what evidence is there for root pressure causing movement of water up the xylem

presence of starch grains in cells adjacent to the xylem in the roots, experiments using exudation

how does the presence of starch grains in cells adjacent to the xylem in the roots support root pressure theory

the starch is hydrolysed to glucose which is used for aerobic respiration which provides the ATP for the active transport of the minerals pumped into the xylem

how do experiments using exudation (water appearing at the top of a cut stem when roots are intact) support root pressure theory when there is no oxygen

no aerobic respiration so no ATP for the active transport of the minerals pumped into the xylem

how do experiments using exudation (water appearing at the top of a cut stem when roots are intact) support root pressure theory when there is a low temperature

rate of reaction for ATP production lowered, no ATP for the active transport of the minerals pumped into the xylem

how do experiments using exudation (water appearing at the top of a cut stem when roots are intact) support root pressure theory when there is a metabolic poison introduced

affect mitochondria, stops aerobic respiration, no ATP produced for the active transport of the minerals pumped into the xylem

what are the different experiments using exudation used to provide evidence to support root pressure

no oxygen, low temperature, metabolic poison

what is the general structure of the xylem and phloem in the roots

xylem in a cross with phloem in the gaps

what are the tissue layers in the root

epidermis, cortex, endodermis, pericycle, xylem

what is the name for the hairs on the root cells

epidermal root hair cells

what is the pericycle

tissue layer in the root, between endodermis and xylem, where lateral root grow

where do lateral roots grow from

the pericycle

what are the pathways for water across root cells called

apoplast pathway and symplast pathway

how does the apoplast pathway help the movement of water in the root

through the cell walls and intercellular space, movement through cell wall is prevented at the Casparian strip in the cell wall of the endodermis, forces the passage of the water into the endodermal cells to join the symplast pathway

what forces the water to leave the symplast pathway in the roots

the Casparian strip

what is the Casparian strip made of

suberin- very impermeable to water

what is the purpose of the Casparian strip

endodermal cells check for entry of pathogens through selectively permeable cell surface membrane, help bring about plant defences

how does the symplast pathway help the movement of water in the roots

moves through the cytoplasm of adjacent cells connected by plasmodesmata, water travels down its water potential gradient: soil to epidermis to cortex to endodermis to pericycle to xylem

how does the uptake of water by root hair cells happen

the water potential of the soil is greater than the water potential of the root hair cells, water is absorbed by osmosis down the water potential gradient, water also enters the apoplast pathway

how are root hairs adapted as exchange surfaces

• microscopic size so can penetrate between soil particles and ensure constant contact with water

• large SA:volume

• thin cellulose cell walls for rapid diffusion and osmosis

• concentration of solute in the cytoplasm maintains water potential gradient between soil water and cell

• many mitochondria for active transport of ions

what are different types of plant in relation to water availability

mesophyte, xerophyte, hydrophyte, halophyte

what is a mesophyte

plants adapted to a habitat with adequate availability of water

what is a xerophyte

plants adapted to a habitat with low availability of water

what is a hydrophyte

plants adapted to a habitat with plentiful freshwater

what is a halophyte

plants adapted to a habitat with saline water

what are plants adapted to a habitat with saline water called

halophyte

what are plants adapted to a habitat with adequate availability of water called

mesophyte

what are examples of xerophytes

marram grass, cacti

what are features of marram grass

hinge cells cause leaf to roll up, upper epidermis is inside the leaf, sunken stomata, hairs, thick cuticle on lower epidermis