Transport in plants

chapter 9

describe the plant cells metabolic demands

• the cells that are underground don’t photosynthesise→ waste products need to be removed and oxygen and glucose need to be provided

• hormones need to be transported to areas where they have an effect

• mineral ions absorbed by roots need to be transported to all cells to make proteins

describe the size of cells in transport systems

• some are very large 

• need very effective systems to move substances up and down the whole plant

describe SA: vol ratio in plant transport systems

• leaves have a large ratio

• whole plant has a small ratio

• cannot rely on diffusion alone to provide cells with everything they need

where are xylem and phloem found

• vascular bundles

which part of the plant are sugars made in

leaves

why are vascular bundles in the middle of the roots

to withstand the tugging forces as a plant is blown in the wind

which is stronger: xylem or phloem tissues

xylem

how are vascular bundles arranged in the root

• xylem forms an x shape

• phloem is in the gaps in the x shape

how are vascular bundles arranged in stems and why

• in a ring around the outer part of the stem

• helps the stem withstand bending forces

describe vascular tissues in stems

• xylem is located on the inside of each vascular bundle

• in between the xylem and the phloem there is a layer of meristem cells (cambium)

• the cambium can divide to form new xylem and phloem

what is the midrib

• main vein carrying the vascular tissue and support to the leaf

• smaller branching veins also spread through the leaf functioning both in transport and support

describe the vascular bundle in the leaf

• xylem is located on top of the phloem

• only applies to dicotyledonous plants→ other plant types have different structures

describe the structure of xylem vessels

• dead cells stacked end-on-end to make a continuous tube

• cells contain no cytoplasm- forms a hollow lumen

• the walls are strengthened with lignin (a woody material)

describe the development of xylem vessels

• made when living cells become elongated

• cytoplasm produces lignin which becomes embedded in the cell walls

• gradually kills the cells

• end walls disintegrate to form hollow tubes

describe lignin

• forms different patterns to provide support whilst allowing flexibility

• rings (annular), spiral, broken rings (reticulate)

what are the layers of a leaf in order from top to bottom

waxy cuticle, upper epidermis, palisade mesophyll layer, spongy mesophyll layer, lower epidermis, waxy cuticle

what are root hair cells

• the exchange surface in plants where water is taken into the body of the plant from the soil

what is a root hair

a long, thin extension from a root hair cell

describe rhc’s microscopic size as an adaptation

• allows them to penetrate between soil particles

describe rhc’s root hairs as an adaptation

has a large SA: vol ratio→ allows more efficient and faster exchange of substances

describe rhc’s thin surface layer as an adaptation

diffusion and osmosis can take place quickly

describe the cytoplasm and vascular sap of the root hair cell

contains many different solvents so the water potential is lower

how does water cross the cortex and enter the xylem in roots

• root hair cells take up mineral ions by active transport

• this lowers the water potential in the cytoplasm, so that water is taken up by osmosis

describe the apoplast pathway

• diffusion (as there are no membranes)

• movement of water into the cell wall and intracellular spaces

• travels from cell wall to cell wall

• cohesion and tension forces act of the cell walls to pull the water upwards.

• fastest movement of water in cells

describe the symplast pathway

• movement of water through the cytoplasm of a cell (living spaces) 

• goes through the plasmodesmata (pores)

• each cell further away from the original root hair cell has a lower water potential→ so water is drawn up the plant

describe the vacuolar pathway

• water is able to enter and pass through vacuoles

• slowest route as it has to pass through more membranes

what is the Casparian strip

• a waterproof strip in the cell walls

why is water forced to take the symplast pathway through the cytoplasm before it enters the xylem of the root

• defence mechanism for the plant

• any toxins in the water would not be able to reach living cell tissues as they cant pass through the semi-permeable membrane

• water still uses the apoplast pathway as it is the fastest

what is root pressure caused by

• minerals actively being transported into the xylem to reduce the water potential

• allows water to move in by osmosis

what is root pressure

• helps give water a push up the xylem

• not the major factor in water transport

what is the definition of transpiration

the process where plants use water vapour from their leaves, primarily through stomata

what is transpiration reliant on

• stomata and guard cells

what is the route of water in transpiration

root hairs→ guard cells→ stem→ xylem vessels→ open stoma

what causes the transpiration pull

• diffusion of water through the stomata

what does the transpiration pull lead to

• water molecules to move from the xylem in the stem to the xylem in the leaves

what is the cohesion-tension theory

• the combination of cohesion and tension that moves water up plants against the force of gravity

what pulls water up the plants

• water evaporates from the leaves

• tension is created which pulls more water into the leaf

• leading whole column of water in the xylem to move upwards

describe how water’s cohesive properties aid transpiration

• when some water molecules are pulled into the leaf others follow

• leads to a continuous stream of water molecules in the xylem

how does water’s adhesive properties aid transpiration

• polar water molecules are attracted to the non-polar cellulose molecules in the xylem vessel walls

what is capillary action

• the combined effects of adhesion and cohesion 

• process in which water can rise up a narrow tube against the force of gravity

describe the stomata when it is open

• turgid

• outer wall is more flexible than the inner wall- so the cell bends and opens the stoma

describe the stomata when closed

• flaccid

• outer wall is more flexible than the inner wall- allows the cell to bend and close the stoma

explain why the xylem is described as a tissue

it has multiple types of specialised cells that all work together to perform a specific function

what is a hydrophyte

a plant that live completely/partially submerged in water

give examples of hydrophytes

duckweed, watercress

describe size of roots in hydrophytes

• small

• water can diffuse directly into the stems and leaf tissue

• less need for water uptake by roots

describe SA of roots in hydrophytes

• large SA of stems and roots

• maximises area for photosynthesis and for O2 to diffuse into submerged plants

describe pneumatophores in hydrophytes

• special aerial roots

• grow upwards into the air

• have many lenticels allowing air in

what is aerenchyma

• specialised parenchyma (packing tissue)

• forms in the leaves, stems and roots of hydrophytes

• has many large air spaces (formed in part through apoptosis)

what is the function of aerenchyma

• makes the leaves and stems more buoyant

• forms a low resistance internal pathway for the movement of substances (e.g. O2 and tissues below the water)

• helps the plant cope with anoxic (extreme low O2), conditions in the mud, and transporting oxygen to the tissues

• provides a low resistance pathway so methane produced in rice plants can be vented into the atmosphere

describe how hydrophytes being thin/ no waxy cuticle aids waterlogging

• do not need to conserve water→ constant water availability

• water loss by transpiration is not an issue

describe how hydrophytes having many stomata aids waterlogging

• maximises the amount of gaseous exchange

• stomata are open the entire time and guard cells are inactive

describe how hydrophytes having reduced structure to the plant waterlogging

• the water supports the leaves and flowers→ no need for strong supporting structures

describe how hydrophytes having air sacs aids waterlogging

• enables leaves and flowers to float to the surface of water

describe how hydrophytes having wide flat leaves aids waterlogging

• spread across the surface of water

• captures as much light as possible

what are xerophytes

• live in areas where water loss via transpiration is greater than taken up by roots

give examples of xerophytes

cacti, conifers, marram grass

describe the waxy cuticle in xerophytes

• thick waxy cuticle

• 10% water loss through the cuticle

• thick cuticle helps to minimise water loss

describe the stomata in xerophytes

• sunken→ located in the pits

• reduces air movement by producing a microclimate of still, humid air

• reduces water vapour potential gradient- so reduces transpiration

describe the number of stomata in xerophytes

• reduced number of stomata

• reduces water loss by transpiration

• also reduces gas exchange capabilities

describe the no. leaves in xerophytes

• reduced amount

• reducing area reduces water loss by transpiration

describe the hairy leaves in xerophytes

• create a microclimate of still, humid air→ reduced the water potential gradient and minimises water loss

describe the curled leaves in xerophytes

• confines all the stomata within a microenvironment of still, humid air

• reduces diffusion of water vapour from the stomata

describe succulents as xerophytes

• store plants in parenchyma tissue- in their stems and roots

• water is stored when it is in a plentiful supply- then used in times of drought

give an example of a succulent

• desert cacti→ aloe vera

describe the leaf loss in xerophytes

• lose leaves when water is not available

• minimises water loss through leaves

describe hinge cells in marram grass leaves

• when flaccid they cause the leaves to roll (creates a humid space)

what is a potometer used for

estimating transpiration rates

what are the limitations of using potometers

• some water would be used for photosynthesis→ so the potometer measures uptake of water rather than rate of transpiration

• doesn’t have roots attached

• cells could get damaged whilst cutting the plant

describe the experimental method of using a potometer

• distance moved by an air bubble

• recorded every minute

• rate= distance moved by bubble/time

how do you increase the validity of a potometer

• use a sharp razor to cut the plant→ damages less cells

• cut the shoot under water→ prevents air bubbles from getting into the xylem

• grease the joints of the potometer with petroleum jelly→ prevents air leaks

• set up the apparatus under water and turn off the tap before taking it out of the water→ prevents air bubbles in apparatus

• allow the shoot to acclimatise to surroundings→ actually shows the effects of the variables

what are mesophytes

able to take up sufficient water to replace transpiration (most plants)