3.1.3 transport in plants

why do plants need transport systems?

• metabolic demands

• SA:V

• size

why do plants need transport systems because of their metabolic demands?

• many internal and underground parts do not photosynthesis

• they need glucose and oxygen transporting to them and need to remove waste products

why do plants need transport systems because of their SA:V?

• plants can’t rely on diffusion alone to supply cells with what they need

• although leaves have relatively hight SA:V

• when stems and roots are taken into account they have low SA:V

why do plants need transport systems because of their size?

• many plants can get very large and so need a substantial transport system

what are dicotyledonous plants?

• plants that produce seeds that contain 2 cotyledons

• these act as food stores for developing embryo to form the first leaves after germination

• they have vascular system made up of xylem and phloem

what is the difference in number of cotyledons in monocots and dicots?

• monocot has 1 cotyledon

• dicot has 2 cotyledons

what is the difference in leaves of monocots and dicots?

• monocots have long, narrow leaves and veins are parallel

• dicots have broad network of veins

what is the difference in vascular bundles of monocots and dicots?

• monocots have scattered vascular bundles

• dicots have a ring of vascular bundles

what is the difference in flower parts of monocots and dicots?

• monocots have flower parts in multiples of 3

• dicots have flower parts in multiples of 5 or 4

what is the function of the xylem?

• transports water and minerals

• support the plant

how is lignin arranged in the xylem?

• lignin is arranged in spiral or small rings

why is lignin arranged in spiral or small rings?

• allows for flexibility

why is there pits in the cell wall, where there is no lignin?

• so that other cells can be provided with water and mineral ions

• to allow lateral movement of water between xylem cells, in case their is blockage/damage

what are the only living cells in xylem called?

• thick walled xylem parenchyma

what does thick walled xylem parenchyma do?

• packs around xylem cells, storing food and tannin deposits

what is tannin?

• chemical that protects plant tissue from attacks by herbivores

• since it tastes really bad

what is the sink in the summer in plants?

• roots turn glucose into insoluble starch to store over the winter

what is the source in plants and when?

• summer/ autumn, the leaf makes glucose by photosynthesis and loads up the phloem to send it to the roots

• spring, the roots hydrolyse starch back into glucose and sends it up to growing parents of the the plant

what is the structure of sieve tube elements?

• have no nucleus

• very little cytoplasm and organelles

what is the structure of companion cells?

• large nucleus

• lots of mitochondria to help load assimilates (sucrose and amino acids) into sieve tubes

what are the cytoplasm of the sieve tube and companion cells connected by?

• plasmodesmata

which way does phloem sap move, when moving through pores in the wall between sieve tubes?

• vertically

where is the xylem found in the roots on a cross sectional?

• central core of xylem often in shape of an X

where is the phloem found in the roots on a cross sectional?

• phloem is found in between the arms of the X

where is the endodermis found in the roots on a cross sectional?

• layer of cells around the vascular bundle

where is the pericycle found in the roots on a cross sectional?

• inside the endodermis is a layer of meristems

where in the steam are the vascular bundles found?

• near the outer edge of the stem to act as scaffolding

what is the layout of the vascular bundles in the stem?

• xylem on inside of each vascular bundle

• phloem towards the outside

• in middle is layer of cambium

what is cambium?

• meristem tissue that divide to produce new xylem and phloem

how do vascular bundles look in the leaf?

explain the how water travels through plant simplified?

1. absorbed by soil through root hairs

2. transported up the stem to the leaves

3. evaporation through the leaves through transpiration

what is the definition of transpiration?

• Transpiration is the loss of water vapour from a plant to the atmosphere, mainly by evaporation from the surface of mesophyll cell and diffusion through stomata

what is the the transpiration stream?

• water moving up the plant against gravity through xylem, fuelled by evaporation

what are the layers of a leaf (GSCE)?

• waxy cuticle

• epidermal tissue

• palisade mesophyll

• spongy mesophyll

• epidermal tissue

what is the epidermal tissue covered with and why?

• waxy cuticle

• to reduce water loss by evaporation

what do palisade mesophyll contain?

• most of chloroplasts

what is the purpose of xylem and phloem tissues in the leaf?

• deliver water and nutrients and take away glucose

what do spongy mesophyll have and why?

• air spaces

• increase the rate of diffusion of gases

what does the lower epidermis have a lot of and why?

• stomata

• for gas exchange

how does water move in the leaf?

• water is pulled along the xylem

• water moves into spongy mesophyll cells down the water potential gradient by osmosis from xylem

• water vapour diffuses into their air down the water potential gradient from spongy mesophyll, lowering water potential

what leaf structures affect transpiration?

• leaf surface area

• thickness of epidermis and cuticle

• stomatal frequency

• stomatal size

• stomatal position

what are the factors affecting transpiration rate? (GSCE)

• light

• temperature

• wind

• humidity

why does light affect transpiration rate?

• in bright light, transpiration increases

• the stomata open wider to allow more CO₂ into leaf for photosynthesis

why does temperature affect transpiration rate?

• transpiration is faster in higher temperatures

• evaporation and diffusion are faster at higher temperatures, this lowers the water potential of the air, so the water potential gradient is steeper

why does wind affect transpiration rate?

• transpiration is faster in windy conditions

• water vapour is removed quickly by air movement, speeding up diffusion of water vapour of of the leaf, lowering water potential of the air, so water potential gradient is steeper

why does humidity affect transpiration rate?

• transpiration is slower in humid conditions

• diffusion of water vapour out of the lead slows down if the lead is already surrounded by moist air

what does transpiration provide plants with water for?

• cooling by losing water by evaporation

• photosynthesis

• turgor pressure (support, provides hydrostatic skeleton) (roots enables plants to force their way through firm material)

• movement of minerals

what are the features of root hair cells?

• long projections increase surface area to absorb water and mineral ions from soil

• thin permeable cell wall

• lots of mitochondria to provide energy for active transport

why does diffusion and osmosis happen quickly in root hair cell?

• root hair cells have thin cellulose wall

how is water transported into the root?

• water moves down the water potential gradient by osmosis because of active transport of the minerals into root hair cells which lowers the water potential in these cells

• water only enters the root near the root tip because there are root hair cells which increases surface area for osmosis

label what the blue, red and purple coloured parts are showing

explain the apoplast route

• water moves through the cellulose wall and intercellular spaces

• the permeable fibres of cellulose don’t resist water flow

• water cannot pass the endodermis route because the casparian strip in the endodermis cell wall is impermeable to water because of waterproof band of Suberin

• so all water must past endodermis via cytoplasm

what is the importance of the casparian strip?

• prevents harmful substances from entering the xylem (since they now have to move via symplast route)

• prevents the leakage of water from xylem vessels

• aids the development of root pressure (an upwards force pushing water up the stem)

explain the symplast route

• through cytoplasm of cells

• water enters the root hair cells across the partially permeable membrane by osmosis from high water potential in the soil to lower water potential in the cell

• it passes from one cell to the other via plasmodesmata down the water potential gradient

what are the features of the source?

• where the assimilates are produced/released

• low water potential

• high pressure

what are the features of the sink?

• where assimilates are needed

• high water potential

• low pressure

what is translocation?

• when leaves produce large amounts of glucose which is converted to sucrose for transport

• when it reaches the cell, it’s converted to glucose for respiration, starch for storage or used to produce other products

• products being transported are known as assimilates and are transported from sources to sinks (tissue that needs them) in process called translocation

how is sucrose transported from leaf cell through companion cell into the phloem?

• H⁺ are actively pumped out of companion cells, as a result a concentration gradient is created

• H⁺ ions diffuse back into companion cells co-transporting sucrose down the concentration gradient

• the concentration of sucrose is now high in companion cell so diffuses into the sieve tube elements through plasmodesmata

why is the process of sucrose/phloem loading important?

• increased pressure

• mass flow

• energy for the plant

explain the reasons why sucrose/phloem loading is important

• increased pressure

the higher concentration of sucrose in the sieve tube elements, lowers the water potential, drawing water in by osmosis from surrounding xylem vessels

• mass flow

this influx of water increases the hydrostatic pressure at the source, creating a pressure gradient that pushes the phloem sap towards the sink

• energy for the plant

this process ensures that the energy rich sucrose produced during photosynthesis is transported to other parts of the plant that require it for growth or storage

what is evidence for translocation

• if mitochondria of companion cells are poisoned, translocation stops

• this is because the flow of sugars in the phloem is about x10000 faster than it would be by diffusion alone suggesting an active process is driving the mass flow

explain how the phloem is loaded

• sucrose entering the sieve tube elements lowers water potential

• this causes water to move into the sieve tube element via osmosis from surrounding cells

• this increases the hydrostatic pressure at the source causing mass flow to area of lower pressures

explain how the phloem is unloaded

• sucrose differs from the sieve tube into companion cells then other cells by diffusion

• sucrose is constantly being used up so a concentration gradient is always maintained

• the loss of sucrose from sieve tubes raises the water potential so water moves out via osmosis, this reduces the hydrostatic pressure at the sink

what are xerophytes?

• plants adapted to a dry habitat

what are the adaptations of xerophytes to reduce water loss?

• thick waxy cuticle to reduce evaporation

• reduced leaf area to reduce SA:V ratio

• hairy leaves as it traps a layer of saturated air

• sunken stomata as the puts above the stomata become saturated

• rolled leaves as this reduced the area expose to the air and keeps stomata on the inside so increasing the water vapour inside the roll

• succulents to store water in specialised tissue in their stems and roots

• leaf loss some plants prevent water loss by losing their leaves when water is not available

what does increasing water vapour around stomata do?

• reduces the water potential gradient

• so slows down water loss

what are hydrophytes?

• plants adapted to wet habitat

what is a problem for hydrophytes?

• water logging

• this is because air spaces need to be full of air, not water

• this will help plants float so they are near the surface so they can get more light for photosynthesis

what are some characteristics of hydrophytes?

• large, flat floating leaves

• long and flexible stem

• roots embedded in the soil at the bottom

what are adaptations of hydrophytes?

• very thin or no waxy cuticle

• floating leaves have their sonata on the upper surface in contact with air, many are always open

• some hydrophytes have air spaces which enable leaves to float to the surface

• no supporting structures as water supports the leaves

• smaller roots as there is less need for uptake by roots

what is aerenchyma? And what is the purpose?

• spongy tissue in roots, leaves and stems

• oxygen from photosynthesis fills them

• this helps to keep the plants buoyant and forms.a low resistant pathway for oxygen to diffuse to tissues below water

what are pneumatophores?

• where roots become water logged

• air is in short supply so they develop special aerial roots

• these roots have lenticels (raised pores) which allows gases into the roots

how does root pressure work?

• water is pushed up the xylem by hydrostatic pressure

• mineral salts are pumped into xylem vessels in the root by endodermal cells which lowers water potential in the xylem

• water moves in from surrounding cells by osmosis which raises the hydrostatic pressure so pushing water up the xylem

what is evidence for roots pressure?

• in certain conditions, such as at night when transpiration is low, some leaves exude water from their tips from their leaves = guttation

• cut stumps of plants exude water from their cut ends

• cyanide and other poisons which interfere with production of ATP, causes root pressure to disappear

• low levels of oxygen means root pressure falls

• root pressure increases with temperature implying it is chemically controlled

what are the limitations of root pressure?

• the pressure measures is not enough to get water to the top of trees

• relies on the plant’s energy (ATP) for active transport

how does capillarity work?

• water rises up narrow tubes due to the adhesive forces between the water molecules and the wall of the tube

• water rises higher in narrower tubes

• xylem vessels are very narrow

what is the limitation of capillarity?

• water will only rise 50mm

what is the cohesion tension theory?

what is evidence for the cohesion tension hypothesis?

• cut stems attached to a tube containing water over mercury almost 1m

• dendrographs record that tree trunks have narrower diameter during the day when transpiration rate is higher, when the most tension is created

explain the variation in trunk diameter and transpiration rate over 24hrs?

• the diameter of the trunk decreases as transpiration rate increases

• evaporation from the leaves draws water from the xylem by osmosis, water is pulled up the xylem causing tension

• the tension pulls the xylem vessel walls in, so the trunk diameter gets smaller

• when the trunk has a larger diameter when there is less transpiration

• this supports the cohesion tension hypothesis but not root pressure