Why do plants need a transport system?
Metabolic demands –
- Transport glucose and oxygen
- Transport waste products
- Transport of hormones and mineral ions
Size – some plants are small, but as they grow year on year some are huge
SA:Vol ratio – Leaves have a large SA:Vol ratio for gas exchange, but when the stem, trunks and roots are included they have a small SA:Vol ratio
what are the two types of flowering plants
what are the two tissues in plants and what od they transport?
Xylem – water and mineral ions upwards
Phloem – organic solutes around the plant (up and down)
Found together in the vascular bundles in the leaves, stem and roots
where are the vascular bundles in stem cells and why are they there?
around the edge to gives strength and support
the phloem are in the wall and the xylem stick out
where are the vascular bundles in roots and why are they there?
In the middle to help plant withstand the tugging strains that result as the stem and leaves are blown in the wind
where are the vascular bundles in leaf cells and why are they there
The midrib is the main vein carrying the vascular tissue through the organ.
It also helps to support the structure of the leaf.
Many small branching veins spread through the leaf functioning both in transport and support
what is the structure of xylem and how are they adapted for the function of xylem
thick cell wall containing lignin - waterproof and makes vessels extremely strong and prevents them from collapsing
wide lumen and linked end to end to create a long hollow tube - means transport of large volumes of water and ions
bordered pits (unlignified) - allows lateral movement of water
what do the thick walled parenchyma and xylem fibre tissues do in the xylem?
Thick walled parenchyma – packs around the xylem, storing food and contains tannin deposits
Xylem fibre – long cells with a lignified secondary wall – provides strength, doesn’t transport water
what is the structure of phloem and how are they adapted to the function?
sieve tube and sieve plates - cells connected end to end like xylem but contains living cells, often lacks a nucleus to allow maximum transport
companion cells -contains the organelles which the sieve tube doesn’t have. plasmodesmata links them. controls movement of substances
parenchyma and fibres - provides support
what is water potential?
The pressure exerted by water molecules as they collide with a membrane or container
Measured in pascals (Pa) or kilopascals (kPa)
Symbol is Ѱ (Greek letter psi)
what is the water potential of pure water?
what is the water potential of solutes?
pure water = 0kPa
Presence of solutes lowers Ѱ, below zero
All solutions have a negative water potential
water can move but NOT solutes
what is osmosis?
Net movement of water from a high water potential to a low water potential
This continues until an equilibrium is reached
what happens if you put a plant cell in pure water?
the water potential is higher outside the cell than in the cell so this causes osmosis into the cell.
Water inside exerts hydrostatic pressure on cell membrane pushing it against cell wall, called the turgor pressure – as turgor pressure increases it resists entry of further water
Cell is said to be TURGID
what happens if you put a plant cell in concentrated salt solution
Water moves out by osmosis because the water potential of the cell is higher (less negative) than the water potential of the solution.
Cell loses its turgidity
Volume of cytoplasm reduced
Cell membrane pulls away from the cell wall = PLASMOLYSIS
Cell is said to be PLASMOLYSED
define hydrostatic pressure
define oncotic pressure
Hydrostatic pressure: pressure exerted by a liquid
Oncotic pressure: tendency of water to move into blood by osmosis due to plasma proteins
what are the features of a root hair cell?
Microscopic – penetrate between soil particles
Large SA:Vol ratio
1000s on each root tip
Hair has thin surface layer
Concentration of solutes in cytoplasm maintains water potential gradient between soil, water and cell
how does water move from the soil to the root hair cell
Mineral ion absorbed by active transport (into root hair cells)
This reduces water potential of cells
Water potential is lower in cell than soil
Water moves from soil to RHC by osmosis
how does water move across the root the symplast way?
- Cytoplasm connected by plasmodesmata
- RHC has higher Ѱ than next cell along
- Water moves across by osmosis
- Process continues until xylem reached
how does water move across the root the apoplast way?
- Water fills spaces between fibres of cell wall
- Water moves into xylem by cohesive forces between water molecules and as a result of transpiration pull up the xylem, water is pulled through
define casparian strip
Layer of cells surrounding the vascular tissue
Band of waxy material (suberin) that runs around each endodermal cell
how does water pass through the endodermis into the xylem
The waterproof layer in the cell walls of the endodermis, means water can no longer pass through via the apoplast route
Instead, it enters the cytoplasm and passes through the symplast route.
This way, it is forced to pass through the cell surface membrane before entering the xylem.
what is the point of the casparian strip
to regulate the water and to stop any potential toxic solutes in the soil water entering the living tissues
why does the water move from the endodermal cells into the xylem
Higher water potential in endodermal cell than xylem
Plus endodermal cells actively transport mineral ions out of themselves and into the xylem (thus furthering increasing the water potential in the cell)
So water moves into xylem by osmosis.
what happens when the water is at the base of the xylem
Root pressure - the entry of water into the base of the xylem, physically pushes any existing water above it upwards.
how did scientist find out active transport was involved in water transport in xylem
- some poisons like cyanide affect the mitochondria and prevent production of ATP. You can see this because there is no energy supply to the roots meaning the pressure disappears
- root pressure increases with an increase in temperature and falls with a fall in temperature suggesting chemical reactions are involved
- if levels of oxygen or respiratory substrates fall, root pressure falls
- guttation - xylem sap may discharge from the cut end of stems at certain times. In the natural world, xylem sap is forced out of special pores at the end of leaves in some conditions
what is transpiration
Transpiration is the evaporation of water vapour from the surface of leaves of a plant.
Most of this occurs from the underside of a leaf, where there are many stomata.
Guard cells open and close the stomata to control water loss.
Diffusion of CO2 and O2 also occurs through the stomata.
what is a transpiration stream
Flow of water from soil into the root hair cells,
through the cortex by osmosis,
into the xylem and up the stem by cohesion of water molecules,
across the leaf by osmosis and
out of the leaf by evaporation and diffusion.
what are the 3 ways that cause the water to move up the xylem
what is the cohesion-tension theory
Adhesion - Water forms hydrogen bonds with the carbohydrates in the xylem, so attracted to wall
Water molecules attracted to each other by cohesion – hold molecules together in a column due to slight electrical charge
Adhesion and cohesion results in capillary action, so water can rise against the pull of gravity.
Transpiration pull - Water is drawn up by a continuous stream to replace water lost
This creates tension so xylem contains lignin to prevent it from collapsing and pulls water in from root.
how do stomata control the rate of transpiration
When turgor is low (no pressure pushing back, so less water in guard cells), guard cell closes pore
In favourable environments, guard cells pump solutes in increasing turgor as water follows by osmosis.
Cellulose hoops prevent swelling in width, so extend lengthways, becoming bean shaped opening pore
what is the evidence for the cohesion-tension theory
- changes diameter of trees - tension increases as rate of transpiration increases hence shrinking the diameter
- when a xylem vessel is broken air is drawn into the xylem rather than water leaking out.
- cohesive forces break when the stem breaks so water no longer moves up the stem
how do you measure the rate of transpiration
- Measures the rate of water uptake
- As this is proportional to the rate of transpiration, it can be taken as a measure of transpiration rate if we assume that the rate of water loss equals the rate of water uptake.
- The connection must be airtight otherwise water will not be drawn up into the capillary tube and no air bubble movement will be possible.
- The shoot must be cut at an angle to increase surface area and remain under water to prevent air bubbles entering the xylem in the shoot
whats the equation for the rate of water uptake and what are the units
Rate of water uptake (cms^-1) = distance moved by air bubble / time taken for air bubble to move that distance
how does light affect the rate of transpiration
more light opens more stomata which enables more gas exchange. this means more water vapour diffuses out therefore increasing rate of evaporation from the surfaces of the leaf, hence increasing the rate of transpiration
how does humidity affect the rate of transpiration
a very high relative humidity will lower the rate of transpiration because of the reduced water vapour potential gradient between the inside of the leaf and the outside air.
so very dry conditions increases the rate of transpiration
how does temperature effect the rate of transpiration
1) increasing temp increases the kinetic energy of water molecules and therefore increases the rate of evaporation from the spongy mesophyll cells into the air spaces of the leaf
2) increasing temp increases the concentration of water vapour that the external air can hold before it becomes saturated (so decreases the humidity hence increasing rate of transpiration)
how does air movement effect the rate of transpiration
each leaf has a layer of still air around it trapped by the shape of the leaf and features like hairs on the surface of the leaf decrease air movement close to the leaf.
the water vapour that diffuses out of the leaf accumulates here so the water vapour potential around the stomata increases which reduces the diffusion gradient.
anything that increases the diffusion gradient increases the rate of transpiration
so wind increases the rate of transpiration
how does soil-water availability effect the rate of transpiration
the amount of water available in the soil can affect transpiration rate. If it is very dry the plant will be under water stress and the rate of transpiration will be reduced.
what is glucose converted to when travelling around the plant
what is an assimilate
Any product of photosynthesis that is carried around the phloem is called an assimilate. Sucrose is the main assimilate in plants.
They move from source to sink
Source: a region of the plant that has excess carbohydrate
Sink: a region of the plant that needs carbohydrate
The movement of assimilates in the phloem from source to sink
what happens in the symplast route of the transport of glucose
Sucrose moves from source through cytoplasm and into sieve tubes by diffusion through plasmodesmata
Water follows by osmosis
No energy required
what happens in the apoplast route of the transport of glucose
- H ions are actively (uses ATP) pumped out of companion cell into surrounding tissue
- H ions returns to companion cell down concentration gradient via co-transport protein with sucrose
- Lots of sucrose in companion cell, low sucrose in sieve tube
- Therefore sucrose moves from high to low concentration via diffusion through plasmodesmata
- Water moves into phloem from xylem as lower water potential
what are the main sinks in a plant
meristems that are actively dividing
developing seeds, fruits or storage organs
what are the main sources in a plant
green leaves or green stems
food stores in seeds when they germinate
define mass flow
the transport of substances in a fluid from a high to low pressure
define turgor pressure
the pressure exerted by the cell-surface membrane against the cell wall
process in which 2 substances are simultaneously transported across a membrane by one protein
how does sucrose unload at the sink end
- sucrose diffuses out of the sieve tube into surrounding cells from high to low concentration of sucrose
- the loss of sucrose from the sieve tube increases the water potential in the tube
- Water moves from the sieve tube into the xylem by osmosis from a high to low water potential
what is the evidence for translocation
Companion cells have many mitochondria
The flow of sugars in the phloem is 10 000 times faster than it would be by diffusion alone
Let an aphid feed on the phloem. Remove the aphid body and leave the mouthpart in the phloem. Sap flows out the mouthpart faster nearer the leaves than further down the stem - flow rate in phloem is lower closer to the sink than it is near the source
Add a metabolic inhibitor and translocation stops - shows chemical reactions are involved because if it was natural then the inhibitor wouldn’t stop anything
xerophytes - Plants in dry habitats have evolved to enable them to live and reproduce in places where water is very low
hydrophytes - Live in water and so need adaptations to cope with growing in water or in permanently saturated soil
how are xerophytes adapted for survival and explain how
thick waxy cuticle sunken stomata reduced numbers of stomata reduced leaves hairy leaves curled leaves succulents (parenchyma stores water) leaf loss root adaptations avoiding the problems
how are hydrophytes adapted for survival and explain how
very thin or no waxy cuticle many, always open stomata on the upper surface reduced structure wide, flat leaves small roots large surface area of stem and roots under the water air sacs aerenchyma
what is aerenchyma (hydrophytes)
in leaves, stems and roots
large air spaces
makes leaves and stems more buoyant
forms a low resistance internal pathway for movement of substances like oxygen to tissues below the water. helps plant cope with extreme low oxygen conditions
Give examples of xerophytes
Give examples of hydrophytes
what’s the difference between the term transpiration and the transpiration stream
loss of water vapour / evaporation of water ;
from, aerial parts of plant / leaves / stomata
movement of water (up xylem vessels) ;
from roots to, leaves / air surrounding leaves