Transport in plants Flashcards
Why do plants need a transport system
- larger plants have a small surface area to volume ratio so they need specialised exchange surfaces and transport systems as they do not have enough surface area to take everything they need in by diffusion
- absorb water and mineral ions from roots, they need to make sugars so need to move water and minerals up the leaves and sugar down to the rest of the plant
What does the xylem and phloem do
xylem = moves mineral ions and water upwards phloem = moves sugars up and down
Where is the vascular tissue distributed
distributed evenly throughout the plant and found in vascular bundles
What doe the vascular tissue contain
contains sclerenchyma and collenchyma these give the plant strength
what plant does the vascular tissue and xylem and phloem in
dicotyledonous plant
Where is xylem and phloem found in the young root
vascular bindle is found at the centre of a young root, the central core of the xylem is found as an x shape and phloem found between the arms of x shape, this provides strength to withstand pulling forces to which the roots are exposed to
What is around the vascular bundle
a special sheath of cells called the endodermis this gets water into the xylem vessel, inside the endodermis is a layer of cells called the pericycle
Where is xylem and phloem in the leaf
vascular bundles for the midrib and veins of a leaf, the xylem is on top of the phloem - in a dicotylendonous lead it has a network of veins that get smaller as they spread away from the midrib
Where is xylem and phloem in the stem
vascular bundle is found near the outer edge of the stem
- in non-woody plants the bindles are separate and discrete but in woody plant they are separate in young stems but a
- continuous ring in older stems, a complete ring provides strength and flexibility
- xylem Is to the inside and phloem is to the outside
What is the structure and function of xylem
vessels carry water and dissolved mineral ions
- fibres help support the plant
- living parenchyma cells act as packing tissue to separate and support the vessels
What do the xylem contain
ligin - makes it waterproof, strengthens the walls, prevents it from collapsing this keeps the vessel open at all times even when the water is in short supply
- they are spiral, annular or reticulate in shape which prevents the stem from being to rigid and allows flexibility
- in some places the ligin is not complete this is called bordered pits
- long column of dead cells with no content
Why isn’t the flow of water stopped in the xylem tube
- no cross walls
- no cell content, nucleus or cytoplasm
- thick ligin prevents walls from collapsing
What are the adaptions of xylem to its function
- made from dead cells to form an continuous column
- tubes are narrow so water column does not break easily and capillary action can be effective
- bordered pits allow water to move sideways from one vessel to another
- ligin patterns allow xylem to stretch as plant grows and allows stem to bend
what does the sucrose dissolve in
it dissolves in water to form sap
What does the phloem consist of
consists of sieve tube elements and companion cells
Describe the role of the companion cells
between sieve tubes are small cells with large nucleus and dense cytoplasm - lots of mitochondria to produce energy this is needed for active processes, they load assimilates actively into the sieve tube
Describe the sieve tube elements
elongated sieve tube elements and lined up end to end to form sieve tubes
- contain no nucleus and very little cytoplasm this leaves space for the mass flow of sap
- sieve plates at the end of sieve tubes this allows movement of sap from one element to the next
- thin walls
What is a property of cellulose
cellulose cell walls of a plant cell are permeable to water this allows the water molecules to freely move between the cellulose molecules or gaps between cells
What are plant cells joined by
joined by cytoplasmic bridges these are cell junctions at which one cell is connected to that of another through a gap in the cell wall these junctions are called plasmodesmata
Describe the apoplast pathway
water passes through the spaces in the cell wall and between the cells
- water moves by mass flow rather than osmosis therefore dissolved mineral ions an salts can be carried
- does not pass through any plasma membrane into the cells
Describe the symplast pathway
water enters the cell cytoplasm through the next plasma membrane, it can pass through the plasmodesmata from one cell to the next
Describe the vacuolar pathway
similar to the symplast pathway but the water is not confined to cytoplasm of the cell and can pass through the vacuoles at the same time
What is the water potential always do
water potential always moves from a high water potential to a low water potential
What is the water potential of pure water
pure water =o
What is the water potential
a measure of the tendency of water molecules to move from one place to another
What does a plant cell have that will reduce its water potential
mineral ions and sugars
What happens when you place a plant cell in pure water
it will take in water via osmosis as the cell has a more negative water potential than the pure water, the water molecules will move down the concentration gradient
How does a plant cell become more turgid
cells will not continue absorbing until it bursts this is because it has a strong cellulose cell wall, the water inside the cell exerts pressure on the cell wall this is called the potential pressure as it increases the pressure potential and decreases water influx and the cell becomes full of water this is called turgid
What happens when placed in a sugar solution
has to have a lower negative water potential then it will loose water as the water will go down the potential gradient
How does the tissue become flaccid
as water loss increases the cytoplasm and vacuole shrink and eventually the cytoplasm no longer pushes against the cell wall therefore the cell is no longer turgid
if the water continues to leave the cell then the plasma membrane will loose contact with the cell wall this is called plasmolysis and the tissue is now flaccid
How does movement of water between cells happen
When plant cells are touching each other the water molecules move from one cell to another
it moves from a negative water potential to a less negative water potential via osmosis and down the water potential gradient
What is transpiration
the loss of water vapour from the aerial parts of a plant mostly through the stomata of the leaves
What limits the loss of water
waxy cuticle
Where is most of the water lost and when
through the stomata which open for gaseous exchange for photosynthesis
most lost during the day when photosynthesis happens as its light
Why is transpiration important
- transports useful mineral ions up the plant
- maintains turgidity
- supplies water for cell growth
- cell elongation and photosynthesis
- supplies water that as it evaporates it can keep the plant cool on a hot day
What is the typical pathway taken by most water leaving the leaf
the water enters the lead through the xylem and moves by osmosis is into the cells of the spongy mesophyll, it may also go through the apoplast pathway
water evaporates from the cells of the spongy mesophyll
water vapour moves by diffusion out of the lead through the open stomata this relies on a difference on concentration of water molecules in the leaf compared to outside
How does light effect transpiration rate
the stomata open in light to allow gaseous exchange for photosynthesis, increased light leads to increase transpiration rate
How does water availability effect transpiration rate
little water= plant cannot replace the water loss and this causes the stomata to close and the leafs to wilt
How does temperature effect transpiration
increases temperature increases the transpiration rate,
- increase evaporation rate from cell surfaces so water vapour potential in leaf increases
- increases rate of diffusion through stomata as water molecules have more energy
- decrease relative water vapour potential in the air allowing more rapid diffusion of water molecules out of the leaf
How does humidity effect transpiration
increases humidity this decreases water loss as smaller water potential gradient between air spaces between leaves and between air spaces
How does air movement effect transpiration
it will carry away the water vapour that has diffused out and it increases the water vapour potential gradient
What is a potometer
a device that can measure the rate of water uptake as a leafy stem transpires
How does a potometer work
set up under water to ensure there are no air bubbles in the apparatus
ensure that shoot is healthy
cut stem under water to prevent air entering the xylem
cut stem at an angle to provide a large surface area in contact with the water
dry the leaves
How do you find the volume
V=3.14xR2xL
R= radius of capillary tube
L = length of capillary tube
How do you find the rate
rate = volume/time
What is the transpiration stream
The movement of water from the soil through the plant to the air and surrounding leaves
the water potential gradient is main driving force
describe the pathway of water
- water in the soil has a higher water potential then in the root hair cell therefore the water travels by osmosis into the root hair cell, the root hair cell increases the surface area
- the water then moves into the root cortex where it can go by three pathways, the vacuolar pathway, symplast pathway, or the apoplast pathway
- the vacuolar pathway is through cytoplasm and vacuoles, the symplast is through the cytoplasm and plasmodesmata, the apoplast pathway is through the spaces between cells and cell walls
- the casparian strip in the endodermis prevents entry of water into the xylem from the apoplast pathway therefor the water has to travel through cytoplasm and cell membrane into the symplast and vacuolar pathway
- transporter proteins actively transport ions from the cytoplasm to cortex cells into the xylem
- this decrease water potential in the xylem so a higher water potential is in the cortext therefore the water travels by osmosis into the xylem down the water potential gradient
What is the endodermis
the endodermis is a layer of cells which surround the medulla and xylem
it is also known as starch granules this is a sign that energy is being used for the active processes
What do the plasma membranes contain
plasma membranes contain transporter proteins these actively pump mineral ions from the cytoplasm of the cortex cells into the medulla and xylem
How and why does water move up the stem
by mass flow
same direction, more volume and higher speed
describe root pressure
action of the endodermis moving minerals into the medulla and xylem by active transport draws water into the medulla by osmosis, pressure in the medulla build ip and pushes the water into the xylem this can push water a few metres but not to the top of the tree
Describe transpiration pull
water must be replaced by water coming up from the xylem, water molecules are attached to each other by cohesion (hydrogen bonds) they are strong enough to hold molecules in a long chain
as molecules are lost the chain is pulled up this creates tension in the column of water, the xylem is strengthened by ligin to prevent vessel from collapsing under tension
this is the tension theory, if chain is broken then water chain can still be maintained through another vessel via bordered pits
describe capillary action
same forces that hold water molecules together attract water molecules to the side of the xylem vessels this is called adhesion
xylem vessels are narrow so the forces of attraction can pull water up the sides of vessel
