3.3 transport in plants Flashcards
what is meristem
a layer of dividing cells
what is the function of the phloem
transports dissolved assimilates up or down
what is vascular tissue
cells specialised for transporting fluids by mass flow
what is the function of the xylem
transports water and minerals upwards
why do plants need a transport system
larger plants have a smaller surface area to volume ratio therefore, they need to have a specialised exchange surfaces and a transport system
what do plants need to move via the transport system?
wat er and minerals from the roots to the leaves
sugar from the leaves to the rest of the plant
what are dicotyledonous plants
those that have two seed leaves and a branching pattern of veins in the leaf
describe the structure of xylem and phloem in the young root
it is found at the centre of the young root and there is a central core of xylem, and the phloem is found between the arms of the x-shaped xylem tissue
how is the structure of the xylem and phloem in the young root adapted for its function
the x-shaped arrangement provides strength to withstand the pulling forces to which roots are exposed to
describe the structure of phloem and xylem in the stem
the vascular bundles are found near the outer edge of the stem.
The xylem is found towards the inside of the vascular bundle and the phloem towards the outside and in between there is a layer of cambium which is a layer of meristem cells that divide to produce new phloem and xylem
describe the structure of xylem and phloem in the leaf
the vascular bundles form the midrib and veins of the leaf, within each vein the xylem is located on top of the phloem
describe the development of xylem vessels
as they develop lignin impregnates the walls of the cells making the walls waterproof. This end walls and contents of the cell decays leaving a long column of dead cells
describe the structure and function of lignin in the xylem vessels
the lignin thickening may form spiral or reticulate patterns in the cell wall which prevents the vessel from being too rigid allows flexibility to the branch
describe bordered pits
this is where lignification is not complete, and gaps are left in the cell wall. They are aligned which allows water to leave one vessel and pass onto the next.
what are the adaptations of xylem to its function?
continuous column
the tubes are narrow so that the water column doesn’t break easily, and capillary action can be effective
bordered pits allow water to move
patterns of lignin deposits allow the xylem to stretch as the plant grows and enables the stem or branch to bend
the flow of water is not impeded because there are no cross walls or cell contents
describe the structure of the phloem
phloem tissue consists of sieve tubes made up of sieve elements and companion cells
describe tsieve tube elements
they are lined up end to end to form sieve tubes
they contain no nucleus and very little cytoplasm leaving space form mass flow of sap to occur and at the ends of the sieve tube elements are perforated cross walls called sieve plates
describe companion cells
in between the sieve tubes are small cells each with a large nucleus and dense cytoplasm
They also have numerous mitochondria to produce the ATP needed for active processes
what is the function of the companion cells
the companion cells carry out the metabolic processes needed to load assimilates actively into the sieve tubes
define plasmodesmata
gaps in the cell wall containing cytoplasm that connects two cells
what are the different pathways water can take through a plant
the apoplast pathway
symplast pathway
vacuolar pathway
describe the apoplast pathway
water passes through spaces in the cell walls and between cells and doesn’t pass through any plasma membranes so therefore it moves by mass flow rather than osmosis
describe the symplast pathway
water enters the cell cytoplasm and passes through plasmodesmata from one cell to the next
describe the vacuolar pathway
similar to the symplast pathway but the water is able to enter and leave the vacuoles
define water potential
the tendency of water molecules to move from one place to another
define osmosis
the movement of water from a region of high water potential to an area of low water potential across a partially permeable membrane
what is the endodermis
a layer of cells surrounding the medulla and xylem and it contains granules of starch
what is the role of the endodermis
it contains the Casparian strip which blocks the apoplast pathway between the cortex and the medulla ensuring that water and dissolved mineral ions have to pass into the cell cytoplasm through the plasma membranes
how does the endodermis block the apoplast pathway
the plasma membranes contain transporter proteins which actively pump mineral ions from the cytoplasm of the cortex cells into the medulla and xylem this makes the water potential of the medulla and xylem more negative so that water moves from the cortex cells into the medulla and xylem by osmosis
define transpiration
the loss of water vapour from the upper parts of the plants
describe the typical pathway of water leaving the leaf
water enters the leaf through the xylem and moves by osmosis into the spongy mesophyll
water evaporates from the cell walls of the spongy mesophyll
water vapour moves by diffusion out of the leaf through the open stomata
this relies on a steep water vapour potential gradient
what is the importance of transpiration
transports useful mineral ions up the plant
maintains cell turgidity
supplies water for growth
cools the plant
what environmental factors affect transpiration rates
light intensity
temperature
relative humidity
air movement
water availability
how does light intensity affect transpiration rates
in light stomata open to allow gaseous exchange for photosynthesis so a larger light intensity increases the transpiration rate
how does temperature affect transpiration rate
a higher temperature will increase the rate because
it will increase the rate of evaporation from the cell surfaces so water potential in the leaf rises
increases the rate of diffusion because the water molecules have more kinetic energy
decreases relative water vapour potential in the air allowing more rapid diffusion out of the leaf
how does humidity affect transpiration rate
this will lower transpiration rates because there will be a smaller water vapour potential gradient between the air spaces in the leaves and the outside
how does air movement affect transpiration rate
wind will carry away water vapour that just diffused out of the leaf maintaining a steep water vapour potential gradient
how does water availability affect transpiration rate
if there is little water int the soil then the plant cannot replace the water that it had lost then the stomata close and the leaves wilt
how does water move up the stem
mass flow
what is root pressure
the action of the endodermis moving minerals into the medulla and xylem causes pressure in the roots to build up and forces water into the xylem
what is transpiration pull
the loss of water from by evaporation from the leaves must be replaced by the water coming up the xylem
water molecules are attracted to each other by forces of cohesion which are strong enough to hold the molecules together in a long chain or column. So as molecules are lost from the top of the column the whole column is pulled up as a chain
what is capillary action
adhesion forces attract water molecules to the sides of the xylem vessels and as xylem vessels are very narrow adhesion forces can pull water up the sides of the vessel
define translocation
the transport of assimilates throughout a plant
describe active loading during translocation
H+ ions are actively pumped out of the companion cell using energy from ATP
this creates a higher concentration of H+ ions outside of the companion cell - this causes facilitated diffusion back into the companion cell of H+ ions that carry sucrose through cotransport proteins in the plasma membrane
increased concentration of sucrose in companion cell causes it to diffuse through the plasmodesmata into the sieve tube elements
what causes the movement of sucrose
sucrose moves through mass flow
the flow is caused by a difference in hydrostatic pressure
when sucrose is actively loaded into the sieve tube element, and this reduces water potential, causing water to follow in by osmosis increasing the hydrostatic pressure
sap then moves down from the source to the sink - high hydrostatic pressure to low hydrostatic pressure- and sucrose is then removed from the sieve tube increasing its water potential and water follows out reducing hydrostatic pressure
what is a source in translocation
sucrose entering the sieve tube e.g. the roots
what is a sink in translocation
anywhere that removes sucrose from the phloem sieve tubes which could be used for respiration
what is a hydrophyte
a plant adapted to living in water or where the ground is very wet
what is a xerophyte
plant adapted to living in dry conditions
how do most terrestrial plants reduce their water loss
waxy cuticle
stomata on under face of leaf
stomata close at night
deciduous plants lose their leaves in winter
what are some adaptations of xerophytes such as marram grass
leaf is rolled longitudinally so that air is trapped inside
thick waxy cuticle on the outer side of the rolled leaf
stomata are on inside of the rolled leaf (lower epidermis)
dense spongy mesophyll with few air spaces
widespread roots
what are some adaptations of halophytes
many large air spaces in the leaf to keep them afloat
stomata are on upper epidermis so that they are exposed to air to allow gaseous exchange
leaf stem has many large air spaces for buoyancy
