transport in plants Flashcards
what substances do plants need to transport over long distances
- sucrose and amino acids
- xylem transports water and mineral ions, from soil (roots) to photosynthesising cells.
what substances do plants need to transport over short distances
- gases are small and non - polar so can diffuse easily across the plasma membrane.
- oxygen and carbon dioxide diffuses in through the stomata
what is the function of the xylem vessels
water conduction and transport from the roots to the leaves
what is the function of the xylem tracheid’s
lined with lignin for support and mechanical strength
what are fibres
dead elongated cells, which are lignified
what is the function of the parenchyma
packing tissue
what are the functions of the xylem
- conduct water ( for photosynthesis) and mineral ions from the roots to the leaves
- provide mechanical support, for stems, roots, shoots due to the lignin in the walls.
- (lignin reinforces the cellulose in cells walls.)
what is the role of the plasmodesmata
- connects companion cells and sieve tube elements in the phloem
- allow water to move between plant cells and the xylem
- allow the movement of signalling molecules from cell to cell. e.g. plant hormones such as auxin.
what is the role of the large lumen in the xylem
to reduce the resistance to water flow
what is the role of the thick lignified wall in the xylem
add mechanical strength + rigidity to prevent the collapse of the xylem vessel, under high pressure and tension, caused by the transpiration stream.
features of meristematic cells
- have thin cell walls ( very little cellulose)
- do not have a vacuole
- no chloroplast
what is meristem
an area, of unspecialised cells (meristematic cells) which can divide ( cell cycle + mitosis) + differentiate into other cell types
water always moves from a region of
higher water potential, to a region of lower water potential
what is the function and structure of a pit
- no lignin, allows lateral movement of water between xylem vessel.
- allows water to leave the xylem completely and move into e.g. the leaf.
what are two features, that distinguish sieve tubes from xylem vessels
- sieve plates
- no pits
- xylem vessels are hollow and thin walled
- xylem vessels are lignified.
what is the role and function of the plasmodesmata
- connect companion cells and sieve elements in the phloem
- allow water to move between plant cells and the xylem
- allow the movement of signalling molecules from cell to cell, e.g. plant hormones such as auxin
why are vascular bundles, arranged around the periphery of the stem
to allow flexibility and bending.
in a plant, the cytoplasm contain dissolved salts and sugars (solutes) that will
- decrease the water potential inside the cell (make it more negative)
water molecules, move from less negative regions (higher water potential)
to more negative regions (lower water potential)
what pressure does osmosis in plant cells result in
turgor pressure
what does trugor drive
cell expansion
what is the apoplast pathway
through the cellulose cell wall (by mass flow (not osmosis))
what is the symplast pathway
the movement of water by osmosis through the cytoplasm and the plasmodesmata.
describe the route of the apoplast pathway
water moves in via mass transport (not osmosis) through the cell walls until reaching the endodermis and impermeable Casparian strip.
what is the Casparian strip
it is a band of waxy material called suberin, and it runs around each of the endodermal cells, forming a waterproof layer.
why is the transpiration stream important?
- it carries water for photosynthesis to the palisade cells in the leaves
- the water carries essential mineral salts in solution
- evaporation from the leaves has a cooling effect (latent heat of vaporisation)
what is the role of the air space in the leaf?
it allows H20 (gas) to accumulated then diffuse out of the leaf.
what is transpiration pull
- air flow around the leaf takes humid air away, creating a diffusion gradient between air spaces and the air.
- water evaporates and diffuses out of the leaf.
- this reduces the water potential inside the spongy mesophyll cells compared to the xylem and so water moves from the xylem down a concentration gradient via the process of osmosis.
- as water leaves, the whole column of water in the xylem is pulled up the xylem, by cohesion and adhesion.
what is capillary action
- continuous columns of water in the xylem are held together by cohesion (water molecules bond to each other) + adhesion (water molecules bind to each other and the wall of the xylem.
- this creates a force know as capillary action
- cohesion tension theory, combined with transpiration pull explain how the water moves
what is root pressure
- root hair cells actively transport mineral ions in and water follows by osmosis down a water potential gradient (symplast).
- water molecules have to move across the cortex and down a concertation gradient.
- mineral ions, are actively transported into xylem vessels using ATP therefore the water potential in the xylem decrease and water enters via osmosis.
- this increase in hydrostatic pressure, at the bottom of the xylem creates root pressure.
know the comparison table between the xylem vessels and the phloem sieve tubes on page 19
what is cohesion
when water molecules form hydrogen bonds with each other
what is adhesion
when water molecules forms hydrogen bonds, with each other and the walls of the xylem.
what are the 8 factors, that can affect water loss
- number of leaves
- size, number and position of the stomata
- presence of a cuticle
- light
- temperature
- relative humidity
- air movement or wind
- water availability
how can the number of leaves affect water loss
- increases the number of stomata
- so increased transpiration rate
how can size, number and position of the stomata affect water loss
- open stomata means a greater rate of transpiration
- larger stomata means water vapour, is lost more quickly
how does the presence of cuticle affect water loss
- the thicker the cuticle, the slower the rate of transpiration
how does light affect water loss
- increased light intensity, means an increase in the rate of transpiration
how does temperature affect water loss
- increase transpiration rate
- increase evaporation from cell surfaces
- increases the rate of diffusion, through stomata
- decrease the relative water potential in the air, allowing rapid diffusion of water particles out of the leaf.
how does the relative humidity, affect water loss
- decrease in humidity means an increase in the rate of transpiration (steeper the concentration gradient, the more rapid the diffusion)
how does air movement or wind affect water loss
- increase wind speed means greater rate of transpiration (steeper the concentration gradient, the more rapid the diffusion)
how does water availability affect water loss
- decrease in water supply, may lead to a decrease in transpiration rates
why has the cactus had to adapt to its enviroment
to reduce water loss by transpiration in dry climates
how do the adaptions of a cactus reduce water loss?
- thick waxy cuticle, on the epidermis reduces water loss by evaporation.
- spines instead of leaves, reduces surface area for water loss. Photosynthesis occurs in the stem
- stomata are closed on the hottest times when transpiration rates would be highest
what are the main sinks, for the process of translocation
- actively diving meristems
- plants laying down food stores, such as developing fruits/seeds
- metabolically active tissues, such as those actively absorbing mineral ions in roots
translocation, background knowledge
- sugars are produced in the leaves during photosynthesis.
- translocation, is the movement of dissolved substances (solutes), like sucrose and amino acids to where they are needed.
- assimilates, are dissolved substances that are transported and become incorporated into plant tissue. the main transported assimilate is sucrose. Sucrose is soluble and metabolically inactive.
- substances, move from source to sink. remember, that a substance is normally at a higher concentration, when it is at it’s site of production, e.g. green leaves and stems.
what are assimilates
assimilates, are dissolved substances that are transported and become incorporated into plant tissue. the main transported assimilate is sucrose. Sucrose is soluble and metabolically inactive.
what are the 3 stages of translocation
- phloem loading (active), pumping hydrogen out of companion cells, into source cell (proton pump)
- pressure flow/ mass flow (passive), bulk movement of sucrose as a result of a difference in hydrostatic pressure, between 2 areas.
- phloem unloading, sucrose is transported passively, into sink cells + either hydrolysed before repaired or stored in the form of starch.
State two precautions that should be taken to ensure no air bubbles are in the potometer setup
- set up underwater
- cut stem underwater, to prevent air entering the xylem
Explain the significance of the Casparian strip
Blocks the apoplast pathway between the cortex and the medulla, ensuring water and dissolved mineral ions have to pass through the cell cytoplasm via the plasma membranes
Describe 3 adaptations of marram grass (xerophyte) and explain their importance
- Leaf rolled longitudinally trapping air inside (air becomes humid and reduces water loss from the leaf)
- thick waxy cuticle on upper epidermis (reduces evaporation)
- stomata on lower epidermis inside rolled leaf (protected by enclosed air space)
- stomata are in pits in lower epidermis which is folded and covered by hairs (reduces air movement and hence water loss)
- spongy mesophyll very dense with few air spaces (less surface area for evaporation of water)
Why is it advantageous for some xerophytes to have a low water potential inside their leaf cells?
Reduces evaporation of water from the cell surfaces as the water potential gradient between the cells and the leaf air spaces is reduced
what is the difference, between active transport and active loading
Active transport is the movement of particles against their concentration gradient using metabolic energy (ATP). Active loading is a more extensive process which involves active transport at some stage. In this case, active loading uses active transport to pump hydrogen ions out of the companion cells. This results in movement of sucrose molecules by facilitated diffusion and diffusion.
Describe the role of hydrogen ions in active loading
The hydrogen ions are pumped out of the companion cells, creating a hydrogen ion concentration gradient across the cell membrane. The hydrogen ions can diffuse back into the companion cells through special transport proteins – but they only move if sucrose is carried in with them (cotransport). 8
Distinguish between the term transpiration and the transpiration stream.
- Transpiration is the loss of water vapour from the leaves or stem.
- Whereas the transpiration stream is the movement of water through the xylem tissue and mesophyll cells.
