Transport in Plants Flashcards
what is the pressure in the phloem?
around 2000kPa
why do multicellular plants need transport system?
- metabolic demands
- size
- SA:V ratio
why is metabolic demand reason for plants needing transport systems?
many internal and underground parts of plants do not photosynthesise and so don’t make their own nutrients. They need O2 and glucose transported to them and waste product taken away
why is size a reason for plants needing transport systems?
plants need very effective transport systems to move substances up and down from the tip of the roots to the topmost leaves and stem in order for them to grow properly
why is SA:V ratio a reason for plants needing transport systems?
Stems, trunks and roots have relatively small SA:V ratios. This means they cannot rely on diffusion alone to supply their cells with everything they need
what are dicotyledonous plants?
plants that make seeds that contain two cotyledons, organs that act as food stores for the developing embryo plant and form the first leaves when the seed germinates
what is a vascular system?
a series of transport vessels running through the stem, roots and leaves, present in dicotyledonous plants
paper flashcard for stem vascular bundle
paper flashcard for root vascular bundle
paper flashcard for leaf vascular bundle
what is the xylem of a plant?
a largely non-living tissue that transports water and mineral ions and supports the stem
what is the structure of the xylem?
they are long hollow structures made by several columns of cells fusing together end to end
what is xylem parenchyma for?
it packs around the xylem vessels, storing food and containing tannin deposits
what is tannin?
a bitter astringent-tasting chemical that protects plant tissues from attack by herbivores
what are xylem fibres?
long cells with lignified secondary walls that provide extra mechanical strength but do not transport water
what can lignin do?
it can be laid down in the walls of the xylem vessels in several different ways
what are the different ways lignin can be laid down in the xylem vessel?
it can form rings, spirals or relatively solid tubes with lots of small unlignified areas called bordered pits
what happens in bordered pits?
it is where water leaves the xylem and moves into other cells of the plant
what is the phloem?
it is a living tissue that transports food in the form of organic solutes around the plant from the leaves where they are made by photosynthesis
what does the phloem supply the cell with?
with sugars and amino acids needed for cellular respiration and for the synthesis of all other useful molecules
what is the direction of flow in the phloem?
can go up or down
what are the main transporting vessels of the phloem?
sieve tube elements
what are the sieve tubes made up of?
many cells joined end to end to form a long hollow structure, however they are not lignified
what do sieve plates do?
let phloem contents flow through
what breaks down as the large pores in the cell walls appear?
the tonoplast, nucleus and other organelles
what is the structure of companion cells?
linked to sieve tube elements by many plasmodesmata
what is plasmodesmata?
microscopic channels through the cellulose cell walls linking the cytoplasm to adjacent cells
what is the function of companion cells?
they are a ‘life support’ for the sieve tube cells, which have lost most of their normal cell functions
what supporting tissues do phloems contain?
fibres and sclereids
what are sclereids?
cells with extremely thick cell walls
what does turgor pressure provide as a result of osmosis?
a hydroskeleton to support the stems and leaves
what does turgor also drive?
cell expansion- it is the force that enables plant roots to force their way through tarmac and concrete
what does water do to help a plant keep cool?
loss of water by evaporation
what are root hair cells?
the exchange surface in plants where water is taken into the body of the plant from the soil
what is the structure of a root hair?
long, thin extension from a root hair cell, a specialised epidermal cell found near the growing root tip
what are adaptations of root hairs that make them good exchange surfaces?
- small size means they can penetrate easily between particles
- each hair has a large SA:V ratio
- each hair has a thin surface layer through which diffusion and osmosis can take place
- the concentration of solutes in the cytoplasm of root hair cells maintains a water potential gradient between the soil water and the cell
what is a symplast?
a continuous cytoplasm of the living plant cells that is connected through the plasmodesmata
how does water enter and leave a root hair cell?
through osmosis
what is the apoplast?
the cell walls and the intercellular spaces
what is the casparian strip?
a band of waxy material called suberin that runs around each of the endodermal cells forming a waterproof layer
what is some evidence for the role of active transport in root pressure?
- some poisons affect the mitochondria and prevent the production of ATP. If it is applied to root cells so there is no energy supply, the root pressure disappears
- root pressure increases with a rise in temperature and vice versa, suggesting chemical reactions are involved
if o2 levels or respiratory substrates fall, root pressure falls
what is the movement of water into the xylem?
- water moves across the root in the apoplast and symplast pathways until it reaches the endodermis
- at this point water in the apoplast pathway can go no further and is forced into the cytoplasm, joining the water in the symplast pathway
- the solute concentration in the cytoplasm of the enodermal cells is relatively dilute compared to the cells in the xylem
- as a result, the WP of the xylem cells is much lower than the endodermal cells
- this increases the rate of water moving into the xylem by osmosis down a WP gradient from the endodermis through the symplast pathway
what happens after water enters the vascular bundle?
- water returns to the apoplast pathway to enter the xylem itself and move up the plant
- the active pumping of minerals into the xylem to produce movement of water by osmosis results in poor root pressure and it is independent of any effects of transpiration
- root pressure gives water a push up the xylem
why is leaves having a waxy cuticle an important adaptation?
it prevents the leaf cells losing water rapidly by evaporation from their surfaces
how does CO2 and O2 move in and out of a leaf?
CO2 moves into the leaf and oxygen moves out of the leaf by diffusion down concentration gradients through microscopic pores in the leaf called the stomata
how can the stomata be opened and closed?
guard cells, which surround the stomata opening
what is transpiration?
loss of water vapour from the leaves and stems of plants
what happens when the stomata is open?
they allow an exchange of CO2 and O2 between the air inside the leaf and external air. water vapour also moves out by diffusion and is lost
why do stomata open and close?
to control the amount of water lost by a plant
why do some stomata need to be open all the time?
during the day plants need to take in CO2 for photosynthesis and at night when noO2 is being produced it needs to take O2 for cellular respiration
what is the transpiration stream?
where the water vapour moves into external air through the stomata along a diffusion gradient
what does the transpiration stream do?
moves water up from he roots of a plant to the highest leaves
how does water leave a leaf?
- water molecules evaporate from the surface of mesophyll cells into air spaces in the leaf, and move out of the stomata into the surrounding air by diffusion down a concentration gradient
- the loss of water lowers the WP of the mesophyll cells, so water moves into the cell from an adjacent cell by osmosis
what is the cohesion theory?
a model of water moving from the soil in a continuous stream up the xylem and across the leaf
how does the stomata open and close?
- when the turgor is low the asymmetrical configuration of the guard cell walls closes the pore
- when the environmental conditions are favourable guard cells pump in solutes by active transport, increasing their turgor
- cellulose hoops prevent the cells from swelling in width, so they extend lengthways
- because the inner walls of the guard cell is less flexible than the outer wall, the cells become bean-shaped and open the pore
- when water becomes scarce, hormonal signals from the roots trigger turgor loss from the guard cells, which close the stomatal pore and conserve water
what factors effect the rate of transpiration?
- light
-humidity - temperature
- wind
how does light effect the rate of transpiration?
- light is required for photosynthesis and in the light stomata open for gas exchange
- increasing light means more stomata will open, increasing the rate of water vapour diffusing out
how does humidity effect 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
how does temperature effect the rate of transpiration?
- an increase in temp increases the kinetic energy of the water molecules and therefore increases the rate of evaporation from the spongy mesophyll
- an increase in temp increases the concentration of water vapour that the external air can hold before it becomes saturated
how does wind effect the rate of transpiration?
- each leaf has a layer of still air around it trapped by the shape of the leaf, and its feature such as 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 and so the water vapour potential around the stomata increases, in turn reducing diffusion gradient
- so, air movements of wind will increase the rate of transpiration, and conversely a long period of still air will reduce transpiration
what is translocation?
a process that transports compounds in the phloem from sources to sinks
what are assimilates?
the products of photosynthesis that are transported
what are the main sources of assimilates in plants?
- green leaves and green stems
- storage organs such as tubers and tap roots that are unloading their stores at the beginning of a growth period
- food stores in seeds when they germinate
what are the main sinks in a plant?
- roots that are growing and/or actively absorbing mineral ions
- meristems that are actively dividing
- any parts of the plant that are laying down food stores, such as developing seeds, fruits and storage organs
what is the main carb transported in the phloem?
sucrose
what are xerophytes?
plants in dry habitats that have evolved a wide range of adaptations that enable them to live and reproduce in places where water availability is very low
what are some ways that xerophytes conserve water?
- thick waxy cuticle
- sunken stomata- reduces air movement
- reduced numbers of stomata
- reduced leaves- e.g needles
- curled leaves- confines stomata within still humid air
what are hydrophytes?
plants that live in water
what are some adaptations of hydrophytes?
- no waxy cuticle- don’t need to conserve water as there isn’t a lack of it
- wide flat leaves- get as much sunlight as possible
- small roots- so water can diffuse directly into the stem
do more on pathways and pores
