transport in plants

Question 1

Why do plants need a specialised transport system?

Answer 1

• to move products of photosynthesis, water, and oxygen around from their place of origin (high metabolic demand)
• most plants are large and so have to transport substances huge distances from root to tip
• SA: vol is small in plants (even though for leaves SA:vol high)

Question 2

where is the vascular bundle in the roots

Answer 2

the xylem is in the centre surrounded by phloem to provide support to the roots as it pushes through the soil
it helps the plant withdtand the tugging strains that teults as the stems and leaves ate blown in the wind

Question 3

where are the vascular bundle in the stem

Answer 3

the xylem and the phloem are around the edges to provide strength and support and prevent bending

Question 4

where are the vascular bundles in the leaves

Answer 4

xylem and phloem make up a network of veins which support the thin leaves

Question 5

how are xylem adapted for transport of water and mineral ions

Answer 5

they are very long tuble like structures formed from cells (vessle elements) joined end to end

no end walls on these cells making an uninterupted tube that allows water to pass up through the middle easily

cells are dead so no cytoplasm

walls are thickened by lignin which helps support the xylem vessel and stops them collapsing inwards. lignin can be diposited in xylem walls in different ways e.g in a spiral or as distinct rings

amount of lignin increases as cells get older

water and mineral ions move into and out of the vessel through small pits in the wall where theres no lignin

Question 6

what do xylem transport

Answer 6

water and mineral ions

Question 7

what do phloem vessels transport

Answer 7

solutes(dissolved substances) mainly sucrose

Question 8

features of sieve tube elements

Answer 8

they are living cells that form the tube for transporting solutes through the plants

they are joined end to end to form sieve tubes

the sieve parts are the end walls which has lots of holes to allow solutes to pass through

they have no nucleus, a very thin layer of cytoplasm and a few organelles

they cytoplasm of adjacent cells is connected through the holes in the sieve plates

Question 9

features of companion cells

Answer 9

the lack of nucleus and other organelles in the sieve tube element means they cant survive on their own so theres a companion cell for every sieve tube element

they carry out the living functions for both themselves and their sieve cell

for example they provide the energy for active transport of solutes

Question 10

how does water go to the xylem

Answer 10

enters the roots via the root hair cells

then passes through the root cortex including the endodermis to reach the xylem

Question 11

how does water enter the roots

Answer 11

by osmosis
water mives from an area of high water potential to an area of low water potential down a water potential gradient
soil generally has high water potential and leaves have low water potential as water constantly evaporates of them
thtis creates a water potential gradient that keeps water moving in a plant in the right direction from roots to the leaves

Question 12

what is the symplast pathway

Answer 12

water flows through the cytoplasm by osmosis . the cytoplasm of neighbouring cells are connected through the plasmodesmata

Question 13

what is the apoplast pathway

Answer 13

movement of water through the cellulose cell wall. the walls are very absorbant and water can simply diffuse through them as well as pass thriugh the space between them. the water can carry solutes and go from an area of high hydrostatic pressure to an area of low hydrostatic pressure. this is an example if mass flow

Question 14

what happens when water gets to the endodermis cells

Answer 14

water in the apoplast path is blocked by the casparian strip which is a band of suberin that runs around the endodermis cells forming a waterproof layer so water enters the symplast pathway
this is useful as water has to go through the cell membrane which is partially permeable do can control whether or not substances can pass through
once it has passed this barrier it moves into the xylem

Question 15

what is the main pathway

Answer 15

apoplast as it provides the least resistance

Question 16

what happens when water reaches the xylem

Answer 16

xylem vessel transports the water all around the plant
at the leaves water leaves the xylem and moves into the cell mainly by apopolast pathway
water evaporates from the cell wall into the spaces between cells in the leaf
when the stomata is open water diffuses out of the leaf down a water potential gradient into the surrounding air
the loss of water from a plants surface is called transpiration
the loss of water lowes the water potential of the cell so water moves into the cell from an adjacent cell by osmosis along both apoplasy and symplasy pathway

Question 17

how does cohesion and tension help water move up a plant

Answer 17

water evaporates from the leaves at the top of the xylem
this creates tension which pulls more water into the leaves
water molecules are cohesive so stick together so when some are pulled into the leaf more follow. this means the whole column of water in the xylem from the leaves down to the roots moves upwards
water enter the stem through the root cortex cells

Question 18

how does adhesion help the movement of water

Answer 18

water is attracted to the walls of the xylem vessel. this helps water rise up through the xylem vessel

Question 19

how does water move into the xylem

Answer 19

the solute concentration in the cytoplasm of the endodermal cells is relatively dilute compared to the cells in the xylem and the endodermal cells move mineral ions into the xylem by active transport.

As a reult the water potential of the xylem cells is much lower than the water potential of the endodermal cells. this increases the rate of water moving into the xylem by osmosis down a water potential gradient from the endodermis through the symplast pathway

Question 20

what happens once the water enters the xylem

Answer 20

it returns to the apoplast pathway to enter the xylem itself and move up a plant.
the active pumping of minerals into the xylem to produce movement of water by osmosis results in root lressure and it is independent of any effects of transpiration.
root pressure gives water a push up the xylem but is not the major facot in the movement of water up from the roots to the leaves

Question 21

evidence for the role of active transport in root presure

Answer 21

some poisons such as cyanide affect the mitochindria and prevent the production of ATP. if cyanide is applied to root cells so theres no energy supply the root pressure disappears

root pressure increases with a rise in temperature and decreases with a fall in temperature suggesting chemical reactions are involved

if levels of oxygen or respiratory substances fall root pressure falls

Question 22

why is transpirationa consequence of gas exchange

Answer 22

a plant needs to open its stomata to let carbon dioxide in for photosynthesis
but this lets water out as theres a high concentration of water inside the leaf then in the air so water moves out of the leaf down a water potential gradient when the stomata open

Question 23

evidence for the cohesion tension theory

Answer 23

changes in the diameter of trees. when transpiration is at its height during the day the tension in the xylem vessel is st its highest too. as a result the tree shrinks in diamtere. at night when transpiration is at it’s lowest the dianeter of the tree increases

when a xylem vessel is broken. when you cut flower stems to put them in water in most circumstances air is drawn into the xylem rather than water leaking out

if a xylem vessel is broken and air is pulled in the plant can no longer move water up the stem as the continuous stream of water molecules held together by cohesive forces has been broken

Question 24

how does light affect the transpiration rate

Answer 24

light- the lighter it is the faster the transpiration rate. this is beacuse the stomata open when it is light so carbon dioxide can diffuse into the leaf for photosynthesis. when it is dark the stomata are usually closed so little transpitation

Question 25

how does temperature affect the transpiration rate

Answer 25

the higher the temperature the faster the transpiration rate. increase in temperature increases the kinetic energy o lf the water molecues so increase the rate of evaporation from the spongy mesophyll cells into the air. the increase in water potential gradient between the inside and outside of the leaf makes water diffuse out of the leaf faster

Question 26

how does himidity affect the rate of transpiration

Answer 26

the lower the humidity the faster the rate of transpiration . if the air around the plant is dry the water potential gradient between the leaf and the air is increased which increases the transpiration

Question 27

how does wind affect the rate of transpiration

Answer 27

the winder the faster the rate of transpiration as rhe lots of air mivement blows away the water molecules from around the stomata. this increases the water potential gradient which increases the rate of transpiration

Question 28

what are xeorphytes and example

Answer 28

plants that live in dry habitats where water availability is very low and have evolved a wide range of adaptations that enable them to survive

example cati and marram grass

Question 29

how are xerophytes adapted

Answer 29

thick waxy cuticle- to help minimise water loss through the cuticle

sunken stonata- stomata located in pits which reduces air movement providing a microclimateof still hunid air that reduces water vapour potential gradient and so reduced transpiration

reduced number of stomatas- reduces water loss by transpiration and reduces their gas exchange capabilities

hairy leaves- traps moist air around the stomata which reduces the water potential gradient between the leaf and the air so slows down transpiration

cacti have spines instead of leaves which reduces the surface area for water loss

curled leaves- creates a micro environment of still humid air to reduce diffusion of water vapour from the stomata

Question 30

what are hydrophytes and example

Answer 30

plants that live in water and need special adaptations to cope with growing on water
eg water lilies

Question 31

how are hydrophytes adapted

Answer 31

very thin or no waxy cuticle- dont need to conserve water so water loss by transpiration is not an issue

many open stomata on the upper surface- maximised gaseous exchange

wide flat leaves that spread across the surface of the water to capture as much light as possible

air sacs to enable the leaves to float on the surface of water

Question 32

what is translocation

Answer 32

movement of dissolved substances ( amino acids and sucrose) to where they are needed
dissolved substance are called assimilates

Question 33

where are sources of assimilates in a plant

Answer 33

usually in the leaves, stems
storgae organs like tubers and tap roots
food store in seeds when they germinate

Question 34

where ate the sinks in a plant

Answer 34

roots that are growinf and or acitvely absorbing mineral ions
meristems that are actively dividing
any parts of the plant that are laying down food store eg developong seeds, fruits or storage organs

Question 35

mass flow hypothesis

Answer 35

active transport is used to actively load solutes into the sieve tubes of the phloem at the source
this lowers the water potential inside the sieve tubes so water enters the tubes by osmosis from the xylem and companion cells
this creates a high pressure inside the sieve tubes at the source if the phloem

at the sink end solutes are removed from the phloem to be used up
this increases the water potential inside the sieve tubes so water leaves the tubes by osmosis
this lowers the pressure inside the sieve tubes

this reults in a pressure gradient from the source end to the sink end
this gradient lushes the solutes along the sieve tube to where they are needed

Question 36

active loading into the phloem

Answer 36

in the companion cells ATP is used to actively transport hydrogen ions out of the cell and into the surrounding tissue cells
this sets up a concentration gradient as thers more hydrogen ions in the surrounding tissue than in the companion cells
a hydrogen ion binds to a co transport protein in the companion cell membrane and renters the cell down a concentration gradient
a sucrose molecule binds to the co transport protein at the same time and the movement of the hydrogen ion moves the sucrose molecule into the cell against the concentration gradient
sucrose molecules are then transported out of the companion cells and into the sieve tubes by the same process

Question 37

how to do potomoter pag

Answer 37

1. cut a shoot under water to prevent air entering the xylem. cut it at a slant to increase surface area available for water uptake
2. assemble the potometer in water and insert the shoot under water so no air can enter
3. remove the apparatus from the water but keep the end of the capillary submerged in a beaker of water
4. check that the apparatus is water tight and airtight
5. dry the leaves, allow time for the shoot to acclimatise and then shut the tap
6. remove the end of the capillary tube from the beaker of water until one air bubble has formed, then put the end of the tube back into the water
7. record the start position of the air bubble
8. start a stopwatch and record the distance moved by the bubble per unit time. the rate of air bubble movement is an estimate of the transpiration rate
9. only change one variable at a time. all other conditions must be kept constant