plants and transpiration

Question 1

why do plants need a transport system?

Answer 1

• they have a small surface area to volume ratio so they can’t rely on diffusion alone to supply them with everything they need
• they have metabolic demands- only green parts of the plants photosynthesise and so glucose needs to be transported to other parts
• waste products need to be removed quickly
• hormones need to be transported to their target sites
• plants continue to grow throughout their lives

Question 2

What is the main transport system that plants have and what does this comprise of?

Answer 2

• vascular bundle
• comprised of xylem and phloem vessels
• these run through the leaves, stems and roots as they are the main organs involved in transport

Question 3

What are dicotyledonous plants?

Answer 3

• they have two cotyledons which are organs which act as food stores for the developing embryo
• they have a series of transports vessels running through the stem, leaf and roots known as the vascular system

Question 4

How many epidermis layers does a leaf have?

Answer 4

• two

- the lower epidermis contain pores called stomata

Question 5

What is the purpose of the endodermis layer in the root?

Answer 5

• contains the Casparian strip which has Suberin which forces water into the symplast pathway
• water has to now pass the cell surface membrane via osmosis so the plant can control what enters
• prevents the entering of pathogens

Question 6

What is the parenchyma tissue?

Answer 6

• it forms the forex in the stems and roots and the pith in the leaves

Question 7

What is the function of a xylem tissue in a plant?

Answer 7

• to transport water and minerals
• for support
• food storage

Question 8

Where is the Casparian strip located?

Answer 8

• between endodermal cells

Question 9

What causes root pressure?

Answer 9

• endodermal cells actively pump ions into the xylem vessels
• the decrease in water potential in the xylem vessels cause water to diffuse in via osmosis
• there is an increase in pressure in the xylem and so to move down pressure gradient, this results in the water getting a little push up

Question 10

What is the cohesion tension theory?

Answer 10

• when water vapour evaporates from leaves, there is a tension created in the xylem
• due to the cohesive forces between water molecules, a continuous stream of water moves up the xylem to replace the water lost

Question 11

What is the cohesion tension theory?

Answer 11

• when water vapour evaporates from leaves, there is a tension created in the xylem
• due to the cohesive forces between water molecules, a continuous stream of water is pulled up the xylem to replace the water lost

Question 12

What is capillary action?

Answer 12

• the water has adhesive properties with the wall of the xylem and cohesive properties with each other
• this allows water to move up the xylem against the force of gravity

Question 13

what is the mechanism for the loading and transport of sugars in the phloem?

Answer 13

• hydrogen ions are actively pumped out of the companion cells
• this creates a concentration gradient
• so they move down concentration gradient via facilitated diffusion using contransport proteins
• this also transports sucrose so sucrose is loaded into companion cells next to source
• sucrose diffuses into sieve tube elements through plasmodesmata so sucrose is loaded into phloem
• water follows via osmosis down water potential gradient
• this creates a higher hydrostatic pressure at the source
• causing the assimilates to travel to the sink via mass flow as the hydrostatic pressure at the sink is low
• assimilates are unloaded at the sink

Question 14

What is the role of a sink in the mechanism of translocation?

Answer 14

• unloading of sucrose happens there( sucrose is removed from phloem) which causes a higher water potential in the sieve tube elements causing water to diffuse into sink via osmosis which decreases the hydrostatic pressure in the sieve tube elements of phloem of that part
• they decrease the concentration of sucrose as they use it for respiration

Question 15

What procedures need to be taken to ensure that valid measurements can be made from the potometer?

Answer 15

• ensure that the healthy shoot is cut under the water
• ensure it is cut at a slant
• place the shoot in potometer underwater
• remove the potometer out of the water ensuring it is airtight
• ensure the leaves are dry
• make sure the ruler is fixed in position beside the capillary tube
• record the distance moved by the capillary bubble at regular intervals of 30 minutes.

Question 16

Why does wind increase the rate of transpiration.

Answer 16

• wind moves the water vapour in air spaces
• this increases the water potential gradient
• increased transpiration rate because faster diffusion of water vapour from air spaces to surroundings

Question 17

Why does the potometer only give an estimate of the fate of transpiration?

Answer 17

• water is used for photosynthesis
• water is used to maintain turgor pressure in plant cells
• transpiration is the loss of water vapour from the leaves whereas the potometer only measures the rate of water uptake and not all water taken in is lost

Question 18

How does the water travel through the root cortex in the apoplast is and symplast is pathways?

Answer 18

• in apoplast pathway it travels through the cellulose walls by diffusion
• in symplast pathway it travels from cell to cell through the cytoplasm by either crossing the cell surface membrane by osmosis or by diffusing through the plasmodesmata
• once the Casparian strip is reached, as it contains Suberin which is impermeable to water, it forces all the water in the symplast pathway

Question 19

What is meant by the term source and sink?

Answer 19

Source is where assimilates are loaded into phloem

Sink is where assimilates are unloaded from sieve tube elements so that they can be used for respiration

Question 20

If a complete ring of bark is removed, why does the trunk swell above the cut if when the bark is removed, the phloem is also removed?

Answer 20

• there will be a buildup of assimilates like sucrose above the cut because they can’t pass the cut
• could be that cut causes an infection so due to the formation of tylose
• as water can’t travel up, the water potential above the cut decreases causing water to move into cells

Question 21

Why is the lignin essential in the wall of a xylem vessel?

Answer 21

• it keeps the xylem open
• provides strength
• prevents it from collapsing when under negative pressure
• it is impermeable to water so prevents lateral movement of water
• to allow a continuous column of water as it allows adhesion between water molecules and the wall of the xylem

Question 22

Why is it important that the xylem cell walls are impregnated with lignin in a spiral way?

Answer 22

• allows the xylem vessels to be flexible so it doesn’t break when water travels through them

Question 23

What is the function of the bordered puts in the xylem vessel?

Answer 23

• allows for the lateral movement of water from the xylem vessels to cells that need them to photosynthesise of maintain turgor pressure

Question 24

What are the adaptations of the phloem that allow efficient transport of assimilates?

Answer 24

• it has very few organelles so less resistance to mass flow
• perforated sieve plates so the continuous flow of assimilates with less obstruction
• has plasmodesmata connecting the sieve tube elements and companion cells allowing for the rapid diffusion of assimilates
• has a thin cytoplasm for also very little resistance
• the sieve tube elements are joined end to end so a continuous column can be formed