4.7 Transport in Plants

Question 1

name the two main transport tissues in plants

Answer 1

xylem and phloem

Question 2

function of xylem

Answer 2

transport water and dissolved minerals from roots to photosynthetic parts of the plant
movement is upwards

Question 3

function of phloem

Answer 3

transports substances like glucose to leaves for growth and storage
movement can be up or down

Question 4

what is cambium?

Answer 4

a layer of unspecialised cells that divide, giving rise to more specialised cells that can form xylem + phloem

Question 5

how is xylem formed?

Answer 5

1. starts as protoxylem (living tissue)
   arranged in column + can grow
2. as stem ages, lignin is deposited in cell walls so cells die
   and xylem becomes a hollow metaxylem-impermeable

Question 6

function of lignin

Answer 6

to strengthen and prevent collapse

Question 7

what does xylem tissue consist of?

Answer 7

xylem vessels
xylem fibres
xylem parenchyma cells

Question 8

what happens when lignification is incomplete?

Answer 8

bordered pits can form which allows water to leave and pass elsewhere

Question 9

what does the phloem consist of + how does it work?

Answer 9

-phloem sieve tubes
many cells joined to make long tubes that become perforated to form sieve plates (living)
-specialised sieve plates
phloem contents flow through holes
sieve tube becomes a tube of phloem sap and phloem clls have no nucleus
-companion cells
active cells linked to sieve tube elements by plasmodesmata
lots of infoldings and have lots of mitochondria - ATP

Question 10

compare xylem and phloem

Answer 10

xylem
made of dead cells
thick cell wall made of lignin - impermeable
no cross walls or cytoplasm
transports water + minerals to leaves
flow is upwards

phloem
made of living cells
thin cell wall of cellulose -permeable
perforated cross walls
plasmodesmata
transports food for growth + storage
flows up and down

Question 11

evidence for movement of water

Answer 11

• eosin dye
• ringing experiments
• autoradiography

Question 12

how is water taken up through root hairs?

Answer 12

water moves from soil into a root hair cell down a concentration gradient by osmosis
this makes the root hair cell more dilute than its neighbour so water moves from cell to cell by osmosis across root to the xylem

Question 13

what is the symplast pathway?

Answer 13

water moves by diffusion down the concentration gradient from the root hair cells to the xylem through plasmodesmata

Question 14

what is the apoplast pathway?

Answer 14

water pulled by attraction between water molecules across adjacent cell walls from root hair to xylem
due to open network structure of cellulose, half of the cell wall can be filled with water
water moves across the cells of root in the cell walls until it reaches endodermis

Question 15

what happens when water meets the Casparian strip?

Answer 15

they temporarily enter the cytoplasm of the cell.

end result of all pathways is a continuous stream of water cross the root to xylem

Question 16

define transpiration

Answer 16

evaporation of water from the surface of the spongy mesophyll cells + the loss of water by diffusion down a concentration gradient from a leaf

Question 17

describe the process of transpiration

Answer 17

1. water lost by transpiration from leaves - moves by osmosis across leaf from cell to cell all the way to xylem
2. when molecules of water leave xylem to enter cell, creates tension in column of water in xylem + tension is sent to roots
3. due to polar nature and h-bonds, water molecules ‘stick’ together, giving column of water a high tensile strength
4. loss of water molecule from evaporation causes tension through plant because of cohesiveness, so more water is pulled up by xylem to replace what is lost

Question 18

explain what cohesion is

Answer 18

forces of attraction between like-molecules
water is strongly cohesive - each molecule can make up to 4 h-bonds
cohesion is responsible for surface tension

Question 19

explain what adhesion is

Answer 19

force of attraction between unlike- molecules

water forms h-bonds with other surfaces including tiny pores in mesophyll cell walls

Question 20

define translocation

Answer 20

movement of solutes in phloem of plants

Question 21

describe mass-flow hypothesis

Answer 21

• dissolved sucrose moves in phloem by pressure gradient between sources and sinks
• high concentration of sucrose reduces mesophyll + sieve cell water potential so water is drawn in - high hydrostatic pressure
• forces mass flow of phloem sap towards sink where hydrostatic pressure is low due to use of sucrose or storage as starch

Question 22

strengths of mass flow hypothesis

Answer 22

• experiments show aphid mouthparts exude sap; supports idea that content of phloem is under pressure
• phloem + xylem are linked in a way that makes sense

Question 23

weaknesses of mass flow hypothesis

Answer 23

• movement can be up or down in same sieve tube at different speeds
• phloem sieve plates have no obvious role in this theory
• experiments show that sieve and companion cells need to be alive but theory does not explain why

Question 24

how does wind affect transpiration rate?

Answer 24

as wind blows, it moves air from near the leaf and increases the diffusion gradient, which increases the concentration gradient

Question 25

how does temperature affect transpiration rate?

Answer 25

as the temperature rises, water molecules gain kinetic energy + move faster, increasing evaporation and transpiration

Question 26

how does light intensity affect transpiration rate?

Answer 26

as it becomes brighter, light causes the stomata to open for photosynthetic gas exchange and so allows more water out. transpiration increases with light intensity until all stomata are open.

Question 27

how does humidity affect transpiration rate?

Answer 27

at lower humidity, the concentration gradient is steeper so the transpiration rate increases. as humidity increases, rate of transpiration decreases

Question 28

how can you work out rate of uptake from a potometer experiment?

Answer 28

rate of uptake = V/ time taken for bubble to move through distance, h