Transport in plants

Question 1

why do plants need a transport system?

Answer 1

small surface area to volume ratio
relatively large – high metabolic rate
exchange of substances by direct diffusion is too slow
need transport system as roots can obtain water and minerals but not all sugars
leaves can produce sugars but not get all water from air

Question 2

what does the xylem and phloem do?

Answer 2

xylem tissue- transports water and soluble minerals upwards
phloem tissue- moves sugars up or down

Question 3

where is the location of the xylem and phloem in the roots?

Answer 3

they are in the centre to provide support for the root as it pushes through the soil

Question 4

where is the location of the xylem and phloem in the stem?

Answer 4

near the outside to provide ‘scaffolding’ that reduces bending
found together in vascular bundles

Question 5

where is the location of the xylem and phloem in the leaves?

Answer 5

make-up a network of veins which support the thin leaves

Question 6

what are the adaptations of xylem vessels?

Answer 6

hollow dead cells
no end walls- water passes up through the middle easily
pits (small holes) in the sides of the walls allow water and minerals to move into neighbouring cells
strengthened with lignin
ring or spiral structures allow flexibility

Question 7

what are the adaptations of phloem tissue?

Answer 7

composed of sieve tube elements and companion cells
sieve tube elements have very few organelles- (no nucleus) thin layer of cytoplasm
At either end they have a sieve plate- lots of holes to allow solutes to pass through
companion cells have organelles to support themselves and the sieve tube elements

Question 8

what is osmosis?

Answer 8

movement of water down a water potential gradient across a partially permeable membrane

Question 9

what is the symplast pathway?

Answer 9

water enters the root hair cell via osmosis
travels through the cortex via the living parts of the cells (cytoplasm) until it reaches the xylem vessels
water moves down a water potential gradient
cytoplasms are connected via plasmodesmata

Question 10

what is the apoplast pathway?

Answer 10

water is absorbed in the cell walls of the root hair cell
travels through the cortex via the non-living parts of the cell (walls and spaces between them) until it reaches the endodermis layer
moving via mass flow from an area of high hydrostatic pressure too low hydrostatic pressure

Question 11

what is the casparian strip?

Answer 11

waxy waterproof layer

stops the water from travelling any further

Question 12

what is the movement of water from roots to leaves called?

Answer 12

the transpiration stream

Question 13

why does transpiration happen?

Answer 13

it happens as a result of gas exchange
a plant needs to open it stomata to let in carbon dioxide so it can produce glucose by photosynthesis
it also lets water out – higher concentration of water inside the leaf than in the air outside so water moves out of the leaf down the water potential gradient when the stomata open

Question 14

how is water transported from the soil to the leaves?

Answer 14

minerals actively transported into root hair cells, lowering water potential in root hair cells
water moves from soil into root hair cells through osmosis, down WP gradient
water moves from cell to cell down WP gradient via symplast OR apoplast pathway
in symplast pathway, water moves through cytoplasm and between cells through plasmodesmata
in apoplast pathway, water moves between spaces in cellulose cell wall
at the casparian strip in the endodermis, water is forced out of apoplast and into symplast pathway
minerals are actively transported from endodermis cells to xylem, lowering WP gradient in xylem
water vapour leaves stomata by diffusion down concentration gradient
water moves up xylem via mass flow / capillary action (cohesive and adhesive)
water is pulled by xylem because of tension created by loss of water vapour via transpiration

Question 15

why isn’t transpiration rate the same as rate of water uptake?

Answer 15

some water is used for photosynthesis
some water is used to create turgor pressure
some water is also produced by respiration

Question 16

what are the factors affecting rate of transpiration?

Answer 16

light intensity
temperature
air movement
humidity
number of stomata

Question 17

how does light intensity affect rate of transpiration?

Answer 17

increasing light intensity- more stomata will open, rate of photosynthesis is greater so more gas exchange occurs
rate of transpiration increases until all stomata are open- allows more water to evaporate out of the leaf
rate of transpiration stays constant

Question 18

how does temperature affect rate of transpiration?

Answer 18

increasing temperature- the faster water vapour molecules will move because they have more kinetic energy
water evaporates from leaf faster
WP inside the leaf increases compared to outside (steeper WP gradient) so water diffuses out of leaf faster
rate of transpiration increases

Question 19

how does air movement affect rate of transpiration?

Answer 19

the windier it is, the faster the transpiration rate
lots of air movement blows away water molecules from around the stomata
increases the WP gradient (maintains steep gradient) which increases the rate of transpiration

Question 20

how does humidity affect the rate of transpiration?

Answer 20

the more humid the air, the more water vapour there is
this reduces the WP gradient between inside and outside of leaf
water molecules diffuse more slowly, decreasing the rate of transpiration

Question 21

how do you measure rate of transpiration?

Answer 21

using bubble potometer

a bubble is incorporated in the setting up so that the movement of water can be tracked

Question 22

how do you use a potometer?

Answer 22

1) cut shoot underwater to prevent air being introduced into the xylem
2) insert shoot into potometer underwater so no air can enter system
3) check system is air and water tight
4) dry both sides of the leaves
5) allows shoot to acclimatise- rate will be constant
6) introduce an air bubble into the capillary tube
7) record start position of bubble, start timer
8) record distance and time taken for the bubble to move
9) use these values to calculate rate (D/T)
10) only alter one variable at a time - keep other factors which affect transpiration constant
11) repeat to eliminate anomalous result
12) calculate mean result and/or carry out statistical analysis

Question 23

what are the limitations of using a potometer?

Answer 23

cutting plant shoot may damage plant

plant has no roots so no resistance to water being pulled

Question 24

how do you calculate transpiration rate?

Answer 24

rate = volume / time
you need to calculate the volume of a cylinder ( length of capillary tube)
V=πr2h

Question 25

what are xerophytes?

Answer 25

plants that have adapted to live in very dry, hot and breezy conditions e.g. cacti and marram grass

Question 26

what is marram grass?

Answer 26

lives on sand dunes
conditions are harsh as water drains quickly and can often be salty
windy conditions

Question 27

describe the adaptation ‘rolled up leaves’

Answer 27

stomata tend to be on lower epidermis
rolling up leaves traps moist air near the stomata
reduces WP gradient between the leaf and the air so transpiration greatly reduces
reduces air movement around stomata

Question 28

describe the adaptation ‘thick cuticle’

Answer 28

some transpiration occurs through waxy cuticle
cuticle is impermeable to water
thick - Less water can escape via evaporation

Question 29

describe the adaptation ‘stomata in pits or grooves’

Answer 29

traps moist air near to leaf surface
reduces WP gradient
reduces air movement near stomata

Question 30

describe the adaptation ‘dense spongy mesophyll’

Answer 30

this leaves few air spaces

therefore there is less SA for evaporation of water

Question 31

what is cacti?

Answer 31

live in deserts

have additional features to overcome harsh conditions

Question 32

describe adaptations that cacti have

Answer 32

store water in their stems- they become fleshy and swollen
stem is often ribbed or fluted so that it can expand when water is available
leaves are reduced to spines- reduces surface area of the leaf and thus transpiration
used to protect the plant from predation
green stems- for photosynthesis
widespread roots-take advantage of any rain that does fall

Question 33

what are hydrophytes?

Answer 33

plants adapted to living in water or where the ground is very wet

Question 34

what are hydrophytes’ problems?

Answer 34

getting oxygen to submerged tissues is problematic
needs to keep afloat
needs to keep leaves in sunlight

Question 35

what are hydrophyte adaptations?

Answer 35

thin/ no waxy cuticle- not needed
large air spaces in the leaf- keeps the leaves afloat so that they can absorb sunlight
always open stomata on upper epidermis- they are exposed to air to allow gaseous exchange
stem has large air spaces- helps with buoyancy, allows oxygen to diffuse quickly to the roots for aerobic respiration

Question 36

what is translocation?

Answer 36

move substances (assimilates) from the source to the sink by mass flow in the phloem

Question 37

what does translocation mainly transport and why?

Answer 37

sucrose

it’s soluble like glucose but it is metabolically inactive so it won’t get used up during transport

Question 38

what is the source and the sink?

Answer 38

source of a substance is where it’s made (high concentration)
sink is the area where it’s used up (low concentration)

Question 39

what happens at the source during translocation?

Answer 39

active transport is used to actively load the solute into the sieve tubes of the phloem
lower the WP inside the sieve tube
water enters tube by osmosis from xylem and companion cells
creates a high pressure inside the sieve tubes at the source end of the phloem

Question 40

what happens at the sink during translocation?

Answer 40

solutes are removed from the phloem to be used up- happens by diffusion because solutes are at a higher concentration in the phloem than in the sink
removal of solute increases the water potential inside the sieve tubes
water leaves the tube by osmosis
this lowers the pressure inside the sieve tubes

Question 41

what happens during the flow?

Answer 41

there is a pressure gradient from the source end to the sink end
gradient pushes solutes along sieve tubes towards the sink
when they reach the sink, solutes will be used (e.g. in respiration) or stored (e.g. as starch)

Question 42

describe the relationship between concentration of sucrose and rate of translocation

Answer 42

the higher the concentration of sucrose at the source, the higher the rate of translocation

Question 43

what is active loading?

Answer 43

used at the source to move substances into the companion cells from surrounding tissues and from companion cells into sieve tubes against concentration gradient
concentration of sucrose usually higher in companion cells than the surrounding tissue cells and higher in sieve tube cells than companion cells

Question 44

how is sucrose moved?

Answer 44

using active transport and co transport proteins

Question 45

what are co transport proteins?

Answer 45

a type of carrier protein that binds to 2 molecules at a time

Question 46

describe the steps of active loading

Answer 46

in the companion cell, ATP is used to actively transport H+ ions out of the cell and into surrounding tissue cells
sets up a concentration gradient- more H+ ions in the surrounding tissue then in companion cell
H+ ion bind to co transporter protein in the companion cell membrane and re-enter the cell (down the concentration gradient)
sucrose molecule binds to the cold transport proteins at the same time
movement of H+ ions is used to move the sucrose molecules into the cell against it’s concentration gradient
sucrose molecules are then transported out of companion cell and into sieve tubes using the same process