mass transport in plants

Question 1

how is pressure generated inside the phloem tube

Answer 1

sucrose is actively transported into the phloem
lowering the water potential
so water moves into the phloem via osmosis from xylem increasing hydrostatic pressure

Question 2

why is phloem pressure reduced during the hottest part of the day

Answer 2

high rate of transpiration
so water is lost through the stomata
causing high tension in xylem so less movement from xylem to phloem of water

Question 3

why would you add oil to beaker

Answer 3

prevent water evaporation

Question 4

why would water move up celery stalks - related to xylem

Answer 4

water evaporates from leaves
reducing the water potential
hydrogen bond adhesion and cohesion maintain a water column

Question 5

how can some plants survive without xylem

Answer 5

surface permeable to water or short diffusion distance to all cells

Question 6

how can heat treatment damage the phloem

Answer 6

movement in the phloem requires ATP. AT respiration
heat treatment damages living cell so respiration and atp cant happen

Question 7

the cohesion tension theory

Answer 7

water lost from leaf via transpiration
through stomata
lowering water potential of mesophyll
Water is drawn out of the xylem to replace the water lost
water pulled up xylem creates tension
water molecules stick together by hydrogen bonding = continuous column
adhesion of water molecules to walls of xylem
Water enters the stem from the roots

Question 8

why might the rate of water uptake in potometer not be the same as the rate of transpiration

Answer 8

water is used for support and turgidity
water is used in photosynthesis and produced in respiration

Question 9

why might the rate of movement through shoot in photometer not be the same as the rate of water movement in the whole plant

Answer 9

shoot in photometer has no roots whereas a plant does
xylem cells are very narrow

Question 10

mass flow hypothesis

Answer 10

In source sugars actively transported into phloem
By companion cells;
Lowers water potential of sieve tube
water enters by osmosis
4. Increase in pressure causes mass movement - source ti sink
5. Sugars used / converted in root for respiration for storage.

Question 11

During their experiment, the scientists ensured that the rate of photosynthesis of
their plants remained constant. why

Answer 11

Rate of photosynthesis related to rate of sucrose production
Rate of translocation higher when sucrose concentration is higher.

Question 12

how does water enter a plant cell

Answer 12

moves down water potential gradient through partially permeable membrane via osmosis

Question 13

advantage of transpiration

Answer 13

The constant stream of water also transports ions around the plant
The evaporation has a cooling effect, stopping the enzymes being denatured if the plant overheats

Question 14

describe the phloem

Answer 14

Elongated cells joined end to end
Holes in end walls - sieve plates
In bundles
Very few organelles/cytoplasm

Question 15

why do phloem cells need companion cells

Answer 15

They have lost many of their organelles e.g nucleus when they are specialised
It helps them repair and maintain themselves
Produces ATP for active transport of H+

Question 16

how does sucrose enter the phloem

Answer 16

Sucrose FD with H+ into the cc from the source cell
H+ actively transported out
Sucrose moves into PST VIA FD

Question 17

how does metabolic inhibitor support mass flow hypothesis

Answer 17

Inhibit respiration
Translocation does not occur
Shows that it required ATP and active transport

Question 18

How do ringing experiments support the mass flow hypothesis?

Answer 18

Ring of bark removed from tree (removes phloem NOT xylem)
Sucrose cannot be transported so collects
Lowers water potential
Water moves out of the xylem and bulges
Shows phloem important in transport of organic molecules

Question 19

How do we know photosynthesis is needed for translocation

Answer 19

plant virus travels up plant in the light not when its dark

Question 20

Why is the rate of transpiration high on a dry, windy day?

Answer 20

Low humidity so low water potential around stomata
Air movement blows away water molecules
Maintains a steep water potential gradient

Question 21

How is the potometer reset?

Answer 21

open reservoir
moves air bubble to original position

Question 22

Why is water usually lost from the leaf?

Answer 22

There is a lower water potential outside the leaf than inside
Water diffuses down the water potential gradient out of the leaf

Question 23

Why is it important that the xylem vessels are narrow tubes?

Answer 23

Increases the surface area for adhesion of water to the walls of the xylem

Question 24

Why is it important to assemble the potometer underwater?

Answer 24

So that no air enters the apparatus
prevents plants from taking up water effectively so affects transpiration
So there are no (more) air bubbles

Question 25

Describe how a potometer works

Answer 25

tube connecting the plant shoot to a beaker of water
air bubble and ruler
As plant takes up water, the air bubbles moves
The distance moved in a given time can be measured

Question 26

Why is the surface area of leaves measured, when investigating the rate of transpiration in different plants?

Answer 26

Different plants have different sized leaves
Rate of transpiration also depends on surface area of leaves
Allows comparison of results

Question 27

Why is the plant left for a while before beginning the experiment into rate of transpiration?

Answer 27

So the shoot can acclimatise
Excess water evaporates from leaves

Question 28

how is high pressure generated in leaves

Answer 28

wp low - water enters by osmosis
increasing volume and pressure

Question 29

2 precautions when setting up photometer to obtain reliable measurements of water uptake by plant root

Answer 29

seal joints - airtight and water tight
cut shoot underwater
dry off leaves
no air bubble present

Question 30

relationship between number of leaves and mean rate of water uptake

Answer 30

less leaves
less surface area
fewer stomata
less transpiration
less cohesion tension
less pulling force
less water uptake

Question 31

stomata on pine trea leaf in pits below leaf surface - how does this reduce water loss

Answer 31

water vapour accumulates increasing humidity so water potential gradient is reduced

Question 32

effect of increasing light intensity on tension in xylem vessels in leaves

Answer 32

greater tension
stomata open more so more transpiration so wp lower
so water moves from xylem to surrounding cells down water potential gradient

Question 33

why does humidity need to be constant in experiment

Answer 33

humidity affects evaporation
increase humidity decreases rate of water loss

Question 34

relation between humidity and transpiration rate

Answer 34

increase humidity decreased transpiration
as more water in air so higher wp
reducing wpg
slower rate of evaporation

Question 35

why is less water lost through upper surface of leaves than lower surface

Answer 35

more stomata on lower surface
thick waxy cuticle on upper surface

Question 36

how does increased transpiration decrease diameter

Answer 36

high transpiration = higher tension
reduces pressure in xylem reducing diameter
adhesion between water molecules and walls of xylem vessels pulling xylem inwards by tension

Question 37

xylem tissue adaptation for functions

Answer 37

cells no end walls so continuous flow
few organelles easier flow
thick cell walls w lignin withstand tension
pits in wall allow lateral movements of water