Plant transport

Question 1

how does water enter the root hair cell from the soil

Answer 1

enters by osmosis
high water potential to lower water potential
down water potential gradient

Question 2

how are root hair cells adapted for the absorption of water

Answer 2

increased SA for absorption
thin surface layer ( cell wall + membrane) - so reduced diffusion pathway

Question 3

how do plants use water

Answer 3

-lost in transpiration
-provide water for photosynthesis turgidity and other metabolic processes

Question 4

in which direction does water move from the root hair cell

Answer 4

move across parenchyma cells (cortex)
the to the xylem

Question 5

what two methods can water move across the root to the xylem

Answer 5

-symplast pathway
-apoplast pathway

Question 6

what is the symplast pathway

Answer 6

-water travels as a result of osmosis
-passes through the cytoplasm of the cells
water enters the symplast pathway by osmosis from high WP in the soil to low WP in the cell through partially permeable mem
passes into cytoplasm and continues from one cell to the next via plasmodesmata - connect the cells of cell wall
water pass along cytoplasm cells by osmosis until the xylem is reached

Question 7

what is the apoplast pathway

Answer 7

movement of water through the cell wall
water is drawn along in a continuous column due to cohesion
this creates a tension which draws water along

Question 8

how does water enter into the xylem vessels

Answer 8

salts from endodermis actively pumped in xylem
lowers WP which draws water in from high WP from endodermis and pericycle - high pressure in xylem - causes root pressure increases and water moves up - more water then can move in - water molecules stick together ( cohesion )

Question 9

what is the evidence for root pressure

Answer 9

-root pressure increases with a rise in temp and decreases with decrease in temp
- metabolic inhibitors cause root pressure to cease - these prevent energy release by respiration
- decrease in availability of oxygen and respiratory substrates causes a reduction in root pressure

Question 10

how does water move up the xylem

Answer 10

Cohesion - stick to wall
Adhesion - xylem narrows
Root pressure - push up
Tension - pull up

Question 11

how does tension work

Answer 11

-water lost in transpiration pulls water up xylem
-sucked up and also pulled down by gravity
-this causes tension
- water moves down pressure gradient from high to low - by mass flow
- pull creates continuous stream of water

Question 12

how is tension caused

Answer 12

when water evaporates water in xylem pulled up as force is generated - can be called negative pressure
- as passes from atmospheric pressure in roots to lower atmos pressure in leaves

Question 13

how does transpiration affect flow of water

Answer 13

greater the water loss the faster water is pulled up - this force causes a decrease in stem diameter and trunk diameter

Question 14

how does weather affect tension

Answer 14

humid - reduced
dry/hot/windy days - maximum

Question 15

how does cohesion work

Answer 15

water molecules stick together - force of attraction created by formation of H bonds

Question 16

what is cohesion-tension theory and how does it work

Answer 16

force combined of transpiration and cohesion - principle mechanism responsible for water movement
1. Water lost from leaf because of transpiration / evaporation of water
(molecules) / diffusion from mesophyll / leaf cells;
OR
Transpiration / evaporation / diffusion of water (molecules)
through stomata / from leaves;
2. Lowers water potential of mesophyll / leaf cells;
3. Water pulled up xylem (creating tension);
4. Water molecules cohere / ‘stick’ together by hydrogen bonds;
5. (forming continuous) water column;
6. Adhesion of water (molecules) to walls of xylem;

Question 17

how does adhesion work

Answer 17

narrowness of xylem means there is a large SA:V and lots of water is in contact with xylem
-attraction of water to another material = adhesion
-the narrower the tube the more water that can stick

Question 18

what does capillarity mean

Answer 18

the narrower the tube the more water that can stick

Question 19

what is root pressure

Answer 19

when salts pumped in actively and lower WP which draws water in by osmosis - pressure pushes water up to the xylem causing root pressure

Question 20

what is transpiration

Answer 20

Transpiration refers to the loss of water vapour via the stomata by diffusion
Note that this is different to the transpiration stream which is the movement of water from the roots to the leaves

Question 21

how is transpiration important for the plant

Answer 21

It provides a means of cooling the plant via evaporative cooling
The transpiration stream is helpful in the uptake of mineral ions
The turgor pressure of the cells (due to the presence of water as it moves up the plant) provides support to leaves (enabling an increased surface area of the leaf blade) and the stem of non-woody plants

Question 22

what are some external factors that effect transpiration

Answer 22

humidity - difference in moisture inside and out of the leaf is reduced - reduces rate of diffusion - low humidity leads to an increase rate of transpiration as max diffusion

temp - high temp increase rate - water molecule shave more kinetic energy so move out leaves

dry soil - stomata close as plants unavible to replace water that is lost in transpiration

light - more light = more photosynthis so stomata open - when less light stomata close so less transpiration

Question 23

internal factors that affect transp

Answer 23

leaf SA - increase SA increase rae sd more surface with stomata is exposed to air

distrubution of stomata - most stomata on lower side as does not face sun or wind

waxy cuticle - waterproof layer to reduce water loss

leaf structure - sunken stomata, curled leaves and hairs which trap layer of air in front of stomata and reduce conc gradient

Question 24

what are xerophytes

Answer 24

plants adapted to live in dry conditions

which have special features

-specialised leaves - reduce water loss
-extensive root system - to increase water uptake
-swollen stem - to store water
- thick waxy cuticle
-small leaf surface - less area for evaporation
- low density - reduced outlets for evap
-sunken stomata - reduced conc gradient
-hairs - trap moist air
-folded or rolled up grass leaves - maintains humid air around stomata

eg marram grass - sand dunes/ costal

Question 25

what are the xylem vessels

Answer 25

-long system of dead cells that have no living contents and connected to form tubes -transport water and provided structural support -have lignin in cell walls - again structural support -narrow to allow adheson and are waterproof - so no water leaves

Question 26

what are the phloem tubes

Answer 26

mixed tissue that contains - made of live cells -sieve tube elements -companion cells -phloem fibres -parenchyma cells

Question 27

evidence for cohesion tension theory

Answer 27

change in diameter of the ree trunk according to the rate of transpiration -transpiration increases = more -ve pressure = xylem walls move inwards = trunk shrinks -transpiration decreases = less negative = xylem out = trunk increases xylem vessel broken = air in = no water enters tree ( continuous column of water so molecules don't stick) also water does not leak out = instead air drawn in as constantly under tension

Question 28

what is translocation

Answer 28

movement of sugars in a plant

Question 29

how is the phloem structured

Answer 29

-sieve tube elements (cells) -companion cells

Question 30

what are the sieve tube elements

Answer 30

-cells that are elongated with pores at each end which make up the sieve plate -have cytoplasm with a few organelles pushed to the side -no nucleus

Question 31

What are companion cells

Answer 31

cells found next to the sieve tube cells contain cytoplasm, a nucleus and more organelles including mitochondria linked to sieve tubes by plasmodesmata

Question 32

what are sources

Answer 32

the sites of sugar production

Question 33

what are sinks

Answer 33

places where sugars will be used directly or stored for future use

Question 34

what is the mass flow hypothesis for translocation

Answer 34

explains the movement of sugars and amino acids in solution from high pressure region to a low pressure region through phloem sieve tubes

Question 35

state and explain the first stage of translocation

Answer 35

transfer of sucrose into sieve elements -leaves make sugar in photosynthesis - they are the source of the sugar -glucose is converted to sucrose - sucrose is higher energy store - so better -however sucrose is too large to simply diffuse across membranes -so companion cells actively load sucrose into the phloem against a conc gradient -H ions pumped out of the companion cell using ATP creating H+ gradient -sucrose along with H+ diffuses into the companion cell by facilitated diffusion -sucrose enters the sieve tube element by facilitated diffusion - using carrier protein-sugar enters phloem sieve tube and lower WP -water then enter sieve tubes by osmosis making them swollen and turgid - causes high hydrostatic pressure in the phloem near the leaves

Question 36

state and explain the 2nd part of translocation

Answer 36

mass flow of sucrose through the sieve tube elements -at respiring cells (sinks) sucrose is used or converted to starch - ie sugar is removed PROCESS -sucrose is too large to simply diffuse out so carries proteins are involved -sucrose is actively transported into these cells from the sieve tubes - lowers the cells WP -water follows by osmosis -this lowers the hydrostatic pressure of the sieve tubes

Question 37

how does hydrostatic pressure differ bwt the sources and sinks

Answer 37

high at source low at sink - move down pressure gradient

Question 38

is translocation active or passive

Answer 38

mostly passive but only occurs due to the active transport of sugar - so affected by temp and metabolic poisons

Question 39

who came up with the mass flow hypothesis

Answer 39

The mass flow hypothesis was modelled by Ernst Münch in 1930. His simple model consisted of: Two partially permeable membranes containing solutions with different concentrations of ions (one dilute the other concentrated) These two membranes were placed into two chambers containing water and were connected via a passageway The two membranes were joined via a tube As the membranes were surrounded by water, the water moved by osmosis across the membrane containing the more concentrated solution which forced the solution towards the membrane containing the more dilute solution (where water was being forced out of due to hydrostatic pressure)

Question 40

evidence for mass flow of translocation

Answer 40

-When the phloem sieve tube is punctured phloem sap oozes out This suggests that it is under pressure -Phloem sap taken from near a source has a higher sucrose concentration than sap taken from near a sink This suggests that different water potentials would result in osmosis into/out of the sieve tubes at those two locations -companion cells possess many mitochondria and readily produce ATP for movement

Question 41

evidence against mass flow translocation

Answer 41

-function of sieve plates are unclear- seem to hinder mass flow -It has been suggested that some sieve tubes translocate at different times The mass flow hypothesis states that nearly all sieve tubes should be involved in translocation at the same time as they are all connected to the same leave -The rate of translocation of different organic substances was measured and the results showed that amino acids appeared to travel more slowly than sucrose The mass flow hypothesis states should be flowing at the same rate

Question 42

what methods are used to investigate translocation

Answer 42

tracer and ringing - the removal of a ring of surface tissues from the stem of the plant while leaving the stem core intact

Question 43

what is the process of the tracer ad ringing experiments

Answer 43

As the phloem is located towards the outside of the stem and the xylem towards the centre, the ring removes the phloem only with the xylem remaining intact The site of ringing is often shown on diagrams as a small black bar dividing the stem - remove bark and phloem After the ringing has been done the plant can then be exposed to a radioactive tracer so that the direction and rate of translocation can be investigated 14C is commonly used for these experiments as it is readily absorbed by the leaves and used in photosynthesis to produce sucrose The sucrose formed will be radioactive so its subsequent movement around the plant via translocation can be traced The amounts of radioactive carbon present in different parts of the plant can be detected If the mass flow hypothesis is correct then the bulk flow of phloem sap should be in one direction (from source to sink) and occur at the same rate in any sieve tube at the same time As leaves are the site of photosynthesis they are the source tissue, sink tissues can be above or below the leaves If the xylem is damaged during the ringing process the plant will not have an adequate supply of water and will wilt

Question 44

what other methods can be used to track transport in plants

Answer 44

using aphids - aphids mouthparts inserted into phloem then cut off the body leaving mouthparts in place -mouthparts then exude the sap which can be collected and analysed for radioactive sugar