CHAPTER 9 - TRANSPORT IN PLANTS

Question 1

What are the main reasons why plants need transport systems

Answer 1

Metabolic demands - transport of substances and growth
Size - from source to sink - eg. leaves to roots
SA:V - cannot rely on diffusion alone

Question 2

What are dicots

Answer 2

plants that contain to cotyledons, organs that act as food stores for the developing embryo plant and form first leaves when the seed germinates

Question 3

What are some traits of herbaceous dicots

Answer 3

Soft tissues, short life cycle (leaves and stem die down at the end of the growing season in the soil level

Question 4

What are the traits of woody dicots

Answer 4

Hard lignified tissues, long life span (eg over a hundred years eg. a tree)

Question 5

What is the vascular system comprised of

Answer 5

Xylem and phloem (arranged in vascular bundles)
(pg 199)

Question 6

What are the two main functions of the xylem

Answer 6

Transport of water and mineral ions

Support

Question 7

What is Xylem made of

Answer 7

Dead hollowed out cells - which have been lignified (which kills them)

Xylem Parenchyma - stores food and tannin deposits (anti-herbivory chemical)

Question 8

Where does water leave the xylem to move into the cells of the plants

Answer 8

The small un-lignified pits which are the gaps in the ring, spiral or solid tubes of the lignin

Question 9

What is Phloem

Answer 9

A living tissue that transports food in the form of organic solutes around the plant (source to sink)

Question 10

Which directions can xylem and phloem contents travel

Answer 10

X: Only up
P: up and down

Question 11

What are the main transport vessels of the phloem

Answer 11

Sieve tube elements - un-lignified cells end to end forming a long hollow structure, with sieve plates on each end of what was previously the cell
(pg 201)

Question 12

What is present in the phloem (apart from solutes) and why

Answer 12

Filled with phloem sap

As large pores of sieve plates form, the tonoplast (vacuolar membrane), nucleus and other organelles break down

Question 13

What are the cells that are closely linked to the sieve tube elements and what do the do

Answer 13

Companion cells, which provide life support systems - hence are very active, linked by plasmodesmata

Question 14

What are plasmodesmata

Answer 14

Microscopic channels between cell walls linking cytoplasms of two adjacent cells

Question 15

How does Phloem tissue provide structural support

Answer 15

Contains supporting tissues, including fibres and sclerids - cells with extremely thick cell walls

Question 16

Explain why multicellular plants need transport systems

Answer 16

Too big for diffusion alone to supply needs as SA : V ratio too small for diffusion to be effective means of transport

transport system required for transporting oxygen and glucose for respiration

waste product removal

water and mineral ions from roots to all the cells

hormones made in one part of a plant to the areas where they have an effect

Question 17

Three differences between transport systems in multicellular animals and multicellular plants

Answer 17

In plants no heart to act as central pump, whereas many multicellular animals have heart

in plants one type of vessel is made of dead cells – all animal vessels made of living tissue

in plants there are two different transport systems carrying different materials – animals have different types of vessels but the same transport medium in both

Question 18

State the positioning of the transport tissue in herbaceous, dicot, stems roots and leaves and explain how this positioning is related to their functions

Answer 18

Stem – vascular bundles around outside

helps give strength and support to structure

roots – vascular bundles in centre

to help give strength against tugging forces when plant blown by wind

leaves – large central vein containing vascular tissue

gives supports to broad structure of leaf

Question 19

Compare contrast the structure and function of the main cell types in xylem and phloem tubes

Answer 19

Similarities:
both transport materials around plant;
both made up of cells joined end to end forming long, hollow structures

Differences:
Xylem largely non-living tissue, phloem living;
xylem transports water, mineral
ions, and supports plant, phloem transports organic solutes around plant from leaves;
in xylem flow of material from roots to shoots and leaves, in phloem flow of material up and down;
xylem cell walls lignified, phloem not;
xylem have wide lumen; mature phloem cells have no nucleus;
other cells associated with xylem in herbaceous dicots include xylem parenchyma and xylem fibres, equivalent in phloem include fibres and scleroid

Question 20

Why is water essential for plants

Answer 20

Raw material for photosynthesis

Mineral ions and products of photosynthesis are transported in aqueous solutions

Evaporation keeps plants cool

Turgor pressure provides hydrostatic skeleton and supports stems and leaves, which also drivers cell expansion

Question 21

How does water move into the root

Answer 21

Via Specialised root hair cells

Question 22

How are root hair cells adapted to their function

Answer 22

Microscopic - can penetrate easily between soil particles

Large SA:V and many thousands on each root tip

Thin surface layer - diffusion/osmosis is v v quick

Conc. solutes in cytoplasm maintains water potential gradient between soil water and cell

Question 23

Why does water move into the root hair cells

Answer 23

Soil water has v low conc of dissolved minerals - very high water potential

Cytoplasm and vacuolar sap of RHC contains many different solvents eg. sugars, minerals and amino acids so the water potential is lots lower

Water moves in by osmosis

Question 24

What are the 3 pathways for water to move across a root

Answer 24

Symplast, Apoplast, vacuolar

Question 25

Describe the symplast pathway

Answer 25

Water moves continuously through the cytoplasm of the living plant cells that are connected by plasmodesmata

Root hair cell has higher water potential than the next cell along and so on, so water diffuses between the cells

Steep concentration gradient
(pg 203)

Question 26

Describe the movement of water through the apoplast pathway

Answer 26

Water moves between the inter cellular spaces, in the open network of fibres in the cellulose cell walls

As water is pulled up the xylem, the cohesive force of water pull the lines of water molecules up the xylem
(pg 203)

Question 27

What is the endodermis

Answer 27

The layer of cells surrounding the vascular tissue of the roots, containing casparian strips
(pg 204)

Question 28

What is the casparian strip

Answer 28

A band of the waxy material Suberin that runs around each endodermal cell forming a waterproof layer, forcing all water in the apoplast pathway to the symplast pathway through a selectively permeable membrane, so toxic solutes in the water dont reach living tissues
(pg 204)

Question 29

How is root pressure generated

Answer 29

Active pumping of minerals into the xylem produces a movement of water independent of transpiration

Question 30

Once the water is in the endodermal cytoplasm, how does it get into the xylem

Answer 30

Solute conc in cytoplasm of endodermal cells is dilute compared to xylem

Endodermal cells move mineral ions into the xylem by active transport

Water potential of xylem much lower than endodermal cells, which increases rate of water from the endodermis down the symplast route.

Once inside the vascular bundle, water returns to the apoplast pathway to enter the xylem and move up the plant
(pg 204)

Question 31

What is the evidence for the role of active transport in root pressure

Answer 31

Some poisons, eg. cyanide, affect the mitochondria and prevent the production of ATP. If Cyanide is applied to root cells, there is no energy hence root pressure disappears

Root pressure increases with rise in temp ORA, suggesting chemical reaction is involved

If level of Oxygen or respiratory substrates fall, root pressure falls

Xylem sap may exude from ends of stems at certain times, out of special pores eg. overnight when transpiration is low - known as guttation

Question 32

Explain how a root hair cell is adapted for its role in the uptake of water from the soil

Answer 32

Very small to penetrate soil particles

large SA : V ratio for water absorption

thin layers for ease of diffusion

high solute concentration in cytoplasm gives low water potential so water moves in from soil by osmosis

Question 33

Explain the differences between the apoplast and symplast pathways for water movement across the root of a plant into the xylem

Answer 33

Symplast pathway:
relies on osmosis as water moves through cell membranes and cytoplasm;
Water moving in from soil by osmosis into root hair cell raises water potential compared to next cell so water moves again by osmosis
Active transport of ions needed to move water from endodermis to xylem by osmosis

Apoplast pathway:
water moves through cellulose cell walls by cohesive forces between water molecules and as result of transpiration pull up xylem;
moves into symplast pathway in endodermis as a result of Casparian strip
and needs active pumping of ions into xylems followed by osmosis
before water moves back to apoplast pathway in xylem

Question 34

Suggest why the effect of temperature on root pressure is not sufficient to prove that active transport is involved in the development of root pressure

Answer 34

Temperature increase increases root pressure BUT although increase in temperature increases rate of chemical reactions

it also increases rate of passive processes such as diffusion and osmosis

Question 35

Explain why the effects of cyanide, oxygen levels, and respiratory substrates on root pressure are taken as evidence that active transport is involved in the development of root pressure

Answer 35

Active transport needs energy in form of ATP

cyanide poisons mitochondria where cellular respiration takes place so cyanide prevents ATP formation

Oxygen and respiratory substrates needed for cellular respiration and ATP production

Factors which interfere with ATP production also interfere with development of root pressure

suggesting that development of root pressure is an active process requiring ATP

Question 36

Describe the process of transpiration

Answer 36

Water is pulled up the plant through the xylem and leaves through the open stomata as water vapour

Question 37

How can a plant control the amount of water lost

Answer 37

opening and closing of stomata, yet still need them open for a sufficient time for gas exchange

Question 38

What is the transpiration stream

Answer 38

Enter of water through the roots by osmosis and is transported up the xylem until it reaches the leaves.

It moves by osmosis across a membrane and by diffusion in the apoplast pathway from the xylem through cells of the leaf where it evaporates from the freely permeable cellulose cell walls of the mesophyll cells into the air spaces

Water then exits through the stomata

Question 39

How does the transpiration stream work

Answer 39

Water molecules evaporate from the surface of mesophyll cells into the air spaces in the leaf and move out of the stomate into the surrounding air by diffusion down a conc gradient

Loss of water by evaporation from a mesophyll cell lowers the water potential of the cell, so water moves into the cell from an adjacent cell by osmosis, along both apoplast and symplast pathways

Repeated across the leaf to the xylem

Water molecules form hydrogen bonds with carbohydrates in cell walls of narrow xylem vessels - adhesion

Water molecules tend to stick together - cohesion, drawn up the xylem tin a continuous stream - transpiration pull

Transpiration pull in a tension in the xylem which in turn helps move water across the roots from soil

Question 40

What is evidence for the cohesion-tension theory

Answer 40

Changes in the diameter of trees - transpiration is at its highest during the day, when transpiration is high

If a xylem vessel is broken, air is drawn into the xylem rather than water leaking out

If this occurs, the plant can no longer move water up the stem as the continuous stream of water molecules held together by cohesive forces has been broken

Question 41

What are the factors that affect transpiration

Answer 41

Light - needed for photosynthesis and in the light stomata are open for gas exchange. Increasing light intensity means more open stomata meaning more transpiration ORA

Relative humidity - high humidity lowers rate of transpiration as more water outside leaf than in it relative to a low humidity, so rate decreases

Temperature - KE, increases rate of evaporation, also increases conc. of water vapour air can hold before it becomes saturated

Air movement - takes water away, maintaining diffusion gradient

Soil-water availability - if dry soil, plant will be under stress so rate of transpiration will be reduced

Question 42

Explain the difference between transpiration and the transpiration stream

Answer 42

Transpiration:
evaporation of water from surface of a leaf

Transpiration stream:
flow of water moved up from soil into root hair and through root cortex by osmosis, into xylem and up through stem by cohesion of water molecules, cross leaf cells by osmosis and out of leaf by evaporation and
diffusion

Question 43

Compare root pressure and the transpiration pull

Answer 43

Root pressure:
active movement of solutes followed by passive movement of water by osmosis, which gives a positive pressure forcing water up the xylem

Transpiration pull:
pulling of a constant stream of water molecules up xylem held together by cohesive forces as a result of evaporation of water from surface of spongy mesophyll cells in leaf
a passive process and a negative pressure

Question 44

Describe how you might use a potometer to investigate the effect of air movements on transpiration rates

Answer 44

All conditions except air movements kept the same
e.g., use of fan for set time
fan placed at different distances from potometer
control readings in still air allow plant recovery time between different distances more than one reading for each distance

Question 45

Describe the cohesion-tension theory of transpiration

Answer 45

Water molecules evaporate from surface of mesophyll cells into air spaces in leaf and move out of stomata into surrounding air by diffusion down concentration gradient

loss of water by evaporation
from mesophyll cell lowers water potential of cell

water moves into cell from adjacent cell by
osmosis, along apoplast and symplast pathways

repeated across leaf to xylem. Water moves out
of xylem by osmosis into cells of leaf

water molecules form adhesive bonds with walls of narrow xylem vessels and also form hydrogen bonds and so tend to stick together – cohesion

as a result of these cohesive forces, along with adhesion to walls of xylem, water is pulled up xylem in continuous stream to replace water lost by evaporation, this is the transpiration pull

transpiration pull results in tension up xylem which helps to move water across roots from soil, movement is partly
by osmosis (change in water potential in cells across root) and partly by cohesion through apoplast pathway

Question 46

Use cohesion-tension theory to explain changes in diameter measured in the trunk of a tree between midday and midnight

Answer 46

Cohesive forces in xylem cause negative pressure
that draws tissues in and reduces
diameter of tree
in daytime when maximum photosynthesis and so maximum transpiration is
taking place
minimum transpiration takes place during night so less tension in the xylem tissue and it expand

Question 47

Use cohesion-tension theory to explain the fact that cut flowers placed in water may drop and die very quickly

Answer 47

Tension and negative pressure in xylem means air may be pulled into xylem when stem is cut

this breaks cohesive stream of water molecules

and prevents more water moving up stem

as a result flowers droop and die very quickly, as air bubble is pulled up the stem

Question 48

What is translocation

Answer 48

The process of plants transporting organic compounds from sources to sinks

Question 49

What are the products transported by translocation called what is the main one

Answer 49

Assimilates
Sucrose

Question 50

What are the main sources of assimilates in a plant

Answer 50

Green leaves and green stems

Storage organs such as tubers and tap roots that are unloading their stores at the beginning of a growth period

Food stores in seeds when they germinate

Question 51

What are the main sinks in a plant

Answer 51

Roots that are growing or actively absorbing mineral ions

Meristems that are actively dividing

Any parts of the plant that are laying down food stores eg. developing seeds, fruits or storage organs

Question 52

What is phloem loading

Answer 52

Loading assimilates into the phloem
Active or passive process

Question 53

What is the symplast route of phloem loading

Answer 53

Assimilates move through the cytoplasm and on into the sieve tubes by diffusion through plasmodesmata

Although Phloem loading and translocations are mostly active processes, this route is largely passive

Osmosis follows sucrose by osmosis, creating pressure of water which moves sucrose up phloem by mass flow

Question 54

What is the apoplast route of phloem loading

Answer 54

Travelling of sucrose through cell walls and inter-cell spaces to the companion cells and sieve elements

By diffusion, maintained by the removal of sucrose into the phloem vessels.

In the companion cells, sucrose moved into the cytoplasm across the cell membrane is an active process

H+ is pumped out of cell using ATP, returning down a concentration gradient via a co-transport protein

(pg 212)

Question 55

How are companion cells adapted for phloem loading

Answer 55

Many infoldings in their membranes to give an increased surface area for the active transport of sucrose into the cytoplasm

Many mitochondria to supply ATP needed for transport pumps

Question 56

What happens as a build up of sucrose in the companion cell and sieve tube element

Answer 56

Water moves in by osmosis

Build up of turgor pressure due to the rigid cell walls. The water carrying the assimilates moves into the tubes of the sieve elements, reducing pressure in companion cells and moves up or down the plant by mass flow to areas of lower pressure

(pg 213)

Question 57

When is sucrose unloaded from the phloem

Answer 57

Any point into the cells that need it

Question 58

Describe the main mechanism of phloem unloading

Answer 58

Diffusion of sucrose from phloem into surrounding cells

Sucrose rapidly moves onto other cells by diffusion or is converted into another substance eg. glucose for respiration

So diffusion gradient is maintained

Loss of sucrose leads to rise in water potential so water leaves phloem and is drawn into the transpiration stream into the xylem

Question 59

What evidence is there for translocation

Answer 59

Advances in microscopy allow us to see the adaptions of the companion cells for active transport

If mitochondria of companion cells are poisoned, translocation stops

Flow of sugars in phloem is 10,000 times faster than it would be by diffusion alone, suggesting active process is driving mass flow

Aphids show there is a positive pressure in the phloem that forces the sap out through the stylet. the pressure and therefore flow rate in the phloem is therefore lower closer to the sink than it is near the source. The conc of sucrose in the phloem sap is also higher near the source than the sink

Question 60

Explain the difference between a source and a sink

Answer 60

Source: supplier of carbohydrates needed by cells of plant
e.g., leaves, stems, storage organs, seeds

sink: area of plant that needs assimilates in phloem sap e.g., roots, meristems, developing fruit, seeds, and storage organs

Question 61

Describe how Aphids can be used to demonstrate translocation, including the effect of light intensity on the process

Answer 61

Aphids pierce plant tissues and push their stylet directly into phloem vessels

if aphid removed, leaving stylet in place, pressure in phloem vessel continues to force sap out of stylet

this can be to show presence and concentration of assimilates in phloem

pressure in phloem

and how these things change with manipulation of other factors such as light intensity

Question 62

Explain the role of active transport in translocation in the phloem

Answer 62

Sucrose moves from cells to companion cells and sieve tube elements by diffusion along a concentration gradient, is moved into companion cells and sieve tube elements by an active process

hydrogen ions (H+) actively pumped out of companion cell into surrounding tissue using ATP

they return into companion cell down a concentration gradient via a co-transport protein carrying sucrose as well

build-up of sucrose in companion cell and sieve tube element means water moves in by osmosis causing a build-up of hydrostatic pressure

Water carrying assimilates moves into tubes of sieve elements, reducing pressure in companion cells and moving up or down the plant by mass flow

Sucrose moves out of phloem into cells which need it by diffusion, followed by water by osmosis.

Question 63

Describe the evidence that supports the aphid translocation model

Answer 63

Companion cells have membrane folding to give large surface area for transport of sucrose

also have many mitochondria to produce ATP needed for active transport

if mitochondria in companion cells are poisoned, translocation in phloem stops

the pH of the companion cells is higher (more alkaline) than the surrounding cells supporting idea of hydrogen ion pump

the flow of sugars in the phloem is about 10 000 times faster than it would be by diffusion alone, suggesting an active process is driving mass flow

Question 64

What are plants that are adapted to conserve water called

Answer 64

Xerophytes

Question 65

What are some examples of Xerophytes

Answer 65

Cactus
Conifer
Marram grass

Question 66

How are xerophytes adapted to conserve water

Answer 66

Thick waxy cuticle - less transpiration

Sunken Stomata - reduce air movement

Reduced number of stomata - less transpiration

Reduced SA:V leaves - less water loss

Hairy leaves - traps still humid air

Curled leaves - Confines stomata within microenvironment of still humid air

Succulents - Water is stored in the parenchyma tissue

Leaf Loss - minimises water loss

Root adaptations - masses of widespread roots both deep and shallow to access lots of rainwater

Avoiding problems - Becoming dormant until summer

Question 67

What are plants that need to remove water called

Answer 67

Hydrophytes

Question 68

What are some examples of hydrophytes

Answer 68

Water Lily, water cress, duckweeds

Question 69

What are some adaptions of hydrophytes

Answer 69

Thin or no waxy cuticle - maximise evaporation

Always open stomata and on upper surface - maximise transpiration

Reduced structure - water supports plant

Wide, flat leaves - Capture lots of light

Small roots - Water can diffuse directly into stem so no need for uptake by roots

Air sacs - so leaves can float on water

Aerenchyma - forming air sacs, making leaves buoyant and forming low resistance pathways

Question 70

In areas where there is plenty of water, and roots become waterlogged, what do plants do to overcome lack of air

Answer 70

Grow aerial roots called pneumatophores which grow upwards into the air, containing many lenticels which allow entry of air into woody tissue

Question 71

State 3 structural adaptions seen in the leaves of xerophytes and explain how these are related to their function

Answer 71

thick waxy cuticle, sunken stomata, reduced leaves, hairy
leaves, curled leaves, succulent leaves.

NOT LEAF LOSS - not structural, give explanation

Question 72

State 3 structural adaptations of hydrophytes and explain how these are related to their function

Answer 72

Air sacs, small roots, large SA of leaves, no cuticle, always open stomata, reduced structure of plant

Question 73

Compare the challenges of plant of living in dry conditions to plant living in water or waterlogged conditions

Answer 73

Dry conditions:
conflict between need to open stomata for gaseous exchange and loss of water by
evaporation from open stomata. Hot dry air so evaporation will take place fast. Ground dry or frozen
so little water available. Tissues need water to maintain turgor and carry out photosynthesis.

In water:
waterlogging of tissues – no access to oxygen. Sinking – need to be near the surface to get
light for photosynthesis. Slow diffusion of oxygen in from water