Plant transport ( Component 3 )

Question 1

Define vascular bundle

Answer 1

Vascular system in herbaceous dicotyledonous plants
Consists of two transport vessels, the xylem and the phloem

Question 2

Describe the structure and function of the vascular system in the roots of dicotyledons

Answer 2

Xylem arranged as X shape to provide resistance against force
Phloem found as patches between the arms.
Surrounded by endodermis, aiding water passage

Question 3

Describe the structure and function of the vascular system in the stem of dicotyledons

Answer 3

Vascular bundles organised around a central pith
Xylem on inside of bundle to provide support and flexibility
Phloem on outside of bundle
Cambium is found between the two

Question 4

Which structure in plants is adapted for the uptake of water and minerals and what adaptations are shown ?

Answer 4

Root hair cells :
large SA
freely permeable

Question 5

How is water taken up from the soil ?

Answer 5

Root hair cells absorb minerals by active transport, reducing the water potential of the root
Water potential of root hair cells is lower than that of the soil
Water moves into root by osmosis

Question 6

Outline how plant roots are adapted for the absorption of water and minerals

Answer 6

Plants are made up of millions of root hair cells which have :
long hairs that extend from the cell body, increasing SA for absorption
many mitochondria which produce energy for the active transport of mineral ions

Question 7

State the 3 pathways by which water moves through the root

Answer 7

Apoplast pathway
Symplast pathway
Vacuolar pathway

Question 8

Describe the apoplast pathway

Answer 8

Water moves through intercellular spaces between cellulose molecules in the cell wall. It diffuses down its water potential gradient by osmosis.

Question 9

Apoplast route spec summary

Answer 9

Soil solution soaks into the walls of epidermal cells and travels across the cortex through the cell walls or through the spaces between cells, drawn by the transpiration stream

Question 10

How can water also cross the plasma membrane ?

Answer 10

Osmosis

Question 11

Describe the symplast pathway

Answer 11

Water enters the cytoplasm through the plasma membrane and moves between adjacent cells via plasmodesmata. Water diffuses down its water potential gradient by osmosis

Question 12

Symplast route spec summary

Answer 12

Water then moves through the cytoplasm of cells via the plasmodesmata;

Question 13

Describe the vacuolar pathway

Answer 13

Water enters the cytoplasm through the plasma membrane and moves between vacuoles of adjacent cells. Water diffuses down its water potential gradient by osmosis

Question 14

What is the endodermis ?

Answer 14

a layer of cells which surround the pericycle within which lies the vascular tissue (stele).

Question 15

Describe the structure and function of the endodermis

Answer 15

Innermost layer of the cortex of a dicot root
Impregnated with suberin which forms the Casparian Strip
Endodermal cells actively transport mineral ions into the xylem

Question 16

Describe the endodermis apoplast route

Answer 16

blocked by the Casparian band located tangentially in the cell wall and made of water-proof suberin.

Question 17

What is the function of the Casparian strip ?

Answer 17

Blocks the apoplast pathway, forcing water through the symplast route
Enables control of the movement of water and minerals across the root and into the xylem

Question 18

What happens at the Casparian band ?

Answer 18

water passes across the plasma membrane and continues along the symplast route.

Question 19

What molecules makes the Casparian strip waterproof ?

Answer 19

Suberin

Question 20

What happens due to the xylem lacking cell contents ?

Answer 20

the water is transferred to the apoplast in the pericycle.

Question 21

What is the role of Nitrogen in the symplast and apoplast routes ?

Answer 21

Nitrogen usually enters the plant as nitrate ions/ammonium ions which diffuse along the concentration gradient into the apoplast stream but enter symplast by active transport against the concentration gradient and then flow via plasmodesmata in the cytoplasmic stream

Question 22

What happens to ions at the endodermis and therefore why is water drawn through the endodermis ?

Answer 22

ions must be actively taken up to by-pass the Casparian band which allows the plant to selectively take up the ions at this point. This lowers the water potential in the xylem, causing water to be drawn through the endodermis.

Question 23

When water is drawn into the endodermis, what is produced ?

Answer 23

This produces a positive hydrostatic pressure inside the xylem, forcing water upwards. This positive pressure is known as root pressure.

Question 24

Draw the structure of the stem of a dicotyledon to illustrate the position of transporting tissue.

Answer 24

…

Question 25

What does the xylem consist of ?

Answer 25

Xylem consists of dead, lignified tracheids and vessels with pits, supporting fibres and living parenchyma.

Question 26

Relate the structure of the xylem to its function

Answer 26

Long continuous columns made of dead tissue, allowing the transportation of water Contain bordered pits, allowing the sideways movement of water between vessels Walls impregnated with lignin, providing structural support

Question 27

What is the role of tracheids and vessels ?

Answer 27

Tracheids and vessels form a continuous system of channels for water transport.

Question 28

What is the route of water through a plant ?

Answer 28

Water passes through the root to the xylem, up through the stem to the leaves where most evaporates

Question 29

Define transpiration

Answer 29

The loss of water vapour from the parts of a plant exposed to air due to evaporation and diffusion Consequence of gaseous exchange, occurs when the plant open stomata to exchange O2 and CO2

Question 30

What is the transpiration stream ?

Answer 30

The flow of water from the roots to the leaves in the plants, where it is lost by evaporation to the environment

Question 31

How does water move up the stem ?

Answer 31

Root pressure Cohesion tension theory Capillarity

Question 32

What is root pressure ?

Answer 32

The force that drives the water into and up the xylem by osmosis due to the active transport of minerals into the xylem by endodermal cells

Question 33

Describe the different forces acting between the water molecules and the xylem

Answer 33

The columns of water in the xylem are held up by the cohesive force between water molecules and the adhesive forces between the water molecules and the hydrophilic lining of the xylem vessels.

Question 34

Explain the cohesion tension theory

Answer 34

Water molecules form hydrogen bonds with each other, causing them to "stick" together Surface tension of water also creates this sticking effect Therefore as water is lost through transpiration, more is drawn up the stem from the roots

Question 35

Define capillarity

Answer 35

The tendency of water to move up the xylem, against gravity, due to adhesive forces that prevent the water column from dropping back

Question 36

Define transpiration ( spec )

Answer 36

Transpiration is the loss of water from the leaves which gives rise to the transpiration stream.

Question 37

What causes a pull on the xylem column ?

Answer 37

The continued removal of water molecules from the top of the xylem vessels results in a tension causing a pull on the xylem column

Question 38

State the factors that affect the rate of transpiration

Answer 38

Light Temperature Humidity Air movement

Question 39

How does temperature affect the rate of transpiration ?

Answer 39

A rise in temperature provides additional kinetic energy for the movement of water molecules. This additional energy accelerates the rate of evaporation of water from the walls of the mesophyll cells. A decrease in relative humidity of the air outside the leaf. A combination of these two results in a steeper concentration gradient of water molecules from leaf atmosphere to air outside.

Question 40

How does light affect the rate of transpiration ?

Answer 40

Higher light intensity increases rate of photosynthesis so more stomata open for gas exchange. Stomata usually open in the light and close in the dark. Therefore an increase in light intensity tends to increase transpiration rates

Question 41

How does humidity affect the rate of transpiration ?

Answer 41

High humidity Water content of air next to leaf is high Decreased concentration gradient Reduces rate of transpiration This makes the diffusion gradient of water vapour from the leaf atmosphere to the atmosphere less steep.

Question 42

How does air movement affect the rate of transpiration ?

Answer 42

Large amounts of air movement blow moist air away from the leaves, creating a steep conc grad. Increases rate of transpiration. In still air a layer of air saturated with water vapour builds up around the leaf, reducing the steepness of the diffusion gradient between the leaf atmosphere and the external air. Windy conditions blow this layer away and therefore increase the rate of transpiration.

Question 43

What alters water loss through transpiration ?

Answer 43

The opening and closing of stomatal pores can alter water loss through transpiration.

Question 44

What are plants classified upon the basis of ?

Answer 44

Structure in relation to the prevailing water supply

Question 45

What are the three classifications of plants ?

Answer 45

Hydrophytes, mesophytes, xerophytes

Question 46

What is a hydrophyte ? ( give an eg )

Answer 46

A plant that is adapted to live and reproduce in very wet habitats eg water lillies

Question 47

Describe the roots and leaves in hydrophytes ( and give an example of one )

Answer 47

eg water lily Live with their roots submerged in the mud at the bottom of the pond Have floating leaves on the surface

Question 48

In hydrophytes, describe : the need for support or transport tissues cuticle and stomata air spaces

Answer 48

Have little need for support or transport tissues Little or no cuticle only on the upper surface of leaves Stomata absent or reduced on submerging leaves Large air spaces present in both stem and leaf tissue

Question 49

How are hydrophytes adapted to living in wet conditions ?

Answer 49

Thin or absent cuticle Stomata often open Wide flat leaves Many air spaces for buoyancy

Question 50

What is a xerophyte ? ( give an eg )

Answer 50

A plant that is adapted to live and reproduce in dry habitats where water availability is low eg marram grass or cacti

Question 51

What conditions have xerophytes adapted to ?

Answer 51

Living under conditions of low water availability

Question 52

What do xerophytes have to prevent excessive water loss ?

Answer 52

Modified structures eg : Succculent stems for water storage Leaves reduced to spines Close stomata during daylight Needle like leaves to reduce SA over which water can be lost Can survive on hot dry deserts or cold regions where soil is frozen for much of the year

Question 53

What type of plant does marram grass demonstrate ?

Answer 53

Role of a xerophyte

Question 54

How does marram grass demonstrate the role of a xerophyte and why ?

Answer 54

With its leaf shape, sunken stomata - humid air trapped outside of stomata so reduces water potential grad between leaf and atm so less transpiration thick cuticle - lowers rate of cuticular transpiration hairs - trap water vapour and reduce water potential grad rolled leaves - large thin walled epidermal cells shrink when they lose water by transpiration causing leaf to roll in on itself - reduces leaf area over which transpiration can occur to reduce water loss

Question 55

How are xerophytes adapted to living in dry conditions ?

Answer 55

Small / rolled leaves Densely packed mesophyll Thick waxy cuticle Stomata often closed Hairs to trap moist air

Question 56

What are mesophytes ?

Answer 56

Terrestrial plants adapted to live in environments with average conditions and an adequate water supply They have features that enable them to survive at unfavourable times of the year. They are plants of temperate regions that flourish in habitats with adequate blood supply

Question 57

What conditions do mesophytes need to survive and how do they do this ?

Answer 57

They need to survive unfavourable times of the year They do this by shedding their leaves, surviving underground or as dormant seeds.

Question 58

What does the phloem consist of ?

Answer 58

sieve tubes and companion cells linked by plasmodesmata with fibres and parenchyma

Question 59

Relate the structure of the phloem to its function

Answer 59

Sieve tube elements transport sugars around the plant Companion cells designed for active transport of sugars into tubes Plasmodesmata allow communication and the exchange of substances between sieve tubes and companion cells

Question 60

How are the products of photosynthesis transported ?

Answer 60

in soluble form (sucrose) to all parts of the plant in the phloem.

Question 61

What are cytoplasmic strands ?

Answer 61

Small extensions of the cytoplasm between adjacent sieve tube elements and companion cells

Question 62

Describe the function of cytoplasmic strands

Answer 62

Allow communication and the exchange of materials between sieve tube elements and companion cells Hold the nucleus in place

Question 63

What are the leaves a source of ?

Answer 63

Sugars

Question 64

What do the growing tissues act as ?

Answer 64

A sink

Question 65

Define translocation

Answer 65

The movement of organic compounds in the phloem, from sources to sinks

Question 66

Describe the 7 main steps in translocation

Answer 66

1) H+ ions actively transported out of companion cells 2) H+ ions return to companion cell with sucrose down the diffusion gradient - occurs through co transport proteins 3) Sucrose diffuses into sieve tube elements through plasmodesmata 4) Water potential inside sieve tube decreases, water moves into sieve tube by osmosis 5) Hydrostatic pressure in the sieve tube at the source increases. Sugary fluid moves down sieve tube from higher hydrostatic pressure to lower hydrostatic pressure - sugary fluid moves from sink to source 6) Sucrose moleules move from sieve tube into surrouding cells by facilitated diffusion / active transport. Sucrose enters root cell ( sink ) to be used in respiration or to be converted to starch for storage. 7) Water moves out of the sieve tube by osmosis, hydrostatic pressure at sink drops

Question 67

Summarise the mass flow hypothesis of translocation

Answer 67

Sugar loaded into sieve tubes via active transport Lowers water potential, causing water to move in from the xylem Hydrostatic pressure causes sugars to move towards sink

Question 68

Give evidence for mass flow hypothesis

Answer 68

Sap released when stem is cut therefore must be pressure in phloem Sap exuding from the stylet ( mouthpart ) of an aphid inserted into sieve tubes provides evidence that sugars are carried in the phloem There is a higher sucrose concentration in leaves than roots Autoradiographs produced using carbon dioxide labelled with radioactive carbon provide evidence for translocation in the phloem

Question 69

What is autoradiography ?

Answer 69

A technique used to record the distribution of radioactive material within a specimen

Question 70

What is a potometer ?

Answer 70

An apparatus used to measure water uptake from a cut shoot

Question 71

How was early evidence about translocation of solutes obtained and explain the experiment ?

Answer 71

Early evidence about translocation of solutes was obtained from ringing experiments. 1) In a ringing experiment, a ring of bark was scraped away, removing the phloem 2) After some time sugar tries to move down stem but is stopped by ring 3) Bulge of sugar forms above ring Suggests sugar moves down stem in phloem

Question 72

What experiment shows translocation is a rapid process ?

Answer 72

The technique of radioactive tracing combined with using aphid mouthparts

Question 73

Describe the technique of radioactive tracing combined with using aphid mouthparts

Answer 73

1) Aphids have specialised mouthparts called stylets used to penetrate phloem tubes 2) If aphids are anaethetised with carbon dioxide and the stylet remains cut off so it remains in the phloem, pure phloem sap can be collected through stylet for analysis 3) Fluid collected contains many sugars

Question 74

What experiment shows sucrose travels bidirectionally to sinks

Answer 74

Radioisotope labelling using carbon dioxide combined with autoradiography

Question 75

Describe radioisotope labelling in aphids

Answer 75

1) Radioactive labelled CO2 placed into a bad surrounding an illuminated individual leaf 2) CO2 is incorperated into sugars and transported in phloem 3) Aphids feeding on sugar in phloem can be used to trace movement of sugar in plant from source to sink

Question 76

Describe radioisotope labelling in autoradiography

Answer 76

1) Radioactive labelled CO2 placed into a bad surrounding an illuminated individual leaf 2) The source and sink leaves are placed on photographic film in the dark for 24 hours. When the film is developed, the presence of radioactivity shows up as fogging of the negatives Therefore sugar transport both up and down the stem

Question 77

What does the mass flow hypothesis suggest ?

Answer 77

That there is a passive flow of sucrose from source to sink

Question 78

Does the mass flow hypothesis account for all observations ?

Answer 78

No

Question 79

Give a observation that the mass flow hypothesis does not account for ?

Answer 79

movement in opposite directions at the same time and at different rates.

Question 80

What other hypothesis apart from mass flow have been proposed ?

Answer 80

diffusion and cytoplasmic streaming