3.1.3 Transport In Plants

Question 1

Why do some plants need a transport system?

Answer 1

Small SA:V ratio
Long diffusion distance
Need a system to transport water, sucrose and mineral ions to all parts of the plant
Although low demand for oxygen/low metabolic rate so sometimes this can be met by diffusion alone

Question 2

What are features of dicotyledonous plants?

Answer 2

Two seed leaves
Vascular tissue distributed throughout, xylem and phloem in vascular bundles, also with collenchyma and sclerenchyma -> strength and support

Question 3

How is xylem and phloem organised in young roots? (What is endodermis, medulla, pericycle, cortex?)

Answer 3

• bundle at center - central core of xylem, often in X shape
• phloem found between arms of X-shaped xylem
• this arrangement provides strength to withstand pulling forces
• around the vascular bundles medulla, and then endodermis
• just inside endodermis is a layer of meristem cells called pericycle
• cortex outside endodermis

Question 4

How is xylem and phloem organised in the stem? (What’s cambium, sclerenchyma, collenchyma?)

Answer 4

Near outer edge

Xylem towards inside and phloem towards outside -> in between is cambium.
Just outside of phloem is sclerenchyma, then cortex, then collenchyma

Question 5

How is xylem and phloem organised in the leaf?

Answer 5

Form midrib and veins
Dicotyledonous leaf has branching network of veins that get smaller as they spread away from midrib
Each vein has xylem located on top of phloem

Question 6

How do you do plant dissections?

Answer 6

Staining
Cut a thin transverse/cross section or longitudinal section
View at low power first, then increase magnification

Question 7

How does xylem develop and what other adaptations does it have?

Answer 7

• lignin impregnates walls
  End walls decay, leaving a long continuous vessel
• patterns of lignin allow flexibility and give support
  Bordered pits allow lateral water movement
• narrow tubes to ensure effective capillary action
• lignin prevents collapse
  No cell contents
  No cross walls

Question 8

What are the adaptations of sieve tube elements?

Answer 8

• elongated
  Little cytoplasm, no nucleus to leave space for sap
• perforated cross walls: sieve plates allow movement

Question 9

What are adaptations of companion cells?

Answer 9

• large nucleus, numerous mitochondria, dense cytoplasm
• carry out metabolic processes needed to actively load assimilates

Question 10

What are the junctions between 2 plant cells?

Answer 10

Plasmodesmata

Question 11

What 3 ways does water move?

Answer 11

APOPLAST: mass flow, passes through spaces in cell walls
SYMPLAST: enters cell cytoplasm through plasma membrane
VACUOLAR: able to pass through vacuoles

Question 12

Define transpiration - why is it important?

Answer 12

Evaporation of water from the leaves via the movement of water from roots to leaves

Transports mineral ions, maintains cell turgidity, supplies water for growth, water evaporates to keep it cool

Question 13

How does light intensity affect transpiration?

Answer 13

Increase = more stomata open = inc rate

Question 14

How does temperature affect transpiration?

Answer 14

Increased evaporation from cell surfaces, maintains water potential gradient so inc rate
Increases rate of diffusion through stomata

Question 15

How does relative humidity affect transpiration?

Answer 15

High = Decreases water loss due to decreases water potential gradient

Question 16

How does air movement affect rate of transpiration?

Answer 16

High = maintains high water potential gradient - inc rate

Question 17

How does water availability affect rate of transpiration?

Answer 17

Little water in soil = plant cannot replace water lost = stomata closes and leaves wilt

Question 18

Describe what happens in the roots during transpiration e.g. role of casparian strip and root pressure

Answer 18

1. Mineral ions actively transported into roots
2. Water follows, through the root cortex, by apoplast pathway
3. Mineral ions actively transported via transporter proteins into medulla from surrounding cells
4. Casparian strip blocks apoplast pathway so resorts to symplast
5. Root pressure is where water moves into the medulla and pressure builds up, forcing water up xylem

Question 19

Describe the role of adhesion, and cohesion-tension

Answer 19

Adhesion between water molecules and cellulose walls allows water to easily move up xylem by capillary action
Cohesion-tension theory due to H bonding - continuous stream of water pulled up by mass flow
Pull puts xylem under tension - narrows xylem walls and allows adhesion

Question 20

Describe what happens in the leaf during transpiration

Answer 20

1. Osmosis from xylem into spongy mesophyll
2. Evaporates from walls
3. Water vapour diffuses out via open stomata from a higher water vapour potential to a lower one
4. Osmosis from surrounding cells, or via apoplast, replaces water lost

Question 21

List adaptations of terrestrial plants (4)

Answer 21

Waxy cuticle to dec evaporation
Stomata closed at night
Stomata on undersurface
Deciduous plants lose leaves in winter

Question 22

List adaptations of marram grass (5)

Answer 22

XEROPHYTE

Leaf rolled so air is trapped, dec WV potential grad
Thick waxy cuticle
Stomata on inner side of rolled leaf so protected by enclosed air space
Stomata in hairy pits to dec water vapour loss
Dense spongy mesophyll so less SA for evaporation

Question 23

List adaptations for cacti (3)

Answer 23

Succulents store water in stems
Spines dec SA = dec water loss
Roots very widespread

Question 24

List other xerophytic adaptations (3)

Answer 24

Closing stomata when water availability is low
High ion conc in leaf cells = dec water potential grad = dec evaporation
Long roots

Question 25

List adaptations of hydrophytes

Answer 25

Stomata on upper epidermis to be exposed to air
Leaf stem has many air spaces for buoyancy and so oxygen can diffuse to roots
No waxy cuticle - this is not needed and is a waste of energy
Tips of leaves have hydathodes to release water droplets

Question 26

Describe the process of translocation

Answer 26

Glucose -> sucrose
Active loading
Hydrogen ions actively transported out of companion cells via hydrogen pumps, into source cells
Hydrogen ions and sucrose transported together via a cotransporter protein
Sucrose then diffuses through plasmodesmata into sieve tube element from a higher conc to lower conc
Apoplast = active, symplast = passive
Increases hydrostatic pressure in phloem = dec WP = water moves in from xylem via OSMOSIS. Inc hydrostatic pressure
Sap moves due to mass flow (from higher hydrostatic pressure to lower) - BIDIRECTIONAL
At sink cells, hydrostatic pressure is lower since sucrose is removed via diffusion into companion cells, (water moved OUT into xylem via OSMOSIS)
Sucrose moves by diffusion or active transport via plasmodesmata
Sucrose -> glucose or starch

Question 27

Meristem cells in plants are used to generate new plant tissues. When …………………………………… tissue is formed, …………………………………… impregnates the cell walls, making them impermeable to water. All cytoplasm is lost. When …………………………………… tissue is formed, cytoplasm remains, but the …………………………………… become elongated and lose most of their cytoplasm.

What is the correct order of missing terms?
A sclerenchyma, phloem, lignin, xylem vessels
B xylem, lignin, parenchyma, phloem vessels
C phloem, collenchyma, xylem, sieve tube elements
D xylem, lignin, phloem, sieve tube elements.

Answer 27

D

Question 28

State and explain three adaptations of plants like T.natans (hydrophyte) which allow them to survive in water. (3)

Answer 28

• air spaces in leaves/stems, allows them to float so they can access light for photosynthesis (1)
• stomata on upper leaf surface to allow access to air/ absorption of carbon dioxide (1)
• stem has air spaces so oxygen can diffuse to roots for aerobic respiration (1)
• roots may be reduced, idea of wasting resources (1)
• reduced vascular tissue, idea of not wasting resources (1)