3.9 - Transport in Plants

Question 1

Why do plants need transport systems?

Answer 1

• SA:V Ratio is often small
• To distribute materials
• Large distances needed to transport (diffusion is not enough)

Question 2

What are dicotyledonous plants?

Answer 2

A plant with 2 food stores in the seed. They have transport systems arranged as vascular bundles.

Question 3

Outline the structure of the root

Answer 3

• Vascular tissue situated centrally
• Roots are subjected to pulling forces
• Vascular tissue position gives strength
• Pericycle and endodermis surround vascular tissue

Question 4

What is the pericycle?

Answer 4

Point of lateral growth in roots/branches.

Question 5

What is the function of the endodermis?

Answer 5

Regulates movement of water and mineral ions into vascular tissue.

Question 6

Outline the structure of the stem

Answer 6

• Vascular bundle arranged towards outside of root
• VBs and collenchyma and sclerenchyma provide support.
• Ring of support is discontinuous allowing flexibility
• Cambium separates phloem and xylem.

Question 7

What are parenchyma, collenchyma and sclerenchyma?

Answer 7

Cells with thickened cell wall used for support.

Question 8

Outline the structure of the leaf

Answer 8

• Network of tiny bundles throughout the leaf
• Tiny bundles fuse into side veins which run parallel to each other
• Side veins merge into the main vein
• Main vein runs along the centre of the leaf, getting wider towards the leaf stalk.

Question 9

Outline the structure of xylem

Answer 9

• Parenchyma and sclerenchyma cells in xylem walls
• Xylem fibres
• Vessels
• Tracheids

Question 10

How is xylem’s structure related to its function?

Answer 10

• Cells are long and thin, arranged end-to-end
• Cell contents die when they are mature
• Cell walls are thickened with lignin
• Annular, reticular and spiral thickening
• Pits throughout cells
• Narrow lumen.

Question 11

Name the three pathways for movement of water in plants

Answer 11

• Apoplast (dead cell walls)
• Symplast (cytoplasm)
• Vacuolar (vacuole)

Question 12

How does water move via the apoplast pathway?

Answer 12

Water moves through cell walls and intracellular spaces which offer low resistance to the flow of water.

As water molecules move through the network of cellulose fibres in the cell walls, water molecules are pulled through as a result of cohesion.`

Question 13

How does water move via the symplast pathway?

Answer 13

water moves through the cytoplasm of the root cells which are connected by plasmodesmata.

Question 14

What is the casparian strip?

Answer 14

A band of waxy, waterproof material called suberin found in the walls of the endodermis.

Question 15

What is the function of the casparian strip?

Answer 15

Casparian strip forces water travelling via the apoplast pathway into the cytoplasm of the cells (symplast pathway).

Question 16

What is the function of endodermal cells?

Answer 16

Endodermal cells actively transport mineral ions into the xylem vessels lowering their water potential, so that water enters via osmosis.

This generates root pressure which pushes water a short way up the xylem.

Question 17

Outline the evidence for root pressure

Answer 17

• Pressure in xylem increases with temp. increase and decreases with temp. decrease. -> suggests that an active process is involved.
• Metabolic inhibitors cause root pressure to cease -> “”
• Decreasing O₂ and glucose levels causes a decrease in root pressure -> Active transport requires ATP, made in respiration ∴ decreasing O₂ slows rate of respiration.

Question 18

Outline cohesion-tension theory

Answer 18

• Cohesion hold water molecules in a long chain
• Loss of water vapour from top of plant lowers pressure at top of xylem pulling more water up.
• This creates tension in the column of water

Question 19

What evidence is there supporting cohesion-tension theory?

Answer 19

Tree trunk diameter decreases as rate of transpiration increases

• Evaporation from the leaves draws water from xylem via osmosis water is pulled up - creating a tension.
• Tension pulls xylem vessel walls in creating a negative pressure.

Question 20

What is capillary action?

Answer 20

Adhesion attracts water molecules to sides of xylem vessel - pulls water up the sides.

Question 21

Define transpiration

Answer 21

Loss of water vapour from aerial parts of the plant via the stomata.

Question 22

How do stomata open and close?

Answer 22

Guard cells have irregular thickness in cell walls.
When guard cells are flaccid, stomata are closed.
When guard cells are turgid, stomata are open.

Question 23

Outline the process of transpiration

Answer 23

1. Water diffuses from xylem to mesophyll cells via osmosis
2. Evaporation of water from the surface of mesophyll cells into intracellular air spaces
3. diffusion of water vapour from intracellular air spaces out through the stomata.

Question 24

List the factors affecting transpiration

Answer 24

• Number of leaves
• Number, size, positioning of stomata
• Presence of a cuticle
• Temperature
• Relative humidity
• Air movement or wind
• Light intensity
• Water availability

Question 25

What is a xerophyte?

Answer 25

Plants which have adaptations for living and reproducing in dry environments which enable them to conserve water or to acquire as much as possible.

Question 26

List the adaptations of xerophytes

Answer 26

- Thick waxy cuticle - Sunken stomata - Reduced stomatal density - Reduced leaves - Hairy leaves - Curled leaves - Water storage tissues - Loss of leaves - Root adaptations (long tap roots/shallow network) - Folded stems (like in cacti) - Avoidance (become dormant/die completely leaving seeds behind to germinate upon rainfall)

Question 27

What is a hydrophyte?

Answer 27

Plants which live in water (submerged or on the surface or at the edges of bodies of water) - need special adaptations to cope with growing in water or permanently saturated in oil.

Question 28

List the adaptations of a hydrophyte

Answer 28

- Very thin/no waxy cuticle - Many always-open stomata on upper surfaces - Reduced plant structure - Wide, flat leaves - Small roots - Large SA of stems and roots underwater - Air sacs - Aerenchyma - speacialised parenchyma tissue formed in the leaves, stems and roots of hydrophytes. It makes the leaves and stems more buoyant.

Question 29

Define translocation

Answer 29

The movement of sucrose and other substances up and down the plant (from source to sink).

Question 30

What are assimilates?

Answer 30

Organic compounds made when Carbon from CO₂ is incorporated into new substances as a result of photosynthesis.

Question 31

Define sink

Answer 31

- Part of the plant where sucrose is removed from phloem - Any part of the plant which is growing - Where sugar is used for respiration or stored as starch.

Question 32

Define source

Answer 32

- Any part of the plant where sucrose is released into phloem - Where sugar is produced by photosynthesis or by breakdown of storage products.

Question 33

What is the phloem?

Answer 33

Plant tissue used to transport dissolved sugars and other assimilates.

Question 34

How are the sieve tube elements related to their function?

Answer 34

- Elongated, little cytoplasm, no nucleus to save space - Joined end-to-end forming a column - End walls have many pores to allow sap to flow (sieve plates).

Question 35

How are companion cells related to their function?

Answer 35

- Small with nucleus and other elements - Many mitochondria to provide ATP for active processes e/g/ sucrose loading - Cytoplasm linked to sieve tubes by plasmodesmata

Question 36

Define mass flow

Answer 36

The bulk movement of substances from one area to another due to differences in pressure.

Question 37

What is phloem loading?

Answer 37

The transfer of sucrose into sieve elements from photosynthesising tissue.

Question 38

Outline the process of phloem loading

Answer 38

1. Active transport of H⁺ ions out of companion cell (requires ATP) 2. Creates H⁺ ion concentration gradient 3. Facilitated diffusion of H⁺ back into companion cell 4. Sucrose (assimilates) move back into the companion cell with H⁺ through cotransport proteins 5. Sucrose diffuses through plasmodesmata from companion cell into sieve tube (symplast route).

Question 39

Outline the process of pressure flow theory

Answer 39

1. Loading sugar into phloem from source increases solute conc. and decreases Ψ inside sieve tube cells. This causes them to take up water from surrounding tissues 2. Water reabsorption creates a hydrostatic pressure which forces the sap to move along the tube. 3. Gradient of pressure in the sieve-tube reinforces by the active unloading of sugar and consequent loss of water by osmosis at sieve. 4. Xylem reabsorbs water from sink to source.

Question 40

Outline the evidence supporting translocation of organic molecules occurring in the phloem

Answer 40

- Pressure in the sieve tubes (sap released when cut) - Concentration of sucrose is higher in leaves (source) than in roots (sink) - Downward flow in phloem occurs in daylight, ceases at night or when leaves are shaded - Increases in sucrose levels in leaf are followed by similar increases in phloem a little later - Metabolic poisons / lack of Oxygen inhibit translocation - Companion cells contain many mitochondria and readily produce ATP.

Question 41

Outline the evidence questioning pressure flow theory

Answer 41

- Function of sieve pates is unclear - they seem to hinder mass flow - Not all solutes move at the same speed - they should do so if movement is by pressure flow - Sucrose is delivered at more or less the same ratio to all regions rather than going more quickly to the ones with the lowest sucrose levels.

Question 42

What is the puncture experiment?

Answer 42

When phloem is punctured with a hollow tube, sap oozes out showing there is high pressure (compression) inside the phloem. If the xylem is punctured then the air is sucked in, showing that there is low pressure (tension) inside the xylem.

Question 43

What evidence is there supporting translocation being an active process?

Answer 43

- Metabolic poisons including the formation of ATP stop translocation - Sugars move up to 10,000x faster by mass flow than they could by diffusion - pH of companion cells is higher than that of surrounding cells (due to H⁺ ion pump).