Transport in plants

Question 1

Transport systems in plants

Answer 1

-water and mineral ions: xylem- up only
-assimilates e.g. sucrose- phloem- up and down

Question 2

Transport systems needed to:

Answer 2

-Meet metabolic demands:
Only areas with photosynthetic pigments can produce glucose
All cells need supply of O2 and glucose
All cells need to remove waste products of metabolism
Hormones need to be transported to site of use
Mineral ions need to be transported to cells for protein and nucleic acid production

-Cope with SA:V
Whole plants have a small SA:V ratio & too small to meet demand by simple diffusion (move substances up at quick rate)

Question 3

Roles of water within a plant

Answer 3

-Photosynthesis
-Tugor pressure: provide hydrostatic skeleton to support stems & leaves, causes cell expansion- force enables roots to force way through the soil
-Main component of cytosol
-Main component of cell sap
-Enables mineral ions and assimilates to be transported to where they are needed

Question 4

Xylem

Answer 4

-Made of hollow dead xylem vessels supported by living parenchyma cells
- Dissolved mineral ions and water up only
-No end walls, uninterrupted tube
-Walls thickened with lignin- more lignified as they age
-Cells linked by pits- cellulose but no lignin- allows lateral movements of water and mineral ions

Question 5

Phloem

Answer 5

Consist of sieve tubes and companion cells

Question 6

Phloem sieve tube element (PSTE)

Answer 6

-Living, narrow, connected end to end to form tubes
-End walls known as sieve plates- perforated to form sieve pores- allow solutes to mass through unimpeded
-Lack nucleus, RER, central vacuole, Golgi- reduces resistance to flow of phloem sap
-Connected to companion cells by plasmodesmata between sieve plates

Question 7

Companion cell

Answer 7

-Living, carry out all living functions required by CC and PSTE e.g. ATP for active transport, so many mitochondria
-1 CC per PSTE
-Very metabolically active

Question 8

Monocotyledons (eg. cereals and grasses)

Answer 8

-Stem:
Vascular bundles distributed evenly across all stem. Phloem outside, cambium middle, xylem inside
-Roots:
vascular tissue is at the centre arranged as a circle of xylem and phloem. pericycle around it. Endodermis around circle
-Leaves:
Veins run parallel along length of leaves. Xylem small circle, phloem around
1 type of mesophyll.

Question 9

Vascular bundles

Answer 9

Where the xylem and phloem are. On the outside of them is the cortex, and the pith on the inside

Question 10

Dicotyledons (broad-leaved crops e.g. carrots, potatoes)

Answer 10

-Stem:
Vascular bundles are in a ring just of circles below epidermis. Phloem on the outside, xylem on the inside
-Roots:
Vascular bundles occur in central Steele. X is xylem. Between arms is phloem. Pericycle a single layer of cells below endodermis.
-Leaves. Xylem upper, phloem lower. 2 types of mesophyll

Question 11

waxy cuticle function

Answer 11

reduces water vapour loss from the leaf surface

Question 12

Transpiration definition

Answer 12

-the loss of water vapour from the underside of the leaf via the open stomata

(water enters cells through osmosis in the root hair cells)

Question 13

Transpiration stream

Answer 13

-the movement of water from roots to leaves via mass flow and transpiration. Also known as cohesion-tension theory

Question 14

Mechanisms involved in the transpiration stream

Answer 14

-Cohesion: water molecules are attracted to each other via hydrogen bonds
-Adhesion: water molecules are attracted to walls of xylem which aids mass flow of water up xylem
-Tension: the evaporation of h2o from PMC to mesophyll air spaces generates a suction that draws more h2o into leaf

Question 15

1. Transpiration at the leaves

Answer 15

-water evaporates from cellulose cell walls of MC to the mesophyll air spaces
-Water vapour then diffuses from mesophyll air spaces to the sub-stomatal air spaces
-Water vapour then diffuses down the water potential gradient through the open stomata to surrounding external air
-This loss of water vapour is called transpiration

Question 16

2. Movement of water up the xylem

Answer 16

-xylem vessels transport water up the plant stem from roots to leaves in a continuous column.
-water is drawn up the xylem by capillary action (cohesion forces) due to hydrogen bonds between adjacent water molecules (polar+dipoles)
-at leaves, water leaves xylem by the apoplast pathway

-Adhesion of h2o molecules to xylem walls also aids in resisting gravity

Question 17

3. Water moves from root hairs to xylem tissue

Answer 17

-Different pathways exist for the water to move from RHC to xylem tissues
transmembrane route, apoplast route, symplastic route

Question 18

Transmembrane route

Answer 18

via water channels

Question 19

Apoplast route

Answer 19

Via cellulose cell walls
Walls v absorbent so water diffuses along cellulose fibres & osmosis through spaces between fibres
Most water travels this way- little resistance

NB: when water reaches the endodermis, pathway is blocked by Casparian strip (band of cells that have waxy Suberin layer in cell walls) So water has to cross via symplast pathway

Question 20

Symplastic route

Answer 20

Water moves through protoplasm via plasmodesmata

Question 21

Water uptake at RHC

Answer 21

-Water taken up from soil by RHC via osmosis
Adapted for role:
-Extended cytoplasm and cell wall to form protrusions (inc SA)
-Thin cell wall (water by osmosis, mineral ions by AT)
-High mitochondria (for AT)
-Absence of chloroplasts

Question 22

Temperature on transpiration

Answer 22

Higher temp, h2o molecules have more kinetic enegy so greater evaporation from MC to air spaces. Higher diffusion gradient between inside of leaf to external air, so more diffusion from sub-stomatal air to outside

Also, higher temp= higher conc of water vapour can be held in the air, so lower humidity and water potential, which maintains diffusion gradient

Question 23

light intensity on transpiration

Answer 23

higher light intensity, greater number of stomata open to obtain co2 for photosynthesis, so faster evaporation of water vapour through stomata

Question 24

humidity on transpiration

Answer 24

lower humidity, external air is drier, greater conc gradient, increases rate of transpiration

Question 25

wind speed on transpiration

Answer 25

wind moves water molecules away from stomata opening, greater conc gradient, faster rate of evaporation

Question 26

number of leaves on transpiration

Answer 26

larger SA, more stomata, greater transpiration rate

Question 27

cuticle on transpiration

Answer 27

thicker- less transpiration

Question 28

stomata on transpiration

Answer 28

greater density or wider diameter of stomatal pore, more SA, so higher rate

Question 29

Guard cells

Answer 29

stomata open= turgid
stomata closed= flaccid

Question 30

How do stomata cells open and close

Answer 30

Guard cells (GC) control + Plant growth regulator (Absisic acid)

Turgid= stoma open
Flaccid= stoma closed

• Aba > specific Ca+ ions open
• Ca+ flood into guard cell
• ^Ca+ conc in cytosol= activates proton pumps.
• Influx of protons= PMF
• PMF opens K+ channels in membrane > they leave via FD.
• WP inc in cytosol
• H2O leaves GC via O, down WP grad.
• GC flaccid = stoma closes

Question 31

Lenticels in bark of trunks and stems

Answer 31

-Pores that provide direct pathway for gas exchange between interior and external atmosphere
-Necessary as stems become woody and develop bark which is impermeable to gases

Question 32

Rate of transpiration practical:
Procedure
Bubble moving

Answer 32

-Cut shoot underwater- to prevent air lock from forming on xylem
-Cut at 45 angle- increase SA for water uptake
-Connect potometer to shoot underwater
-Ensure apparatus is water tight e.g. seal with vaseline
-Dry leaves
-Record starting position of the bubble and record distance

Question 33

Rate of transpiration practical:
Limitations

Answer 33

Assume all water movement is due to loss of evaporation of water vapour from stomata
however:
-water used in photosynthesis
-water produced in respiration
-water used in condensation reactions
etc

Question 34

Rate of transpiration practical:
Fair test

Answer 34

Ensure all other variables are controlled: humidity, light intensity, temp, wind which are not being investigated

Question 35

Rate of transpiration practical:
Errors

Answer 35

-Error when reading meniscus
-Leaves not being fully dried properly
-Not resetting air bubble to correct position

Question 36

Translocation definition

Answer 36

The movement of dissolved organic solutes through the phloem around the plant from sources to sinks. This is an active process

Question 37

Molecules that are transported by translocation

Answer 37

Sucrose, amino acids

Question 38

Sources

Answer 38

-Site of photosynthesis where sucrose is made (glucose +fructose)
-green leaves, storage organs

Question 39

Sinks

Answer 39

-site of assimilate storage
-Main sinks:
roots that are growing, meristems that are actively dividing, developing seeds or fruits etc

Question 40

Phloem loading
a) Symplast route

Answer 40

-Sucrose moves from source through cytoplasm via plasmodesmata into sieve tubes
-This route is passive
-Lowers WP inside, so water moves in by osmosis
-Generates hydrostatic pressure that moves sucrose through phloem
-mass flow hypothesis

Question 41

Phloem loading
b) uses both passive and active mechanisms

Answer 41

-Protons actively pumped out of companion cells into cell wall
-Using ATP
-Proton gradient builds up
-Protons diffuse back into companion cells via co-protein which also carries sucrose through facilitated diffusion
-Sucorse moved into cc agains its conc gradiet
-Sucrose enter STE by diffusion via plasmodesmata.

Question 42

Phloem unloading

Answer 42

• Sucrose diffuses from phloem into surrounding tissues (sucrose also converted into glucose and starch)
  -Loss of solute raises WP in STE
  -So water exits by osmosis and some is drawn into xylem and enters transpiration stream
  -so WPG between STE and xylem vessels recirculates solution

Source loading increases hydrostatic pressure, sink unloading reduces hydrostatic pressure

Question 43

photosynthesis equation

Answer 43

water + co2-> glucose and O2

Question 44

respiration equation

Answer 44

glucose + oxygen -> carbon dioxide + water + ATP