MTO 3.3 TRANSPORT IN PLANTS

Question 1

What’s a Dicotyledonous Plant?

Answer 1

• make seed that contain two cotyledons (organs).
  -act as food stores for developing embryo plant and form initial leaves in germination.

Question 2

Vascular Bundle [Roots]

Answer 2

• Xylem and Phloem found together.
• found in centre of root.
• ring of endodermis around vascular bundle.
• inside endodermis are ring of meristem cells called pericycle.

Question 3

Vascular Bundles [Stem]

Answer 3

• found near outer edge of stem.
• xylem near inside of bundle.
• phloem near outside.
• in between xylem and phloem -> Cambium. Layer of meristematic cells.

Question 4

Vascular Bundle [Leaves]

Answer 4

• form veins of leaves.
• xylem are above phloem.

Question 5

Xylem Tissue

Answer 5

• transports water and dissolved minerals up the plant. (Transpiration)
• consists of xylem vessels, fibres and parenchyma cells.

Question 6

Structure of Xylem Vessels

Answer 6

Continuous, hollow tubes with no end walls —> water movement much faster. More space for water to flow due to lack of content.

Walls reinforced with Lignin —> strengthens wall (prevents collapse under tension). Water-proof walls reduce lateral flow of water. Increases capillarity.

Lignification in Spiral Pattern —> allows flexibility and stretching of stem.

Bordered pits (pores) in walls of vessels —> allow lateral flow of water between vessels to get around blockage (e.g. air bubbles).

Narrow Lumen —> more effective capillary action.

Question 7

Phloem

Answer 7

• transports sucrose up and down the plant.
• made of sieve tube elements and companion cells.
• sucrose transported as sap (dissolved in water).

Question 8

Structure and Function of Sieve Tube Elements

Answer 8

Little Cytoplasm + most organelle absent —> less resistant for transport and more space for transport.

Sieve Plates —> connect sieve tube elements to allow sucrose through.

Joined end to end to form tube —> allows continuous transport.

Bidirectional Flow —> allows sucrose to go both up and down plant.

Living —> allows active transport.

Question 9

Structure and Function of Companion Cells

Answer 9

Many Mitochondria —> lots of respiration. Provides large amounts of ATP for active processes.

Nucleus —> controls functions of both companion cell and sieve tube elements.

Plasmodesmata —> allows continuation of cytoplasm between companion cell and sieve tube element - transport of molecules such as protein and ATP.

Question 10

Osmosis

Answer 10

Passive Movement of water molecules from region of higher water potential to region of lower water potential across a partially permeable membrane.

Question 11

Turgid

Answer 11

Plant cells swell as lots of water enters it through osmosis.

Question 12

Plasmolysed

Answer 12

Cell membrane pulls away from cell wall.

Question 13

Root Hair Cells Adaptations to their Functions

Answer 13

• found on epidermal layer of plant roots.
• hair-like projection in soil —> large SA for osmosis and mineral uptake.
• thin wall —> short diffusion pathway.
• many mitochondria —> energy for active transport of minerals.

Question 14

Pathways for Osmosis [Apoplast]

Answer 14

Apoplast Pathway (Cell Wall)
-> water travels through the cell walls in gaps.
-> doesn’t cross membranes and so can’t enter the cytoplasm.

Question 15

Pathways for Osmosis [Symplast]

Answer 15

-> water crosses the cell surface membranes via osmosis (through aquaporins) and can then move through plasmodesmata.

Question 16

Pathways for Osmosis [Vacuolar Pathway]

Answer 16

-> similar to symplast but water also moves through through the vacuoles not just the cytoplasm.

Question 17

The Casparian Strip

Answer 17

1. Water pot lowest in xylem than anywhere else in root. Causes osmosis of water from root hair cell from cortex to endodermis.
2. Lowers water pot in xylem. Water now crosses the cell surface membranes from the cortex to the endodermis.
3. On cell walls of the cells of the endodermis strip of waterproof material called Suberin known as Capsparian Strip.
4. Casparian Strip blocks Apoplast pathway between the cortex and xylem - water now must take the symplast pathway.
5. Minerals must be actively transported from the cytoplasm into the xylem - water must take the symplast pathway.

Question 18

Transpiration

Answer 18

Loss of water by evaporation out of plants leaves via stomata. Happens at same time as gaseous exchange through the stomata - as oxygen leaves the leaf, so does water.

Question 19

3 Ways Water is helped to move up the xylem vessels from the root:

Answer 19

1. Root Pressure (push of water entering xylem vessel in roots. Doesn’t move water far.)
2. Capillary Action (adhesion (forces of attraction) of water molecules to lignin in narrow xylem vessels can pull the water up the sides of the vessel.)
3. Transpiration Pull (most of the driving force.)

Question 20

Why water molecules are cohesive and adhesive

Answer 20

• Adhesive because they form hydrogen bonds with lignin.
• Cohesive because they form hydrogen bonds with each other.

Question 21

Factors affecting Transpiration

Answer 21

Number of Leaves -> more leaves = more water loss. Larger Surface Area for Evaporation.

Number and Size of Stomata -> more stomata = more water loss.

Waxy Cuticle Present -> less water loss. Hydrophobic.

Light -> more water loss. Stomata open wider for photosynthesis so more SA for water loss by diffusion.

Temperature -> more water loss when higher temp. More kinetic energy. Water diffuses faster.

Humidity -> more water loss = high humidity. Air more saturated with water therefore water potential gradient is shallower.

Wind -> more water loss = more wind. Carries water that has diffused out away maintaining high water potential gradient.

Water Availability -> more water loss (water in soil). Can’t replace water that’s been lost.

Question 22

Xerophytes

Answer 22

Plant adapted to reduce water loss by transpiration so that it can survive in very dry conditions. E.g. Maram Grass.

Question 23

Adaptations of Plants to reduce Water Loss

Answer 23

Waxy Cuticle (hydrophobic)
Stomata on underside of Leaf (reduce evaporation due to sun)
Stomata close at Midnight (no light for photosynthesis)
Deciduous Plant lose leaves in winter (when may not be able to photosynthesise.)

Question 24

Hydrophyte

Answer 24

Plant that is adapted to living in water or where the ground is very wet.

Question 25

Adaptations of Hydrophotic Leaf

Answer 25

-> Large Air Spaces in leaf (so can stay afloat so in air and can absorb sunlight.)
-> Stomata on upper epidermis (exposed to the air to allow gaseous exchange.)
-> Leaf Stem has Large Air Spaces (buoyancy + oxygen diffuse quickly to roots for aerobic respiration.)

Question 26

Translocation

Answer 26

Transport of assimilates between the sources and sinks of a plant in phloem tissue. Requires energy.

Question 27

[Translocation] How Sucrose enters the phloem from the source by active loading

Answer 27

1. H+ ions actively transported out of companion cells.
2. Produces diffusion gradient for H+ ions.
3. Move back into companion cell via facilitated diffusion through the sucrose - H+ transporter.
4. Sucrose been actively loaded into companion cell.
5. High conc of sucrose in companion cell compared to sieve tube element. Diffuses down concentration gradient through plasmodesmata.

Question 28

[Translocation] At the Source

Answer 28

• sucrose actively loaded into Sieve Tube Elements at the Source.
• reduces water potential in Sieve Tube Element.
• water enters sieve tube elements by osmosis.
• increases hydrostatic pressure in sieve tube element near source.

Question 29

[Translocation] At the Sink

Answer 29

• sucrose unloaded at sink by diffusion/active transport used in respiration/stored.
• increase water potential in sieve tube element.
• water moves into the sink via osmosis down the water pot gradient.
• reduces hydrostatic pressure in the sieve tube element near the sink.
• water in sieve tube element at source moves down the hydrostatic gradient from source to sink.
• creates flow -> carries sucrose + other assimilates along the phloem via mass flow either up or down plant.

Question 30

Evidence For & Against this Mechanism of Translocation

Answer 30

FOR
-> aphids feeding on plant stems inserts mouthparts into phloem.
-> conc of sucrose is higher in source than sink.

AGAINST
-> not all solutes in phloem move at same rate.
-> role of sieve plates is unclear.
-> sucrose moved to all parts of plant at same rate. Doesn’t go to lower conc faster.