3.1.3 - Transport In Plants

Question 1

Transport of water: Roots

Answer 1

• There is a lower water potential in root hair cells due to a higher concentration of solutes
• Water moves into root hair cells via osmosis

Question 2

Transport of water: Roots to xylem

Answer 2

• Apoplastic pathway: Through cell wall, until it reaches the casparian strip (waterproof layer in endodermis, which forces the water into the symplastic pathway)
• Symplastic pathway: Through cytoplasm, passes cell to cell via the plasmodesmata into the xylem

Question 3

Cohesion-Adhesion-Tension theory

Answer 3

• High hydrostatic pressure in the roots
• Low hydrostatic pressure in leaves
• As water evaporates through the stomata; tension is created in the xylem
• Water moves up the xylem along the hydrostatic pressure gradient in a continuous column
  This is due to;
• Adhesion: Water is attracted to xylem walls
• Cohesion: Hydrogen bonds between water molecules
• Capillary action: Natural movement of water up a narrow tube
• Mass flow: Bulk movement of water

Question 4

Transpiration

Answer 4

Evaporation of water from the leaves

Question 5

Transpiration stream

Answer 5

Movement of water up the xylem, from the roots to the leaves

Question 6

The need for a transport system in plants

Answer 6

• Large SA:V ratio
• Rate of diffusion into plant tissues is too slow
• High metabolic rate

Question 7

Vascular tissues in plants

Answer 7

Xylem: Water & soluble minerals upwards
Phloem: Sugars up or down

Question 8

Factors that affect transpiration

Answer 8

Temperature
- Higher temperature = increased kinetic energy of water, more evaporation of water vapour

Humidity
- Higher conc. of water vapour surrounding the stomata = less steep diffusion gradient, less water leaves the stomata via evaporation

Light intensity
- Higher rate of photosynthesis = More gas exchange required, the stomata opens which allows water to escape

Air movement
- Water vapour is blown away = Steeper conc. gradient, more water leaves

Water availability
- More water available = Higher transpiration rate

Question 9

Potometer set up

Answer 9

• Select a healthy plant
• Cut stem under water; at an angle (to prevent air entering)
• Dry the leaves (Humidity decreases concentration gradient)
• Use the same age/species of plant
• Same SA of leaves
• Assemble underwater to prevent air bubbles
• Introduce an air bubble once set up

Question 10

Why does a potometer not accurately measure the rate of water uptake?

Answer 10

Some water might be used in;
- Turgor pressure
- Photosynthesis

Question 11

Calculate the rate of water uptake on a potometer

Answer 11

(Surface area of a circle (A = πr^2) x length the bubble has moved) / time

Question 12

Adaptations of Xerophytes

Answer 12

Rolled leaves eg marram grass
- Reduces SA for evaporation; Traps a layer of water vapour outside stomata, decreasing the water potential gradient & Reducing evaporation from the leaf

Hairy leaves
- Traps a layer of water vapour outside stomata, decreasing the water potential gradient

Sunken stomata
- Trap layer of water (can’t be blown away by wind); Creating a higher water vapour potential outside the stomata; reducing the water vapour potential gradient

Needle like leaves
- Reduces SA, less evaporation

Dense spongy mesophyll layer
- Smaller SA for evaporation from the vascular bundle

Also;
- Fewer stomata
- Thick waxy cuticle
- High solute conc. in roots

Question 13

Adaptations of hydrophytes

Answer 13

Aerenchyma
- Air spaces = allows buoyancy

Large leaves
- Increase SA for photosynthesis

Pneumatophore (roots)
- Roots that grow out of the water to aid with gas exchange; increases the rate of photosynthesis

Also
- Many stomata on upper surface of leaves
- Thinner waxy cuticle
- Short root system

Question 14

Translocation: Source

Answer 14

Where sugars are made/released from a carbohydrate source (eg starch) and are therefore in high concentration eg leaves or roots

Sucrose is less reactive than glucose and is more commonly transported

Question 15

Translocation: Sink

Answer 15

Where sugars are used and therefore in low concentration

Question 16

Translocation: Active loading

Answer 16

• H+ ions are actively transported out of companion cells into surrounding cells
• H+ ion move back into the companion cell with a sucrose or an amino acid, using a co-transporter protein. This is facilitated diffusion
• Sucrose diffuses through plasmodesmata into the sieve tube elements

Question 17

Translocation: Mass flow hypothesis

Answer 17

• Sucrose lowers the water potential of the sieve tube elements therefore water moves into the sieve tube elements from the xylem via osmosis
• This increases the hydrostatic pressure inside the sieve tube elements at the source

Question 18

Translocation: At the sink

Answer 18

• Sucrose leaves the sieve tube elements via diffusion, and therefore the water potential of the sieve tube elements increases. Therefore water leaves the sieve tube elements via osmosis
• This decreases the hydrostatic pressure inside the sieve tube elements at the sink

**Therefore assimilates move from source to sink down the hydrostatic pressure gradient via mass flow **