Chapter 3: Transport in plants

Question 1

What is the role of the parenchyma in roots?

Answer 1

To store starch

Question 2

Explain the difference between the role of xylem and phloem

Answer 2

Xylem: Transports water and dissolved mineral salts
Phloem: Transports products of photosynthesis eg. sugars

Question 3

What is the casparian strip?

Answer 3

Located in the endodermis in the root, the cell wall helps to contain water movement into the xylem

Question 4

Out of the xylem and phloem, which is on the outside of which?

Answer 4

Phloem

Question 5

State three characteristics of a xylem vessel

Answer 5

• Transports water and mineral salts
• Transported from roots to leaves and shoots
• Transport is passive and driven by transpiration
• Cells are elongated and tubes are hollow and dead
• Cells are strengthened with lignin

Question 6

What is the tracheid in the xylem?

Answer 6

Tapered ends with bordered pits that line up to allow water to pass through

Question 7

Give three features of the sieve tube element in the phloem

Answer 7

• Transports organic solutes eg. glucose
• Sugars are transported from leaves to the roots and growing regions
• Thin cytoplasm, no nucleus and few organelles
• Perforated end wall helps flow (sieve plate)

Question 8

Give two features of a companion cell in phloem

Answer 8

• Contains a nucleus, cytoplasm and all other organelles
• Connected to sieve tube elements by plasmodesmata

Question 9

Why are mitochondria particularly important in companion cells?

Answer 9

They provide all the energy requirements for translocation

Question 10

What is the plasmodesmata?

Answer 10

Gated plant cell wall channels that allow the trafficking of molecules between cells

Question 11

Give two examples of ions absorbed in roots

Answer 11

Potassium and nitrate

Question 12

What happens to the water potential in a root when ions are absorbed?

Answer 12

It lowers

Question 13

What’s the apoplast pathway?

Answer 13

Water passes through the spaces in the cell wall and the cohesive properties of water pulls more along behind it

Question 14

In three points, what’s the symplast pathway?

Answer 14

• Water passes through the plasmodesmata and into cells, making the water potential higher
• Higher potential adjacent cell = water passes down potential gradient into next cell
• Lowers water potential in first cell so water enters by osmosis from the soil

Question 15

What’s the vacuolar pathway?

Answer 15

Water passes down water potential gradient through the vacuoles of adjacent cell

Question 16

How does water enter the xylem?

Answer 16

Water enters endodermis via apoplast pathway - stops going further due to casparian strip
Nitrates are pumped into the xylem by active transport, lowering the water potential

Question 17

Define transpiration

Answer 17

Consequence of gaseous exchange as water evaporates through the stomata

Question 18

What are the four aspects of transpiration?

Answer 18

• Osmosis from xylem to mesophyll cells
• Evaporation from the cells into the intracellular spaces
• Diffusion of water vapour out of the leaf via stomata
• Water moves through the plant via the transpiration stream (water lost at the top is replaced by water from the bottom)

Question 19

What are two unique properties of water and what do they mean?

Answer 19

Cohesive: Water molecules form hydrogen bonds with each other and stick together
Adhesive: Water molecules stick to the side of xylem vessels - the water is pulled up the sides of the vessels

Question 20

What are mesophyll cells role in gaseous exchange?

Answer 20

• Covered by a thin layer of water
• Gases dissolve in this water as they move into and out of the cells. When the plant is photosynthesising during the day, these features allow carbon dioxide to diffuse into the spongy mesophyll cells, and oxygen to diffuse out of them.

Question 21

Outline the cohesion tension theory

Answer 21

• Water evaporates from mesophyll cells - lowering their water potential
• Water moves down the water potential gradient
• By the forces of cohesion, continuous columns of water are pulled down from the xylem towards the mesophyll cells
• Water adhesives to the sides of the xylem vessels, forming a continual transpiration stream

Question 22

In the potometer PAG, why is the shoot cut at an angle?

Answer 22

Prevents the crushing of xylem and increases the surface area

Question 23

What is a Xerophyte? And give two examples of one, and where they can be located

Answer 23

Plants adapted to living in dry places eg. cactus, marram grass
Places found: Tundra, Deserts

Question 24

Give three adaptations and their purpose of a hydrophyte

Answer 24

• Thick waxy cuticle - reduces water loss
• Rolling up their leaves - to conserve water
• Hairy leaves - hairs trap a layer of water vapour

Question 25

What is a hydrophyte? And give two examples of it

Answer 25

A plant adapted to living in fresh water

Question 26

Outline three adaptations of a hydrophyte

Answer 26

• Stomata mainly on top to maximise gaseous exchange
• Large air sacs to store oxygen and help with buoyancy
• Thin waxy cuticle

Question 27

What’s a potometer?

Answer 27

A device for measuring loss of water from a plant

Question 28

In 4 steps, outline the first stage of translocation (loading)?

Answer 28

1. Hydrogen ions actively pumped out of companion cell using ATP
2. H+ ions return to companion cell through co-transporter protein
3. Increased concentration of sucrose in companion cell as it’s co-transported with H+ ion
4. Sucrose diffuses into sieve tube, causing water to move into the phloem - increases turgor

Question 29

Is mass flow passive or active?

Answer 29

Passive

Question 29

In translocation, outline the process of mass flow

Answer 29

• Increase in hydrostatic pressure causing pressure gradient to build up
• Where sucrose is unloaded = lower pressure at sink

Question 30

Outline the process of unloading

Answer 30

• Unloaded in sinks
• Sucrose leaves phloem via diffusion
• Invertase breaks down sucrose into glucose + fructose which is either respired or metabolised
• This reduces the solute concentration, increases water potential and helps maintain pressure gradient