Mass Transport In Plants

Question 1

Describe transpiration

Answer 1

Evaporation of water from the stomata of leaves due to heat energy from the sun that forces water up through the xylem vessels in the stem

Question 2

Movement of water across cells of a leaf

Answer 2

• Mesophyll cells lose water to air spaces due to evaporation
• lower w.p so water enters cell via osmosis from neighbouring cell
• loss of water from neighbouring cell lowers w.p
• water moves in via osmosis
  = w.p gradient formed pulling water from xylem across leaf into atmosphere

Question 3

Movement of water out through the stomata

Answer 3

When stomata is open, water vapour molecules diffuse out of air spaces through the stomata in to the air because humidity of atmosphere is lower

Question 4

Describe the cohesion tension theory

Answer 4

• Evaporation of water from mesophyll cells causes transpiration
• H2O molecules form H bonds so they stick together (cohesion)
• Forms a continuous unbroken column of water across mesophyll cells
• more water evaporating from leaf so more water is drawn up through xylem
• called transpiration pull
• puts xylem under tension

Question 5

What happens to the diameter of a tree trunk during the day?

Answer 5

Decreases - transpiration is at its greatest so there’s more tension in xylem pulling the walls of the vessel inwards

Question 6

What are the four main factors affecting transpiration?

Answer 6

Light - stomata open to let in CO2 for respiration
Hot - water molecules have more energy
Dry - conc grad between leaf and air is increased
Windy - physically blows water molecules away

Question 7

What’s the difference between xylem and phloem

Answer 7

Xylem - transports water and mineral ions (only moves up)

• made of dead cells
• no end walls to allow water to pass through easily

Phloem - transports organic substances like sugars in both directions

Question 8

What are the two important cell types in phloem tissue

Answer 8

Sieve tube elements and companion cells

Question 9

What’s the function of the sieve tube elements

Answer 9

They’re living cells that form the tube for transporting solutes (have no nucleus and few organelles)

Question 10

What’s the function of the companion cells?

Answer 10

They carry out living functions for sieve cells (provide energy for active transport of solutes)

Question 11

What is translocation?

Answer 11

Movement of solutes to where they’re needed in plants (from source to sink)
Source - where solutes are produced
Sink - where they’re used up

Question 12

Describe the mass flow hypothesis

Answer 12

• Active transport actively loads solutes from companion cells to sieve tubes
• Lowering the w.p inside sieve tubes so water is drawn in via osmosis
• Creating a high h.p inside sieve tube elements
• At sink end, solutes are removed from phloem and used up
• increasing the w.p inside sieve tube elements so water leaves via osmosis
• lowering the h.p
• creates a pressure gradient forcing solutes from source to sink where they’ll be used up during respiration or stored as starch