3.3.4.2 mass transport in plants

Question 1

What are the two types of vascular tissue in plants?

Answer 1

Xylem and phloem

Question 2

Role of xylem

Answer 2

transports water & mineral ions in a solution in one direction, UP (from roots to leaves)

Question 3

Features of xylem vessels

Answer 3

• Long tube like structures
• Hollow thick-walled tubes
• formed from dead cells
• can be spiral, annular, reticulate or pitted
• provide structural support to stem

Question 4

Role of phloem

Answer 4

transporting organic substances like sugars in both directions in plant

Question 5

Cohesion definition (for transpiration)

Answer 5

When water molecules stick to other water molecules

Question 6

Adhesion definition

Answer 6

When water molecules stick to the walls of the xylem vessels

Question 7

Explain the cohesion-tension theory.

Answer 7

• water evaporated from the surface of mesophyll cell in leaf
• diffuses out of stomata
• replaced by water pulled up from xylem vessels. (Move between vessels via pits)
• tension is created which pulls more water into the leaf
• water molecules are cohesive so the whole column of water in the xylem moves upwards
• water then enters the stem via the roots

Upward movement so it maintains water potential gradient.

Question 8

What are the three components of

phloem vessels?

Answer 8

● Sieve tube elements= form a tube to transport
sucrose in the dissolved form of sap.
● Companion cells= involved in ATP production for
active loading of sucrose into sieve tubes.
● Plasmodesmata= gaps between cell walls where
the cytoplasm links, allowing substances to flow.

Question 9

Name the process whereby organic
materials are transported around the
plant.

Answer 9

Translocation

Question 10

process of transpiration

Answer 10

• water evaporates from a plants surface and build up in air spaces of leaf.
• stomata open—-> water vapour diffuses out of leaf (down a concentration gradient)

Question 11

factors that affect transpiration rate

Answer 11

light intensity
humidity
temperature
wind

Question 12

light intensity on transpiration rate

Answer 12

Increased light=increased rate of photosynthesis
light causes stomata to open to let in CO2 for photosynthesis.
Therefore, increase rate of transpiration

Question 13

temperature on transpiration rate

Answer 13

Increased temp= increase rate of transpiration

more energy so evaporate out of leaf faster increases concentration gradient, making water diffuse out of the leaf faster.

Question 14

humidity on transpiration rate

Answer 14

Increased humidity=decrease in transpiration

Decreased humidity= increase in transpiration

Question 15

wind on transpiration rate

Answer 15

More wind= increased transpiration rate

Question 16

what is translocation?

Answer 16

movement of solutes (e.g. sucrose) to where they’re needed in a plant - requires energy to move solutes from source to sink

Question 17

what is the source?

Answer 17

where the assimilates (products of photosynthesis) are produced (there is a high concentration)
e.g. leaves

Question 18

what is the sink?

Answer 18

where the solutes are used up (low concentration)

Question 19

enzyme role in translocation

Answer 19

maintain a concentration gradient from source to sink by changing the solutes at the sink (e.g. using them up or breaking them down) so that there is always a lower concentration at the sink

Question 20

Root Pressure Theory

Answer 20

Evidence to say water is pushed up from roots.

• cut near base of stem- water leaks
• Active transport- pumps ions across endodermis into xylem of roots
• enters xylem by osmosis pushing upwards creating root pressure

Question 21

process of using a potometer

Answer 21

• cut shoot underwater & at a slant
• assemble the potometer underwater & insert shoot underwater
• remove apparatus from underwater but keep end of capillary tube submerged
• check apparatus is water & airtight
• dry leaves & allow time for shoot to acclimatise then close tap
• remove submerged end of capillary tube until 1 air bubble has formed then put tube back in water
• record starting position for air bubble
• start a stopwatch & record distance moved by bubble per unit time - rate of bubble movement is estimated transpiration rate

Question 22

why is the shoot cut underwater (potometer)

Answer 22

prevent water entering xylem (prevents cohesion)

Question 23

what is the reason for sealed joints (potometer)

Answer 23

prevent water& air entering xylem

Question 24

why is the shoot cut at a slant (potometer)

Answer 24

increases surface area

Question 25

evidence for cohesion theory

Answer 25

• broken xylem vessel doesn’t leak water so water not under pressure
• broken xylem vessels means that the water column and cohesive forces between water molecules are broken - stops flow of water
• tree trunk diameter decreases when transpiration rate is high due to negative pressure caused by cohesion between water molecules

Question 26

mass flow hypothesis process

Answer 26

SOURCE

• active transport is used to actively load solutes from companion cells into sieve tube elements at the source
• lowers water potential of sieve tubes so water enters by osmosis from xylem & companion cells
• high pressure created inside sieve tubes at the source

SINK

• solutes are removed from phloem and used up
• increases water potential so water moves out by osmosis
• low pressure created inside sieve tubes

FLOW

• a pressure gradient between source & sink is created
• the gradient pushes solutes from source to sink along sieve tubes
• solutes used up at sink

Question 27

mass flow evidence - supporting

Answer 27

• pressure in sieve tubes- leaks when cut
• concentration of sucrose higher in sources than in sink
• downward flow in daylight but slows/stops in dark
• increased sucrose in leaves is followed by increased sucrose in phloem
• metabolic poison/lack of O2 inhibits translocation
• companion cells produce lots of ATP

Question 28

mass flow evidence - against

Answer 28

• function of sieve plates unknown - do not affect mass flow other than forming a barrier
• not all solutes move at same rate
• sugar travels to many different sinks not just those with the highest water potential

Question 29

ringing experiements

Answer 29

• if a ring of bark (including phloem) is removed from a woody stem, a bulge forms above the ring
• the fluid from the bulge has a higher concentration of sugars than the fluid below the ring - sugars cannot flow past areas where bark has been removed

Question 30

tracer experiment

Answer 30

• radioactive isotopes (14C) is used to make radioactive 14CO2
• a plant is grown in presence of 14CO2 will carry out photosynthesis and incorporate 14C into its glucose & sugar molecules
• radioactive molecules can be traced round the plant using autoradiography
• thin cross sections of plant are produced and exposed to xray radiation using xray film

Question 31

why is there a layer of oil (potometer)

Answer 31

To stop water evaporating from surface

Question 32

aphids experiment

Answer 32

• aphids use their mouthpiece to pierce the phloem and extract sugary sap
• the contents show daily variations of sugar concentration in leaves which is mirrored in short time by identical changes in phloem sugar concentration

Question 33

puncture experiment

Answer 33

Xylem- air gets sucked into the xylem

phloem- cell sap oozes out