3.6 Mass Transport in Plants

Question 1

What vessel transports water and mineral ions in plants?

Answer 1

Xylem

Question 2

What vessel transports sugars in plants?

Answer 2

Phloem

Question 3

Name and explain adaptations of the xylem.

Answer 3

1. Thick cell walls → Withstand tension/negative pressure
2. Lignin in cell walls → Walls waterproof / withstand tension/negative pressure
3. Xylem cells have no end walls → So a continuous column of water
4. Xylem vessels are stacked on top of each other → So a continuous column of water
5. Have no cytoplasm/hollow → Reduces resistance to flow of water, so a continuous column of water
6. Xylem cells have pores → Enable sideways flow, allows entry or exit of water
7. Narrow tubes → Increased surface area for adhesion
8. Molecules in cell walls → Allows adhesion

Question 4

What is cohesion?

Answer 4

Attraction of water molecules to each other allowing water to travel ho the chemo in a continuous water column

Question 5

What is adhesion?

Answer 5

Attraction of water molecules to other substances

Question 6

Explain the cohesion-tension theory.

Answer 6

1. Transpiration of water through stomata/from leaves
2. Lowers water potential of mesophyll
3. Water pulled up xylem (creating tension)
4. Water molecules cohere by hydrogen bonds forming continuous water column
5. Adhesion of water molecules to walls of xylem

Question 7

What is transpiration?

Answer 7

The absorption of water via the roots which then travels up the plant through the xylem followed by the evaporation of water through the stomata.

Question 8

What factors effect transpiration? (how do they effect transpiration)

Answer 8

• Light intensity → positive correlation, more light causes more stomata to open= large SA for evaporisation, ** transpiration rate is faster**
• Humidity → negative correlation, more water vapour in the air makes water potential more positive outside the leaf so transpiration rate is slower
• Temperature → positive correlation, more heat= more KE, faster moving molecules and therefore more evaporation, MORE transpiration
• wind → positive correlation, removes water vapour from the environment maintaining water potential gradient, MORE transpiration

Question 9

What is the mass flow hypothesis?

Answer 9

• In source sugars actively transported into phloem by companion cells
• Lowers water potential of sieve cell and water enters by osmosis
• Increase in hydrostatic pressure causes mass movement (towards sink) or to storage tissue or organs/respiring cells
• Sugars converted in root for respiration for storage (unloaded from phloem by AT)

Question 10

Evidence for mass flow: If a ring of bark ringing experiment

Answer 10

• this removes the phloem in the tree
• this makes the trunk above the section removed swell
  This shows that movement of sucrose can occur downwards as sugars can’t move past the area where the bark has been removed as there is no phloem.

Question 11

Evidence for mass flow: An aphid’s mouth can pierce the phloem, allowing it to feed. Scientists can snap off the aphid’s body leaving their mouth parts in the plant. If this is done near the leaves, the phloem sap flows out quicker than if it was further down the stem

Answer 11

This shows that there is a pressure gradient (higher pressure at source and lower pressure at sink) due to movement of water into and out of the phloem by osmosis.

Question 12

Evidence for mass flow: Carbon-14 can be used as a radioactive tracer. Radioactive CO2 can be supplied to a plant which is used in photosynthesis to form sucrose, which will then be moved around the plant in translocation. The radioactivity can be traced using autoradiography , where the plant is placed onto photographic film. Radioactivity is observed more at the leaves at first, then further away from the leaves over time.

Answer 12

• plants are provided with radioactively labelled CO2, which is absorbed by plant and used in photosynthesis to create sugars
  Tis shows that the leaves photosynthesise, forming radioactive sucrose at the source. The radioactive sucrose is then actively transported into the phloem and translocated towards the sink via mass flow.

Question 13

Structure and feature of the phloem

Answer 13

• Made out of columns of living elongated cells
• carries food up to grown new leaves/ stems and carries food down to the roots for safe storage or to provide energy for active transport

Question 14

What is the apolost pathway

Answer 14

Series of space running through the cellulose walls, dead cells and the hollow tubes of the xylem

Question 15

What is the casparian strip

Answer 15

A barrier in the cell wall that makes it impossible for water to pass through the apolost pathway meaning water must now take the symplast pathway

Question 16

What is the symplast pathway

Answer 16

Water moves by osmosis into the cell across a partially permeable membrane

Question 17

What is capillarity?

Answer 17

The narrower the xylem= better impact of capillarity

• a narrower xylem will help move water up the column

Question 18

Regulations needed when setting up an experiment measuring transpiration

Answer 18

• sample of plant must be cut from a plant underwater → prevents air from entering the xylem and breaking the water column
• photometer is filled with water → ensures all air bubbles are removed
• completely air tight → ensures no air bubbles can get in and effect accuracy of uptake of water

Question 19

How does a potmeter measure transpiration

Answer 19

• it measures the rate of uptake of water from a plant
• one air bubble is introduced into the equipments and the distance the air bubble has ,paved towards the plan is recorded

Question 20

Formula for rate of transpiration

Answer 20

Rate of transpiration = volume of water taken up/ time taken

Question 21

Explain why apparatus must be set up snd plant shoot should be cut underwater ,

Answer 21

1. Due to cohesion - tension creating a negative pressure in the xylem tube
2. Would break continuous water column and prevent transpiration
3. Cutting underwater will draw only water into the column ( no air bubbles)

Question 22

What is translocation ?

Answer 22

Movement of dissolved substances from parts of the plant tissue where substances are made to other parts of the plant where they are needed

Question 23

Structure of the phloem

Answer 23

Sieve tube elements: end walls are perforated to allow continuous flow of sugar solution
- living cells, contain no nucleus, contain few organelles → so hollow take to ensure maximum flow of sugary solutions

Companion cells:
Provide ATP required for active transport of sucrose

Question 24

Describe the key stages of translocation

Answer 24

1. Photosynthesis occurring in the chloroplast of leaves create organic substances e.g sucros
2. This creates Hugh conc of sucrose at source cell -leaves, so sucrose diffuses down conc gradient into companion cell via facilitated diffusion
3. Co transport of sucros with H+ to transport sucros to sieve tube element of phloem
4. Increase of sucros in sieve tube element lowers the water potential so water moves into phloem from xylem via osmosis
5. This creates a high hydrostatic pressure in sieve tube elements that causes the liquid to be forced down to roots (sink cell)
6. Sucrose is actively transported into the root cell, causing the water potential in sieve tube element to decrease
7. Results in osmosis of water from the sieve tube element into the xylem, this lowers the hydrostatic pressure at the lower section of the phloem

Question 25

Phloem vs xylem

Answer 25

In xylem the cells are dead, in phloem they are living Xylem moves water and mineral ions, phloem moves sucrose and amino acids Xylem has a one way flow, phloem has flow in all directions