mass transport in plants

Question 1

what is the role of the xylem in the mass transport of water through plants?

Answer 1

• plants get water and mineral ions from soil via roots.
• water enters root hair cells in epidermis
• water passes up roots to stem
• passes across cells of cortex, across endodermis and into xylem vessel which transports it to leaves
• water leaves plant through stomatal pores by diffusion (transpiration)

Question 2

what is the role of the phloem in the mass transport of organic molecules in plants?

Answer 2

• transports sugars & organic substances (AAs & sucrose) from leaves where formed (by photosynthesis) to where needed e.g. roots

Question 3

Explain how water enters xylem from the endodermis in the root and is then transported to the leaves. (6)

Answer 3

(In the root)
1. Casparian strip blocks apoplast pathway / only allows symplast pathway;
2. Active transport by endodermis;
3. (Of) ions/salts into xylem;
4. Lower water potential in xylem / water enters xylem by osmosis /down a water potential gradient;

(Xylem to leaf)
5. Evaporation / transpiration (from leaves);
6. (Creates) cohesion / tension / H-bonding between water molecules / negative pressure;
7. Adhesion / water molecules bind to xylem;
8. (Creates continuous) column of water

Question 4

Root pressure moves water through the xylem. Describe what causes root pressure. (4)

Answer 4

1. Active transport by endodermis;
2. ions/salts into xylem;
3. Lowers water potential (in xylem);
4. (Water enters) by osmosis;

Question 5

Name a factor that can affect transpiration

Answer 5

Light (intensity) / temperature / air movement / humidity;

Question 6

how are root hair cells adapted for their function?

Answer 6

• hair like extensions that increase SA so more water uptake & to increase area for channel & carrier proteins
• thin cell wall so shorter diffusion pathway

Question 7

how does water enter root hair cells(RHCs)?

Answer 7

• roots surrounded by soil particles & O2 (which contains solutes & low conc. of ions)
• RHCs actively transport ions from soil across membranes into cytoplasm via carrier proteins
• lowers water potential of cytoplasm, compared to soil
• water enters RHCs by osmosis down water potential gradient

Question 8

what is osmosis?

Answer 8

movement of water from a region of higher water potential to a region of lower water potential across a partially permeable membrane through AQUAPORINS

Question 9

what are the 2 pathways that water passes from RHCs, across cortex cells into xylem?

Answer 9

1. symplastic pathway: through cytoplasm & across membranes, through plasmodesma from 1 cell to another (by osmosis). slower(resistance to movement of water)
2. apoplastic pathway: along adjoining cellulose cell walls by diffusion. faster(less resistance to water as larger gaps between cellulose molecules)

Question 10

when is the symplastic pathway vs apoplastic pathway used to transport water?

Answer 10

• apoplastic until reach endodermis (has waterproof strip - casparian strip so water can’t move through walls of endodermis).
• has to pass symplastic way

Question 11

how does water move from root hair to centre of root?

Answer 11

• cells of endodermis actively transport ions into xylem using carrier proteins
• lowers PO2 in xylem, water pass into xylem from endodermal cells down O2 potential gradient
• enables movement of O2 from root hairs to centre of root, by osmosis

Question 12

what adaptations do the xylem have?

Answer 12

• dead hollow tubes (no cytoplasm)
• contain lignin, makes rigid & waterproof
• thick walls, to withstand -ve O2 pressure
• narrow, enable O2 travel up by capillary action
• pits, so O2 can move laterally between xylem vessels (in case parts of stems get damaged)

Question 13

describe how the xylem transporting water from the roots to the leaves is driven by transpiration (evaporation of water from a plant)

Answer 13

1. Stomata open (CO2 enter leaf for photosynthesis) causes H2O diffuse from air spaces at higher water potential in leaf to lower water potential of air outside leaf. loss of water from leaf = TRANSPIRATION. (Evaporation)
2. loss of water from air spaces causes water move down water potential gradient from mesophyll cells to air spaces.
3. lowers the water potential of the mesophyll cells, so water moves by osmosis from adjacent mesophyll cells
4. sets up water potential gradient across leaf to xylem vessels.
5. Water from xylem enters leaf, causes water pulled up under tension through xylem from roots
6. Water forms continuous column in narrow xylem vessels.
7. Water molecules form weak H bonds between them, so stick together (cohesion)
8. attracted to walls of xylem – forces of ADHESION between water & xylem.
9. great pulling force is great, column of H2O under tension
10. this is transpiration stream (cohesion-tension theory)

Question 14

Describe how a high pressure is produced in the leaves. (3)

Answer 14

1. Water potential becomes lower/becomes more negative (as sugar enters phloem);
2. Water enters phloem by osmosis;
3. Increased volume (of water) causes increased pressure;

Question 15

factors affecting rate of transpiration?

Answer 15

1. light - stomata open in light, close in dark
2. temp. - increase rate water diffusion (higher KE)
3. humidity - greater diff in humidity between air spaces in leaves & air outside = greater diffusion rate
4. air movement - moves water vapour away from stomatal pores, higher water potential gradient in & out of leaf

Question 16

what are limitations of a potometer

Answer 16

• volume of water taken up doesn’t always equal water lost through transpiration
• only measure uptake through stem(attached) , not consider uptake through roots normal plant
• plants also use water for:
  photosynthesis, storage(vacuoles), support (turgiduty /opening guard cells), hydrolysis

Question 17

Give two precautions the students should have taken when setting up the potometer to obtain reliable measurements of water uptake by the plant shoot.

Answer 17

1. Seal joints / ensure airtight / ensure watertight;
2. Cut shoot under water;
3. Cut shoot at a slant;
4. Dry off leaves;
5. Insert into apparatus under water;
6. Ensure no air bubbles are present;
7. Shut tap;
8. Note where bubble is at start / move bubble to the start position;

Question 18

how do xerophytes reduce water loss in dry environments?

Answer 18

• reduce transpiration
  1. less stomata
  2. stomata in pits
  3. hairs to trap water
  4. rolled leaves
  5. leaves reduced to spines
  6. thick waxy xuticles

Question 19

what is the phloem and mass flow hypothesis for translocation?

Answer 19

• Phloem transports solutes around plants
• Phloem formed from cells arranged in tubes
• Sieve tube elements & companion cells form tube for transporting solutes(no nucleus, few organelles)
• companion cell for each sieve tube element. carry out living functions for sieve cells. contain many mitochondria to synthesise ATP through aerobic respiration for active transport of solutes.
• Sieve tubes connected to each other through sieve plates

Question 20

what is translocation?

Answer 20

• movement of solutes to where they are needed in a plant. Solutes called assimilates (substances incorporated in plant tissue).
• requires energy, happens in phloem
• moves solutes from sources (assimilates produced, so high conc.) to sinks where used up, so in low conc.
• Enzymes maintain conc. gradient from source to sink by converting solutes at sink to other storage substances.
• so always lower conc. at sink than source

Question 21

Describe the mass flow hypothesis for the mechanism of translocation in plants.

Answer 21

1. In source/leaf sugars actively transported into phloem;
2. By companion cells;
3. Lowers water potential of sieve cell/tube and water enters by osmosis;
4. Increase in pressure causes mass movement (towards sink/root);
5. Sugars used/converted in root for respiration for storage;