transport in plants

Question 1

what are the functions of xylem?

Answer 1

support: provides mechinal support to the plant
transport: transports water and dissolved mineral salts from the roots to the rest of the plant

Question 2

what cells does the xylem contain?

Answer 2

vessel elements and tracheids, parenchyma cells, fibres

Question 3

what are vessel elements and tracheids?

Answer 3

they are cells involved in the transportation of water

Question 4

what are parenchyma cells?

Answer 4

they have unthickened cellulose cell walls and contain all the organelles of a typical plant cell except for chloroplasts. this is because they are not exposed to sunlight. they are found in large numbers, have a variety of shapes and are of similar sizes

Question 5

what are fibres?

Answer 5

elongated cells with liginified walls to provide plant with mechanical support. they are dead cells and do not contain any living content within it

Question 6

what is lignin?

Answer 6

it is a very hard and strong substance that is impermeable to water

Question 7

how are xylem vessels formed?

Answer 7

they are formed by having elongated elements arranged end to end. Each vessel element was a normal plant cell until lignin was laid down. Lignin builds up around the cell and the contents of the cell breaks down, resulting in a empty space aka lumen. end walls of the neighbouring vessel elements break down, forming a continuous tube which is the xylem vessel

Question 8

are there opened pores in the xylem vessel?

Answer 8

no, certain parts of the vessel may not be covered with lignin and these regions are called pits. Pits are crossed over with a fully permeable, unthickened cellulose cell wall.

Question 9

what are tracheids?

Answer 9

they are dead cells with lignified walls but have no open ends. Water passes through each tracheid through the pits in their walls

Question 10

What are the structural adaptations of xylem and what are their functions?

Answer 10

1. Long and hollow lumen: no protoplasm or cross-walls, reduces resistance and allows for continuous and increased rate of transport of water and mineral ions from roots to other parts of the plant
2. Lignified walls: provide mechanical support when xylem vessels are bundled together, prevents collapse

Question 11

water from soil to root hair

Answer 11

1. root hair grows in close contact with soil particles
2. soil particles have a thin film of diluted solution of mineral salts surrounding it
3. cell sap of the root hair cell is more concentrated due to the presence of sugars and mineral salts. thus it has a lower water potential than the soil solution and water enters the root hair cell through osmosis
4. root hair cell sap has a higher water potential than the inner cells so water is passed down to the inner cells by osmosis

Question 12

mineral salts from soil to root

Answer 12

1. diffusion when concentration of ions is higher in the soil solution than the cell sap of root hair cell
2. active transport when concentration in root hair cell sap is higher than that of soil solution. ions taken in with the use of energy

Question 13

what are the structural adaptations and functions of the root hair cell?

Answer 13

1. Long and narrow protrusion, increases surface area to volume ratio to increase rate of abssorption of water molecules and mineral ions
2. lower water potential in cell sap due to presence of sugars, salts and amino acids in the cell sap. maintains perpetual lower water potential in cell sap which allows for water from soil solution to continuously enter the cell sap by osmosis down its concentration gradient
3. contains numerous mitochondria to allow for greater rate of aerobic respiration that releases energy for active transport of mineral ions from soil solution to cell sap against its concentration gradient
4. uniformly thin cell wall which decreases the distance needed for movement of substances, increases rate of absorption of mineral ions and water into the cell sap

Question 14

from root hair cell to xylem

Answer 14

water moved from cell to cell in the roots by osmosis down its concentration gradient until it reaches the xylem vessels. the xylem vessels will always have a lower water potential than the root cells as water is constantly being pulled up the xylem to be transported to other parts of the plant.

Question 15

what is root pressure?

Answer 15

When dissolved mineral salts and sugar have been actively transported into the apoplast of the stele, it creates osmotic pressure in the xylem sap which then create root pressure. Root pressure account for the movement from root to xylem in some plants

Question 16

from xylem to leaf

Answer 16

water in the leaves are constantly being lost either during photosynthesis or evaporation of water from the cell walls of the spongy mesophyll cells. water is constantly being drawn from the xylem vessels in leaf to replace the loss of water. this results in lower hydrostatic pressure at the top region of the xylem vessels at the leaves as compared to the bottom region of the xylem near the roots. the difference in pressure results in water moving up the xylem vessel

Question 17

how do water molecules travel up the xylem vessels

Answer 17

1. cohesion: water molecules attract each other and fill the column in the xylem, acting as a huge single water molecule
2. capillary action: is the movement of liquid across a solid due to adhesion between the two. the narrower the xylem vessel, the higher the water can travel on its own by capillary action. adhesion forces water up along the columns of cells and through fine tubes in the cell walls

Question 18

from leaf to atmosphere

Answer 18

1. water moves out from the mesophyll cells and forms a thin film of moisture around the cells.
2. water evaporates from this film of moisture to form water vapour which accumulates in the air space near the stomata
3. water vapour diffuses out of the stomata and into the atmosphere (from higher concentration of water vapour in the air spaces of the leaf to less humid air outside leaf)
4. water moves out of the mesophyll cells to replace the film of moisture, resulting in lower water potential in their cell sap
5. water is passed down to the mesophyll cells from inner cells by osmosis
6. the inner cells obtain water from xylem vessels by osmosis, resulting in a suction force that pulls water up the xylem

Question 19

transpiration pull

Answer 19

generated by transpiration or loss of water through the stomatal pores of the leaves. loss of water creates negative pressure in the xylem vessels from the leaves to the roots, and water present in the xylem is pulled as a single column. cohesion forces between the water molecules and adhesion forces of water molecules to cell walls prevents column of water from splitting

Question 20

factors affecting transpiration

Answer 20

1. humidity of external environment: more humid> gentler concentration gradient> rate of transpiration decreases
2. air movement: wind helps to remove water vapour lingering outside the stomata> steeper concentration gradient> rate of transpiration increases
3. temp of external environment: more water evaporated from mesophyll cell surfaces, increase in evaporation increases rate of transpiration
4. light intensity: brighter> increases size of stomatal opening, increases rate of transpiration

Question 21

what is translocation?

Answer 21

it is the transport of assimilates within a plant through sieve elements

Question 22

what does phloem tissue contain?

Answer 22

sieve elements, companion cells, parenchyma cells and fibres

Question 23

what are sieve elements?

Answer 23

living typical plant cells with cellulose cell wall, cell membrane and cytoplasm containing organelles like endoplasmic reticulum and mitochondria. it does not contain nucleus and ribosomes. sieve elements join together to form sieve tube and when two elements meet, a sieve plate is formed. sieve plate is a thick wall perforated by large pores that allows assimilates to pass through

Question 24

what are companion cells?

Answer 24

companion cells are typical plant cells but have much more ribosomes and mitochondria as they are very metabolically active. each sieve elemnt has at least one companion cell near it

Question 25

structural adaptation and functions of phloem

Answer 25

1. sieve tube elements have very little protoplasm and are joined end to end by cytoplasmic strands. reduces resistance and facilitates ease of flow of substances through a continuous column
2. pores in sieve plates allows for rapid flow of manufactured products through sieve tubes
3. large number of mitochondria in companion cell: greater rate of aerobi respiration allows for more energy to be released for active transport of sugars from mesophyll cells into sieve tubes

Question 26

how does translocation occur?

Answer 26

phloem sap moves by pressure flow in the phloem sieve tube. the pressure difference is generated by the active loading of sucrose into the sieve elements from the leaf cell. This lowers the water potential in the leaf cell sap, causing water to follow the sucrose into the sieve element by osmosis. when sucrose is loaded into another part of the plant, water also follows. The pressure difference between the area where sucrose is loaded into the phloem(source) and the area where sucrose is removed from the phloem(sink) results in mass flow of assimilates.

Question 27

how is sucrose loaded and unloaded into the phloem?

Answer 27

sucrose is loaded into the companion cell by active transport and moves from the companion cell into the phloem through plasmodesmata . It moves out of the phloem by diffusion which reduces pressure at the sink, promoting mass flow of more sucrose solution from the source

Question 28

what is wilting?

Answer 28

when rate of transpiration exceeds rate of water absorption by the roots

Question 29

what happens during wilting?

Answer 29

turgor pressure is lost in the mesophyll cells of the leaf and cells become flaccid and are unable to support the leaf and other structures of the plant