4.4 - transport in plants

Question 1

describe how a xylem vessel is formed

Answer 1

-the first xylem formed is the protoxylem
-is living
-can stretch and grow because the cell wall isn’t fully lignified
-cellulose microfibrils in the cell wall of xylem cells are laid down vertically, to withstand the weight of the plant
-xylem cells stop growing
-lignin build up in the cell walls making the metaxylem
-the metaxylem is impermeable to water, strong, the organelles in the cells die
-the end walls between cells break down forming a hollow tube

Question 2

what are the main features of xylem vessels

Answer 2

-transports water and dissolved minerals from the roots to the leaves
-water only travels in 1 direction up the plant
-dead
-cell walls are lignified

Question 3

what are the main features of phloem vessels

Answer 3

-transports food e.g. sucrose
-substances can travel in both directions up and down the phloem
-living
-sieve cells
-companion cells
-active transport

Question 4

what are phloem sieve tubes

Answer 4

-phloem sieve tubes are made up of many cells joined together to make very long tubes, forming the phloem

Question 5

how do sieve plates form

Answer 5

-the cell wall and cell membrane between phloem cells become perforated, forming sieve plates
-contents in the phloem cells exit through the holes in the sieve plates

Question 6

how does the formation of sieve plates cause the there to be no organelles in the phloem sieve tubes

Answer 6

-when sieve plates are formed the organelles in phloem cells (nucleus, tonoplast and others) break down and exit through the holes in the sieve plates
-the phloem sieve tubes fills with phloem sap
-they are supported by companion cells

Question 7

how are companion cells adapted to support phloem sieve tubes

Answer 7

-companion cells are linked to the sieve tube elements through plasmodesmata
-the cell membrane of companion cells have infoldings, increasing surface area over which sucrose can be transported into the cell
-many mitochondria to supply the ATP needed for active transport

Question 8

what are the 2 pathways water travels from root hair cells to the xylem

Answer 8

symplast pathway
apoplast pathway

Question 9

describe the symplast pathway

Answer 9

-water moves by diffusion from the root hair cells to the xylem through the symplast
-the symplast travels through the plasmodesmata of cells
-water then travels through the cytoplasm of cells, following the symplast

Question 10

describe the apoplast pathway

Answer 10

-water is pulled by the attraction between water molecules across adjacent cell walls
-water then reaches the endodermis and the Casparian strip
-when water molecules reach the xylem, attraction between them ensures more water will be drawn up from the adjacent cell wall

Question 11

what is the endodermis and Casparian strip

Answer 11

-the endodermis is a single layer of cells that acts like a barrier between the cortex and a single layer of cells called the pericycle endodermis, then there is the vascular tissue containing xylem
-the Casparian strip is a waterproof layer on the endodermis
-whichever route water takes, when it reaches the Casparian strip water enters the cytoplasm of the endodermis cell
-this allows the plant to control the amount of water moving from the soil into the xylem

Question 12

what is the definition of translocation

Answer 12

a general term used for the movement of substances

Question 13

describe the transpiration stream

Answer 13

-water moves from the soil into the root hair cells by osmosis
-water moves across the cortex either through the symplast or apoplast pathway
-water enters the xylem in the stem and travels up to the leaves
-water moves into the xylem of the veins fo leaves
-water leaves the xylem and moves into the spongy mesophyll cells through osmosis
-water moves from the cell wall of the spongy mesophyll into air gaps
-then water diffuses out of the leaf through open stomata into a layer of still air outside the leaf

Question 14

explain the cohesion-tension theory of transpiration

Answer 14

-water molecules experience cohesion
-because water molecules are polar and form hydrogen bonds between molecules
-therefore water molecules stick together giving the column of water in the xylem high-tensile strength (hard to break)
-there is adhesion between water molecules and the channels and pores within the cell wall
-the combination between
-the loss of a water molecule from a spongy mesophyll cell causes tension throughout the plant
-so more water is pulled up from the xylem to replace it

Question 15

what 4 factors effect the rate of transpiration

Answer 15

temperature
light intensity
air movement / wind
humidity

Question 16

how does temperature affect the rate of transpiration

Answer 16

-increase in temperature increases the rate of evaporation from the surface of the spongy mesophyll cells
-increasing the concentration gradient between air inside and outside the leaf
-increasing the rate of transpiration
-increase in temperature also increases the kinetic energy of molecules, increasing the rate of diffusion out of the leaf, increasing the rate of transpiration

Question 17

how does light intensity affect the rate of transpiration

Answer 17

-at higher light intensities more stomata will open for a greater rate of gas exchange, because the rate of photosynthesis increases
-water diffuses out of the stomata at a greater rate
-so the transpiration rate will increase

Question 18

how does humidity affect the rate of transpiration

Answer 18

-at high humidity there will be a lower concentration gradient of water molecules inside and outside the leaf
-so the rate of transpiration increases
-at low humidity there will be a greater concentration gradient of water molecules inside and outside the leaf
-so rate of transpiration increases

Question 19

how does air movement affect the rate of transpiration

Answer 19

-increases air movement decreases the layer of still air around the leaf and stomata
-increasing the concentration gradient between water molecules inside and outside the leaf
-increasing the rate of transpiration

Question 20

what is guttation

Answer 20

-at night when transpiration rate can be very low, drops of water may be forced out of the leaves
-this process is called guttation
-guttation is evidence for the role of root pressure in water movement through the plant

Question 21

explain what causes root pressure

Answer 21

-the active secretion of minerals into the xylem sap increases the concentration gradient across the root
-meaning the water potential of the root increases and water potential of the xylem decreases
-so water moves by osmosis from the root into the xylem
-this generates root pressure

Question 22

when are the effects of root pressure seen the most

Answer 22

when transpiration rates are low, e.g. at night, there is still a continuous stream of water up the plant due to root pressure

Question 23

what are the substances that the phloem transports called

Answer 23

assimilates
the main assimilate in the phloem is sucrose

Question 24

why is glucose converted into sucrose to be transported in the phloem

Answer 24

-sucrose has a lower osmotic effect than glucose
-sucrose can then be converted back into glucose to use for respiration
-or be converted into starch for storage

Question 25

briefly describe the transport of assimilates in the phloem

Answer 25

-assimilates are transported from sources -such as the green parts of the plant where there are high rates of photosynthesis -sucrose from these sources moves into the sieve tube elements and is transported to the sinks -which includes the meristems where cells are actively dividing

Question 26

describe the symplast pathway in the translocation of sucrose

Answer 26

-sucrose moves by diffusion from source cells to other cells through the symplast and plasmodesmata, and then into the cytoplasm -then sucrose diffuses into companion cells and into the phloem sieve tubes -this is mostly a passive process -due to the high concentration of sucrose in the phloem, water moves into the sieve tubes by osmosis -creating a positive hydrostatic pressure which moves the phloem sap, containing sucrose, towards sink cells -maintaining a constant diffusion gradient

Question 27

describe the apoplast pathway in the translocation of sucrose

Answer 27

-sucrose moves by diffusion into the cell wall of companion cells -in the companion cells, sucrose moves from the cell wall into the cytoplasm by active transport -creating a high concentration of sucrose in the cytoplasm of companion cells -water moves into companion cells, creating a positive hydrostatic pressure that moves assimilates and water into the phloem sieve tubes -sucrose moves into the sieve tubes through plasmodesmata by diffusion -the hydrostatic pressure moves the xylem sap towards sink cells -near the sink cells companion cells have a lower concentration of sucrose, therefore there is a lower hydrostatic pressure -so less movement of water into the phloem sieve tubes

Question 28

what are the disadvantages of the mass flow model

Answer 28

-it doesn't take into account the active loading of sucrose from companion cells by the source into the phloem sieve tubes -translocation is continuous, it doesn't end with equal concentrations, because the model doesn't take into account the continuous loading of sucrose at the source and removal at the sink -water can move into the phloem sieve tubes at any point -the return route for water to cells is through the xylem

Question 29

describe the mass flow hypothesis

Answer 29

-container A contains a concentrated sugar solution -container C contains a dilute sugar solution -water moves into container A at a higher rate than C -causing a flow of solution from A to C -the hydrostatic pressure created forces water out of container C -this flow continues until both containers have the same concentration of solution

Question 30

describe the pressure flow hypothesis

Answer 30

-sucrose is actively loaded into sieve tube elements -osmotic gradient generated causing water to move into sieve tube elements -hydrostatic pressure rises in the phloem sieve tubes -mass flow of water down the hydrostatic pressure gradient -sucrose moves with the water -sucrose and water are removed from sieve tube elements and move into the sink cells