mass transport in plants

Question 1

plant transport - xylem

Answer 1

water and mineral ions

water is needed as a reactant in photosynthesis and hydrolysis
also maintain turgor pressure in plant cells

Mineral ions: NO3- to make proteins and DNA, RNA, ATP
				PO43- to make PL, DNA, RNA, ATP
				K+ to open stomata
				Mg2+ to make chlorophyll
				H+ to synthesise ATP in resp and photosyn

Question 2

plant transport - phloem

Answer 2

sucrose and amino acids (products of photosynthesis / organic molecules)

Glucose / sucrose needed as respiratory substrate to produce ATP to release energy for endothermic reactions / AT etc.
Other molecules such as Glycerol, FA, AA etc can be synthesised from glucose (actually from Triose Phosphate - next year).
AA needed for protein synthesis.

Question 3

xylem

Answer 3

made of dead cells to form hollow tubes - less resistance to flow of water

passive process driven by transpiration

unidirectional - roots to leaves

molecule called LIGNIN - makes it woody - is deposited on the inside of the xylem vessels

Question 4

phloem

Answer 4

made of living cells with cytoplasm - to dissolve sucrose and amino acids

active process by translocation - a form of mass flow (movement down a pressure gradient)

bidirectional - from source (where photosynthesis occurs) to the sink (where products of photosynthesis are used or stored)

sieve tube cell - contains cytoplasm, mitochondria, chloroplasts but NO nucleus/ribosomes/ER - this makes space in cytoplasm for sugar

companion cell - normal plant cell - metabolic processes for sieve tube cell

gaps in cell wall act as a sieve allow flow of cytoplasm

Question 5

cohesion-tension theory

Answer 5

stomata open in response to light

water molecules evaporate out of stomata

this pulls the next water molecule and the next upwards - due to cohesion force (H-bonds) between water molecules

pulls the entire transpiration stream upwards against gravity

this causes negative water potential in the root

the water column does not collapse due to gravity because of the adhesive force / H-bonds between water and lignin

Question 6

evidence to support cohesion-tension theory

Answer 6

the diameter of a tree trunk is smaller during the day (transpiration occurring causing xylem walls to be pulled inwards and upwards due to adhesion) and larger at night when no transpiration

when the stem is cut, the water column very quickly contracts on both sides because the water column is under tension

Question 7

factors affecting transpiration

Answer 7

1. External Factors:
   a. Temperature
   b. Light Intensity
   c. Wind speed
   d. Humidity
   e. Soil water content
   
   2. Internal Factors:
      a. SA of leaf
      b. Position / location of stomata
      c. Leaf structure
      d. Root:Shoot ratio
      e. Orientation of leaves

Question 8

studying transpiration

Answer 8

use a potometer to measure rate of water uptake

• the stem / shoot must be cut underwater and inserted into the potometer
  (to prevent air bubbles forming - disrupt H-bonds and interrupt transpiration stream)

dye/air bubble introduced into tube - as water is taken up by the plant, the bubble moves towards the plant

internal diameter of tube (d) r=d/2

Question 9

translocation

Answer 9

Translocation in the Phloem - Mass Flow Hypothesis
At the source:
1. Sucrose is actively transported from the companion cell into the sieve tube cell (active loading).
2. Water potential of the sieve tube cell decreases (becomes more negative).
3. Water moves in from the xylem by osmosis.
4. This causes high hydrostatic pressure in the source.
5. Therefore the cytoplasm / sucrose moves down the pressure gradient from the source to the sink.

At the sink:
Sucrose is actively transported from the s.t.c to the c.c. (active unloading).
y increases, water moves out to the xylem by osmosis.
Causes a low hydrostatic pressure at the sink.

Question 10

evidence for translocation in the phloem

Answer 10

radioactive glucose/aa/organic molecules given to plant - took a cross section of the stem and exposed to x-ray film - radioactivity detected only in phloem
- proves that phloem transports organic molecules

aphids - droplets ooze out (not sucked out by insect) - proof that movement occurs down a pressure gradient
- can test for sugars, aa - but cannot tell which specific sugar
- chromatography - use Rf values to identify the components accurately

Question 11

ringing experiments

Answer 11

• Phloem is removed / stopped halfway down the tree.
• Sucrose is transported to that point, and then accumulates just above the ring.
• Lower y, therefore water accumulates - causing a bulge.
• No sucrose supplied to the roots: roots initially use up any stored carbohydrates as respiratory substrates.
• When no carbs are left, they stop making ATP, so no active transport of ions into the root cells.
• y increases, so no osmosis of water into the roots.
• No transpiration - plant wilts, cannot supply of water for photosynthesis or hydrolysis reactions. Plant dies.

Question 12

how is water transported in plants

Answer 12

through xylem vessels; long, continuous columns that also provide structural support to the stem

Question 13

explain cohesion-tension theory

Answer 13

water molecules form hydrogen bonds to stick each other, causing them to stick together (cohesion)

the surface tension of the water also created this sticking effect

therefore as water is lost through transpiration - more can be drawn up the stem

Question 14

what are three components of phloem vessels

Answer 14

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

phasmodesmata = gaps between cell walls where the cytoplasm links, allowing substances to flow

Question 15

how does sucrose in the lead move into the phloem

Answer 15

sucrose enters companion cells of the phloem vessels by active loading

which uses ATP and a diffusion of hydrogen ions

sucrose then diffuses from companion cells into the sieve tube elements through the plasmodesmata

Question 16

how do phloem vessels transport sucrose around the plant

Answer 16

as sucrose moves into the tube elements, water potential inside the phloem is reduce

this causes water to enter via osmosis from the xylem and increase hydrostatic pressure

water moves along the sieve tube towards areas of lower hydrostatic pressure

sucrose diffuses into surrounding cells where it is needed

Question 17

evidence for the mass flow hypothesis of translocation

Answer 17

sap is released when a stem is cut, therefore there must be a pressure in the phloem

there is a higher sucrose concentration in the leaves than roots

increasing sucrose levels in the leaves results in increased sucroses in the phloem

Question 18

evidence against the mass flow hypothesis of translocation

Answer 18

the structure of sieve tubes seem to hinder mass flow

not all solutes move at the same speed, as they would in mass flow

sucrose is delivered at the same rate throughout the plant, rather than to areas with the lowest sucrose concentration first

Question 19

how can ringing experiments be used to investigate transport in plants

Answer 19

the bark and phloem of a tree are removed in a ring, leaving behind the xylem

eventually the tissues above the missing ring swells due to accumulation of sucrose as the tissue below begins to die

therefore sucrose must be transported in the phloem

Question 20

how can tracing experiments be used to investigate transport in plants

Answer 20

plants are grown in the presence of radioactive CO2 which will be incorporated into the plant’s sugars

using autoradiography, we can see that the areas exposed to radiation correspond to where the phloem is