3.5 Mass transport in plants Flashcards
Describe 4 ways that the xylem is adapted for its function
- Long hollow vessel with no end walls => allows for movement of continuous water column
- No organelles or cytoplasm => prevents disruption of water flow
- Supported by rings of lignin => to resist the tension of water and provide strength/support
- Has pits (gaps in cell wall) => to allow lateral movement of water (from vessel to vessel)
What happens during transpiration
- Water vapour diffuses through the air spaces into the surrounding air, via the stomata
- Water vapour lost from air spaces is replaced by evaporation of water from surrounding mesophyll cells
- Water potential in mesophyll cells decreases => osmosis of water from adjacent cells into mesophyll cells
- Water from xylem replaces water lost from mesophyll cells
What is the name of the theory used to explain transpiration
The cohesion-tension theory.
Describe cohesion in the xylem during transpiration
- Water molecules in mesophyll cells and xylem form hydrogen bonds
- This causes water to form a continuous, unbroken column
Describe tension in the xylem during transpiration
- Water lost from mesophyll cells, causing water to move into the mesophyll cells from xylem vessels by osmosis
- Causes a column of water to be pulled up the xylem - transpirational pull
- Transpirational pull puts the xylem under tension and xylem has negative pressure (xylem vessels pushed inwards)
Describe adhesion in the xylem during transpiration
There are attractive forces between water molecules and xylem walls/lignin, causing the water molecules to adhere to the walls of the xylem.
Describe the symplast pathway
The cytoplasm’s of adjoining cells are connected through little pores (gaps) in the cell wall called plasmodesmata which water can travel through.
What are 2 ways water can be transported from cell to cell
- Symplast pathway (through cytoplasm)
- Apoplast pathway (through cell wall)
Where does translocation occur
In the phloem vessels
What is translocation
- The transport of biological molecules around the plant (e.g. sucrose, amino acids, etc.)
- Occurs from the site of production (source) to the site of use/storage (sink)
What are the 2 types of cells present in the phloem vessels
- Sieve tube elements
- Companion cells
Describe the structure of the sieve tube elements
- Elongates
- Perforated cell walls at ends - sieve plates (not completely open like in the xylem)
- No nucleus and very few organelles for easier flow
- Thick cell walls to resist the high pressure
Describe the structure of companion cells
- High mitochondrial density to increase ATP synthesis for active transport
- Have nuclei
- High ribosome density for a greater rate of carrier protein synthesis
How are sieve tube elements and companion cells linked
By plasmodesmata in the cell walls.
Why is sucrose transported around the plant rather than glucose
- Less soluble than glucose
- Less reactive than glucose (not going to be used in respiration)
How does sucrose enter the phloem
- H+ ions (protons) are actively transported out of the phloem into the mesophyll cells
- H+ ions combine with sucrose in the mesophyll cells, causing them to move into the companion cells by co transport which increases the concentration of sucrose in the companion cells
- This causes sucrose to diffuse into the sieve tube elements through the plasmodesmata
How is sucrose transported to the rest of the plant
- Concentration of sucrose in the sieve tube elements increases which decreases the water potential
- This causes water to move into the sieve tube elements by osmosis from the xylem, which increases the hydrostatic pressure
- Water moves down the phloem from an area of high to low hydrostatic pressure
- Sucrose is then unloaded into the sink cells by active transport via carrier proteins
- Sucrose is used by the sink cells for respiration, or converted to starch for storage
- Loss of sucrose from the sieve tube elements increases the water potential, causing water to move back into the xylem vessels by osmosis
What is used to measure the rate of transpiration
The rate of transpiration can be estimated using a potometer
Why can a potometer be used to estimate the rate of transpiration
- Approximately 99% of water taken up by a plant is lost via transpiration
- Therefore, the rate of water uptake is approximately equal to the rate of transpiration
Describe a method for preparing a potometer
- Leafy shoot cut underwater (to avoid air bubbles in xylem)
- Potometer and reservoir is filled with water
- Remove from water and seal all joints with waterproof jelly (e.g. vaseline)
- Introduce an air bubble into the capillary tube
- Measure the distance moved by the air bubble in a given time
- Tap can be opened and reservoir allows air bubbles to return to start point and repeat
- Repeat and calculate a mean time
What is the formula used to calculate the volume of water lost
- V = 𝝅r^2l
- r is the radius of the tube
- l is the length of the tube
What is the evidence for part 1 of the mass flow theory
Tree ringing
Describe the process of tree ringing
- Cut off a ring of tree bark (including phloem)
- Over days, accumulation of sugary sap visible just above the cut
How does tree ringing provide evidence for part 1 of the mass flow theory
The accumulation of sugary sap visible just above the cut shows that sucrose is moving via mass flow from source to sink (from photosynthetic cells to non-photosynthetic cells).
What is the evidence for part 2 of the mass flow theory
Tracers
Describe the process of using tracers
- Plant exposed to CO2 containing C-14 radioisotope
- Plant fixes 14-CO2 during photosynthesis to produce 14-glucose
- Plant converts this glucose to 14-C containing sucrose (sucrose containing 14-C radioisotope)
- Plant stem cut and extracted after various time intervals
- Autoradiograph detects 14-C radioisotopes in stem
- Movement of sucrose in stem observed from source to sink, via phloem
What are aphids
- Small green insects farmed by ants
- Require sugar water to survive
- No jaws to bite
How do aphids get sugar sap from plants
- Pierce into sieve tube vessels
- Extract sap (sucrose solution)
- Able to extract as the sap is under higher pressure than atmospheric air, so is ‘squeezed’ out on its own
Describe the relationship between time of day and sucrose concentration in phloem compared to leaves and why this provides evidence for mass transport
- Lower overall concentration (lower peak and trough)
- Shallower gradient due to smaller range between minimum and maximum
- Trough and peak both shifted approximately 1-2 hours later in the day
- Shows mass transport takes time
