Topic 3.4 - Mass Transport in Plants Flashcards
What is the function of the xylem?
- Xylem tissues transports water and minerals in solution. Allows substances to move up the plant from roots to leaves
What are xylem vessels apart of?
Part of xylem tissue that transports water and ions
Describe the structure of xylem vessels
- They’re long, tube-like structures formed from dead cells joined end to end
- There’s no end walls on these cells
Why are there no end walls on the cells that make up xylem vessels?
Makes uninterrupted tube = allows water to pass up through middle easily by cohesion
Describe how water gets from the soil to the roots
Water enters root hair cells by osmosis due to the active uptake of mineral ions which has created a water potential gradient (conc. of solutes in soil is lower than in roots)
What happens to the water once it has entered the roots?
Water moves through the cortext by osmosis down a water potential gradient gradient
Name the mechanisms that help the movement of water up the xylem vessels
- Mass flow
- Cohesion tension theory
- Adhesion
Describe mass flow of water through the xylem vessels.
- Whole body of water moving together
- Pressure from water moving into roots = high pressure at base of xylem
- Pressure is higher than top therefore water forced upwards
e.g. similar to water moving up straw
Describe cohesion tension theory
- Water molecules are held together by hydrogen bonds (weak bonds)
- Therefore if one molecule moves, it drags another with it
- Attraction between them = cohesion
Describe adhesion of water which helps mass flow
- Same bond between water molecules will bind molecules to side of xylem vessels
- Therefore water almost “crawls” up side of vessels
Describe how water in the roots moves up the stem
- Water evaporates from the leaves, creates water potential gradient (lower water potential in leaves)
- Water drawn out of xylem by osmosis
- Creates tension on water in xylem
- Water molecules cohesive, so column of water in xylem move upwards
- Due to h-bonding, column doesn’t break because of adhesion with xylem walls
What is transpiration?
Evaporation of water from plant’s surface through leaves
Describe Transpiration
- Water evaporates from moist cell walls and accumulates air spaces in leaf
- When stomata open, water vapour diffuses out leaf down concentration gradient
- (Higher concentration of water vapour inside leaf than outside leaf)
Describe the transpiration pull/stream
- Water evaporates from cell walls of mesophyll
- Water from xylem vessels replaces this
- Water moving out of xylem reduces pressure. Therefore water is at a higher pressure so it can move up the xylem vessels
Name 4 factors that affect the rate of transpiration
- Light Intensity
- Temperature
- Humidity
- Wind
Describe how temperature affects the rate of transpiration
- Higher temp. = faster transpiration rate
- Water molecules have more kinetic energy so therefore evaporate from cells inside leaf faster. This increases water potential gradient so water diffuse out leaf faster
Describe how humidity affects the rate of transpiration (3)
- As humidity increases = transpiration rate decreases
- because a higher humidity causes a reduced water potential gradient (Less evaporation)
Describe how wind affects the rate of transpiration
- Windier = faster transpiration rate
- Lots of air movement blows away water molecules around stomata
- Increases water potential gradient
What does a potometer do?
estimate transpiration rates
How does a potometer estimate transpiration rates?
Measures water uptake by plant but assumes water uptake by plant is directly related to water loss by leaves
What is the function of the phloem?
Phloem tissue transports solutes (mainly sugar e.g. sucrose) round plants
What is the phloem formed from?
It’s formed from cells arranged in tubes
Name the 2 cells the phloem is formed from
- Sieve tube elements
- Companion cells
Describe sieve tube elements
- Living cells that form tube for transporting solutes
- No nucleus and few organelles
Describe companion cells
- Companion cell for each sieve tube element
- Carry out living functions for sieve cells, e.g. providing energy needed for active transport of solutes
What is translocation?
Movement of assimilates (solutes) from one area of a plant to another (where they’re needed) by mass flow
Translocation requires ____
energy
Why does translocation require energy and where does it come from?
Companion cells produce ATP to actively load assimilates into and out of sieve tube elements
Translocation moves solutes from ______ to _____
‘sources’ to ‘sinks’
What is a source?
Area where sucrose is moved into phloem (high concentration)
Give an example of a source
e.g. source for sucrose is leaves
In winter: roots - convert stored starch back to sugars when needed for growth
What is a sink?
Area where sucrose is removed from phloem (low concentration)
Give an example of a sink
e.g. food storage organs, meristems (area of growth) in roots, stems and leaves
(In winter: leaf)
What do enzymes maintain in translocation & how?
- Enzymes maintain concentration gradient from source to sink by changing solutes at sink
- (e.g. breaking them down or making them into something else)
- Makes sure there’s a lower concentration at sink than at source
What does the mass flow hypothesis explain?
Explains how solutes transported from source to sink by translocation
Describe how solutes are transported from source to sink by translocation
- Active transport used to actively load solutes from companion cells into sieve tubes of phloem at source (e.g. leaves)
- This decreases water potential inside sieve tubes, water enters tubes by osmosis from xylem and companion cells
- This increases pressure inside sieve tubes = mass movement (towards sink)
- At sink end, solutes removed from phloem to be used up
- This increases water potential inside sieve tubes which causes water to leave tubes by osmosis
- pressure decreases inside sieve tubes
- Result is pressure gradient from source end to sink end
- Gradient pushes solutes along sieve tubes towards sink
- At sink, solutes used for respiration or stored
Name 4 pieces of experimental evidence of translocation
- Ringing experiment
- Tracer experiment
- Use of aphids
- Metabolic Inhibitor
Describe the ringing experiment
- Ring of bark/phloem removed from woody stem
- = bulge forms above ring
- Fluid from bulge has higher concentration of sugars than fluid from below ring
- = evidence that there’s downward flow of sugars
Describe the tracer experiment
- Supply part of plant (e.g. leaf) with carbon-14
- Carbon-14 incorporated into organic substances produced by leaf (e.g. sucrose) = moved around plant by translocation
- Movement of substances tracked using autoradiography
- Results show translocation of substances from source to sink
Describe how aphids are used to find evidence of translocation
Pressure in phloem can be investigated using aphids
- They pierce the phloem, leave mouthparts behind which allows sap to flow out
- Sap flows out quicker nearer leaves than further down stem
- Evidence for pressure gradient
Describe how a metabolic inhibitor is used to find evidence of translocation
- Stops ATP production, put into phloem and translocation stops
- Evidence that active transport involved
Name and describe 2 objections to the mass flow hypothesis
- Sugars travel to many different sinks, not just to one with highest water potential, as model would suggest
- Sieve plates would create barrier to mass flow
- Lots of pressure would be needed for solutes to get through at reasonable rate
Explain why the values for the pressure in the xylem are negative
(Inside xylem) lower than atmospheric pressure / (water is under) tension
Why is less water lost through upper surface of leaves than through the lower surface?
- More stomata on the lower surface
- (thicker) waxy cuticle on the upper surface
Describe and explain 5 adaptions of xerophytic plants to reduce water loss
(xerophytes = plants adapted for life in warm, dry or windy habitats)
- Stomata sunk in pits
- Increases humidity in pits/reduces exposure to the wind
- Reduced water potential gradient
- Layer of ‘hairs’ on epidermis
- Traps layer of moist air around stomata
- Reduced water potential gradient
- Curled leaves
- Reduced SA for water loss / stomata covered
- Reduced number of stomata
- Reduced SA for water loss
- Thick waxy cuticle on leaves & stems
- Waterproof = reduces evaporation
Explain why the diameter of a tree trunk is smallest at midday (on a sunny, summer day) (6)
- Midday = warmest & brightest time of day
- Stomata open in light = more water loss
- More heat energy for water evaporation
- Cohesion between water molecules
- Adhesion between water molecules and walls of xylem vessels
- Xylem pulled in by tension (faster flow of water)
Summary: ↑ Transpiration = produce higher tension in xylem = reducing diameter
Explain why increasing light intensity increases tension in the xylem vessels in the leaves (6)
- More stomata open
- Increased evaporation/transpiration
- Ψw of leaves becomes lower
- ∴ more water moves from xylem to surrounding cells
- Down Ψw gradient
- Cohesion between water molecules
Xerophytic Leaf
Explain how the leaf being rounded helps reduce water loss (1)
Smaller SA to volume ratio
