A-level Biology - 3.2.8 Mass Transport Systems in Plants
The function of the xylem is to transport water and mineral ions from the roots to the leaves in one direction — upwards. This movement supports photosynthesis and maintains cell turgor for structural support.
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
Key Terms
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
Describe how water gets from the soil to the roots
Water enters root hair cells by osmosis
∵ active uptake of mineral ions has created a Ψw gradient (conc. of solutes in soil is lower than in ...
What happens to the water once it has entered the roots?
Water moves through the cortext by osmosis down a Ψw gradient
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| Term | Definition |
|---|---|
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 |
Describe how water gets from the soil to the roots | Water enters root hair cells by osmosis ∵ active uptake of mineral ions has created a Ψw 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 Ψw gradient |
Name the 2 possible pathways across the cortex to the endodermis | Apoplastic pathway Symplastic pathway |
Describe how water travels through the apoplastic pathway | Water soaks into cellulose walls of cells in cortex Water seeps towards xylem |
Where and why is the water in the apoplastic pathway stopped & what does this mean? | Pathway stopped at endodermis as there’s the Casparian strip Which is impermeable Water is forced into cytoplasm, enters cell by osmosis |
Describe how water travels through the symplastic pathway | Cells in cortex joined by plasmodesmata (small pores) Water moves by osmosis through cytoplasm + vacuoles of each cells and through plasmodesmata between cells |
Name the mechanisms that help the movement of water up the xylem vessels | Mass flow Cohesion tension theory Adhesion |
Transport through Xylem Vessels Describe mass flow | Whole body of water moving together Pressure from water moving into roots = high pressure at base of xylem Pressure is higher than top ∴ water forced upwards e.g. similar to water moving up straw |
Transport through Xylem Vessels Describe cohesion tension theory | Water molecules are held together by hydrogen bonds (weak bonds) ∴ if one molecule moves, it drags another with it Attraction between them = cohesion |
Transport through Xylem Vessels Describe adhesion | Same bond between water molecules will bind molecules to side of xylem vessels ∴ water almost “crawls” up side of vessels |
Describe how water in the roots moves up the stem (5) | Water evaporates from the leaves, creates Ψw gradient (Ψw ↓ 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 ∵ of adhesion with xylem walls |
What is transpiration? | Evaporation of water from plant’s surface – 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 = water at 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 light intensity affects the rate of transpiration | Lighter = faster transpiration rate ∵ stomata open when there’s light to let in CO2 for photosynthesis |
Describe how temperature affects the rate of transpiration | Higher temp. = faster transpiration rate Water molecules have more kinetic energy = evaporate from cells inside leaf faster Increases water potential gradient = water diffuse out leaf faster |
Describe how humidity affects the rate of transpiration (3) | As humidity increases = transpiration rate decreases ∵ higher humidity = 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 essentially 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 |
Describe how you would investigate the rate of water uptake of plant using a potometer | Cut shoot underwater Cut it at a slant to increase SA available for water uptake Assemble potometer in water and insert shoot underwater = no air can enter Remove apparatus from water but keep end of capillary tube submerged in beaker of water Check apparatus if watertight and airtight Dry leaves, allow time for shoot to acclimatise and shut tap Remove end of capillary tube from beaker of water until one air bubble forms & put tube back in water Record starting position of air bubble Start stopwatch and record distance moved by bubble per unit time Rate of air bubble movement = estimate of transpiration rate |
Potometer Pratical Name the 2 formulas you need for calculations | Rate = Distance moved (mm) / Time (s) Volume of water = Distance moved x Area of circle |
Potometer Pratical Is it a direct measurement of water lost from the stomata? Explain your answer. | No, water is used for photosynthesis & to maintain turgor pressure |
Potometer Pratical Suggest how reservoir allows repeat measurements to be made | Allows bubble to be returned to start |
Potometer Pratical Why should you use only a healthy shoot? | To ensure stomata are open and transpiration can happen |
Potometer Pratical Why should the leaves be dried? | Moist leaves would affect rate of diffusion (water vapour potential gradient) from the stomata and make results invalid |
What is the function of the phloem? | Phloem tissue transports solutes (mainly sugar e.g. sucrose) round plants |
What is the phloem is 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 |
Give an example of how enzymes maintain the concentration gradient from source to sink | e.g. in potatoes, sucrose converted to starch in sink areas ∴ always lower concentration of sucrose at sink than inside phloem Makes sure constant supply of new sucrose reaches sink of phloem |
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) ↓ Ψw inside sieve tubes, water enters tubes by osmosis from xylem and companion cells ↑ pressure inside sieve tubes = mass movement (towards sink) At sink end, solutes removed from phloem to be used up ↑ Ψw inside sieve tubes = water leaves tubes by osmosis ↓ pressure 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 |
Tracer Experiment Describe how the movement of substances can be tracked using an autoradiography | To see where carbon-14 has spread, plant killed and whole plant is placed on photography film Radioactive substances is present wherever film turns black |
Tracer Experiment Results show translocation of substances from source to sink. Suggest what the results would look like. | e.g. autoradiographs of plants killed at different times show overall movement of solutes form leaves to roots |
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 |
Potometer Pratical Why should you cut the shoot underwater? (2) | To prevent air bubble being trapped in xylem Would slow/stop water uptake |
Potometer Pratical How do you reset the air bubble to the start? | Open tap on the reservoir to release water & move air bubble to start |
Potometer Pratical Describe a method the students could use to find the rate of water uptake in mm min-1 mm-2 of leaf surface Explain why this is a more valid comparison | Measure surface area using graph paper Divide transpiration/water uptake rate by surface area Takes into account surface area, some shoots may have bigger leaves |
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 |
Lignin is present in the xylem cell walls. Explain how it is related to the function of the xylem tissue. (1) | Resists tension in water / provides support |
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 |
If you hang leaves on some thread, why does their mass decrease and then plateau if you leave them for several hours? (3) | Stomata open = water evaporates Water potential gradient reduces as water not being replaced (i.e. not water supply) Stomata close |
Xerophytic Leaf Explain how the leaf being rounded helps reduce water loss (1) | Small SA to volume ratio |