Mass transport in plants and aphids Flashcards
(12 cards)
Movement of water into the xylem
Soil → root hair cells
Water moves into the root hair cells by osmosis down the water potential gradient
Root hairs are long and branched to provide large surface area for water uptake
Root hair cells have thin surface layer across which materials can move faster
Root hair cells → endodermis
Water travels through the cortex and endodermis along apoplastic and symplastic pathways
Apoplastic pathway – through cell walls, by cohesion, symplastic pathway – through cell cytoplasm and plasmodesmata, by osmosis
At the casparian strip water from the apoplastic route joins that from the symplastic route as the casparian strip is a waxy layer in the cell wall
Endodermis → xylem
Active transport of salts from endodermal cells into the xylem, along carrier proteins, lowers the water potential of the xylem, so water travels in by osmosis
This creates root pressure – one of the two driving forces behind transpiration
Movement of water from the xylem
Xylem → mesophyll cells
Water travels along apoplastic and symplastic pathways through the mesophyll cells
Water loss to the air spaces establishes a water potential gradient for osmosis to occur
Mesophyll cells → air outside of the leaf
Evaporation of water from mesophyll cells into the air spaces
Stomata open for CO2 to diffuse in for photosynthesis
Water is lost through stomata into the surrounding air down the water potential gradient
Movement of water up the stem
Root pressure
Water moving into the xylem at the roots down the water potential gradient
Cohesion-tension
Cohesion – hydrogen bonds form between water molecules so they tend to stay together
As water evaporates from the mesophyll cells, more water molecules are drawn up as a result of cohesion
Water is pulled up the xylem = transpiration pull
Transpiration pull puts the xylem under tension – negative pressure in the xylem
Factors affecting transpiration
↑ Light - ↑ rate of transpiration
Stomata open in the light and close in the dark
↑ Temperature - ↑ rate of transpiration
Alters the kinetic energy of the water molecules and the relative humidity of the air
↑ Humidity - ↓ rate of transpiration
Affects the water potential gradient between the air spaces in the leaf and the atmosphere
↑ Air movement - ↑ rate of transpiration
Changing the water potential gradient by altering the rate at which moist air is removed from around the leaf
Energy for transpiration comes from evaporation of water from leaves, which is mostly the result of the Sun’s energy. Therefore, Sun ultimately drives transpiration and water movement in the xylem up the stem.
Measurement of water uptake using a potometer
- Cut the leafy shoot under water to stop air entering the xylem
- Dry the leaves using tissue paper
- Rubber bung and petroleum jelly used to seal all the junctions
- Syringe pushed down to push the air bubble back to its initial position
- Distance travelled by the air bubble measured repeatedly
- Volume of water lost plotted against time taken
Mechanism of translocation
Source → phloem
Sucrose is made from products of photosynthesis at the source
Sucrose travels down the concentration gradient by facilitated diffusion into the companion cells
Hydrogen ions actively transported from companion cells into spaces within cell walls using ATP, then diffuse into the sieve tube elements through carrier proteins
Sucrose is transported along with hydrogen ions by the process called co-transport
Mechanism of translocation
In the phloem
At the source:
As sucrose enters the phloem, the water potential of the phloem is lower
Water moves from the xylem into sieve tubes by osmosis
High hydrostatic pressure is created
At the sink:
Cells have a low sucrose content as it’s constantly used up
Sucrose enters the cells, decreasing their water potential, so water also moves into the cells, from sieve tubes, by osmosis
The hydrostatic pressure of the sieve tubes is lowered
Mass flow of sucrose solution down the hydrostatic gradient occurs in the sieve tubes
Mechanism of translocation
Phloem → sink
At the respiring cells (sink), sucrose is either used up in respiration or converted into starch for storage
These cells have a lower sucrose content, so sucrose is actively transported by the companion cells out of the sieve tubes and into the sink cells
The water potential of the cells lowers, so water leavers the phloem and moves into these cells by osmosis
Xylem vs Phloem
xylem
- transpiration
- water and salts
- up the stem, from roots to leaves
- non-living tissue
- no end walls
- stiffened with lignin
phloem
- translocation
- sucrose and amino acids
- in any direction, from source to sink
- living cells, have companion cells
- sieve plates - perforated cell walls
- not lignified
How do aphids feed on plants?
- insert their mouthpiece, stylet, into the phloem
- extract the phloem sap which is forced into their body under the pressure
*some sap is exuded from the rear of the aphid
3 ways aphids reduce yields in crop plants
- by removing phloem sap and depriving the plant of sugars and amino acids
- by encouraging the growth of mould on leaves, which reduces photosynthesis and looks unattractive
- by transmitting plant viruses
How can plant viruses spread?
- plants are immobile
- viruses cannot penetrate plant cell walls
so they enter the plant cells
- through damage sites
- by using vectors
