9.1 Transport in the Xylem of Plants Flashcards
Transpiration if the inevitable consequence of gas exchange in the leaf
1) Define Transpiration
2) Outline gas exchange that occurs through leaf stomata
Co2 is needed for photosynthesis to take place in the leaves and oxygen is released as a by-product. Gas exchange occurs through the stomata (when open gas exchanges) - water also lost through transpiration. Guard cells regulate opening and closing of the stomata depending on the turgor pressure of the surrounding guard cells. When guard cells take in water the turgor pressure increases opening the stomata, when the guard cell loses water pressure decreases and stomata close. Adhesion and Cohesion help to maintain the water column through from the root to leaf
Plant transport water from the roots to the leaves to replace losses from transpiration
1) outline structures and mechanisms involved in the flow of water from roots to leaves
Evaporation occurs when some of the light energy absorbed by leaf converted to heat - water vapour. Transpiration if the evaporation of water form leaves stems and flowers. Majority water lost through stomata. Transpirational pull results when water evaporated from leaves and stems. Water is drawn up through the plant to replace water lost. results from the combined forces of adhesion and cohesion. Water moves into the roots by osmosis through the cell walls (apoplast) and through the cytoplasm (symplast) as the concentration of solutes inside the cell is greater than outside due to active transport of minerals and ions in.
The cohesive property of water and the structure of the xylem allow transport under tension
1) describe the structure of the xylem
2) outline how xylem is able to maintain rigidity even under low pressure
2) outline polarity of water molecules
3) define cohesion
xylem vessels are transport tissue found in vascular plants composed of a number of different types of cell, including long continuous thin usually dead cells (tracheids and vessel elements). Xylem vessels have cell wall strengthen with lignin which thickens and makes them stronger. Since atmospheric pressure is greater than the pressure inside the xylem vessel, ridged structure prevent them from collapsing.
The xylem is responsible for the transport of water and soluble materials from roots to different parts of plant that use water. This allows minerals absorbed from the soil to be transported through the xylem to the leaves
Water molecules cling tofether by hydrogen bonds between the molecules. This is known as cohesive forces and helps water to be pulled through the plant. Along with the adhesive forces of water molecules to the xylem walls- a strong tension force is created within the xylem vessel. The cohesive property would stop all flow of water through the xylem vessels
The adhesive property of water and evaporation generates tension forces in leaf cell walls
1) explain the decrease in pressure and transpiration-pull that results from evaporation of water from the leaf
adhesion :
- the cell walls of xylem vessels are charges, attracting water molecules
- the adhesive attraction of water to xylem vessels moves them up the stem against gravity
- adhesion is important when sap starts to rise in plants that were leafless through the winter
- adhesion also helps prevent the column of water-filled xylem vessels from breaking
Active uptake of mineral ions in the roots causes absorption of water by osmosis
1) Explain why roots are hypertonic relative to the soil
2) Outline the role of active transport in maintaining root tonicity
3) Describe how water enters roots from soil
4) Compare the symplastic and apoplastic pathways of water transport through the root
If the mineral conc of certain ion greater inside root than out - mineral ions must be actively transported into the root cell. In addition : cannot directly cross the cell membrane because of the non polar regions inside the bilayer. Proton pumps use energy (ATP) to pump protons out of root cell - results in higher conc of protons outside root cell creating electrochemical and conc gradient
H can combine with sucrose NO3 -, PO4 3- and other - bring them back into the root cell through protein channels (following the conc gradient established)
Water will move into thr root cell by osmosis
Adaptations of plants in deserts and in saline soils for water conservation
1) Define xerophyte and halophytic
2) outline strategies used by these plants to reduce water loss
Xerophytes : plants adapted to grow in very dry conditions
1) reduced leaves = decreases SA for transpiration decreasing water loss
2) Rolled leaves = creates local humidity in the leaf decreasing the leafs exposure to air currents as water evaporates into the rolled leaf rather than air
3) Reduced number of stomata = water loss through transpiration is decreased as there are fewer holes for evaporation to take place
4) Thickened waxy cuticle = makes the leaves more waterproof and impermeable to water . preventing loss
5) Stomata sunken in pits and presence of hair creates humidity by trapping moist air close to leaf - decreases exposure to air currents and hair reduces air flow around the stomata
Halophytes : plants that can tolerate salty conditions due to the presence of a number of adaptations
1) cellular sequestration - sequester toxic ions and salts within the cell wall
2) tissue partitioning - concentrate salts in particular leaves which then drop off
3) root level exclusion - structured to exclude 95% of the salt in soil solutions
4) salt excretion - certain parts of the plant may contain salt glands which actively eliminate salt
Models of water transport in xylem using simple apparatus including blotting or filter paper, porous pots and capillary tubing
1) explain use a models in science
2) describe simple models of water transport, inclusive of evaporation, adhesion and cohesion
movement of water up length of xylem modelled using simple apparatus - capillary tubing, filter or blotting paper and porous pots
1) models allow one factor/ aspect to be studied independently
2) capillary tubes to model adhesion between water and xylem vessel walls
3) porous pot to model flow in a xylem vessel due to transpiration from leaf
4) blotting paper to model capillary action
Drawing the structure of primary xylem vessels in sections of stems based on microscope images
1) Draw a vessel tube labelling cellulose wall and helical lignin
drawing the structure of primary xylem vessels is it important to remember the following features:
1) vessel elements should be drawn as a continuous tube
2) remnant of the fused end wall can be represented as indents
3) xylem wall should contain gaps which allows exchange of water
4) lignin can be represented by spiral or annular arrangement
Measuring the rate of transpiration rates using a potometer
1) Describe the use of a potometer to measure transpiration rates
potometer- device used to measure the rate of water uptake by leafy plant shoot.
Light : guard cells close in darkness so transpiration much greater in light
Temp: doubles for every 10*C increase in temp
Wind: removes water vapour from leaf , reducing potential around leaf
Humidity : as humidity decreases = decreased rate of transpiration water loss
