Chapter 36 Flashcards
Which direction is negative pressure and what goes in that direction?
Water and minerals are pulled up the plant
Which direction is positive pressure and what goes in that direction?
Sugars are pushed both ways
What was the purpose of the evolution of xylem and phloem?
Made long-distance transport of water, minerals, and products of photosynthesis possible
Apoplast
Major transport pathway ; consists of everything external to the plasma ; includes cell walls, extracellular space, interior of dead cells
Symplast
Major transport pathway ; consists of the entire cytosol of all living cells in a plant ; plasmodesmata
Short distance active transport
Transport from root hairs to the xylem
How can water cross the cortex?
Via the symplast or apoplast or through the transmembrane route
Aquaporins
Transport proteins in the cell membrane that facilitate the passage of water
What is the last checkpoint for the selective passage of minerals from the cortex to the vascular tissue
Endodermis surrounding the vascular cylinder
What does the waxy casparian strip of the endodermal wall do?
It blocks apoplastic transfer of water and minerals from the cortex of the vascular cylinder
What is bulk flow?
Long-distance transport ; the movement of a fluid driven by a pressure gradient
How do water and solutes move together?
They move through Tracheids and vessel elements of the xylem and the sieve-tube elements of the phloem
What do the branching veins in leaves do?
They ensure that all cells are within a few cells of the vascular tissue
How does bulk flow move?
Goes from areas of high pressure to areas of low pressure independently of the difference in solute potential
What is transpiration?
The movement of xylem sap (water and minerals) against gravity, from the soils to the leaves, without using any energy
What does transpiration depend on?
The evaporation of H2O from the leaves pulls water upwards from the roots(tension) ; Physical properties of water(cohesion and adhesion)
Cohesion-tension hypothesis
Transpiration provides the pull for the ascent of xylem sap and water cohesion transmits this pull along the entire length of the xylem from shoots to roots
What kind of pressure is xylem pressure normally under?
Negative pressure or tension
What do guard cells do?
Balance water conservation with the need for gas exchange by opening and closing stomata
Characteristics of leaves and what it does
Large surface area and high surface-to-volume ratio ; increases rates of both photosynthesis and water loss
How do guard cells change the diameter of the stoma?
By changing shape ; when turgid they bow outward and the pore between them opens ; when flaccid they become less bowed and the pore closes
Where does the majority of the water a plant loses escape from?
The stomata ; water loss is limited elsewhere on leaf by waxy cuticle ; amount of water lost per leaf depends largely on stomata density and average stoma size
Stomata opening is triggered at dawn by
Light ; co2 depletion ; an internal “clock” in guard cells
Circadian rhythms
Internal clock ; 24-hour cycle
What causes a change in turgor pressure?
The absorption and loss of potassium ions (K+) by the guard cells
Water potential
Tendency of water to move from one area to another
How does free water move?
From regions of higher water potential to regions of lower water potential if there is no barrier
Water potential equation
Ψ= Ψs (solute potential) + Ψp (pressure potential)
Process of cohesion-tension theory
Water vapor diffuses out of leaf, water evaporates inside leaf, water is pulled out of xylem, water is pulled up xylem, water is pulled out of root cortex, water moves from soil into root
How is bulk flow driven?
Driven by water potential different at opposite ends of xylem tissue ; doesn’t need energy
Translocation
Process by which products of photosynthesis are transported through the phloem
What is phloem sap and how does it travel?
It is an aqueous solution high is sucrose ; travels from sugar source to sugar sink through sieve tube elements
Sugar source
Organ that is a net producer of sugar (ex: mature leaves)
Sugar sink
An organ that is a net consumer or depository of sugar (ex: roots, buds, fruits)
What kind of pressure if bulk flow?
Bulk flow by positive pressure is the mechanism of translocation in angiosperms
What are the different ways a sugar can travel?
May move by symplastic or both symplastic and apoplastic pathways
What do companion cells do?
Enhance solute movement between the apoplast and symplast
Pressure flow hypothesis
Predicts that phloem sap near sugar sources should have higher sugar content than phloem sap near sinks
What is sugar transport based on?
Turgor pressure near sink tissues to generate the necessary force (not based on transpiration)
