Lecture 19-â Transport
Lecture 19-‐ Transport Vascular system multifunctional and complex • Vascular systems multifunctional o Distribute § Nutrients § Water § Gases § Hormones o Heat management o Waste management • Plants use two systems o Xylem: concentrates on water, moving some nutrients around and heat management o Phloem: deals with sugar and hormones
Xylem for water transport: brilliant and brainless • Brainless: plants’ H20 system wasteful> incomplete circle o Water leaks out of stomates • Brilliant: unlike many animals, H20 transport in plants requires no metabolic energy
Leaf
A lot of vein endings in leaves One square centimetre of leaf may have up to 6,000 vein endings Spongy mesophyll speeds movement of CO2 to main site of photosynthesis (palisade mesophyll) but increased water loss is the cost • 90-‐95% of plants’ water loss occurs through the leaves through the stomates • 90% of this via stomata • 10% via cuticle
• • •
Two pathways to epidermis
1. Water enters pores in cell wall 2. Wall pores, then across plasma membrane into root hair
Three properties that are really important: These pathways utilise one or more of 3 properties of H2O
1. Capillary-‐ water likes to stick to water and water likes to stick to other things
Water is drawn through the system by adhesive/ cohesive reactions between water molecules and pore surfaces.
Cohesion-‐ water is attracted to water Adhesion-‐ water is attracted to other substances
2. Hydrostatic pressure-‐ positive and negative
3.
“Diffusion” pressure due to H2O
How does water potential dictate water flow?
•
The total water pressure (or “potential”) of a solution is the sum of its o Hydrostatic pressure o Diffusion pressure (proportional to H2O)
If water can flow: • It will always flow from a region of higher total water “potential” to one with lower “potential”
HIGH → LOW
By convention Water potential (Ψ) of pure water is zero-‐ so lower potentials are -‐ve
Through the cortex: two possible routes • Intracellular spaces are one CONTINUUM in plants = SYMPLAST (across plasma membranes) • Extracellular spaces operate as a second continuum= APOPLAST (outside plasma membranes)
Onwards: the endodermis: guardian of the stele • An endodermal cell might be the only cell a H2O molecule need pass through between root and stem. • At endodermis H2O must switch to the symplastic route WHY? 1. Forced passage through plasma membrane allows screening of unwanted materials: a. Toxic minerals b. Microbes 2. Prevents backflow of water from stele via extracellular route: . A one-‐way valve a. Allows pressure build-‐up in stele
Tension-‐ cohesion theory • Water is constantly lost by transpiration in the leaf. When one water molecule is lost another is pulled along by the processes of cohesion and adhesion. • Water moves from high water potential to low • Water molecules stick together and are pulled out of leaves
Xylem for water transport: brilliant and brainless • Brainless: plants’ H20 system wasteful> incomplete circle o Water leaks out of stomates • Brilliant: unlike many animals, H20 transport in plants requires no metabolic energy
Leaf
A lot of vein endings in leaves One square centimetre of leaf may have up to 6,000 vein endings Spongy mesophyll speeds movement of CO2 to main site of photosynthesis (palisade mesophyll) but increased water loss is the cost • 90-‐95% of plants’ water loss occurs through the leaves through the stomates • 90% of this via stomata • 10% via cuticle
• • •
Two pathways to epidermis
1. Water enters pores in cell wall 2. Wall pores, then across plasma membrane into root hair
Three properties that are really important: These pathways utilise one or more of 3 properties of H2O
1. Capillary-‐ water likes to stick to water and water likes to stick to other things
Water is drawn through the system by adhesive/ cohesive reactions between water molecules and pore surfaces.
Cohesion-‐ water is attracted to water Adhesion-‐ water is attracted to other substances
2. Hydrostatic pressure-‐ positive and negative
3.
“Diffusion” pressure due to H2O
How does water potential dictate water flow?
•
The total water pressure (or “potential”) of a solution is the sum of its o Hydrostatic pressure o Diffusion pressure (proportional to H2O)
If water can flow: • It will always flow from a region of higher total water “potential” to one with lower “potential”
HIGH → LOW
By convention Water potential (Ψ) of pure water is zero-‐ so lower potentials are -‐ve
Through the cortex: two possible routes • Intracellular spaces are one CONTINUUM in plants = SYMPLAST (across plasma membranes) • Extracellular spaces operate as a second continuum= APOPLAST (outside plasma membranes)
Onwards: the endodermis: guardian of the stele • An endodermal cell might be the only cell a H2O molecule need pass through between root and stem. • At endodermis H2O must switch to the symplastic route WHY? 1. Forced passage through plasma membrane allows screening of unwanted materials: a. Toxic minerals b. Microbes 2. Prevents backflow of water from stele via extracellular route: . A one-‐way valve a. Allows pressure build-‐up in stele
Tension-‐ cohesion theory • Water is constantly lost by transpiration in the leaf. When one water molecule is lost another is pulled along by the processes of cohesion and adhesion. • Water moves from high water potential to low • Water molecules stick together and are pulled out of leaves
