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Lecture 5

BIOL 1202 Lecture Notes - Lecture 5: Casparian Strip, Sieve Tube Element, Water Potential

Biological Sciences
Course Code
BIOL 1202

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CH. 36 – Resource Acquisition & Transport
Acquiring Resources
Adaptations for acquiring resources were key steps in the evolution of vascular plants.
The broad category of nutrition includes all of the materials and energy needed for an organism to live and reproduce
The “materials” required for life are nutrients
Nutrition in all organisms involves four main steps:
Acquisition of nutrients
Digestion, if required for a useable form of nutrients
Distribution of nutrients throughout the body.
Synthesis of molecules for the organism's body.
Plants and animals acquire their nutrition differently
Starting material Elements and small molecules Large organic molecules
Digestion Usually none Required to break large molecules down to building
Distribution Driven by osmosis and evaporation Pumping mechanism for most
Synthesis of molecules Can do it all Must acquire some through diet
Plants acquire all their nutrients from soil (minerals), water (hydrogen and oxygen), or air (CO2)
Transport in vascular plants occurs on three scales:
1) Transport of water and solutes by individual cells
2) Short-distance transport of substances from cell to cell at the level of tissues and organs
3) Long-distance transport (bulk flow) within xylem and phloem
First Scale
Transport of water (by osmosis) and solutes (by diffusion or active transport) by individual cells occurs via a transmembrane route
Very short distances across a membrane
Plasma membrane controls movement of solutes into and out of the cell
Passive transport
Straight through phospholipid bilayer
Facilitated diffusion
Using transport proteins
▪ Co-transport
Active transport
“Pay” for the movement AGAINST concentration gradient
Water Movement
Concentration of solutes controls the movement of water
▪ Osmosis
Pressure can regulate water movement
Water potential (Ψ) affected by
Solute potential (ΨS) – determined by concentration of dissolved molecules
Concentration goes up, ΨS goes down
Will ALWAYS be negative
Pressure potential (ΨP) – determined by the physical pressure on a solution
Negative pressure is sucking up, has to exceed the force of gravity
Ψ = Ψs + Ψp
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When the water potential inside the cell is lower than the water potential outside the cell, water moves in (osmosis).
Because the cell expansion is limited by the cell wall the pressure potential increases and the cell becomes turgid (see fig 36.9)
Second Scale
Short-distance transport of substances from cell to cell at the level of tissues and organs can occur via the symplastic route (see figure 36.6).
This route uses plasmodesmata to flow from one cell to the next.
Third Scale
The symplastic route can also be used for long-distance transport (bulk flow) within xylem and phloem.
Bulk flow: something moving and whatever is moving gets to move for free, like cytoplasm
Example: water moving in street, but the trash and dirt in the water goes for free
The apoplastic route allows movement of material (water and solute) around the outside of cells and can occur over long distances as long as
no barrier to the route exists
Only minerals dissolved in soil water are accessible to roots.
Four step process of mineral absorption by roots (see fig 36.10)
1) Active transport into root hairs.
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2) Diffusion through root hair cytoplasm to endodermis cells via either apoplastic or symplastic route.
3) Active transport from endodermis cytoplasm into cell or the extracellular space of the vascular cylinder
4) Diffusion into the xylem
Using this process the plant may concentrate minerals inside their tissues.
Role of Casparian strip
The Casparian strip "leakproofs" the vascular cylinder, retaining the concentrated mineral solution within the extracellular space of the vascular
Water moves straight in from outside of root hairs into xylem by osmosis.
The movement of solute has resulted in a lower water potential in the cells.
So by moving minerals into the root, the plant has created conditions to drag in water as well.
How this works:
1) Higher water potential in soil relative to the root cells.
2) The waterproof Casparian strip blocks movement of water between cells at the endodermis.
3) The cell membranes of the endodermis act as a gate separating the outer low mineral (higher water potential) solution in the cortex from
the inner high mineral (lower water potential) solution in the vascular cylinder.
4) Therefore, water moves from the cells and the extracellular space outside the Casparian strip, through the endodermal cells, and into the
extracellular space inside the vascular cylinder by osmosis.
5) Water moves from the extracellular space of the vascular cylinder into tracheids and vessel of xylem through the cell wall pits.
6) Water is then pulled up the xylem, powered by evaporation of water from the leaves.
Only this last step is not osmosis.
Water and dissolved minerals move from roots to stems and leaves by a process known as bulk flow where water and mineral move together.
Not a big deal when you think of a small seedling, but what about a huge oak, or a giant sequoia over a football field high.
The cells at the top of the plant are the most metabolically active (that's where growing is taking place), need water and minerals.
Minerals are dissolved in water, so the question becomes: How does water move up a plant against gravity?
This problem is answered by the "Cohesion-tension theory".
Called a theory because you cannot actually watch it occur, you can only see the effects of it
Cohesion-Tension Theory
Two fundamental parts of the cohesion-tension theory:
1) Cohesion - water within xylem tubes sticks together (by hydrogen bonds).
Water molecules in the xylem tube resist being pulled apart
2) Tension - water is pulled up by negative pressure.
Because water molecules bind to each other, they can pull water up like a chain.
Water is pulled up the xylem powered by the force of evaporation of water from the leaves.
This process is transpiration
Transpiration - How it works (see fig 36.13)
1) Water evaporation occurs through stomata of leaves.
This creates the pull of water
2) Water leaving leaf makes it “dry”, so it pulls in more water from xylem (see fig 36.12).
3) The water molecule leaving the xylem is “stuck” to other water molecules, so it pulls up on those molecules, thereby pulling the "chain" of
water molecules up the tree.
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