Transport In Plants

Transport In Plants

Chapter 32 – Transport in Plants

Transport at the cellular level

	Hydrolyze ATP and use the released energy to pump hydrogen ions out of the cell resulting in a proton gradient. Sets up a membrane potential to help move things into the cell.
	Plant cells store energy in the proton gradient to drive the transport of solutes
	Cotransport also helps move some of these solutes through by moving the anions with the hydrogen ions at the same time.
	This is referred to as chemiosmosis because of the transmembrane proton gradient.

Differences in water potential

	Water potential is the ability of water molecules to move from one place to another based on a gradient. If the solute concentration is high, the water potential is low and vice versa.
	Measured in megapascals
	The more negative the number, the lower the water potential.

Aquaporins

	Water specific transport proteins.
	Cannot actively transport water but facilitate diffusion and increase the transport rate.
	May form from a gated channel

Vacuolated plant cells

	First compartment is between the cell wall and the cell membrane
	The second is between the cell membrane and the vacuole
	The tonoplast, membrane that surrounds the vacuole, is the third compartment.
	Plasmodesmata connect the cells together for movement of molecules between the cells.
	Symplast moves “stuff” through the plasmodesmata and within the cell itself
	Apoplast moves stuff along the cell walls and never enters the cell itself.

Absorption of Water by roots

	Increase surface area
	Most occurs near the root tips where the epidermis is permeable.
	Root hairs are extensions of the epidermis
	Soil particles are usually covered in water and mineral and adhere tightly to the hairs.
	The soil solution flows into the hydrophilic walls of the epidermis and passes freely along the apoplast into the root cortex.
	As it moves along the cell walls, some of the water and solutes are taken up by the cells of the epidermis and cortex by switching pathways.

The Endodermis

	Water and minerals pass from the soil into the root cortex and cannot be moved to the rest of the plant until it reaches the stele.
	The endodermis is the innermost layer of cells in the root cortex and surround the stele as the last checkpoint.
	Minerals in symplast stay that way and continue on into the vascular bundle by the plasmodesmata.
	Each endodermal cell has a Casparian strip, which is a belt, made of suberin, a waxy material that is impervious to water and dissolved minerals.
	Water and minerals cannot cross this so they cannot get into the stele via apoplast.

Transpiration

	Loss of water vapor from the leaves that must be replaced for the plant to continue photosynthesizing.
	At night, when transpiration is low, the root cells are still expending energy to pump minerals into the xylem. The endodermis prevents back spills.
	The accumulation of minerals lowers the water potential and water moves into the cortex creating a positive pressure that forces fluid into the xylem. This is call root pressure.
	Root pressure causes guttation which is the exudation of water droplets that can be seen in the morning on the tips of grass blades. During the night, the root pressure forces water out the specialized hydathondes which function as escape valves.

Pulling Xylem Sap

	Stomata are connected to airspaces that are saturated with water. When the air outside contains less water than the leaf, water diffuses out of the leaf readily.
	This depends on the generation of a negative3 pressure in the leaf.
	Cohesion and Adhesion of water pulls more up because as one molecule leaves, another must enter the system.

Controlling Transpiration

	Controlled through the stomata
	Open and close based on the amount of water in the leaf and the uptake and loss of potassium ions.
	More potassium, more water and vice versa

Translocation of Phloem Sap

	Transport of sugar
	Phloem sap is an aqueous solution that differs from xylem sap by the amount of sucrose dissolved.
	May also contain minerals, amino acids, and hormones in transit
	The direction phloem sap travels may very.
	Moved from a sugar source to a sugar sink.

	The sugar source is a plant organ where sugar is being produced like mature leaves.
	The sugar sink is an organ that consumes or stores sugar. Growing roots, shoots, stems, and fruits.
	A sink usually receives sources from the nearest source.

Loading and unloading

	Sugar from the Mesophyll cells must be loaded.
	Sometimes it moves by symplast and other times, it uses a combination of pathways.
	Companion cells and transfer cells are often used.