AP Biology, Chapter 36
Transport in Plants 

SUMMARY
INTRODUCTION

AN OVERVIEW OF TRANSPORT MECHANISMS IN PLANTS
	Introduction
		1. List three levels in which transport occurs in plants.
			a. Cell  environment
				i. Photosynthetic gases
				ii. Water and minerals from soil 
			b. Cell  cell
				i. Sugar from parenchyma into sieve tubes
				ii. Water from root hair to the xylem in the stele
			c. Organ  organ
				i. Water in xylem in the roots  leaves
				ii. Sugar in leaf phloem  roots
	Transport at the cellular level depends on the selective permeability of membranes
		2. Compare the processes of passive and active transport. Distinguish between the two 
		main categories of transport proteins.
			a. Passive
				i. Does not require the use of cellular energy; higher  lower
				ii. Ex.: osmosis, CO2 into parenchyma
			b. Active
				i. Requires the use of cellular energy; lower  higher
				ii. Ex.: sugar into phloem, protons out of the cell
			c. Transport protein mechanisms
				i. Undergo a conformational change; ex.: ??
				ii. Form a selective channel; ex.: K+ channels, aquaporins
	Proton pumps play a central role in transport across plant membranes
		3. Describe the role and importance of proton pumps in transport across plant 
		membranes.
			a. ATP is used to pump H+ out
				i. Chemical gradient since H+ is higher outside
				ii. Voltage or potential positive outside
			b. Uses
				i. Positive ions like K+ are attracted into the cell
				ii. Diffusion of H+ can power transport of other materials
		4. Define cotransport and chemiosmosis.
			a. Cotransport = H+ is allowed in with NO3- moving from lower to higher
			b. Chemiosmosis = linkage of any energy-consuming process to a proton 
			gradient
	Differences in water potential drive water transport in plant cells
		5. Define osmosis and water potential. Explain how water potential is measured.
			a. Osmosis = diffusion of water through a selectively permeable membrane
			b. Water potential
				i. Tendency of water to move taking into account concentration and 
				pressure
				ii. Measured in megapascals (MPa), a unit of pressure
		6. Explain how solutes and pressure affect water potential.
			a. Water pressure (Ψ) = solute potential (ΨS) + pressure potential (ΨP)
			b. Adding solutes makes ΨS negative, lowering water potential
			c. Increasing the pressure increases water potential
		7. Explain how the physical properties of plant cells are changed when the plant is placed 
		into solutions that have higher, lower, or the same solute concentrations. Define flaccid, 
		plasmolyze, turgor pressure, and turgid.
			a. Solution effects: hypo, bigger; hyper, smaller; iso, the same
			b. Definitions
				i. Flaccid: not exerting outward pressure
				ii. Plasmolyze: lose water, membrane shrinks away from cell wall
				iii. Turgor pressure: pressure of the cell against the cell wall
				iv. Turgid: solute potential balances pressure potential, stiff with water
		8. Explain how aquaporins affect the rate of water transport across membranes.
			a. Aquaporins are proteins forming water channels; speed up osmosis
	Vacuolated plant cells have three major compartments
		9. Describe the three major compartments in vacuolated plant cells, noting their 
		interrelationships.
			a. Cell wall: soaked in extramembraneous fluid; continuous with neighbors
			b. Cytoplasm: modified by transport through membranes and plamsodesmata
			c. Vacuole: contents = cell sap; not continuous from cell-to-cell
	The symplast and apoplast both function in transport within tissues and organs
		10. Describe the three routes available for lateral transport in plants.
			a. Apoplast: transport through extramembraneous fluid
			b. Symplast: cytoplasms are continuous by plasmodesmata
			c. Cell-to-cell across cell membranes
	Bulk flow functions in long-distance transport
		11. Define bulk flow and describe the different types of forces that generate pressure.
			a. Bulk flow = fluid driven by pressure
			b. Forces
				i. Phloem
					.Sugar transported into leaf sieve tube members
					.Water follows by osmosis raising the pressure
				ii. Xylem
					.Evaporation in leaves lowers the pressure in leaf xylem
					.Cohesive water column is pulled upward
		12. Relate the structure of sieve-tube cells, vessel cells, and tracheids to their functions 
		in bulk flow.
			a. Sieve-tube members are cells
				i. Must maintain sugar concentration
				ii. Simplified for lower resistance
			b. Vessel elements and tracheids are dead and empty for lower resistance

ABSORPTION OF WATER AND MINERALS BY ROOTS
	Introduction
	Root hairs, mycorrhizae, and a large surface area of cortical cells enhance water and 
	mineral absorption
		13. Explain how the structure of root hairs promotes their functions. Explain how 
		mycorrhizae facilitate the functions of roots.
			a. Root hairs
				i. Have a large surface area
				ii. They cling to soil particles
				iii. Water flows over the particles and along the cells
			b. Mycorrhizae further increase surface area 
	The endodermis functions as a selective sentry between the root cortex and vascular 
	tissue
		14. Explain how the endodermis functions as a selective barrier between the root cortex 
		and vascular tissue.
			a. Apoplast is blocked by suberin in the Casparian strip around each endodermal 
			cell
			b. Water and minerals must pass through the symplast to enter stele
			c. Minerals are pumped out of the endodermal cells into the stele apoplast
			d. Endodermis especially prevents leakage back out of the stele

TRANSPORT OF XYLEM SAP
	Introduction
	The ascent of xylem sap depends mainly on transpiration and the physical properties 
	of water
		15. Describe the potential and limits of root pressure to move xylem sap. Define root 
		pressure, transpiration, and guttation.
			a. Root pressure
				i. At night, transpiration stops
				ii. Roots are still absorbing water
				iii. Xylem in the stele has higher solutes
				iv. Pressure builds up in root xylem, pushing the water up
			b. Transpiration is the evaporation of water out of the leaves
			c. If the root pressure is high enough drops of water come out the leaves = 
			guttation.
		16. Explain how transpirational pull moves xylem sap up from the root tips to the leaves.
			a. Water evaporates from spaces in spongy parenchyma
			b. Water film forms strong menisci in the corners of the spaces
			c. Surface tension of the menisci pulls water through apoplast
			d. Cohesion of water molecules pulls on the entire water column
	Xylem sap ascends by solar-powered bulk flow: a review

THE CONTROL OF TRANSPIRATION
	Guard cells mediate the photosynthesis-transpiration compromise
		17. Describe the role of guard cells in photosynthesis-transpiration.
			a. Plant must balance needs to exchange gases and save water
			b. Guard cells can open and close stomata
		18. Explain the advantages and disadvantages of the extensive inner surface area of a 
		leaf.
			a. Advantages 
				i. Increases surface area for absorption of CO2
				ii. Most photosynthetic cells are close to air
			b. Disadvantages
				i. Increases surface area for water loss
				ii. 90% of water loss is through stomata
		19. Explain how the transpiration-to-photosynthesis ratio is calculated and what it 
		indicates about a plant.
			a. Calculation: water lost per gram of fixed CO2
			b. Indicates efficiency of water usage: C4 plants use half the water
		20. Explain how transpiration changes the temperatures of leaves and why this is 
		adaptive. Explain how plants with low transpiration rates compensate for higher 
		temperatures.
			a. Transpiration is evaporative cooling
			b. Keeps photosynthetic enzymes from denaturing
			c. Desert plant enzymes are adapted to be stable at higher temperatures
		21. Explain how and when stomata open and close.
			a. During the day: guard cells are turgid, more curved, stomata open
				i. Proton pumps export H+
				ii. K+ flows in, water follows by osmosis
			b. At night: guard cells are flaccid, less curved, stomata closed
			c. At dawn
				i. Light receptors turn on proton pumps
				ii. Photosynthesis begins, CO2 in mesophyll goes down
				iii. Circadian rhythm
			d. Close in the daytime in response to stress
	Xerophytes have adaptations that reduce transpiration
		22. Explain how xerophytes reduce transpiration.
			a. Small thick leaves with a thick cuticle
			b. Stomata in depressions with trichomes
			c. CAM plants store CO2 at night; close stomata in the day

TRANSLOCATION OF PHLOEM SAP
	Introduction
	Phloem translocates its sap from sugar sources to sugar sinks
		23. Define and describe the process of translocation. Trace the path of phloem sap from 
		the primary sugar source to common sugar sinks.
			a. Translocation is the transport of food
			b. Phloem sap
				i. Flows through sieve tube members
				ii. As much as 30% sucrose; also minerals, amino acids, hormones
			c. Translocation mechanism =  pressure flow
				i. Sugar is transported into sieve tube members
				ii. Water follows by osmosis
				iii. Increase pressure pushes sap through sieve plates
			d. Pathway
				i. Sources: leaves, storage tissues
				ii. Through sieve tube members
				iii. Sinks: growing tissues, storage tissues, any living tissue
		24. Describe the process of sugar loading and unloading.
			a. Various combinations of symplast and apoplast
			b. Into sieve tube members by cotransport with protons
	Pressure flow is the mechanism of translocation in angiosperms