Transport Within Plants Packet #36 Chapter #36

Introduction I Transport Movement of materials between roots and shoots Occurs on three levels Cellular Uptake and loss of water, and solutes, by individual cells Tissue/Organ Level “short distance” transport of substances Whole Plant Level Long distance transport of sap within xylem and phloem

Cellular Level Most Already Covered Packet #9 “Water” Packet #12 “Cell Membrane & Movement of Molecules” Packet #13 “Cell to Cell Communication”

Review I Diffusion Proton Pumps Regular Diffusion Facilitated Diffusion Active Transport Proton Pumps Use of ATP Formation of the proton gradient

Review II Water Potential Also, review A measure of free energy of water. Measured in megapascals Pure water has a water potential of 0 megapascals Water with dissolved solutes has a negative water potential Water moves from an area of higher water potential (less negative) to an area of lower water potential (more negative) water potential. Also, review Solute potential Pressure potential Osmotic Pressure  = s + p

Review III Osmotic Pressure Recall Determined by the concentration of dissolved substances (solutes) in a solution. Cells regulate this pressure to prevent shrinking or bursting Recall Plant cells withstand high internal hydrostatic pressure because of their cell walls Prevents the cell from expanding and bursting When water moves into the cell, via osmosis, the central vacuole is filled Cell swells Turgor pressure is built up along the rigid cell walls.

Review IV Flaccid describes a limp cell. Plasmolyze Plasmolysis The shrinking and pulling away of the plasma membrane from the cell wall Plasmolysis Process of shrinking and pulling away from the cell wall in a hypertonic environment Turgor Pressure Hydrostatic pressure Describes the swelling of the cell and the associated pressure Turgid cells are ones in which the solute concentration is greater than their surroundings. Water flows in to reach an isotonic state. Cell begins to swell and apply pressure on the cell wall.

Review V Facilitated Transport The movement of a substance, including water, from an area of high concentration to an area of low concentration, using a protein but no ATP The protein NOW has a name.

Aquaporins Transport proteins that facilitate the passive movement of water across a membrane Does not effect water potential but increases the rate at which water diffuses.

Tissue/Organ Level Lateral Transport Short Distance along the Radial Axis of Plant Organs.

Introduction I Short distances along the radial axis of plant organs Lateral Transport Three routes Cell to cell Packet #12 & 13 Symplastic Pathway The symplast is a continuum of living cytoplasm, which is connected from one cell to the next by cytoplasmic bridges called plasmodesmata. Packet #11 Chapter 34 of Biology by Solomon

Introduction II Apoplastic Pathway The apoplast consists of interconnected pourous cell walls of a plant, along which water and nutrient mineral ions move freely. Water and minerals diffuse freely without ever entering a living cell. Chapter 34 of Biology by Solomon

Whole Plant Level Long Distance Transport

Introduction Vocabulary Diffusion Efficient process for moving solutes over short distances Bulk Flow Movement of fluid driven by pressure Functions in long distance transport Roots to leaves Hydrostatic Pressure Difference in pressure, generated in the phloem, between the top if the tube to the bottom Transpirational Pull Tension, or negative pressure, due to the evaporation of water from a leaf

Tension Cohesion Model & Transpiration Pull Explains the rise of water in even the largest plants. Transpiration pull Tension or negative pressure due to the evaporation of water from a leaf Result of the water potential gradient that ranges from the slightly negative water potentials in the soil and roots to the very negative water potentials in the atmosphere. Rate of flow depend on diameter of the leaf.

Functions of Transport

Functions of Transport Absorption of water and minerals by roots Root hair/epidermis  cortex endodermis  pericycle  root xylem Transport (ascent) of xylem sap Xylem sap moves upwards to the veins that branch out each leaf How? Root Pressure Control of transpiration Guard cells control the size of stomata and help balance the photosynthesis-transpiration compromise Transport of organic nutrients within phloem Translocation Phloem sap moves via bulk flow

Root Pressure

Root Pressure Caused by the movement of water into roots from the soil as a result of the active absorption of nutrient mineral ions from the soil Pushes water up through the xylem. Causes guttation HW assignment Guttation—the exudation of water droplets. More water entering the system than leaving via transpiration so the excess is forced out.

Root Pressure II Transpiration low at night Root cells still pump nutrients into the xylem Minerals accumulate in the steele Water potential gets low Water moves in from the cortex of the root generating a positive pressure Pressure forces fluid up the xylem

Transpiration

Transpiration Part II Loss of water vapor from inside the leaf surfaces. Transpiration Pull Negative pressure or tension pulls the water out of the leaf through the mesophyll and ultimately through the stomata. Pressure is transmitted from leaves, to root tips and even into the soil solution

Control of Transpiration Guard cells control the size of stomata and help balance the photosynthesis- transpiration compromise Cues for stomata to open at dawn Light Depletion of CO2 within air spaces Circadian rhythms Environmental stress can cause stomata to close during day

Sugar Translocation & Pressure Flow Hypothesis

Translocation I The phloem transports sugars throughout the plant via translocation Dissolved sugar is translocated upward or downward in phloem Sucrose is the predominant sugar translocated Dissolved sugar originates from a source to a sink Direction of flow may change with the season or the developmental stage of the plant.

Translocation II Sugar Source Sugar Sink Storage Organ Plant organ where sugar is produced by either photosynthesis or the breakdown of starch. Area of excess sugar Usually a leaf—mature leaf Sugar Sink Plant organ that is a net consumer or storer of sugar Growing roots Shoot tips Apical meristems Fruits Seeds Storage Organ Can either be a source or sink depending on the season.

Bulk (Pressure) Flow Hypothesis Movement of materials in phloem Companion cells actively load sugar into the sieve tubes at the source ATP is required for this process ATP supplies energy to pump protons out of the sieve tube elements Proton gradient drives the uptake of sugar by the cotransport of proteins back into the sieve tube elements Sugar therefore accumulates in the sieve tube element Causes the movement of water into the sieve tube via osmosis

Bulk Flow II Research suggests that the ability to transport sugars and not photosynthetic rates limits crop production. A better understanding of the factors that limit bulk flow of sugars may help to improve agricultural technology.