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Transport in Plants. What is the tallest tree on the planet?. Sequoia sempervirens - The coastal redwood (115m = 379 feet).

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Transport in Plants

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Transport in plants

Transport in Plants

What is the tallest tree on the planet

What is the tallest tree on the planet?

Sequoia sempervirens - The coastal redwood (115m = 379 feet)

Seems like it would require a pump, like you and I have, but a much larger one to transport substances from roots to leaves. Trees as we know do not have any “pumps” of that nauture. So how do they do it?

Maybe plants push xylem sap root pressure

Maybe Plants Push Xylem Sap: Root Pressure

  • Water flows in from the root cortex generating a positive pressure that forces fluid up the xylem. This is upward push is called root pressure

  • Root pressure sometimes results in guttation, (the exudation of water droplets on tips of grass blades or the leaf margins of some small, herbaceous dicots in the morning). More water enters the leaves than is leaves it (transpired), and the excess is forced out of the leaf.

Plant transport mechanisms solve a fundamental biological problem

Plant transport mechanisms solve a fundamental biological problem:

  • The need to acquire materials from the environment and distribute them throughout the entire plant body



  • Clear nail polish

  • Leaves

  • Activity 2

  • Flaccid carrot and cucumber slices

  • Bowl

  • dH2O, bottled water, tap water

  • Salt

Precursor 1 water chemistry and characteristics

Precursor 1: Water chemistry and characteristics

  • Polarity

    *H-bonds (Strong or weak? Can you draw and H-bond between 2 or more water molecules?)

  • Consequences include: Cohesion, Adhesion, Surface Tension…etc (properties of water)

Activity 3 a mini experiment demonstration

Activity 3: A mini-experiment/demonstration

  • Indirect and relative measure of H-bond strength (as well as cohesion andadhesion)

    • Glass slides

    • Plastic cups

    • Water

    • Pennies

    • Masking Tape (Thumbs)

Precursor 2 selective permeability of membranes

Precursor 2. Selective Permeability of Membranes

  • The selective permeability of a the plasma membrane controls the movement of solutes into and out of the cell AND the role of:

  • Specific transport proteins are involved in movement of solutes (and water too!)

  • Passive Transport – Diffusion, Facilitated Diffusion, Osmosis (Differences?)

  • Active Transport (Features of?)

Proton pumps










Proton pump generates

membrane potential

and H+ gradient.








Proton Pumps

Proton pumps create a hydrogen ion gradient that is a form of potential energy that can be harnessed to do work

They contribute to a voltage known as a membrane potential (Plant cytoplasm is (-) compared to extracellular fluid)

Consequences include:

Fac diffusion of other cations

Cotransport: symport and antiport (secondary active transport)

Membrane potential and cation uptake





Cations ( , for

example) are driven into the cell by themembrane potential.











Transport protein


(a) Membrane potential and cation uptake

Membrane potential and cation uptake

  • Plant cells use the proton gradient and membrane potential to drive the transport of many different solutes (e.g. cation (+) uptake: opposites attract)

Cotransport symport




NO3 –




Cell accumulates

anions (, for

example) by

coupling their transport to theinward diffusion









of through a



NO3 –


NO3 –








(b) Cotransport of anions

Cotransport (symport)

  • In cotransport a transport protein (known as a symport) couples the passage of one solute to the passage of another in the same direction

Cotransport antiport

Cotransport (Antiport)

  • Energy released as a molecule (e.g.H+) diffuses back into the cell and powers the active transport of a second molecule (ex. Ca++ or Na+) out of the cell

Sucrose uptake







Plant cells can

also accumulate a

neutral solute,

such as sucrose

( ), by


down the

steep proton






















(c) Cotransport of a neutral solute

Sucrose uptake

  • The cotransport is also responsible for the uptake of the sugar sucrose (a neutral solute) by plant cells

An important membrane protein side note

An important membrane protein side note

Water potential

Water Potential

  • To survive plants must balance water uptake and loss

  • Water potential is a measurement that combines the effects of solute concentration and physical pressure (due the presence of the plant cell wall) It is a measurement of the FREE amount of water molecules and the direction of movement of water (i.e. water’s potential to do work).

  • Water flows from regions of high water potential (areas of more free water molecules) to regions of low water potential (less free water molecules)

Ex of water doing work on an organismal level

Ex. of water “doing work” on an organismal level

Which has the greatest water concentration

Which has the greatest water concentration?

  • A or BA or B

  • Water potential is essentially not much different

Getting a little technical the water potential equation don t freak out think p o s e i don

Getting a little technical - The water potential equation. Don’t freak out! Think Poseidon!

Transport in plants

By convention, plant physiologists measure water potential in units of pressure called megapascals (MPa). Note: bars is acceptableFor a baseline, the water potential for pure water at 1ATM is expressed as having 0 Mpa or 0 bars

Breaking it down

Breaking it down….

Cont d


Consider this u tube examples ap loves them

Consider this (U-tube Examples – AP Loves them)

  • An artificial model

Cont d addition of solute example

Cont’d: Addition of Solute example

Cont d positive pressure example

Cont’d – Positive Pressure Example

Cont d a negative pressure example

Cont’d: A negative pressure example

Connection to plants

Connection to plants:

Ap will not be thrilled however if that was your response to an explain what happens prompt

AP will not be thrilled however if that was your response to an “Explain what happens” prompt

  • So what’s a better answer?

Ap explain what happens prompt possible answers

AP “Explain what happens” prompt possible answers

  • 1 star = The cell gains water

  • 2 stars = Since water moves from high water potential to low water potential, it will enter the cell.

  • 3 stars = (include the data if provided) – Since the water potential for the cell is -0.7 bars and the surrounding environment has a water potential of 0 bars, water moves into the cell.

  • 4 stars = (include consequences ) Since the water potential for the cell is -0.7 bars and the surrounding environment has a water potential of 0 bars, water moves into the cell making it turgid.

Cont d produce 4 star answer for scenario b at home not now

Cont’d – Produce 4 star answer for scenario B(At home, not now )

  • Note: The original cell has a starting water potential of -0.7 bars

Transport in plants

Compare each situation with respect to the cytoplasm’s water potential and the surrounding environment’s water potential

cell env. cell env. cell env.

Water Pot:

Bonus info, free of charge: What could you say about each situations: cell env cell env cell env

Water concentration?

Solute Concentration?

Osmotic potential?

Collaborative review study break

Collaborative Review/Study Break

  • On mini-poster paper

    • 1. Explain the role(s) of a gradient of protons in moving substances across a plant cell’s plasma membrane

    • 2. How do symports and antiports differ? Give an example of key substances each mechanism transports.

    • 3. What is “water potential” and discuss why it is important with respect to plant cells

Check your vegetable and your fruit


  • Evaluate your slices

  • Explain what has happened to them to a classmate (or to a teacher)

Next step how do roots take in water and minerals from the soil

Next Step: How do roots take in water and minerals from the soil

  • Water and mineral salts from the soil enter the plant through the epidermis of roots and ultimately flow to and through the shoot system (xylem tissue) by bulk flow and active transport respectively.

  • Bulk flow – the group movement of molecules in response to a difference in pressure between two locations (see more later)

  • Soil solutionRoot Hair EpidermisRoot Cortex Root Xylem

Cont d1


  • Root Hairs

    • Much of the absorption of water and minerals occurs near root tips, where the epidermis is permeable to water and where root hairs are located

    • Root hairs account for much of the surface area of roots

A mutulaistic symbiotic relationship and a surface area multiplier

A mutulaistic symbiotic relationship. and a surface area multiplier

Plant cell structure more info for understanding transport

Cell wall

Transport proteins in

the plasma membrane

regulate traffic of

molecules between

the cytosol and the

cell wall.

Transport proteins in

the vacuolar

membrane regulate

traffic of molecules

between the cytosol

and the vacuole.




Cell compartments. The cell wall, cytosol, and vacuole are the three main

compartments of most mature plant cells.

Vacuolar membrane



Plasma membrane

Plant Cell Structure- more info for understanding transport

  • The vacuole is a large organelle that can occupy as much as 90% of more of the protoplast’s volume

  • The vacuolar membrane (the tonoplast)

    • Regulates transport between the cytosol and the vacuole

Water travels to the root xylem by one of three pathways




Transmembrane route


The symplast is the

continuum of

cytosol connected

by plasmodesmata.

The apoplast is

the continuum

of cell walls and




Symplastic route

Apoplastic route

Transport routes between cells. At the tissue level, there are three passages:

the transmembrane, symplastic, and apoplastic routes.

Water travels to the root xylem by one of three pathways

Water and minerals can travel through a plant by one of three routes

  • Out of one cell, across a cell wall, and into another cell (transmembrane route)

  • Via the symplast (symplastic route)

  • Along the apoplast (apoplastic route)

Lateral transport of minerals and water in roots

Casparian strip

Endodermal cell

Pathway along






Uptake of soil solution by the

hydrophilic walls of root hairs

provides access to the apoplast.

Water and minerals can then

soak into the cortex along

this matrix of walls.

Casparian strip





Minerals and water that cross

the plasma membranes of root

hairs enter the symplast.







As soil solution moves along

the apoplast, some water and

minerals are transported into

the protoplasts of cells of the

epidermis and cortex and then

move inward via the symplast.







Vascular cylinder




Endodermal cells and also parenchyma cells within the

vascular cylinder discharge water and minerals into their

walls (apoplast). The xylem vessels transport the water

and minerals upward into the shoot system.

Within the transverse and radial walls of each endodermal cell is the

Casparian strip, a belt of waxy material (purple band) that blocks the

passage of water and dissolved minerals. Only minerals already in

the symplast or entering that pathway by crossing the plasma

membrane of an endodermal cell can detour around the Casparian

strip and pass into the vascular cylinder.

Lateral transport of minerals and water in roots

The endodermis

The Endodermis

  • Is the innermost layer of cells in the root cortex

  • Surrounds the vascular cylinder and functions as the last checkpoint for the selective passage of minerals from the cortex into the vascular tissue

  • Water can cross the cortex via the symplast or apoplast

  • The waxy Casparian strip of the endodermal wall blocks apoplastic transfer (but not symplastic) of water and minerals from the cortex to the vascular cylinder

  • Ascent of xylem sap

    Ascent of Xylem Sap

    • Plants lose an enormous amount of water through transpiration (the loss of water vapor through the stomata) and the transpired water must be replaced by water transported up from the roots

    • Xylem sap rises to heights of more than 100 m in the tallest plants

    Pulling xylem sap

    Pulling Xylem Sap

    The Transpiration-Cohesion-Tension Theory

    • Transpirational Pull

      • Water transport begins as water evaporates from the walls of the mesophyll cells inside the leaves and into the intercellular spaces

      • Driven by the

    Cohesion and adhesion in the ascent of xylem sap

    Cohesion and Adhesion in the Ascent of Xylem Sap

    • The transpirational pull on xylem sap:

    • Solar Powered

    • Bulk Flow (pressure differences created by water potential differences)

      Is transmitted all the way from the leaves to the root tips and even into the soil solution

      It is facilitated by the cohesion and adhesion properties of water

      Narrow diameter of xylem

    Cont d2


    • Transpiration produces negative pressure (tension) in the leaf which exerts a pulling force on water in the xylem, pulling water into the leaf

  • This water vapor escape through the stomata

  • Transport in plants

    • The Transpiration Dance


    • Transpiration animations


    Stomata and transpiration control

    20 µm

    Stomata and Transpiration Control

    • Stomata help regulate the rate of transpiration

    • Leaves generally have broad surface areas and high surface-to-volume ratios. Good and bad:

      •  increase photosynthesis;

      •  Increase water loss through stomata

    Check your nail polished spinach leaves

    Check your nail-polished spinach leaves

    • Tear your leaf as to produce a “lip” of dried nail polish

    • Peel off as large a section of the dried nail polish only

    • Microscopic observation reveals imprint of the organization of the leaf surface – specifically stomata (guard cell) arrangement

    Stomata cont d

    Cells turgid/Stoma open

    Cells flaccid/Stoma closed

    Radially oriented

    cellulose microfibrils




    Guard cell

    Stomata cont’d

    • About 90% of the water a plant loses escapes through stomata (lenticel, cuticle other 10%)

    • Each stoma is flanked by guard cells which control the diameter of the stoma by changing shape

    Guard Cells

    Shape changes due to multiple factors including





    Role of potassium in stomatal opening and closing.

    The transport of K+ (potassium ions, symbolized

    here as red dots) across the plasma membrane and

    vacuolar membrane causes the turgor changes of

    guard cells.








    Shape changes due to multiple factors including:

    • Changes in turgor pressure that open and close stomata result primarily from the reversible uptake and loss of potassium ions (K+) by the guard cells

    • Creates water potential differences

    Xerophyte adaptations that reduce transpiration


    Upper epidermal tissue

    Lower epidermal




    100 m


    Xerophyte Adaptations That Reduce Transpiration

    • Xerophytes are plants adapted to arid climates

      • They have various leaf modifications that reduce the rate of transpiration

    • The stomata of xerophytes

      • Are concentrated on the lower leaf surface

      • Are often located in depressions that shelter the pores from the dry wind

      • Possess thicker waxy cuticles

      • Sunken stomata

      • Trichomes (“hair)

    Stomata in recessed crypts of Oleander plant

    Second major plant tranport event translocation of phloem sap

    Second Major Plant Tranport Event -Translocation of Phloem Sap

    • Organic nutrients are translocated through the phloem (translocation is the transport of organic nutrients in the plant)

    • Phloem sap

      • Is an sucrose solution

      • Travels from a sugar source to a sugar sink

      • A sugar source is a plant organ that is a net producer of sugar, such as mature leaves

      • A sugar sink is an organ that is a net consumer or storer of sugar, such as a tuber or bulb or a leaf too!

    Translocation of phloem sap cont d the pressure flow hypothesis

    Translocation of phloem sap cont’d: The pressure-flow hypothesis

    Seasonal changes in translocation

    Seasonal Changes in Translocation

    • A storage organ such as a tuber or bulb may be a sugar sink in summer as it stockpiles carbohydrates.

    • After breaking dormancy in the spring the storage organ may become a source as its stored starch is broken down to sugar and carried away in phloem to the growing buds of the shoot system

    Phloem loading

    Phloem loading

    • Sugar from mesophyll leaf cells must be loaded into sieve-tube members before being exported to sinks

    • Depending upon the species, sugar moves by symplastic and apoplastic pathways

    In many plants phloem loading requires active transport.

    Proton pumping and cotransport of sucrose and H+ enable the cells to accumulate sucrose.

    Answers to first study break sesssion

    Answers to first study break sesssion

    • 1. After an H+ gradient is established (by pumping protons out of the cell) the resulting inward flow of H+ down its concentration gradient provides energy to actively transport other substances into the cell

    • 2. In symport, two substances move in the same direction through a cell membrane; in antiport two substances cross the cell membrane in opposite directions

    • Sample AP FR and Key

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