Transport in Plants

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

• 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:

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

Activity1• Clear nail polish• Leaves

• Activity 2• Flaccid carrot and cucumber slices• Bowl• dH2O, bottled water, tap water• Salt

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

• 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

• 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 PumpsProton pumps create a hydrogen ion gradient that is a form of potential energy that can be harnessed to do workThey contribute to a voltage known as a membrane potential (Plant cytoplasm is (-) compared to extracellular fluid)

Consequences include:Fac diffusion of other cationsCotransport: symport and antiport (secondary active transport)

CYTOPLASM EXTRACELLULAR FLUID

ATP

H+

H+ H+

H+

H+

H+

H+

H+

Proton pump generates membrane potentialand H+ gradient.

–

–

–

–

– +

+

+

+

+

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)

+CYTOPLASM EXTRACELLULAR FLUID

Cations ( , for example) are driven into the cell by themembrane potential.

Transport protein

K+

K+

K+

K+

K+ K+

K+

K+

–

–

– +

+

(a) Membrane potential and cation uptake

–

–

+

+

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

H+

H+

H+

H+

H+

H+

H+H+

H+

H+

H+

H+

NO3–

NO 3 –

NO3

–

NO 3

–

NO3

–

NO3 –

–

–

– +

+

+

–

–

– +

+

+

NO3–

(b) Cotransport of anions

H+of through a

cotransporter.

Cell accumulates anions ( , for example) by coupling their transport to theinward diffusion

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

•

H+

H+

H+

H+

H+H+

H+

H+ H+

H+

SS

S

S

SPlant cells canalso accumulate a neutral solute,such as sucrose

( ), bycotransporting

down the

steep protongradient.

S

H+

–

–

–

+

+

+

–

–

++–

H+ H+S+

–(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

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

Which has the greatest water concentration?

• A or B A or B• Water potential is essentially not much different

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

By convention, plant physiologists measure water potential in units of pressure called megapascals (MPa). Note: bars is acceptable

For a baseline, the water potential for pure water at 1ATM is expressed as having 0 Mpa or 0 bars

Breaking it down….

Cont’d

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

• An artificial model

Cont’d: Addition of Solute example

Cont’d – Positive Pressure Example

Cont’d: A negative pressure example

Connection to plants:

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

• 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 )

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

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• 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

• 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 solutionRoot Hair EpidermisRoot Cortex Root Xylem

Cont’d• 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

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

Transport proteins inthe plasma membrane

regulate traffic ofmolecules betweenthe cytosol and the

cell wall.

Transport proteins inthe vacuolarmembrane regulatetraffic of moleculesbetween the cytosoland the vacuole.

Plasmodesma

Vacuolar membrane(tonoplast)

Plasma membrane

Cell wall

Cytosol

Vacuole

Cell compartments. The cell wall, cytosol, and vacuole are the three maincompartments of most mature plant cells.

(a)

Water travels to the root xylem by one of three pathways

Key

Symplast

Apoplast

The symplast is thecontinuum of

cytosol connectedby plasmodesmata.

The apoplast isthe continuumof cell walls andextracellularspaces.

Apoplast

Transmembrane route

Symplastic routeApoplastic route

Symplast

Transport routes between cells. At the tissue level, there are three passages: the transmembrane, symplastic, and apoplastic routes.

Water and minerals can travel through a plant by one of three routes1. Out of one cell, across a cell wall, and into another cell (transmembrane

route)2. Via the symplast (symplastic route)3. Along the apoplast (apoplastic route)

Lateral transport of minerals and water in roots

1

2

3

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.

Minerals and water that crossthe plasma membranes of roothairs enter the symplast.

As soil solution moves alongthe apoplast, some water andminerals are transported intothe protoplasts of cells of theepidermis and cortex and thenmove inward via the symplast.

Within the transverse and radial walls of each endodermal cell is the Casparian strip, a belt of waxy material (purple band) that blocks thepassage 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.

Endodermal cells and also parenchyma cells within thevascular cylinder discharge water and minerals into theirwalls (apoplast). The xylem vessels transport the waterand minerals upward into the shoot system.

Casparian strip

Pathway alongapoplast

Pathwaythroughsymplast

Plasmamembrane

Apoplasticroute

Symplasticroute

Root hair

Epidermis Cortex Endodermis Vascular cylinder

Vessels(xylem)

Casparian strip

Endodermal cell

4 5

2

1

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•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 SapThe 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

• 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 solutionIt is facilitated by the cohesion and adhesion properties of waterNarrow diameter of xylem

Cont’d• 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

• The Transpiration Dance

and

• Transpiration animations

• https://www.youtube.com/watch?v=U4rzLhz4HHk

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

20 µm

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• 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

Cells flaccid/Stoma closedCells turgid/Stoma open

Radially oriented cellulose microfibrils

Cellwall

Vacuole

Guard cell

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

H2O

H2O

H2OH2O

H2O

K+

Role of potassium in stomatal opening and closing. The transport of K+ (potassium ions, symbolized here as red dots) across the plasma membrane andvacuolar membrane causes the turgor changes of guard cells.

H2O H2O

H2O

H2O

H2O

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)

Lower epidermaltissue

Trichomes(“hairs”)

Cuticle Upper epidermal tissue

Stomata 100 m

Stomata in recessed crypts of Oleander plant

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

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• 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• 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