9.1 Transport in the Xylem of Plants

An Brief Intro to PlantsAll living organisms require chemical energy

(ATP) to run the various chemical reactions that sustain life.

In the process of cellular respiration, organisms convert simple sugars (i.e. glucose) into that chemical energy.

Animals such as ourselves obtain sugars from the food we eat.

But how do plants obtain the sugars required for cellular respiration to sustain life?

Photosynthesis!

The chemical process in which plants make sugar using light energy, water and carbon dioxide, making oxygen as a side product.

The sugar made during photosynthesis can then be used for cellular respiration.

Equation for Photosynthesis:

6 CO2 + 6 H2O + Light Energy → C6H12O6 + 6O2

From the atmosphere

From the surrounding environment

From the Sun

A variety of simple sugars may be formed, though glucose (C6H12O6) is one of the most common.

If any of the reactants are lacking or are limiting, photosynthesis may not occur and the plant may die.

The LeafThe majority of photosynthesis in a plant

occurs in the leaf.

Leaves are specialized for photosynthesis. They regulate the flow of gases and capture light energy for photosynthesis.

The structure and arrangement of leaves maximize the surface area exposed to sunlight and limits the distance gases need to travel.

Maple leaves are thin and broad with a large surface area

Pine leaves are thin and narrow. A single needle does not provide a sizable surface area, but a branch of needles do.

3-D Cross-Section of a Leaf

Protects the leaf from excessive absorption of light and evaporation of water

A transparent colourless layer that allows light to pass through to the mesophyll cells

Where most of the photosynthesis

takes place (abundant in

chloroplasts).

A system of vessels that transport water, minerals, and carbohydrates within the plant.

Photosynthetic epidermal cells that create microscopic openings called stomata.

Regulates the exchange of gases in the atmosphere

Obtaining the Materials for PhotosynthesisLight is captured by the leaves – specifically by the

chloroplasts of the mesophyll cells. (We will discuss this further in our photosynthesis unit next year).

Gas exchange happens via the stomata (stoma = singular form), which are small pores in the lower epidermis of the leaf

The stomata allows CO2 into the plant, and O2 out.

Water is absorbed through the roots, not the leaves.

StomataEach stoma is surrounded by a pair of guard

cells that control the control the size of a stoma by changing their shape in response to water movement by osmosis in the cells.

When water moves into guard cells, the cells become turgid (swollen) and the stoma opens.

When water move out of the guard cells, the guard cells become flaccid (limp), and the stoma closes.

O2CO

2

Cell Turgor Pressurethe pressure inside the cell that is exerted on the cell wall by the plasma membranecreated by water entering the cell via osmosis

Stomata OpeningIn general, stomata are open in the daytime

and closed at night.

When the Sun comes out in the morning, it activates receptors in the guard cell membranes, stimulating proton pumps that pump H+ out of the guard cells.

K+ move into the cells, followed by water (via osmosis)

Stomata ClosingHormone absicis acid (ABA) causes the

stomata to close.

Also, changes the particles in the guard cells of the stomata will cause the guard cells to lose water and become flaccid, closing the stomata.

H+ are pumped out of guard cellsK+ diffuses into guard cells H2O diffuse into cells by osmosisGuard cells swell and open CO2 enters stoma

RootsMain function

is mineral ion and water uptake for the plant.

RootsRoot hairs increase the surface area over

which water and mineral ions may be absorbed.

The Root cap is important in protecting the apical meristem during primary growth of the root through the soil.

How do mineral ions and water move into the root?WATER- must pass through the epidermis

and cortex to get to the vascular tissue.

Water moves into the root hairs via osmosis.

There is a higher solute concentration and a lower water concentration than the surrounding soil.

How do mineral ions and water move into the root?

IONS (i.e. nitrates, ammonium, potassium, phosphates, calcium) enter through:

DiffusionFungal HyphaeActive Transport

DIFFUSION – when the concentration of minerals is higher in the soil than in the root. They dissolve in water and then move into the root.

May also come in with water during MASS FLOW in which the plant takes in large volumes of water.

FUNGAL HYPHAE – some plant species have developed a symbiotic relationship (mutualism) with fungus to help absorb minerals.

They can grow into the plant roots and transport minerals to the roots that the plant cannot absorb without it. Also creates a larger surface area for absorption

ACTIVE TRANSPORTUsed when the concentration of minerals is higher

inside the root than outside.Requires energy and protein pumps, specific to

certain mineral ions.Mineral ions can only be absorbed by active

transport if they make contact with the appropriate protein pump

Proton pump uses energy from ATP to pump H+ out of the cell.

Higher [ H+] outside the cell than inside creating a negative charge inside the cell and an ELECTROCHEMICAL GRADIENT.

Now the positive ions can move into the cell via diffusion.

Water transport in the PlantOnce in the plant, water is transported by the

vascular tissue known as xylem.

The other type of vascular tissue is the phloem

XylemLong continuous hollow tubes.Made of dead cells, responsible for transporting

water.Water flows in one direction (up!)Reinforced by lignin.

Lignin is a highly branched polymer that strengthens the walls so they can withstand low pressure without collapsing

(Pressure in the xylem is usually much lower than in the atmosphere)

Xylem

Cross Section of a Stem

(see page 411 on DRAWING XYLEM VESSELS)

EpidermisCortexPhloem VascularXylem BundleCambium

Pith

TranspirationThe loss of water vapour from leaves through

the stomata. Often leaves are exposed to direct sunlight. They have a large surface area to capture

light for photosynthesis but also creates a large surface for water to be evaporated out.

(A medium sized tree can evaporate +1000L on a hot, dry day.)

TranspirationWhen water evaporates from the surface of

the wall in a leaf, adhesion causes water to be drawn through the cell wall from the nearest available supply to replace the lost water.

The nearest available water supply is the xylem vessels in the veins of the leaf.

TranspirationThe water that is lost by transpiration is replaced

by the intake of water in the roots.

TRANSPIRATION PULL is a continuous stream of water against gravity from the roots to the upper parts of the plant, aided by cohesion and adhesion.

COHESION: H bonds between water molecules

ADHESION: H bonds between water molecules and the sides of the vessels – it counter acts gravity.

Mineral Uptake (long, detailed)http://glencoe.mheducation.com/olcweb/cgi/pluginpop.cgi?it=swf::640::480::/sites/dl/free/0003292010/811349/Mineral_Uptake.swf::Mineral Uptake

Water Uptake (long, detailed) http://glencoe.mheducation.com/olcweb/cgi/pluginpop.cgi?it=swf::640::480::/sites/dl/free/0003292010/811349/Water_Uptake.swf::Water Uptake

Transpiration http://www.youtube.com/watch?v=mc9gUm1mMzc

Factors that affect TranspirationLight – warm leaf and open stomata

Humidity- decrease in humidity increases transpiration

Wind – increases rate – because humid air near the stomata is carried away

Temperature – increases – because more evaporation

Soil water – if intake of water by the roots does not keep up with transpiration, cells lose turgor pressure and stomata close.

Carbon Dioxide – high levels around the plant cause guard cells to lose turgor and the stomata close.

Using a Potometer

http://www.passmyexams.co.uk/GCSE/biology/measuring-transpiration.html

Using a PotometerA device used to

measure transpiration rates.

Consists of:A leafy shoot in a

tubeA reservoirGraduated capillary

tube with a bubble marking zero

As the plant takes up water, the bubble will move along the capillary tube

Time to move along the tube can be measure

Adaptations for Water Conservation XEROPHYTESPlants that can tolerate dry conditions (such as deserts)

Adapted to increase rate of water uptake and reduce water loss

Less competition in these environments

Xerophyte AdaptationsReduced leaves – smaller surface area

reduces transpirationRolled Leaves – reduces stoma exposure to

air and sun thus reduces transpirationSpines – decrease in surface area

Xerophyte AdaptationsThickened waxy cuticle

– less water can escape

Low growth form – closer to the ground and thus less wind exposure

Fleshy stems – with water stored from rainy seasons

Xerophyte AdaptationsReduced number of stomata Sunken stomata in pits surrounded by hairs

– the water vapour stays in the pit reducing the concentration gradient.

Xerophyte AdaptationsHair like cell on leaf surface – trap a layer of

water vapour maintaining a higher humidityShedding leaves in driest months

CAM photosynthesis – stomata are open at night when it is cooler so less water loss.

C4 photosynthesis

- involves a specialized leaf

structure to maximize

photosynthesis

Adaptations for Water ConservationHalophytesPlants that live in saline soils (high salt

concentrations)They require adaptations for water

conservation (otherwise water loss will occur because of osmosis)

Halophyte AdaptationsReduced leaves or spinesShedding of leaves when water is scarce (and

then stem takes over photosynthesis)Water storage structures in leaves (away

from saline root environment)Thick cuticle; multiple epidermal layersSunken stomataLong roots to search for waterStructures to remove salt build up.

9.2 Transport in the Phloem of Plants

Phloemvessel transporting “food” or organic material (i.e.

sucrose, amino acids) via TRANSLOCATION

Materials can move in either direction in the phloem

Phloem tissue is found throughout the plant (stem, roots, leaves)

It is composed of sieve tubes which are sieve tube cells separated by perforated walls called sieve plates

Sieve tube cells are closely associate with companion cells

Phloem Sieve TubesThe sieve tubes are composed of columns of

specialized cellsRemember the cells that make up the xylem

are dead.These cells are living (though no nucleus)

because they need to be able to undergo active transport to transport materials in and out of the phloem

The sieve plates are remnants of cells walls that separated the adjacent sieve tube cells

Phloem Sieve Tube CellsSieve tube cells are closely associated with

companion cells. (They are daughter cells from a mitotic division of one same parent cell)

The companion cell performs many of the genetic and metabolic functions to support the sieve tube cell.

They are abundant in mitochondria for this purpose.

Plasmodesmata connect companion cells with sieve tube cells.

Source and SinkSugars are made in photosynthetic organs (the

leaves) and stored in the root.“source” – where food is made or stored

Made: Green leaves, stems, Stored: seeds, roots

“sink” – where food in usedDeveloping fruits, developing seeds, growing

leaves, developing roots

Organic material moves through the phloem from source to sink

Phloem Loading

Phloem LoadingEx: Sugar is made in the leaves during

photosynthesis. However, it is required throughout the plant for cellular respiration. In many plants, excess sugar is stored in the roots as longer carbohydrates.

How is sugar made in the leaves moved to the roots?Answer: Translocation via the phloem – using the

Pressure Flow HypothesisSource= leaves Sink = roots Remember: 1)materials move from source to sink 2) molecules move from high pressure to low

pressure

Pressure Flow Hypothesis1. At the source, sugar is brought into the phloem

by active transport

2. Water follows, moving into the phloem (from the adjacent xylem) via osmosis (remember H2O follows solutes) to produce sap

High pressure created in this area of the phloem

3. The sap will be pushed to a lower pressure area, a sink

Pressure Flow Hypothesis4. At the sink, the presence of sap now creates

a high pressure situation. Phloem cells move the sugar out.

5. Water will also move out of phloem following osmotic gradient (H2O will move back into xylem)

Low pressure recreated in the sink, resulting in

more sap flowing to the area.

Later in the life of the plant, the plant may require this stored sugar from the roots, for example to grow a fruit.

In this new scenario, now the roots will be the source and the developing fruit would be the sink and the sap would move against gravity up the stem.

Translocation http://highered.mheducation.com/sites/

9834092339/student_view0/chapter38/animation_-_phloem_loading.html

Identifying Xylem and PhloemClues:

Xylem larger than phloemWithin one vascular bundle, phloem cells are

closer to the outside of the plant in stems and roots.

See page 420-421

Cross section of a stem.

Vascular bundles are the coloured clusters

Larger openings xylem, smaller phloem

root of a buttercup (Ranunculus)

HomeworkRead Sections 9.1 and 9.2

Read “Experiments using aphid stylets” on page 417 and do DB Q on page 418

Read “Radioisotopes as important tools in studying translocation” on page 419 and do DB Q on same page