*Plant Transport

*O

verv

iew

* http://www.biology.ualberta.ca/facilities/multimedia/uploads/alberta/transport.html

*Overview…

*Water from the environment (lakes, rivers, soil) is actually a solution of dissolved substances, including nutrients. Will be referred to as “soil water”

*Sugars are exclusively carried by the phloem

*Nutrients in the soil water are carried by the xylem

*Xylem*Xylem- long tube, thickened walls (lignin),

can withstand low pressure without collapsing (cavitation)

*Formed from files of cells- arranged end to end

*Non-living at maturity

*The pressure inside the xylem is less than atmospheric pressure

*Water moves through xylem at low pressure causing water to be pulled upward due to transpiration- called transpiration-pull

EpidermisCortexPhloem VascularXylem BundleCambium

Pith

(see page 411 on DRAWING XYLEM VESSELS)

*Transpiration

*The 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.)

*Transpiration

*When 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.

*Transpiration

*The 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.

*Factors that affect

Transpiration

*Light – 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/measurin

g-transpiration.html

*Using a Potometer

*A device used to measure transpiration rates.

*Consists of:

*A leafy shoot in a tube

*A reservoir

*Graduated 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

XEROPHYTES

*Plants 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 Adaptations

*Reduced leaves – smaller surface area reduces transpiration

*Rolled Leaves – reduces stoma exposure to air and sun thus reduces transpiration

*Spines – decrease in surface area

*Xerophyte Adaptations

*Thickened 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 Adaptations

*Reduced number of stomata

*Sunken stomata in pits surrounded by hairs – the water vapour stays in the pit reducing the concentration gradient.

*Xerophyte Adaptations

*Hair like cell on leaf surface – trap a layer of water vapour maintaining a higher humidity

*Shedding 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 Conservation

*Halophytes

*Plants that live in saline soils (high salt concentrations)

*They require adaptations for water conservation (otherwise water loss will occur because of osmosis)

*Halophyte Adaptations

*Reduced leaves or spines

*Shedding 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 layers

*Sunken stomata

*Long roots to search for water

*Structures to remove salt build up.

*Movement of water…

*Water and nutrient transport involves three stages: (i) from the soil into the roots, (ii) from the roots to the stem, and (iii) from the stem to the leaves.

*Water also returns to the environment, mostly from the leaves (transpiration).

*Transport into the root…

*Water enters via osmosis (high to low water molecule concentration) –less water in cells than in soil or more solute in cells

*Nutrients enter via active transport (low to higher concentration)- more nutrients in plant cells than in the soil and the process requires energy

*The flow

*1. Water enters passively through osmosis and nutrients enter actively into the root hairs and epidermis cells

*Can travel between cell spaces (cytoplasm)- symplast

or through the cell wall apoplast

*2. They then diffuse into the cortex toward the endodermis through interconnecting cytoplasm between cells

*3. At the endodermis they encounter the Casparian strip. The key role of the Casparian strip is to prevent substances from leaking back into the cortex.

*Transport through the stem

to leaves…

*Once passed the Casparian strip, the nutrients and water form a liquid called xylem sap

*As more water enters, root pressure builds that helps push the sap up

*Capillary action, which is the tendency of a liquid in a narrow tube to rise or fall contributes to the rise of the xylem sap.

*The liquid has cohesion or attractive forces between molecules (cling together by H bonding) and adhesion to the sides of the wall. The water molecules in the xylem sap stick to each other are also drawn up the sides of the xylem tubes.

*Can move between xylem tubes to surrounding tissue from pits

*Phloem*Composed of sieve

tubes- which are made up of sieve tube cells

*Individual sieve tubes are separated by perforated walls called sieve plates

*Sieve tube cells are closely associated with companion cells

*Transport organic molecules (sugar/amino acids)- called translocation

*Phloem Sieve Tubes

*The sieve tubes are composed of columns of specialized cells

*Remember 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 Cells

*Sieve 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.

*Transport of sugars…

*Called translocation

*Source: a plant cell with a high concentration of sugars and other solutes, such as a leaf cell

*Sink: a plant cell with a low concentration of sugars; sugars may be converted to starch for storage or used rapidly for energy or as building blocks of other carbohydrates

*Sugars can move up or down

*Source and sinks may change upon season; i.e. leaf growth becomes a sink in the spring and root and stem cells are sources.

*Examples

Sources Sinks

Mature photosynthesizing leaves

Growing roots

Green stems Developing seeds/fruits

Storage tissues in germinating seeds

Growing leaves/developing roots or tubers

*Phloem Loading*Ex: 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 Hypothesis

*Source= leaves Sink = roots

*Remember: 1)materials move from source to sink

2) molecules move from high pressure to low pressure

*Pressure Flow Hypothesis

1. 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 Hypothesis

4. 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.

*Active transport for phloem

loading

1) Proton pump uses energy from ATP to pump H+ out of the cell.2) Higher [ H+] outside the cell than inside creating a negative charge inside the cell and an ELECTROCHEMICAL GRADIENT.3) Now the sugars can move into the cell via diffusion.

*Translocation

*http://highered.mheducation.com/sites/9834092339/student_view0/chapter38/animation_-_phloem_loading.html

*Identifying Xylem and Phloem

*Clues:

*Xylem larger than phloem

*Within one vascular bundle, phloem cells are closer to the outside of the plant in stems and roots.

*See page 420-421