Transport in Angiospermatophytes

Topic 9.2

Assessment Statements9.2.1 Outline how the root system provides a large surface area for mineral ion and water

uptake by means of branching and root hairs.

9.2.2 List ways in which mineral ions in the soil move to the root.

9.2.3 Explain the process of mineral ion absorption from the soil into roots by active transport.

9.2.4 State that terrestrial plants support themselves by means of thickened cellulose, cell turgor and lignified xylem.

9.2.5 Define transpiration.

9.2.6 Explain how water is carried by the transpiration stream, including the structure of xylem vessels, transpiration pull, cohesion, adhesion and evaporation.

9.2.7 State that guard cells can regulate transpiration by opening and closing stomata.

9.2.8 State that the plant hormone abscisic acid causes the closing of stomata.

9.2.9 Explain how the abiotic factors light, temperature, wind and humidity, affect the rate of transpiration in a typical terrestrial plant.

9.2.10 Outline four adaptations of xerophytes that help to reduce transpiration.

9.2.11 Outline the role of phloem in active translocation of sugars (sucrose) and amino acids from source (photosynthetic tissue and storage organs) to sink (fruits, seeds, roots).

Roots and angiosperm transport

• Main function of roots is to provide mineral ion and water uptake for the plant

• Efficiency due to extensive branching pattern and root hairs

• Root hairs increase surface area by a factor of 3

• Root cap protects apical meristem during primary growth

Root Zones

• Zone of cell division – new undifferentiated cells are forming, M phase of the cell cycle

• Zone of elongation – cells are enlarging in size, corresponds to G1 of the cell cycle

• Zone of maturation – cells are becoming functional to the plant

Water

• Epidermis → cortex → vascular cylinder

• Vascular cylinder surrounded endodermis and pericycle

• Endodermis – cylindrical layer of cells that separates the cortex from the vascular tissue

epidermis

cortex

xylem

phloemendodermis

pericycle

• Pericycle – layer of cells that can become meristematic; produces the lateral or branching roots of a plant

• Why must lateral or branching roots originate from this region?

• To allow their connection to the vascular cylinder

epidermis

cortex

xylem

phloemendodermis

pericycle

Water enters a plant through root hairs by osmosis

Symplastic route

• water moves from cell to cell

Apoplastic route

• water moves through the cell walls and the extracellular spaces

Major processes that allow mineral ions to pass from the soil to the root

• Diffusion of mineral ions and mass flow of water in the soil carrying these ions• Occurs when there is a higher concentration of

mineral outside the root than inside• Minerals dissolved in and move via water

• Aid provided by fungal hyphae• Filaments form a cover over the surface of young

roots• Larger surface area is created for water and mineral

ion absorption• Mutualisitic relationship referred to as mycorrhiza

• Active transport• Energy must be expended when the ion the plant

needs cannot cross the lipid bilayer of the membranes

• Ions must therefore pass through a transport protein in the membrane

• Transport proteins are specific for certain ions• They bind to the ion on one side of the membrane

and then release it on the other side

How the proton pump works

1. Proton pump uses energy from ATP to pump hydrogen ions out of the cell

2. Result is higher hydrogen ion concentration outside the cell than inside. This creates a negative charge inside the cell.

3. This gradient results in the diffusion of hydrogen ions back into the cell.

4. The voltage difference is called a membrane potential.

5. The hydrogen ion gradient and the membrane potential represent forms of potential energy that can be used to absorb mineral ions.

(Form of chemiosmosis)

Support in terrestrial plants

Support provided by

• Thickened cellulose• Occurs in cell walls

• Cell turgor pressure• Pressure inside the cell that is exerted on the cell wall by

the plasma membrane due to water that has entered the cell wall by osmosis

• Important in keeping plants upright• If the soil around a plant dries or gets too salty, water may

no longer move into the plant, water content decreases, and the plant wilts

• Lignified xylem• Rings of highly branched polymer

Transpiration

• Loss of water vapour from leaves and other aerial parts of the plant

• Water lost through openings called stomata which are opened and closed due to guard cells

• Transpired water has to be replaced from the roots to the upper parts of the plant by absorption

• Column of water provides minerals and the water needed for photosynthesis

• Water lost cools sun-drenched leaves and stems

Factors affecting transpirationEnvironmental factor Effect

Light Speeds up transpiration by warming the leaf and opening stomata

Humidity Decreasing humidity increases transpiration because of the greater difference in water concentration

Wind Increases the rate of transpiration because humid air near the stomata is carried away

Temperature Increasing temperature cause greater transpiration because more water evaporates

Soil water If the intake of water at the roots does not keep up with transpiration, turgor loss occurs and the stomata close – this decreases transpiration

Carbon dioxide High carbon dioxide levels in the air around the plant usually cause the guard cells to lose turgor and the stomata to close

• Increase light → increase photosynthesis → carbon dioxide used up → increase pH of guard cells →

stimulates conversion of starch to glucose → influx of water into guard cells by osmosis → raises turgor

pressure opening stomata

• So carbon dioxide increase in air, lowers pH, glucose is converted to starch, water leaves guard cells, turgor

pressure is lowered, and stomata close

Xerophyte (plants adapted to arid climates)Modifications to decrease transpirational water loss• Small, thick leaves reduce water loss by decreasing

surface area• Reduced number of stomata decreases the openings

through which water loss may occur• Thickened, waxy cuticle• Hair-like cells on the leaf surface trap a layer of water

vapour, thus maintaining a higher humidity near the stomata

• Shed their leaves in the driest months• Water stored in fleshy stems• Alternative photosynthetic processes

CAM photosynthesis

• Close stomata during the day

• Incorporate carbon dioxide during the night

C4 photosynthesis

• Have stomata open during the day

• Take in carbon dioxide more rapidly than non-specialised plants

Stomata and guard cells

• Closing of stomata is only possible on a short-term basis

• Why?

• Carbon dioxide must enter the mesophyll so that photosynthesis can occur

• Stomata open and close due to changes in the turgor pressure of the guard cells that surround them

• Gain and loss of water in the guard cells is largely due to the transport of potassium ions

• Light triggers the activity of ATP-powered proton pumps in the plasma membrane of the guard cells

• Potassium moves into cell• Higher solute, water follows by

osmosis• When potassium leaves, so

does water• The plant hormone abscisic

acid causes potassium ions to rapidly diffuse out of the guard cells

• The result is stomatal closure• Hormone is produced in the

roots during times of water deficiency

Transport of water and mineral transport by xylem

• Complex tissue composed of:• Tracheids• Vessel elements

• Tracheids• Dead cells that taper at

the ends and connect to one another to form a continuous column

• Ancient plants only had tracheids

• Vessel elements• Most important• Dead cells that have thick,

lignified secondary walls interrupted by areas of primary wall

• Have pits or pores that allow water to move laterally

• Attached end to end• Have perforations allowing

water to move up• Most modern flowering

plants only have vessel elements

How water is carried by the transpiration stream1. Water moves down concentration gradient from high

concentration of water vapor within leaf to the atmosphere which has a lower water vapor concentration.

2. Water lost by transpiration is replaced by water from the vessels.

3. Vessel water column is maintained due to cohesion (water molecules held together with strong hydrogen bonds) and adhesion (water molecules held to sides of the vessels with hydrogen bonds) counteracting gravity.

4. Water is pulled from the root cortex into xylem cells, again due to cohesion and adhesion.

5. Water is pulled from the soil into the roots due to the tension created by transpiration.

The movement of organic molecules in plants

• Organic molecules move via the phloem in plants• Made up of living cells

• Sieve tube members• Companion cells

• Sieve tube members are connected to one another by sieve plates to form sieve tubes

• Sieve plates have pores that allow the movement of water and dissolved organic molecules throughout the plant

• Companion cells are connected to their sieve tube members by plasmodesmata

• Phloem cells transport their contents in various directions

• Direction of movement is from a source to a sink• Source: a plant organ that is a net producer of sugar

either by photosynthesis or by the hydrolysis of starch• Primary source are leaves

• Sink: a plant organ that uses or stores sugar• Ex. Roots, buds, stems, seeds and fruits

• Some structures may be both a source and a sink, such as tubers

Translocation

• The movement of organic molecules in plants • Organic molecules are dissolved in water and the

solution is known as phloem sap• Mostly sugars (sucrose most common)• Amino acids• Plant hormones• mRNA

Pressure-flow hypothesis

1. Loading of sugar into the sieve tube at the source (accomplished by active transport). This reduces the relative water concentration in the sieve tube members causing osmosis from the surrounding cells.

2. The uptake of water causes a positive pressure in the sieve tube that results in a flow of the phloem sap.

3. This pressure is diminished by the removal of the sugar from the sieve tube at the sink. The sugars are changed at the sink to starch. Starch is insoluble and exerts no osmotic effect.

4. Xylem recycles the relatively pure water by carrying it from the sink back to the source.