9.2 Transport in Angiospermophytes

• 9.2.1

Outline how the root system provides a large surface area for mineral ion and water uptake by means of branching and root hairs.

• Roots have tiny root hairs on them, which increase the surface area and allow maximum uptake of water.

• Mineral ions are taken in the root hairs by active transport.

• Branching allows the roots to cover a large amount of area to get a variety of nutrients and more water.

• The cortex cell walls allow for osmosis to occur because they are permeable.

• 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

http://plantandsoil.unl.edu/croptechnology2005/plant_phys/?what=animationList&informationModuleId=1057703469

• Mineral movement– active transport: into root hairs. – passive transport (diffusion) through symplast

to endodermis. – active transport across endodermal cell layer.– passive transport (diffusion) into xylem. – Fungal hyphae/mutualism– Mass flow of water

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

lignified xylem.

Cellulose

• The plant cell wall is made up of cellulose and protein, and, in many cases, lignin.

• It is very firm and prevents any sudden expansion of cell volume, and, without contribution of auxins, any expansion at all.

Turgor pressure

• pressure of the cell contents against the cell wall

• determined by the water content of the vacuole resulting from osmotic pressure

• Turgid plant cells contain more water than flaccid cells and exert a greater osmotic pressure on its cell walls.

Lignified xylem

• Lignified xylem conduits provide structural support to the plant body

• Lignin has several functions. One of them is to prevent implosion of vessels and tracheids that are holding water under negative pressure. 

• 9.2.5 Define transpiration.

http://www.phschool.com/science/biology_place/labbench/lab9/transpull.html

• Transpiration is the evaporation of water from the aerial parts of plants, especially leaves but also stems, flowers and roots.

• Occurs through stomata, and can be thought of as a necessary "cost" associated with the opening of stomata to allow the diffusion of carbon dioxide gas from the air for photosynthesis.

• Transpiration also cools plants and enables mass flow of mineral nutrients from roots to shoots

• 9.2.6

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

• Water movement– In xylem– Passive transport– Evaporation and transpiration cause low pressure

which allows particles to be “pulled” in– Travel with transpiration stream from roots to leaves– Cohesion (result of hydrogen bonding) – Apoplastic pathway through cell walls

(Apoplast: free diffusional space outside the plasma membrane)

http://www.phschool.com/science/biology_place/labbench/lab9/xylem.html

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

http://www.phschool.com/science/biology_place/labbench/lab9/guard.html

Guard Cell FunctionStomata closing

1.Potassium ions move out of the vacuole and out of the cells.2.Water moves out of the vacuoles, following potassium ions.3.The guard cells shrink in size.4.The stoma closes.

Stomata opening 1.Potassium ions move into the vacuoles.2.Water moves into the vacuoles, following potassium ions.3.The guard cells expand.4.The stoma opens.

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

• Abscisic acid (ABA) is the hormone that triggers closing of the stomata when soil water is insufficient to keep up with transpiration.

HOW?• ABA binds to G-protein-coupled receptors at the surface of the

plasma membrane of the guard cells. • The receptors activate several interconnecting pathways which

converge to produce a rise in pH in the cytosol • transfer of Ca 2+ from the vacuole to the cytosol• The increased Ca 2+ in the cytosol blocks the uptake of K+ into the

guard cell while the increased pH stimulates the loss of Cl− and organic ions (e.g., malate2−) from the cell.

• The loss of these solutes in the cytosol reduces the osmotic pressure of the cell and thus turgor

• The stomata close

• 9.2.9

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

• If stomata open, transpiration increases, and vice versa.• Light effects blue-light receptors in the leaves that open stomata by

creating a potassium gradient and causing the guard cells to absorb water.

• Hot temperatures cause stomata to close. • Wind causes the water vapor that is emitted from the stomata to

travel very quickly, causing the air near the leaves to be dryer than it would be without wind. This causes a greater difference between water concentration between the stomatal air and the outside air, causing more transpiration.

• Humidity does the opposite. Because there is less difference between inner and outer air, the water does not travel out of the leaf as much, causing transpiration to decrease.

• 9.2.10

Outline four adaptations of xerophytes that help to reduce transpiration.

• Waxy stomata• Few stomata• Sunken stomata• Stomata open at night• CAM and C4• Larger hairs on

leaves• Curled/Rolled leaves• Reduced leaves

• Spines• Deep Roots• Reduced # of stomata• Low growth

• 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).

• The Pressure-Flow Hypothesis• This is essentially a source and sink hypothesis.

• Phloem is a living tissue with food-conducting cells arranged into tubes that distribute sugar, amino acids, and other organic nutrients throughout the plant.

• This tissue transports food made in the leaves to the roots and to nonphotosynthetic parts of the shoot system, from source to sink.

• Proton pumps do the work that enables the cells to accumulate sucrose. The ATP-driven pumps move H+ concentration across the plasma membrane.

• Another membrane protein uses this energy source to cotransport sucrose in the cell along wih returning hydrogen ions.

• In some plants, the sucrose may be unloaded from phloem by active transport. In other species, diffusion is sufficient to move sucrose from phloem to the surrounding cells of the sink organ.